WO2015088368A1

WO2015088368A1 - Hydrophilic thermo-switchable pressure-sensitive adhesive composition

Info

Publication number: WO2015088368A1
Application number: PCT/RU2013/001107
Authority: WO
Inventors: Mikhail Maiorovich FELDSTEIN
Original assignee: Limited Liability Company "Smart Adhesives"
Priority date: 2013-12-09
Filing date: 2013-12-11
Publication date: 2015-06-18
Also published as: RU2585787C2; RU2013154323A

Abstract

A hydrophilic thermoswitchable pressure- sensitive adhesive composition comprising an adhesive that is capable of reversibly detaching in aqueous media under temperature elevation and has an optimized adhesive ability is claimed. The composition consists of a hydrophilic film-forming polymer possessing Lower Critical Solution Temperature (LCST); and a complementary short-chain non-covalent oligomeric crosslinker of the film-forming polymer; wherein the hydrophilic polymer and the oligomeric crosslinker are capable of forming the hydrogen bond with each other. In spite of the fact that components of an adhesive mixture do not show the adhesion in an unmixed state, the mixture of these components has adhesiveness.

Description

HYDROPH I LIC THERMO-SWITCHABLE PRESSURE-SENSITIVE

ADHESIVE COMPOSITION

TECHNICAL FIELD

This invention relates generally to intelligent adhesive compositions, and more particularly relates to water-absorbent adhesive compositions composed of the blends of polymers, possessing Lower Critical Solution Temperatures (LCTS), which disadhere from a substrate (or contact surface) at temperature elevation above the LCTS or cloud point. The invention additionally relates to methods for formulating such compositions, including methods for their preparation and tuning the adhesion and thermo-responsive behavior, to methods for using the compositions, and to products manufactured with the compositions. The invention can be useful in any area requiring an adhesive composition that adheres to a substrate surface at ambient and low temperatures and different relative humidity of surrounding medium, including in water, and loses tack reversibly upon temperature elevation above the cloud point.

BACKGROUND

Hydrophilic adhesives, particularly hydrophilic pressure-sensitive adhesives ("PSAs"), are used in a wide variety of commercially significant products particularly designed for medical applications, including drug delivery systems, wound dressings, materials for attachment of bioelectrodes, tooth-whitening systems, and the like. A general distinctive feature of hydrophilic PSAs is that they typically adhere to wet substrates, while conventional hydrophobic (rubber- based) PSAs typically lose their adhesive capability when moistened.

A difficult challenge that arises in designing a skin contact pressure sensitive adhesive (PSA) for the medical industry is to balance the needs for achieving high bond strength to skin along with the ease of removal from skin. A medical adhesive that could be optimized for performance on skin, but could be subsequently removed without the potential for skin damage and adhesive trauma, would be highly desirable.

Over the last decade, a number of companies have developed so-called "intelligent" or stimuli-responsive PSA compositions which are capable of being spontaneously detached and, thus, more easily removed from substrates such as skin. Skin friendly removable adhesives which do not cause trauma on removal from delicate wounds have long been sought after as a means to improve patient comfort. [1]

Various means of PSA's detaching have been offered, including UV curing, application of heat or cold or by the use of water dispersible components which can be removed by immersing the adhesive in water [2] . A US based corporation has produced PSA compositions based on side chain liquid crystal technology. This relies on a crystallisable side chain which results in detaching on cooling below its melting point [3]. The melting point of the side chain, and hence the detaching temperature, can be tailored to suit the application. Similar approaches have been described in [4-6].

Yarusso and Hyde of 3M Company have developed an interesting kind of a "smart" PSA based on partially oriented and partially crystallized elastomer, which imparts anisotropic peel behavior to transdermal patch or adhesive tape [7]. The anisotropic peel force is an unusual property because the force necessary to peel the PSA article from a surface to which it is adhered varies along different axes. The peel force measured in the direction parallel to the preferred orientation occurred to be substantially less than that observed for a PSA of the same formulation but whose elastomer component is not oriented. The peel force measured in the direction perpendicular to the preferred orientation is substantially greater than that measured in the parallel direction.

Smith and Nephew in the UK have developed a novel light curing PSA composition which can be removed cleanly from the skin [8-12]. The photoinitiator is built into the polymer chain to make it safe for skin contact. The tape can be removed by removing an opaque liner and exposing to ambient light.

A number of water detachable two-phase compositions based on hydrophobic PSAs have also been developed which rely on at least one dispersed water soluble or swellable component (polymer filler, absorbent of water, tackifier or plasticiser) [1,2,13]. Upon exposure to liquid water and swelling of hydrophilic dispersed phase, the phase separation occurs that inhibits the adhesion [13]. Another type of wetting-sensitive PSA composition has been described by Stewart in [4].

Radically different type of wetting-responsive PSAs is recently developed in our laboratory [14-16]. This is ternary noncovalently crosslinked hydrogen-bonded complex of a film forming polymer with a telechelic oligomer, bearing complementary functional groups at the both ends of short chains, and another complementary polymer, containing reactive groups in monomer units of backbone. In dry state, the material reveals no tack to dry substrates, but adheres aggressively to moistened surfaces. Such interpolymer complex, combining the properties of pressure-sensitive and bioadhesives, has found its application in tooth whitening strips [17].

A switchable adhesive article for attachment to skin was proposed [18]. The PSA comprises one or more water-dispersible sulfonated polyesters and a humectant in physical mixture with the polyesters. When contacted with a liquid of low-ion content, the adhesive undergoes a reduction in peel strength, which allows for easy removal, but remains strongly adhered when contacted with ionic liquids, such as blood, sweat, and other bodily fluids.

There has been extensive literature on the properties of polymers which display a lower critical solution temperature (LCST) [19]. These are polymers which consist of monomers having both hydrophilic and hydrophobic groups in aqueous solution. In solution, such polymers are in a fine balance between their hydrophilic and hydrophobic characteristics. A slight change in temperature or pH is often enough to cause the polymer chains to collapse into a globular form and become insoluble.

The vast majority of work has concentrated on the N-substituted acrylamides, in particular, N-isopropylacrylamide (NIP AM). Poly(NIPAM) displays a dramatic change from a hydrophilic, highly water soluble polymer with gaussian chain conformation, to an insoluble polymer with a globular chain conformation on raising the temperature above 31°C (LCST) [20,21].

Lightly crosslinked hydrogels based on NIP AM and its copolymers have been extensively studied for potential applications such as self cleaning contact lenses, controlled water release materials in agriculture, rheology modifiers, artificial muscle fibres and controlled drug release membranes. There has been significant work carried out on the properties of thermosensitive gels for use as possible drug delivery systems. The interest lies in the response of these gels to small changes in temperature or pH, which cause them to expel aqueous solutions (containing the active material), thus acting as a "smart reservoir." Such hydrogel-like materials have enormous potential as unique drug delivery systems [1].

Large studies have shown that it is possible to produce pressure sensitive adhesive compositions based on PNIPAM, which can be made to swell and de-swell on reaction to an outside stimulus, such as a slight temperature change or shift in pH [ 1 , 13 ,22-29] . The aim of these investigations was to determine if it is possible to formulate pressure sensitive compositions which display an LCST and can, thus, be swollen into a gel below a critical solution temperature. The PSA are produced which are able to detach in water by swelling, but only when it is exposed to water below a certain temperature, thus, giving a "smart" switchable PSA. The switching effect has been shown to be completely reversible. On heating above the critical temperature, the composition releases absorbed water and regains its pressure sensitive adhesion. As a result, pressure sensitive adhesive compositions have been obtained that display different levels of water absorption which could be tailored in combination with temperature sensitivity. Materials like these may have applications as smart drug delivery systems which could potentially be provided in pressure sensitive tape form.

Some of other LCST polymers employed for thermoswitchable PSA preparation include polyethylene oxide - polypropylene oxide - polyethylene oxide triblock copolymers (known as Pluronics and Polaxamers) [30,31 ], polyesters [13]; PNIPAM copolymers with polyethylene glycol (PEG) [1,25], and hybrids with conventional PSAs such as polydimethylsiloxane (PDMS) [24].

In the strict sense, all above listed polymers underlie thermoresponsive PSAs. Thermoswitchable PSAs constitute specific class within the thermoresponsive adhesives. Distinctive feature of the thermoswitchable PSAs is the reversibility of transition from tacky to nontacky state. Thermoresponsive PSAs produced by heat- or photo-curing of elastomers represent covalently cross-linkable adhesives, losing tack irreversibly upon cross-linking.

The pioneer research by Clarke et al. [6] laid the fundamentals for development of thermoswitchable PSAs. By using side chain crystallizable polymers with existing acrylic PSA technology, three different types of products have been developed: 1 ) An adhesive that is not tacky at room temperature but sticks aggressively above its switch temperature, 2) A PSA whose adhesion level can be reduced 60 % - 90 % on cooling, and 3) A PSA whose adhesion is reduced by approximately 90 % on warming. Further developments of thermoswitchable PSAs based on LCST polymer technology gave birth only for the former type of products. To the best of our knowledge, none of thermoswitchable PSAs, described to date, loses tack completely and reversibly in aqueous media under temperature elevation above LSCT.

Meantime it is desirable, with respect to thermoswitchable PSA products for painless removal from skin surface, to have a hydrophilic thermoswitchable adhesive material, utilizing an LCST polymer, providing spontaneous detaching of the adhesive product from skin at slightly elevated temperature in aqueous surrounding, e.g. in a bath. Nonlimiting examples of such thermoswitchable PSA products could be transdermal patches, tacky plasters, wound dressings or surgical drapes. Similar thermoswitchable PSA materials can be demanded also in industry, e.g. for development of rational technology enabling to wash away glass or plastic containers from sticky labels, or for temporary advertising of promotional posters on car and box bodies.

SUMMARY OF THE INVENTION

The invention is addressed to the aforementioned need in the art.

In this way, the challenge of present invention is the development of hydrophilic thermoswitchable adhesive products (both compositions and application materials, employing the LCST polymers) which are capable of reversibly and spontaneously detach under temperature elevation in aqueous media.

Major aspect of the invention pertains to a thermoswitchable pressure-sensitive adhesive composition based on a hydrogen-bonded complex of a hydrophilic film-forming polymer selected from polymers possessing Lower Critical Solution Temperature (LCST) in water with an oligomeric non-covalent crosslinker of the film-forming polymer that contains complementary reactive functional groups at both ends of its short chains.

Another aspect of the invention relates to an oligomeric noncovalent crosslinker that is selected from short-chain polymers capable of crosslinking a hydrophilic film-forming polymer by hydrogen bonds between both terminal complementary functional groups and recurring functional groups in the backbone of the film-forming LCST polymer. The hydrophilic LCST polymer and the oligomeric noncovalent crosslinker are selected from the polymers capable of forming a stoichiometric network complex.

In particular embodiment of this invention the hydrophilic Film-Forming LCST-Polymer is poly(N-vinyl lactam), either homopolymer of poly(N- vinyl caprolactam) (PVCL) or an N-vinyl caprolactam copolymer having a number average molecular weight in the range of approximately 10,000 to 5,000,000 g/mol, more preferably from 500,000 to 1 ,500,000 g/mol. The Oligomer Non- Covalent Crosslinker is selected from the group consisting of polyalcohols, monomeric and oligomeric alkylene glycols, polyalkylene glycols, carboxyl-teminated polyalkylene glycols, amino-terminated polyalkylene glycols, ether alcohols, alkane diols, hydroquinone, diphenyl, bisphenol A, and carbonic diacids, as well as from the group consisting of polyalkylene glycols and carboxyl-terminated polyalkylene glycols. As polyalcohols, a glycerol, sorbitol, xylitol and others similar polyalcohols may be employed. Preferable example of Non-Covalent Crosslinker is poly(ethylene glycol) with molecular weight ranged between 200 and 600 g/mol.

In another embodiment of present invention the composition can contain an Optional Film- Forming Polymer with recurring functional groups which are capable of forming hydrogen bonds with complementary functional groups in the repeat units of the Film-Forming LCST-Polymer. The Optional Film-Forming Polymer is selected from polyacrylic acid, polymethacrylic acid, polymaleic acid, polysulfonic acid, polyalkylene glycols, polyvinyl alcohols, polyvinyl phenols, poly(hydroxyalkyl acrylates), poly(hydroxyalkyl methacrylates), poly(N-vinyl acrylamides), poly(N-alkylacrylamides), polar derivatives of cellulose containing hydroxyl and carboxyl groups, alginic acid, combinations and copolymers thereof. The cellulose derivatives represent a cellulose ester polymer containing unesterified cellulose monomer units, cellulose acetate monomer units, and either cellulose butyrate monomer units or cellulose propionate monomer units. The cellulose derivative is a polymer containing hydroxyalkyl cellulose monomer units or carboxyalkyl cellulose monomer units. The acrylate-based polymer or copolymer is selected from polymers and copolymers of acrylic acid, methacrylic acid, methyl acrylate, ethyl acrylate, methyl methacrylate, and ethyl methacrylate. The Optional Film-Forming Polymer can be selected from water-soluble cellulose derived polymers, homopolymer and copolymers of vinyl alcohols, homopolymer and copolymers of vinyl phenols, homopolymer and copolymers ofalkylene oxides, homopolymer and copolymers of maleic acid, alginates, starches, naturally occurring polysaccharides, and combinations thereof. The molecular weight of the Optional Film-Forming Polymer is preferably to be less than the molecular weight of the Film-Forming LCST-Polymer.

In one version of the invention realization the composition can further contain at least one additive selected from absorbent fillers, preservatives, pH regulators, plasticizers, softeners, thickeners, antioxidants, pigments, dyes, conductive species, refractive particles, stabilizers, toughening agents, tackifiers or adhesive agents, detackifiers, flavorants and sweeteners, therapeutic agents and skin permeation enhancers.

Another facet of the invention relates to a thermoswitchable pressure sensitive adhesive material for application onto various surfaces that capable of reversibly disadhering in aqueous media under temperature elevation and includes a backing film with the surface coated by the layer of above presented adhesive composition. The backing film can be considered as a polymer membrane with designated value of water permeability. The third invention facet represents a preparation method of the thermoswitchable pressure sensitive adhesive material with preassigned detaching temperature from various contact surfaces that includes:

(a) preparation of the adhesive composition;

(b) construction of a diagram phase separation temperature in the mixtures of prepared adhesive composition with water followed by measuring of the amount (fraction) of absorbed water required for phase separation under necessary temperature,

(c) selection of a backing film with predetermined rate of water transport, calculation of the amount of the adhesive composition necessary for coating the backing film, assessing the prepared patch application area, and the time required for the patch detaching from the contact surface of a substrate under a iven temperature following the Equation:

where m - the adhesive composition weight (g), S - the area of adhesive material tape covered by water-permeable backing film, (cm ), VH¾> - the rate of water transport across the backing film ^g/cm² min), WH.¾D and w_poi - weight fractions of absorbed water and the adhesive polymer composition, referring to a critical solution temperature (CST) in the phase separation temperature diagram of the system the adhesive composition - water.

(d) preparation of the patch (adhesive tape) with prearranged value of detaching temperature from a substrate in aqueous medium by means of the coating the preselected backing film with calculated amount of the adhesive composition.

The adhesive compositions produced by the methods of the invention provide a number of significant advantages relative to the prior art. In particular, these compositions provide one or more of the following advantages over the art: provide ease of handling; are readily modified during manufacture so that properties such as adhesion, switching temperature, cohesive strength, water absorption, translucence, and swelling can be controlled and optimized; can be formulated so that tack increases or decreases in the presence of moisture so that the composition is not sticky until moistened and tacky in hydrated state below switching temperature, and becomes non-sticky above LSCT; can be easily removed from the skin surface without pain and skin trauma, and leave no residue; are amenable to extended duration of wear or action; and can provide sustained and controlled release of a variety of active agents.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic representation of supramolecular structure of a stoichometric hydrogen-bonded network complex of film-forming polymer with oligomer, bearing reactive functional groups in backbone recurring units, with an oligomer noncovalent crosslinker, containing complementary functional groups on both ends of its short chains. In the stoichiometric polymer - oligomer complex approximately 60 % of film-forming polymer monomer units form H-bonds with complementary terminal groups of oligomer chains.

FIG. 2 schematically illustrates the structure of interpolymer complex formed by complementary functional groups in recurring units of an obligatory film-forming polymer, possessing LCST, and an optional film-forming polymer.

FIG. 3 shows the impact of PEG molecular weight on mol. % of poly(N- vinyl pyrrolidone) (PVP) recurring units crosslinked by hydrogen bonding through both PEG terminal hydroxyl groups [34].

FIG. 4 compares the effects of blend compositions on mol. % of crosslinked PVP recurring units for PEG molecular weight of 200, 300, 400 and 600 g/mol. Relative independence of the amount of crosslinks on the PVP - PEG blend composition is indication of network complex stoichiometry [34].

FIG. 5 demonstrates effect of temperature on practical work of adhesion for five types of

PSAs: polyisobutylene PSA (PIB), acrylic PSA, PSA based on styrene-isoprene-styrene triblock copolymer (SIS), hydrophilic PSA based on poly(N-vinyl pyrrolidone) (PVP) complex with oligomeric poly(ethylene glycol) of molecular weight 400 g/mol (PEG-400) and amphiphilic PSA based on polyelectrolyte complex (PEC).

FIG. 6 compares the molecular structures of PVP and PVCL poly(N-vinyl lactams).

FIG. 7 displays the isotherms of water vapor sorption by polyvinyl caprolactam (LMW PVCL) and polyvinyl pyrrolidone (PVP) at ambient temperature.

FIG. 8 shows the impact of PEG-400 content in the blends with PVCL on Probe Tack curves.

FIG. 9 illustrsates the effect of PEG-400 concentration on practical work of adhesion, W, and maximum debonding stress, a_max, of PVCL - PEG blends. FIG. 10 demonstrates the behavior of glass transition temperature as a function of PVCL - PEG composition. Line: relationship calculated with the Fox equation (1); Points: experimental data.

FIG. 11 is a relationship between the content of non-covalently crosslinked PVCL recurring units and the composition of LMW PVCL blends with PEG-400, expressed in terms of the amount of terminal hydroxyl groups in PEG short chains per one PVCL monomer units.

Figure 12 compares the composition behaviors of non-covalently crosslinked PVCL recurring units and the practical work of adhesion for PVCL blends with PEG-400.

FIG. 13 represents isotherms of water vapor sorption by PVCL blends with different contents of PEG-400.

FIG. 14 compares the effects of relative humidity (RH) on the adhesion of P VP-PEG blends and LMW PVCL-PEG hydrogels.

FIG. 15 demonstrates the impact of absorbed water on cloud point behavior of PVCL and the PVCL blends with 45 wt. % of PEG-400.

FIG. 16 shows the effects of absorbed water and temperature on peel adhesion of PVCL blends with 45 wt. % of PEG-400.

FIG. 17 illustrates kinetics of liquid water transport across PP backing film at 24 (left) and 60 °C (right).

FIG. 18 exhibits the impact of temperature on the rate of liquid water penetration across the PP backing film.

FIG. 19 represents temperature sweep curves of storage modulus, G', loss modulus, G ", and loss angle tangent, tan δ, of PVCL blend with 45 wt. % of PEG-400.

DETAILED DESCRIPTION OF THE INVENTION

DEFINITIONS AND OVERVIEW:

It is to be understood that, unless otherwise indicated, this invention is not limited to specific polymers, oligomers, crosslinking agents, additives, manufacturing processes, or adhesive products. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting.

In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below. The singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a hydrophilic polymer" includes not only a single hydrophilic polymer but also two or more hydrophilic polymers that may or may not be combined in a single composition, reference to "a plasticizer" includes a single plasticizer as well as two or more plasticizers that may or may not be combined in a single composition, and the like.

A "hydrophobic" polymer absorbs only up to 1 wt. % water at 100% rh, while "hydrophilic" polymers absorb at least 1 wt.% water at 100% rh.

A "water-swellable" polymer is one that is capable of absorbing water in an amount that is at least 50% of its own weight . That is, a water-swellable polymer weighing x grams can absorb at least 0.5x grams of water, to provide a hydrated polymer weighing at least 1.5x grams and having a polymer to water (weight) ratio of at most 3:1.

The term "crosslinked" herein refers to a polymer composition containing intramolecular and/or intermolecular noncovalent bonds. Noncovalent bonding includes hydrogen bonding, electrostatic bonding, and ionic bonding.

The term "polymer" as used herein includes both linear and branched polymers, and homopolymers as well as copolymers, the latter including all types of copolymer structures (e.g., block copolymers, alternating copolymers, random copolymers, etc.) as well as "higher order" copolymers (e.g., terpolymers). Those compounds referred to herein as "oligomers" are polymers having a molecular weight below about 1000 g/mol, preferably below about 800 g/mol.

The term "water-insoluble" is used to refer to a polymer, compound or composition whose aqueous solubility measured at 20 °C is less than 5 wt%, preferably less than 3 wt%, and more preferably less than 1 wt. %. The term "insoluble" is used to refer to a polymer, compound or composition whose solubility in water, polar organic solvents, and possibly nonpolar organic solvents, measured at 20 °C, is less than 5 wt%, preferably less than 3 wt%, and more preferably less than 1 wt. %.

The term "hydrogel" is used in the conventional sense to refer to water-swellable polymeric matrices that can absorb a substantial amount of water to form elastic gels, where the "matrices" are three-dimensional networks of macromolecules held together by covalent or non-covalent crosslinks. Upon placement in an aqueous environment, dry hydrogels swell to the extent allowed by the degree of cross-linking. The term "hydrogel composition" refers to a composition that either contains a hydrogel or is entirely composed of a hydrogel. As such, "hydrogel compositions" encompass not only hydrogels per se but also compositions that comprise a hydrogel and one or more non-hydrogel components or compositions.

The terms "tack" and "tacky" are qualitative. However, the terms "substantially nontacky,"

"slightly tacky," and "tacky," as used herein, may be quantified using the values obtained in a tack determination methods PKI, a TRBT, or a PSA tack determination/Polyken Probe (Solutia, Inc.). The term "substantially nontacky" is used to refer to a composition having a tack value less than about 25 g-cm/sec, the term "slightly tacky" refers to a composition having a tack value in the range of about 25 g-cm/sec to about 100 g-cm/sec, and the term "tacky" refers to a composition having a tack value of at least 100 g-cm/sec.

The term "plasticizer" is used in the conventional sense of the term to refer to a relatively low molecular weight compound that is miscible with a polymer or polymer blend and decreases the glass transition temperature and elastic modulus thereof.

The term "pressure sensitive adhesive" (PSA) relates to the polymer materials, which form a strong adhesive bond to any surface with application of very slight external pressure (1-10 Pa) over a short period of time (e.g., 1-5 seconds).

The term "bioadhesive" means a hydrogel that exhibits a pressure-sensitive character of adhesion toward highly hydrated surfaces such as mucosal biological tissue.

Some polymers bearing attractive groups are called "smart", "intelligent" or "stimuli- sensitive" since they show critical phenomena as, for example, phase transitions that can be induced by external stimuli: changes in temperature, pH, solvent, ionic composition, electric or magnetic fields, light, etc.

The term "thermoresponsive adhesive" defines the adhesive composition changing the strength of its adhesive bond with the change of temperature.

The term "thermoswitchable adhesive" designates the thermoresponsive adhesive changing its adhesive strength reversibly.

Most synthetic macromolecules become more soluble when heated, but some water-soluble polymers separate from solution upon heating. This unusual property, referred to as inverse temperature-dependent solubility, is characteristic of polymers which dissolve when cooled and phase separate when heated above the phase transition temperature, known as a lower critical solution temperature (LCST).

The term "lower critical solution temperature (LCST)" designates the minimum critical temperature below which the components of a mixture are miscible and above which they become immiscible. The LCST value depends only on polymer structure and molecular weight. In contrast, "critical solution temperature (CST)" is also the function of polymer - solvent composition. Depending on blend composition, below CST a polymer is soluble in a solvent, whereas above CST polymer and solvent are incompatible and form two-phase blend.

The "cloud point" of a polymer possessing LSCT solution is the temperature where the mixture starts to phase separate and two phases appear, thus becoming cloudy. This behavior is characteristic of non-ionic surfactants and polymers containing poly(alkyl amide) or polyoxyethylene chains, which exhibit reverse solubility versus temperature behavior in water and therefore "cloud out" at some point as the temperature is raised. Below LSCT the test solution is required to be transparent in layers 40 mm in thickness. A gel phase typically first forms with the appearance of a whitish or milky cloud. The cloud point is the temperature at which this phase first appears.

The term "complex" or "interpolymer complex" refers to the association of macromolecules of two or more complementary polymers that forms as a result of favorable interactions between their functional groups.

The term "stoichiometry" refers to the proportions in which chemical substances enter into chemical reactions or non-covalently interact with each other. In this sense, "stoichiometric complex" is the complex of specified chemical composition, wherein all the complex components are combined in a strictly fixed proportion.

The stoichiometric polymer - oligomer hydrogen-bonded complexes are formed through a mechanism of self-assembling of polymer and oligomer chains into three-dimensional network supramolecular structures. In essence, the stoichiometric interpolymer complex behaves as a new chemical entity and exhibits the properties that are untypical of parent components. The stoichiometric composition of the polymer - oligomer complex has been shown to be governed by the length of oligomer chains. Relatively long-chain oligomers provide substantially sparser H- bonded networks than their short-chains counterparts. The term "cohesion" refers to the intermolecular attraction between like or complementary, self-associating macromolecules. The driving force of an intermolecular cohesion are hydrogen, electrostatic and/or ionic bonding between the complementary macromolecules. In polymer composite materials the long chain entanglements serve as an additional factor contributing to high cohesive strength.

In interpolymer complexes the high cohesion energy is due to the formation of ionic, electrostatic or hydrogen bonds crosslinking the polymer chains into network. Hydrogen bonding is substantially weaker than electrostatic or ionic bonding, whereas ionic bonds are much weaker than covalent bonds. Nevertheless, comparatively weak intermolecular bonds have appreciable advantage over the strong covalent bonds with respect to mechanical strength and adhesion of polymer blend. Being once ruptured, the covalent bond is incapable to reform. In contrast, the ionic, electrostatic and hydrogen bonds have temporary character and demonstrate the capability of rearrangement and reformation at a new place under applied mechanical or debonding stress. As a result, viscoelastic deformation and debonding of the PSA networks involving the ionic, electrostatic and hydrogen bonding can dissipate much more mechanical energy than in covalently crosslinked adhesives. Thus, non-permanent nature of comparatively weaker molecular interaction contributes significantly to the strength of adhesive bond.

Interpolymer complexes are noncovalently crosslinked three-dimensional polymer networks (gels) resulting from ionic, electrostatic or hydrogen bonding between complementary functional groups in their macromolecules. If both complementary polymers contain ionogenic functional groups, their association product is termed polyelectrolyte complex. A distinctive feature of "hydrogen bonding" between proton donating and proton accepting complementary groups is that both the reactive groups and the product of their interaction bear no electric charge. "Electrostatic bonding" is the interaction of proton donating and proton accepting groups, which are initially uncharged, but their interaction is accompanied with proton transfer and occurrence of the charge. And lastly, "ionic bonding" is the interaction of oppositely charged (cationic and anionic) groups with the formation of ionic (salt) bond.

The term "free volume" of a polymer is used to define the unoccupied space, or vacancies, available for segmental motion of macromolecules. The free volume of a material is the difference between the bulk volume and the sum of the hard core and vibrational volumes of the constituent building blocks (atoms). In polymer physics the free volume concept has long been used to interpret and explain the molecular mobility of the macromolecules along with such fundamental properties and quantities as the glass transition and glass transition temperature, viscoelastic, adhesion and relaxation behaviors, diffusion, and other transport properties of polymer systems. Along with the energy of intermolecular cohesion, free volume is a factor controlling the values of cohesive energy density, solubility parameter and the Flory-Huggins interaction parameter.

It is desirable to obtain water-swellable hydrophilic thermoswitchable pressure-sensitive adhesive polymer compositions (adhesive hydrogels) that are capable to form homogeneous films either upon casting a solution to backing layer followed by drying, or under external pressure, e.g. by means of extrusion. The film-forming capability requires that the blend has to be free of covalent crosslinks. Blending the polymers provides a convenient way to obtain composite materials with specifically tailored properties, since the properties of the blend are typically intermediate between those of the unblended components when the components are immiscible or partly miscible. In order to make the composite insoluble in water, water-insoluble materials are usually mixed with water-soluble materials. When this is done, however, a phase separation can often occur that does not favor adhesion. Moreover, the insolubility of blend components may hamper the procedure of blend preparation, which often involves the dissolution of all the components in a common solvent, followed by casting the solution and drying.

Preparation of polymer composite materials whose properties are new and untypical of parent components requires a high skill of a material designer. This challenge may be resolved if the blend components are capable of a strong favorable interaction to each other. More often, such interaction is hydrogen, electrostatic or ionic bonding. In this instance mixing of two or more soluble film-forming polymers with an oligomer bearing its functional reactive groups at both ends of its short chains can give their three-dimensional network complex schematically shown in FIG.l .

In order to resolve these problems, this invention is directed to a method of obtaining water-swellable, thermoswitchable, pressure-sensitive adhesive, film-forming compositions by blending soluble polymers possessing suitable LSCT values, more specifically by blending hydrophilic polymers with complementary macromolecules that are capable of hydrogen bonding, electrostatic or ionic bonding.

By way of overview, the thermoswitchable adhesive compositions of the invention contain at least one film-forming hydrophilic polymer having at least one linear segment with a plurality of recurring polar groups thereon, and at least one oligomer noncovalent crosslinker/plasticizer compatible with (i.e., miscible with) or at least partially compatible with the film-forming polymer. Frequently, but not necessarily, the film-forming polymer is present in a higher concentration than its complementary multifunctional oligomer nocovalent crosslinker, and it is this higher concentration that determines the film-forming characteristics. Therefore, while there may be materials that are suitable for use as either the film-forming polymer or as the complementary multifunctional oligomer crosslinker, their function in the composition is determined by the quantity of the component in the composition. If the recurring polar groups or the recurring functional groups are ionogenic, another factor that controls the performance of composite material is the degree of ionization or pH of the mixture.

For example, polyacids such as acrylate polymers bearing carboxyl proton- donating functional groups or polyols bearing hydroxyl proton-donating functional groups and proton- accepting polymers such as poly(N-vinyl lactams) or polyamines are suited for use as both the film-forming polymer or as the complementary multifunctional noncovalent polymer crosslinker. In a composition having a greater amount of a poly(N-vinyl lactam) or another proton-accepting LSCT-polymer relative to the amount of an acrylate oligomer or polyalcohol, the poly(N-vinyl lactam) or polyamine or another proton-accepting polymer serves as the film-forming polymer, and the acrylate or polyalcohol oligolymer serves as the complementary multifunctional noncovalent crosslinker.

Maintaining a specified pH value in the blend or in an admixture used to obtain the blend provides an additional factor controlling the performance of the blend when one or more ionogenic polymers are present. Ionized groups are capable of ionic, but not electrostatic or hydrogen bonding. Fully or partly ionized polymers are always soluble in water, whereas non-ionized polymers as a rule are insoluble or poorly soluble in water. Consequently, the degree of ionization affects appreciably the solubility and swelling of interpolymer complexes involving ionogenic polymers. Moreover, by varying the pH value and degree of ionization, the adhesive properties of composite materials can be controlled. Indeed, adhesion is a result of specific balance between cohesive interaction energy and free volume. As polymeric components bear opposite charges, cohesion is increased. As two polymers have the same positive or negative charge, cohesion is immediately suppressed and free volume is increased. Moreover, due to electrostatic repulsion between the functional groups of identical charge, the chain rigidity and free volume is usually increased. All these factors dramatically affect the adhesive performance.

The adhesion profile of the water-swellable, thermoswitchable, pressure-sensitive adhesive film-forming compositions of the invention can be tailored based on materials, the ratio of components in the composition, their LSCT values, the degree of ionization and the quantity of water in the blend. The oligomeric noncovalent crosslinker, its ratio to the amount of film- forming polymer, concentration of an optional plasticizer and ionization degree are selected so as to provide the desired adhesion profile with respect to hydration. Generally, the compositions that are relatively slightly crosslinked through comparatively loose hydrogen bonds and demonstrating a large free volume provide initial tack in dry state. When the degree of crosslinking and the cohesive strength of the network in the interpolymer complex is above some critical value, the energy of cohesion dominates over free volume and such compositions are usually non-tacky in the dry state. However, as a free volume is increased in this blend (e.g. by adding a suitable plasticizer), adhesion immediately appears. Because water is a good plasticizer for hydrophilic polymers, absorption of the water leads to some extent to an improvement of adhesion. Electrostatic bonds are appreciably stronger than the hydrogen bonds, therefore cohesion in the blends of polymers bearing carboxyl groups is usually higher than in the materials composed of polymers having hydroxyl groups. Adhesion in such blends appears normally with a higher concentration of absorbed water. Flexible polymers provide higher cohesion than polymers with rigid chains. As an example, for blends of poly(vinyl pyrrolidone) (PVP) as a film-forming polymer, when the noncovalent polymer crosslinker is a rigid-chain cellulose ester bearing OH groups, the composition is generally tacky prior to contact with water (e.g., with a moist surface) but gradually loses tack as the composition absorbs moisture. When the noncovalent polymer crosslinker is an acrylate polymer or copolymer with carboxyl groups, a composition is provided that is generally substantially nontacky prior to contact with water, but that becomes tacky upon contact with a moist surface.

POLYMER COMPONENTS:

In a first embodiment, the invention provides a method for obtaining hydrophilic PSAs by mixing a specific amount of a selected hydrophilic film forming polymer with a specific amount of a selected plasticizer such as complementary short-chain plasticizers. Preferable content of an oligomeric noncovalent crosslinker in the adhesive ranges between 30 and 60 wt. %. Thermoswitchable pressure sensitive adhesive composition on this invention contains typically one or few film-forming hydrophilic polymers, and oligomeric complementary noncovalent crosslinker thereof. For the purposes of this invention, at least one of aforementioned film-forming polymers is selected from the group of hydrophilic polymers, bearing functional reactive groups in recurring units of their backbones, which are selected from the family of polymers, possessing LCST. This obligatory and major polymeric component of a thermoswitchable pressure-sensitive adhesive composition is named herein as Film-Forming LSCT-Polymer".

While at least one of the film-forming polymers always possesses LCST and is responsible for thermoswitching of adhesion, others film-forming polymers are optional and intended for executing auxiliary functions in a composition, such as imparting pH-responsive properties or improvement of rheological behavior. Such film-forming polymers are called herein as "Optional Film-Forming Polymer.

Another obligatory component of thermoswitchable pressure sensitive adhesive composition is complementary nocovalent crosslinker of the film forming polymers. It is an oligomer, bearing reactive functional groups at both ends of its short chains and capable of forming stoichiometric H-bonded network complex with the Film-Forming LSCT-Polymer. This obligatory component of a thermoswithchable PSA composition is termed herein as "Non- Covalent Oligomer Crosslinker"..

Supramolecular structure of such network complex is schematically shown in FIG. 1. The stoichiometry of the polymer - oligomer network complex is non-equimolar, taking into account that not all reactive functional groups of the Film-Forming LSCT Polymer are involved in hydrogen bonding with complementary terminal group of short chains of Non-Covalent Oligomer Crosslinker. Unique feature of the polymer - oligomer complex is that intermolecular cohesion (the result of hydrogen interpolymer bonding) is counterbalanced by large free volume, provided by the length and flexibility of the chains of Non-Covalent Oligomer Crosslinker. Generally, high cohesion and large free volume are conflicting properties which are difficult to provide in a single material. Proper combinations of these conflicting factors is necessary condition of pressure- sensitive adhesion.

While the Non-Covalent Oligomer Crosslinker forms with Film-Forming LCST-Polymer the complex of nonequimolar stoichiometry (FIG. 1), Optional Film-Forming Polymer and the Film-Forming LCST-Polymer, bearing reactive functional groups in recurring units of their backbones, tend to form the complex of equimolar stoichiometry, schematically presented in FIG. 2. Characteristic feature of such interpolymer complex is domination of intermolecular cohesion over free volume that is unfavorable for high adhesion.

The Film-Forming LCST-Polymer and the complementary Non-Covalent Oligomer

Crosslinker, as noted elsewhere herein, are generally selected from the same classes of hydrophilic polymers and copolymers. By definition herein, the polymer that serves as the Film-Forming LCST-Polymer frequently but not necessarily represents a greater weight fraction in the mixtures and compositions of the invention. Typically, the Film-Forming LCST-Polymer represents approximately 20 wt.% to approximately 95 wt.% of the mixtures and compositions of the invention, while the complementary Non-Covalent Oligomer Crosslinker represents approximately 0.5 wt.% to approximately 50 wt.% of the mixtures and compositions of the invention.

Besides a Film-Forming LCST-Polymer and a Non-Covalent Oligomer Crosslinker, a thermoswitchable PSA composition on the invention can also contain an Optional Film-Forming Polymer. Desirably although not necessarily that both film-forming polymers should contain complementary functional groups in their recurring units. Anyway they should be compatible with each other, because often phase separation worsens PSA adhesion.

Generally, although not necessarily, the Film-Forming LCST-Polymer will also have a higher molecular weight than the complementary multifunctional Optional Film-Forming Polymer. The molecular weight of the Film-Forming LCST-Polymer will usually be in the range of about 20,000 to 3,000,000, preferably in the range of about 100,000 to 2,000,000, and most preferably in the range of about 100,000 to 1,500,000.

The recurring polar groups on the Film-Forming LCST-Polymer and the recurring functional groups on the complementary Optional Film-Forming Polymer may comprise backbone heteroatoms, e.g., an oxygen atom in an ether (-0-) or ester (-(CO)-O-) linkage, a nitrogen atom in an amine (-NH-), imine (-N=), or amide (-NH(CO)-) linkage, a sulfur atom in a thioether (-S-) linkage, and the like. The recurring polar groups and the recurring functional groups may also comprise pendant groups, for instance:

hydroxy 1;

sulfhydryl;

Ci-Ci₈ hydrocarbyloxy, preferably Q-Cg alkoxy; C₂-Ci8 acyl, preferably C₂-C₈ acyl (e.g., C₂-C₈ alkylcarbonyl);

C₂-Ci8 acyloxy, preferably C₂-C₈ acyloxy (e.g., C₂-C₈ alkylcarbonyloxy);

C₂-Ci8 hydrocarbyloxycarbonyl (-(CO)-O-alkyl), preferably C₂-C₈ alkoxycarbonyl (-(CO)-

O-alkyl));

carboxy (-COOH);

carboxylato (-COCT);

carbamoyl (-(CO)-NR₂ wherein R is H or Ci-Ci₈ hydrocarbyl, preferably H or Ci-C₈ alkyl); cyano(-C≡N);

isocyano (-N⁺≡C^~);

cyanato (-0-C≡N);

isocyanato (-0-N⁺=C );

formyl (-(CO)-H);

amino, i.e., -NR'R² where R¹ and R² are independently selected from H and Ci-C]₈ hydrocarbyl, preferably selected from H, Ci-C₈ alkyl, and C₅-Ci₂ aryl, or are linked to form an optionally substituted five- or six-membered ring, thus including mono-(C i -C₈ alkyl)-substituted amino, di-(Ci-C₈alkyl)-substituted amino, mono-(C5-Ci₂aryl)-substituted amino, and di-(C₅-Ci₂ aryl)-substituted amino), piperidinyl, pyrrolidinyl, and pyrrolidonyl;

quaternary ammonium, i.e., -[NR³R⁴R⁵]⁺Q^~ where R³, R⁴, and R⁵ are Cj-Cig hydrocarbyl, preferably Ci-C₈ alkyl, and most preferably C1-C4 alkyl, and Q is a negatively charged counterion, e.g., a halogen anion;

C₂-C₁₈ alkylamido, preferably C₂-C₈ alkylamido (-NH-(CO)-alkyl);

C₆-Ci8 arylamido, preferably C₆-Ci₂ alkylamido (-NH-(CO)-aryl);

nitro (-N0₂);

sulfo (-S0₂-OH);

sulfonate (-S0₂-0^_);

C1-C18 hydrocarbylsulfanyl, preferably Ci-C₈ alkysulfanyl (-S-hydrocarbyl and -S-alkyl, respectively, also termed "hydrocarbylthio" and "alkylthio");

phosphono (-P(0)(OH)₂);

phosphonato (-P(0)(0^~)₂);

phosphinato (-P(0)(0^~)); and

phospho (-P0₂), any of which may be substituted as permitted, e.g., with hydrocarbyl groups and/or additional functional groups. The pendant groups may also be directly linked to an atom in the polymer backbone, or they may be indirectly linked through a linking group (e.g., Cj-Cig hydrocarbylene linker such as C₂-C₈ alkylene linker). Additionally, there may be two or more types of polar groups on the Film-Forming LCST-Polymer (which may include backbone heteroatoms as well as pendant polar groups) and two or more types of functional groups on the Optional Film-Forming Polymer (again, which may include backbone heteroatoms as well as pendant polar groups).

Preferred pendant groups are those present on polymers that are readily synthesized or commercially available, typically including hydroxy, C C₈ alkoxy, carboxyl, carboxylato, sulfo, sulfonato, amino, di(Ci-C₈ alkyl)-substituted amino, quaternary ammonium, piperidinyl, pyrrolidinyl, pyrrolidinyl, and phosphono groups.

For the purposes of present invention it is not necessarily that Optional Film-Forming Polymer would have LCST.

While the Film-Forming LCST-Polymer is a major component of a thermoswitchable PSA composition, the Optional Film-forming Polymer is always the minor component. As a rule the content of the Optional Film-Forming Polymer should be less than that of the Non-Covalent Oligomer Crosslinker.

HYDROPHILIC FILM-FORMING LSCT-POLYMER

It will be appreciated by those of ordinary skill in the art that virtually any polymers meeting the aforementioned criteria may be used herein. Suitable LSCT-polymers include, but are not limited, to the following:

Polymers bearing amide groups form the largest group of thermosensitive polymers having LCST. Such polymers include: Poly(N-alkyl acrylamides, e.g. PNIPAM, poly(N- isopropylacrylamide); PDEAAM, poly(N,N,-diethylacrylamide).

Poly(N-vinyl amides, e.g. PVAA, poly(N-vinyl acetamides.

Poly(N-vinyl lactams, e.g. PVVL, poly(vinyl-2-valerolactam), and PVCL, poly(N- vinylcaprolactam);

Poly(2-alkyl-2-oxazoline), e.g. PIOZ, poly(2-isopropyl-2-oxazoline). Polyvinyl ethers), e.g. PVME, poly(vinyl methyl ether), PVEE, poly(vinyl ethyl ether), PVPE, poly(vinyl propyl ether), PVIPE, poly(vinyl isopropyl ether), PVBE, poly(vinyl butyl ether), PVIBE, polyvinyl isobutyl ether).

Poly(alkylene oxides) such as polypropylene oxide (PPO), pluronics (i.e., block copolymers of ethylene oxide and propylene oxide), and poloxamers (i.e., copolymers of ethylene oxide and propylene oxide);

Polylactide and poly(lactide-co-glycolide);

Cellulose esters and other cellulose derivatives, including cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cellulose butyrate, cellulose diacetate, cellulose phthalate, cellulose propionate, cellulose propionate butyrate, cellulose triacetate, hydroxyethylcellulose, hydroxypropylcellulose, hydroxypropyl methylcellulose, hydroxypropyl methylcellulose phthalate, methylcellulose, carboxymethylcellulose, and sodium carboxymethylcellulose;

Block copolymers with blocks displaying different LCSTs.

Random copolymers of above-mentioned monomers with tunable thermosensitivity.

LCST copolymers, providing combination of thermoresponsive and zwitterionic properties: block copolymers with blocks displaying LCST and upper critical solution temperature (UCST).

Thermoreversible hydrogels based on PNIPAM and zwitterionic comonomer.

LCST polymers and compositions providing combination of thermo- and pH-responsive properties, including random copolymers and hydrogels, block- and graft-copolymers, interpolymer complexes.

The molecular weight of the hydrophilic Film-Forming LCST-Polymer is not critical; however, the number average molecular weight of the hydrophilic polymer is generally in the range of approximately 100,000 to 2,000,000, more typically in the range of approximately 500,000 to 1 ,500,000. The hydrophilic Film-Forming LCST-Polymer may or may not be adhesive in nature, as a nonadhesive hydrophilic polymer will become adhesive upon admixture with a predetermined quantity of the Non-Covalent Oligomer Crosslinker, also performing functions of plasticizing agent.

NON-COVALENT OLIGOMER CROSSLINKER

If the Non-Covalent Oligomer Crosslinker has sufficiently low value of glass transition temperature (below -30 °C as is measured with DSC), it also possesses the properties of plasticizer. In this case, the thermoswitchable PSA composition does not require incorporation of special plasticizing agent. Depending on the glass transition temperature of the Film-Forming LCST-Polymer, optimal T_g value of Non-Covalent Oligomer Crosslinker ranges between - 50 and - 100 °C, preferentially between - 65 and - 80 ⁰ C.

The complementary Non-Covalent Oligomer Crosslinker of the Film-Forming LCST-

Polymer, performing also functions of plasticizing agent, is hydrophilic polymer terminated with hydroxyl groups, amino or carboxyl groups, and is typically a monomeric or oligomeric material that has a glass transition temperature T_g in the range of about -100°C to about -30°C and a melting temperature T_m lower than about 20°C. The Non-Covalent Oligomer Crosslinker, acting also as plasticizing agent, may be also amorphous. Generally, the Non-Covalent Oligomer Crosslinkers/plasticizers have a molecular weight in the range from about 45 to about 800 g/mol, preferably in the range of about 45 to about 600 g/mol.

The hydrophilic Film-Forming LCST-Polymer and Non-Covalent Oligomer Crosslinker should be miscible with respect to each other, i.e. capable of forming a homogeneous blend that exhibits a single T_g, intermediate between those of the unblended components.

The Film-Forming LCST Polymer and its Non-Covalent Oligomer Crosslinker should have disparate chain lengths (as may be deduced from the above). The ratio of the molecular weight of the hydrophilic Film-Forming LCST-Polymer molecular weight to that of the short-chain Non- Covalent Oligomer Crosslinker/plasticizer should be within about 200 and 200,000, preferably within about 1,250 and 20,000. Also, the Film-Forming LCST-Polymer and the Non-Covalent Oligomer Crosslinker/plasticizer should contain complementary functional groups capable of hydrogen bonding or electrostatic bonding to each other. Ideally, the complementary functional groups of the Film -Forming LCST-Polymer are located throughout the polymeric backbones, while the functional groups of the Non-Covalent Oligomer Crosslinker / plasticizer are preferably located at the two termini of a linear molecule, and are not present along the backbone, if the Non- Covalent Oligomer Crosslinker / plasticizer is a short-chain polymer or low molecular weight compound. Forming hydrogen bonds or ionic bonds between the two terminal functional groups of the Non-Covalent Oligomer Crosslinker / plasticizer and the corresponding functional groups contained along the backbone of the hydrophilic Film-Forming LCST-Polymer results in a stoichiometric noncovalently linked supramolecular network structure outlined in simplified view in Figure 1. Strong interaction between the complementary groups of the Non-Covalent Oligomer Crosslinker / plasticizer and hydrophilic Film-Forming LCST-Polymer imparts cohesive strength to the network. At the same time, due to the length and flexibility of the Non-Covalent Oligomer Crosslinker / plasticizer molecules, they behave as spacers, creating a free volume between cohesively interacting macromolecules of the hydrophilic polymer. In this way, the apparently conflicting performance properties of pressure-sensitive adhesives are both achieved: a liquid-like fluidity needed for adhesive bonding, coupled with a rubber-like resistance to shear deformation necessary to dissipate the detaching energy under adhesive joint failure.

The difference between the T_g values of Film-Forming LCST-Polymer and Non-Covalent Oligomer Crosslinker / plasticizer has a decisive significance for the adhesive behavior of the polymer-plasticizer blend. Preferably, the difference is greater than about 50° C, preferably greater than about 100° C, and most preferably in the range of about 150° C to about 300° C.

In addition, appropriate Non-Covalent Oligomer Crosslinker should decrease the glass transition temperature of the hydrophilic Film-Forming LCST-Polymer/plasticizer composition to a greater degree than predicted by the Fox equation, which is given by equation (1)

1 ¹ _ w p l . _|_ w ^yvncc

_ _ _ (1)

g predicted _gpo, g„_cc

where T_{g pre}d_iclecj is the predicted glass transition temperature of the hydrophilic polymer/plasticizer composition, w_poi is the weight fraction of the hydrophilic Film-Forming LCST-Polymer in the composition, w_ncc is the weight fraction of the Non-Covalent Oligomer Crosslinker / plasticizer in the composition, T_gpoi is the glass transition temperature of the hydrophilic Film-Forming LCST- Polymer, and T_g „_cc is the glass transition temperature of the Non-Covalent Crosslinker / plasticizer. Generally, the predetermined deviation from T_{g prec}ncted will be the maximum negative deviation, such that this is the point where adhesive strength is maximized.

That is, the weight ratio of hydrophilic Film-Forming LCST-Polymer to Non-Covalent Crosslinker / plasticizer should be of a specified value in order for adhesion to appear in a blend of a non-tacky hydrophilic Film-Forming LCST-Polymer and a short-chain complementary Non- Covalent Crosslinker / plasticizer. As has been earlier reported for PVP blends with PEG-400 [32- 38], adhesion is inherent only in those compositions that demonstrate negative deviations from the T_g value predicted by the Fox equation (1). The larger the negative deviation, the higher the adhesion. This finding is generally applicable and is not limited to PVP-PEG-400 blends. Preferably, the negative deviation is within the range about 30⁰ C to about 150° C, preferentially from about 50 ⁰ C to about 120° C. The extent of the negative T_g deviation depends on the difference between the T_gs of blend compoments, i.e., between T_gpoi md T_{g p\}. In general, the negative T_g deviation should be approximately 20 - 40 % of the difference between the T_g values of unblended polymer and plasticizer.

As is shown in [34], the stoichiometry of polymer - oligomer noncovalent network complexes can be evaluated from the value of negative T_g deviation from the simple rule of component mixing, outlined by the Fox equation (1), using the equation (2): s ₌ - 100 ₍₂₎ Where S is mole percent of crosslinked polymer units, w_po] and MW_poi are weight fraction of polymer and molecular weight of polymer recurring units, MW_ncc is molecular weight of oligomer non-covalent crosslinking macromolecule, and w*_ncc is the negative blend T_g deviation from the value predicted by the Fox equation.

The inventor herein has now discovered that an adhesive composition having an optimized (e.g., maximized) degree of adhesion can be prepared from a hydrophilic polymer and a complementary oligomer non-covalent crosslinker / plasticizer by selecting the components and their relative amounts to give a stoichiometric complex of Film-Forming LCST-Polymer with Non-Covalent Oligomer Crosslinker / plasticizer. Indeed, as our recent unpublished research data have shown, only those blends of Film - Forming Polymers (both thermoresponsive or not) with complementary Non-Covalent Crosslinkers / plasticizers are tacky, wherein the complex of a specified nonequimolar stoichiometry is completely formed, even if each component of the complex individually is non-tacky.

The stoichiometry of polymer - oligomer complexes is governed by the length of short chains of Non-Covalent Oligomer Crosslinler [34,38] and a nature of high molecular weight complementary Film-Forming Polymer. As is seen from FIG. 3, in PVP blends with PEG ranging in molecular weights from 200 to 600 g/mol, the length of PVP chain segment between two neighboring PEG crosslinks is in inverse proportion to the length of oligomer chain. In the PVP blends with PEG 400, each fifth recurring unit of PVP macromolecule is crosslinked by hydrogen bonding through short PEG chain, whereas in the PVP blends with PEG-600, each ninths unit is crosslinked (FIG. 4). As has been established for the first time in present invention, while in PVP blends with PEG-400 approximately 20 % of the PVP units is hydrogen bonded to PEG terminal hydroxyl groups, forming network stoichiometric complex, in PVCL blends with the same oligomer about 60 % of recurring units is crosslinked through PEG-400 chains.

The stoichiometric composition of the polymer - oligomer complex correlates very well with adhesion properties. While in PVP blends with PEG-400 best adhesion is achieved at 36 wt. % of PEG-400 in blends, in PVCL - PEG-400 blends highest adhesion relates to 60 wt. % PEG- 400.

Examples of suitable Non-Covalent Oligomer Crosslinkers/plasticizing agents include, but are not limited to :

Low molecular weight polyalcohols (e.g. glycerol),

Monomeric or oligoalkylene glycols such as ethylene glycol and propylene glycol,

Ether alcohols (e.g., glycol ethers), hydroquinone,

Oligomers containing phenol groups, e.g. diphenyl, bisphenol A,

Alkane diols from butane diol to octane diol includingly,

Carboxyl-terminated and amino-terminated derivatives of polyalkylene glycols such as polyethylene glycol, and carbonic diacids.

Polyalkylene glycols, optionally carboxyl-terminated, are preferred herein, and polyethylene glycol having a molecular weight in the range of about 300 to 600 is an optimal Non- Covalent Oligomer Crosslinker/plasticizing agent.

OPTIONAL FILM-FORMING POLYMER

Suitable hydrophilic Optional Film-Forming Polymers include repeating units derived from an N-vinyl lactam monomer, a carboxy vinyl monomer, a vinyl ester monomer, an ester of a carboxy vinyl monomer, a vinyl amide monomer, and/or a hydroxy vinyl monomer. Such polymers include, by way of example:

poly(N-vinyl lactams),

poly(N-vinyl acrylamides),

poly(N-alkylacrylamides),

substituted and unsubstituted acrylic and methacrylic acid polymers,

polyvinyl alcohol (PVA), polyvinylamine, copolymers thereof and copolymers with other types of hydrophilic monomers (e.g. vinyl acetate).

Poly(N-vinyl lactams) useful herein are preferably noncrosslinked homopolymers or copolymers of N- vinyl lactam monomer units, with N-vinyl lactam monomer units

representing the majority of the total monomeric units of a poly(N- vinyl lactams) copolymer. Preferred poly(N-vinyl lactams) for use in conjunction with the invention are prepared by polymerization of one or more of the following N-vinyl lactam monomers: N-vinyl-2- pyrrolidone; and N-vinyl-2-valerolactam.

Nonlimiting examples of non-N-vinyl lactam comonomers useful with N-vinyl lactam monomeric units include N,N-dimethylacrylamide, acrylic acid, methacrylic acid, hydroxyethylmethacrylate, acrylamide, 2-acrylamido-2 -methyl- 1 -propane sulfonic acid or its salt, and vinyl acetate.

Polyvinyl alcohols, including polyvinyl alcohol per se and polyvinyl phenol;

Poly(oxyethylated) alcohols such as poly(oxyethylated) glycerol, poly(oxyethylated) sorbitol, and poly(oxyethylated) glucose;

Polyacrylamides such as poly(N-methacrylamide), poly(N,N-dimethylacrylamide, poly(N- vinyl acrylamide), and other poly(N-alkyl acrylamides and N-alkenyl acrylamides);

Polymers of carboxy vinyl monomers are typically formed from acrylic acid, methacrylic acid, crotonic acid, isocrotonic acid, itaconic acid and anhydride,

Acrylate and methacrylate polymers and copolymers, including poly(dialkyl aminoalkyl acrylates), poly(dialkyl aminoalkyl methacrylates), poly(hydroxyalkyl acrylates) such as poly(hydroxyethyl acrylate), and poly(hydroxyalkyl methacrylates) such as poly(hydroxyethyl methacrylate) (PolyHEMA). Preferred acrylate polymers are those copolymers available under the tradename "Eudragit" from Rohm Pharma (Germany). The Eudragit series E, L, S, RL, RS, and NE copolymers are available as solubilized in organic solvent, in an aqueous dispersion, or as a dry powder. Preferred acrylate polymers are copolymers of methacrylic acid and methyl methacrylate, such as the Eudragit L and Eudragit S series polymers. Particularly preferred such copolymers are Eudragit L-30D-55 and Eudragit L-l 00-55 (the latter copolymer is a spray-dried form of Eudragit L-30D-55 that can be reconstituted with water). The molecular weight of the Eudragit L-30D-55 and Eudragit L- 100-55 copolymer is approximately 135,000 g/mol, with a ratio of free carboxyl groups to ester groups of approximately 1 : 1. The copolymer is generally insoluble in aqueous fluids having a pH below 5.5. Another particularly suitable methacrylic acid-methyl methacrylate copolymer is Eudragit S- 100, which differs from Eudragit L-30D-55 in that the ratio of free carboxyl groups to ester groups is approximately 1 :2. Eudragit S-100 is insoluble at pH below 5.5, but unlike Eudragit L-30D-55, is poorly soluble in aqueous fluids having a pH in the range of 5.5 to 7.0. This copolymer is soluble at pH 7.0 and above. Eudragit L-100 may also be used, which has a pH-dependent solubility profile between that of Eudragit L-30D-55 and Eudragit S-l 00, insofar as it is insoluble at a pH below 6.0. It will be appreciated by those skilled in the art that Eudragit L-30D-55, L-100-55, L-100, and S-100 can be replaced with other acceptable polymers having similar pH-dependent solubility characteristics. Other preferred Eudragit polymers are cationic, such as the Eudragit E, RS, and RL series polymers. Eudragit E 100 and E PO are cationic copolymers of dimethylaminoethyl methacrylate and neutral methacrylates (e.g., methyl methacrylate), while Eudragit RS and Eudragit RL polymers are analogous polymers, composed of neutral methacrylic acid esters and a small proportion of trimethylammonioethyl methacrylate.

Alginic acid, and poly( sulfonic acids);

A 1 ,2-dicarboxylic acid such as maleic acid or fumaric acid, maleic anhydride, or mixtures thereof, with preferred hydrophilic polymers within this class including polyacrylic acid and polymethacrylic acid, with polyacrylic acid most preferred.

Preferred hydrophilic polymers herein are the following: poly(N-vinyl acetamides), particularly polyacetamide per se; polymers of carboxy vinyl monomers, particularly polyacrylic acid and polymethacrylic acid; and copolymers and blends thereof. PVP and copolymers of acrylic, methacrylic and maleic particularly preferred.

Poly(vinyl amines);

Poly(alkylene imines);

Copolymers of any of the above may also be used herein, as will be appreciated by those of ordinary skill in the art.

PLASTICIZERS:

Suitable optional plasticizers and softeners, used when needed, include, by way of illustration and not limitation: alkyl and aryl phosphates such as tributyl phosphate, trioctyl phosphate, tricresyl phosphate, and triphenyl phosphate; alkyl citrates and citrate esters such as trimethyl citrate, triethyl citrate and acetyl triethyl citrate, tributyl citrate and acetyl tributyl citrate, acetyl triethyl citrate, and trihexyl citrate; alkyl glycerolates; alkyl glycolates; dialkyl adipates such as dioctyl adipate (DOA; also referred to as bis(2-ethylhexyl)adipate), diethyl adipate, di(2- methylethyl)adipate, and dihexyl adipate; dialkyl phthalates, dicycloalkyl phthalates, diaryl phthalates and mixed alkyl-aryl phthalates, including phthalic acid esters, as represented by dimethyl phthalate, diethyl phthalate, dipropyl phthalate, dibutyl phthalate, di(2-ethylhexyl)- phthalate, di-isopropyl phthalate, diamyl phthalate and dicapryl phthalate; dialkyl sebacates such as diethyl sebacate, dipropyl sebacate, dibutyl sebacate and dinonyl sebacate; dialkyl succinates such as diethyl succinate and dibutyl succinate; dialkyl tartrates such as diethyl tartrate and dibutyl tartrate; glycol esters and glycerol esters such as glycerol diacetate, glycerol triacetate (triacetin), glycerol monolactate diacetate, methyl phthalyl ethyl glycolate, butyl phthalyl butyl glycolate, ethylene glycol diacetate, ethylene glycol dibutyrate, triethylene glycol diacetate, triethylene glycol dibutyrate and triethylene glycol dipropionate; hydrophilic surfactants, preferably hydrophilic non- ionic surfactants such as, for example, partial fatty acid esters of sugars, polyethylene glycol fatty acid esters, polyethylene glycol fatty alcohol ethers, and polyethylene glycol sorbitan-fatty acid esters, as well as non-ionic surfactants such as ethylcellosolve; lower alcohols from ethyl to octyl; sorbitol; tartaric acid esters such as dibutyl tartrate; and mixtures thereof.

A preferred plasticizer for use in conjunction with the present invention is a bifunctional oligomer that is "complementary" to the Film-Forming LCST-Ppolymer and Optional Film- Forming Polymer as described in U.S. Patent No. U.S. Patent No. 6,576,712 to Feldstein et al., cited earlier herein [32]. Preferably, the complementary Non-Covalent Oligomer Crosslinker / plasticizer is terminated with hydroxyl groups, amino or carboxyl groups. The oligomer typically has a glass transition temperature T_g in the range of about -100°C to about -30°C and a melting temperature T_m lower than about 20°C. The oligomer may be also amorphous. The difference between the T_g value of the film-forming polymer and that of the complementary oligomer is preferably greater than about 50 °C, more preferably greater than about 100 °C, and most preferably in the range of about 150 °C to about 300° C. Generally, the oligomer will have a molecular weight in the range from about 45 to about 800, preferably in the range of about 45 to about 600. Examples of suitable oligomers include, but are not limited to, low molecular weight polyalcohols (e.g. glycerol), oligoalkylene glycols such as ethylene glycol and propylene glycol, ether alcohols (e.g., glycol ethers), alkane diols from butane diol to octane diol, including carboxyl-terminated and amino-terminated derivatives of polyalkylene glycols. Polyalkylene glycols, optionally carboxyl-terminated, are preferred herein, and polyethylene glycol having a molecular weight in the range of about 300 to 600 is an optimal complementary oligomer.

The compositions of the invention may also include two or more plasticizers in combination, e.g., triethyl citrate and tributyl citrate, triethyl citrate and polyethylene glycol 400, polyethylene glycol 400 and dioctyl phthalate, etc.

OTHER ADDITIVES:

Other additives may also be incorporated into the present thermoswitchable adhesive compositions, so long as they are not detrimental to the composition in any way. For example, the following optional components are often present in adhesive formulations and are presented here for illustrative purposes only and are not meant to limit the adhesive compositions in any way. These additives, and amounts thereof, are selected in such a way that they do not significantly interfere with non-covalent crosslinking or the desired chemical and physical properties of the final adhesive.

The adhesive compositions of the invention may also include one or more conventional additive, which may be combined with the polymers and the plasticizer during adhesive formulation or incorporated thereafter. Optional additives include, without limitation, fillers, pH regulating agents, ionizing agents, tackifiers, detackifying agents, electrolytes, antimicrobial agents, antioxidants, preservatives, colorants, flavors, and combinations thereof.

In certain embodiments, the compositions of the invention may also include a pharmacologically active agent or a cosmeceutically active agent. For instance, transdermal, transmucosal, and topical delivery systems in which an adhesive composition of the invention serves as a drug reservoir and/or skin contact adhesive layer may be formulated for the delivery of a specific pharmacologically active agent. Cosmeceutical products such as tooth whitening gels and strips may be formulated for the delivery of one or more tooth-whitening agents.

Absorbent fillers may be advantageously incorporated to control the degree of hydration when the adhesive is on the skin or other body surface. Such fillers can include microcrystalline cellulose, talc, lactose, kaolin, mannitol, colloidal silica, alumina, zinc oxide, titanium oxide, magnesium silicate, magnesium aluminum silicate, hydrophobic starch, calcium sulfate, calcium stearate, calcium phosphate, calcium phosphate dihydrate, woven and non-woven paper and cotton materials. Other suitable fillers are inert, i.e., substantially non-adsorbent, and include, for example, polyethylenes, polypropylenes, polyurethane polyether amide copolymers, polyesters and polyester copolymers, nylon and rayon. A preferred filler is colloidal silica, e.g., Cab-O-Sil^® (Cabot Corporation, Boston MA).

Compounds useful as pH regulators include, but are not limited to, glycerol buffers, citrate buffers, borate buffers, phosphate buffers, and citric acid-phosphate buffers. Buffer systems are useful to ensure, for instance, that the pH of a composition of the invention is compatible with that of an individual's body surface.

Ionizing agents are also useful to impart a desired degree of ionization to the interpolymer complex within the adhesive compositions of the invention. Suitable ionizing agents are acids and bases, depending on the group to be ionized. The acids and bases may be inorganic (hydrochloric acid, hydrobromic acid, sodium hydroxide, potassium hydroxide, sodium carbonate, ammonium carbonate, etc.) or organic (acetic acid, maleic acid, triethylamine, ethanolamine, etc.).

Preferred thickeners herein are naturally occurring compounds or derivatives thereof, and include, by way of example: collagen; galactomannans; starches; starch derivatives and hydrolysates; cellulose derivatives such as methyl cellulose, hydroxypropylcellulose, hydroxyethyl cellulose, and hydroxypropyl methyl cellulose; colloidal silicic acids; and sugars such as lactose, saccharose, fructose and glucose. Synthetic thickeners such as polyvinyl alcohol, vinylpyrrolidone-vinylacetate-copolymers, polyethylene glycols, and polypropylene glycols may also be used.

The compositions of the invention can be rendered electrically conductive for use in biomedical electrodes and other electrotherapy contexts, i.e., to attach an electrode or other electrically conductive member to the body surface. For example, the composition may be used to attach a transcutaneous nerve stimulation electrode, an electrosurgical return electrode, or an EKG electrode to a patient's skin or mucosal tissue. These applications involve modification of the composition so as to contain a conductive species. Suitable conductive species are ionically conductive electrolytes, particularly those that are normally used in the manufacture of conductive adhesives used for application to the skin or other body surface, and include ionizable inorganic salts, organic compounds, or combinations of both. Examples of ionically conductive electrolytes include, but are not limited to, ammonium sulfate, ammonium acetate, monoethanolamine acetate, diethanolamine acetate, sodium lactate, sodium citrate, magnesium acetate, magnesium sulfate, sodium acetate, calcium chloride, magnesium chloride, calcium sulfate, lithium chloride, lithium perchlorate, sodium citrate and potassium chloride, and redox couples such as a mixture of ferric and ferrous salts such as sulfates and gluconates. Preferred salts are potassium chloride, sodium chloride, magnesium sulfate, and magnesium acetate, and potassium chloride is most preferred for EKG applications. Although virtually any amount of electrolyte may be present in the adhesive compositions of the invention, it is preferable that any electrolyte present be at a concentration in the range of about 0.1 to about 15 wt.% of the hydrogel composition. The procedure described in U.S. Patent No. 5,846,558 to Nielsen et al. for fabricating biomedical electrodes may be adapted for use with the hydrogel compositions of the invention, and the disclosure of that patent is incorporated by reference with respect to manufacturing details. Other suitable fabrication procedures may be used as well, as will be appreciated by those skilled in the art.

Antimicrobial agents may also be added to the compositions of the invention.

Antimicrobial agents function by destroying microbes, preventing their pathogenic action, and/or inhibiting their growth. Desirable properties of antimicrobial agents include, but are not limited to: (1) the ability to inactivate bacteria, viruses and fungi, (2) the ability to be effective within minutes of application and long after initial application, (3) cost, (4) compatibility with other components of composition, (5) stability at ambient temperature, and (6) lack of toxicity.

Antioxidants may be incorporated into the compositions of the invention in lieu of or in addition to any antimicrobial agent(s). Antioxidants are agents that inhibit oxidation and thus prevent the deterioration of preparations by oxidation. Suitable antioxidants include, by way of example and without limitation, ascorbic acid, ascorbyl palmitate, butylated hydroxyanisole, butylated hydroxytoluene, hypophophorous acid, monothioglycerol, sodium ascorbate, sodium formaldehyde sulfoxylate and sodium metabisulfite and others known to those of ordinary skill in the art. Other suitable antioxidants include, for example, vitamin C, butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), sodium bisulfite, vitamin E and its derivatives, propyl gallate, sulfite derivatives, and others known to those of ordinary skill in the art.

Other preservatives that can be incorporated into the present compositions include, by way of example, p-chloro-m-cresol, phenylethyl alcohol, phenoxyethyl alcohol, chlorobutanol, 4- hydroxybenzoic acid methylester, 4-hydroxybenzoic acid propylester, benzalkonium chloride, cetylpyridinium chloride, chlorohexidine diacetate or gluconate, ethanol, and propylene glycol.

It will be appreciated that because the adhesive compositions of the invention are useful in a variety of contexts, the desirability or need for certain additives may differ depending on the intended use. The applications in which the adhesive compositions of the invention are useful include, for example: tacky labels, easily removable advertising adhesive sheets for applications to marine and river hulls, train carriages and cars, sterile surgical drapes, drug delivery systems; topical drug-containing plasters, wound dressings; conductive hydrogels; pressure-relieving cushions for application to the skin including heel cushions, elbow pads, knee pads, shin pads, forearm pads, wrist pads, finger pads, corn pads, callus pads, blister pads, bunion pads, and toe pads, all of which can include active agents; intraoral applications such as tooth whitening strips, breath freshening films, and oral care products to treat sore throat, sores within the mouth, gingivitis, periodontal and oral infections, periodontal lesions, or dental caries or decay; adhesives for affixing medical devices, diagnostic systems and other devices to a body surface; sealants for ostomy devices, prostheses, and face masks; sound, vibration, and impact absorbing materials; carriers in cosmetic and cosmeceutical gel products; and many other uses known to or readily ascertainable by those of ordinary skill in the art, or as yet undiscovered.

MANUFACTURING METHODOLOGIES:

The properties of the compositions of the invention are readily controlled by adjusting one or more parameters during fabrication. For example, the adhesive strength of the composition can be increased, decreased, or eliminated during manufacture, by varying the type and/or quantity of different components, or by changing the mode of manufacture. It should also be noted that compositions prepared using a conventional melt extrusion process generally, although not necessarily, exhibit somewhat different properties relative to compositions prepared using a solution cast technique; for example, melt extrusion is typically more useful for preparing adhesive compositions that having lower tack than corresponding adhesive compositions prepared using solution casting.

The compositions described herein are generally melt extrudable, and thus may be prepared using a simple blending and extruding process. The components of the composition are weighed out and then admixed, for example using a Brabender or Baker Perkins Blender, generally although not necessarily at an elevated temperature, e.g., about 90 to 170°C, typically 100 to 140°C. Solvents or water may be added if desired. The resulting composition can be extruded using a single or twin extruder, or pelletized. Alternatively, the individual components can be melted one at a time, and then mixed prior to extrusion. The composition can be extruded to a desired thickness directly onto a suitable substrate or backing member. The composition can be also extruded first, and then be pressed against a backing member or laminated to a backing member. A releasable liner may also be included. The thickness of the resulting film, for most purposes, will be in the range of about 0.020 to 0.80 mm, more usually in the range of about 0.37 to 0.47 mm.

Alternatively, the compositions may be prepared by solution casting, by admixing the components in a suitable solvent, e.g., a volatile solvent such as ethyl acetate, or lower alkanols (e.g., ethanol, isopropyl alcohol, etc.) are particularly preferred, at a concentration typically in the range of about 35 to 60 % w/v. The solution is cast onto a substrate, backing member or releasable liner, as above. Both admixture and casting are preferably carried out at ambient temperature. The material coated with the film is then baked at a temperature in the range of about 80 to 100°C, optimally about 90°C, for time period in the range of about one to four hours, optimally about two hours.

In selecting the components for incorporation into an adhesive composition of the invention, the hydrophilic Film-Forming LCST-Polymer is selected first. Then, a complementary Non-Covalent Oligomer Crosslinker, with terminal functional groups capable of noncovalent bonding to the recurring polar groups within at least one linear segment of the hydrophilic Film- Forming LSCT-Polymer is selected, which, as noted elsewhere herein, is a bifunctional linear oligomer capable of forming a bridge between a polar group on one film-forming polymer chain and a polar group on a second film-forming polymer chain, thereby forming a crosslinked network complex. Optional Film-Forming Polymer is then selected if necessary. And finally, additional plasticizer is then selected, if glass transition temperature of the composition is too high to provide good adhesion and mechanical properties. The amount of the Film-Forming LSCT-Polymer is greater than the amount of the Optional Film-Forming Polymer. The content of Non-Covalent Oligomer Crosslinker in this case may be less or higher than the amount of the Film-Forming LCST Polymer.

Optional additives, including pharmacologically active agents and cosmeceutical agents, can be combined with the polymers and oligomer during adhesive preparation. Alternately, an additive can be added after the components are mixed and the composition prepared. One method of loading the composition with an active agent, for example, involves providing a layer of the composition on a substrate, coating the layer with a solution of the active agent, placing a release liner on top of the active agent layer, and allowing the active agent to become absorbed by the composition. REPRESENTATIVE COMPOSITIONS:

The purpose of this invention is a hydrophilic thermoswitchable PSA composition, based on a film-forming polymer possessing appropriate LCST, which adheres strongly to a variety of substrates, both dry and moistened, at low and ambient temperatures, but disadheres spontaneously and reversibly as temperature is elevated.

As reference examples, Figure 5 illustrates the effect of temperature on the practical work of adhesion of five PSA types, three conventional, hydrophobic PSAs, and two innovative, hydrophilic adhesives. Examples of conventional PSAs include polyisobutylene PSA (PIB), acrylic PSA DuroTak 87-900A, obtained from Henkel Adhesives,USA, and styrene-isoprene- styrene triblock copolymer (SIS). Details of hydrophobic PSA compositions are disclosed in our recent publication [39]. Examples of hydrophilic adhesives include the PSA based on hydrogen bonded complex of high molecular weight poly(N-vinyl pyrrolidone) polymer, PVP, with PEG- 400 oligomer, described in US Patent 6,576,712 (2003) by Feldstein et al. [32], and the PSA based on polyelectrolyte complex (PEC), disclosed in US Pat. Appl. 2005/0113510 and 2005/0215727 by Feldstein et al. [40,41].

As follows from the data shown in Figure 5, the PSA compositions, based on polymers possessing no LCST, do not relate to thermoswitchable materials, despites they are thermoresponsive. With the rise in temperature, adhesion always goes through the maximum at some specified temperature, which is a characteristic of a PSA material. Moreover, the PSAs never demonstrate zero tack at high temperatures below 100 °C. The temperature of adhesion maximum corresponds to a specific combination of cohesive strength and free volume, which are the factors controlling adhesive behavior [36,39].

As has been shown in [1,13,22-31], thermoswitchable PSA compositions can be only produced based on polymers possessing LCST. However, in all these instances the compositions demonstrate no tack in swollen states below LCST, and adhesion always comes into being as temperature rises above LCST and the composition desorbs water. Meantime, this invention is aimed at development of hydrophilic thermoswitchable PSAs, which should be tacky below certain critical temperature, in swollen state, and become nontacky above such temperature, when gel phase releases absorbed moisture. So far as we know, such thermoswitchable PSA compositions, losing tack under temperature elevation above LSCT, are not described in literature. Hydrophilic PSAs based on polymer - oligomer complexes of glassy poly(N-vinyl pyrrolidone), PVP, with short-chain, liquid poly(ethylene glycols), PEG, are well characterized and described in the publications of our group [32,36-39,42,43]. Both parent polymers, PVP and PEG, are non-tacky in unblended state, but high adhesion is observed in very narrow range of their blend compositions, around 36 wt. % of PEG-400. This behavior makes the PVP - PEG blends a very convenient model system in order to gain a molecular insight into nature of pressure-sensitive adhesion. With this purpose we have compared the structure and properties of the PVP - PEG blends exhibiting different adhesion.

The PVP - PEG blends reveal no thermoswitchable adhesion behavior, probably because calculated value of LCST for PVP is predicted to be too high (~ 175 °C) [44] . As follows from Figure 6, poly(N-vinyl caprolactam), PVCL, is a homologue of PVP, which contains two additional methylene groups in side-chain ring, and demonstrates LCST ranging between 31 and 37° C in dependence of molecular weigh [44 - 46]. As is obvious from comparison of water vapor sorption isotherms by PVP and PVCL, shown in Figure 7, being hydrophilic polymer, the PVCL absorbs much less water than its highly hygroscopic homologue, PVP. As has been earlier reported in [32], the PVCL blends with PEG-400 demonstrate the PSA properties resembling those of PVP - PEG system.

The author of present invention has now discovered that hydrogels based on PVCL - PEG^~ blends demonstrate thermoswitchable adhesive behavior in aqueous media. They have high tack below the temperature of phase transition (cloud point), but become spontaneously and reversibly detached as temperature increases above this critical value, when hydrogel deswelling occur.

An illustrative composition includes herein poly(N-vinyl-2-caprolactam) ("PVCL") as the Film-Forming LCST-Polymer and polyethylene glycol ("PEG") as the Non-Covalent Oligomer Crosslinker. The PVCL-PEG system is considered here as a model, but it is to be understood that the invention is not limited in this regard, and a number of hydrophilic LCST-polymers and Non- Covalent Oligomer Crosslinkers may replace the PVCL and PEG, respectively.

Mixing a PVCL-PEG adhesive blend with an Optional Film-Forming Polymer that is a moderately hydrophilic or water-insoluble polymer (at temperatures above LCST) results in the decrease of blend hydrophilicity and dissolution rate. In order to decrease the dissolution rate further or to obtain insoluble mixtures, the PVCL-PEG blend can be mixed with polymers that bear complementary (with respect to PVCL) reactive functional groups in their repeating units (Optional Film-Forming Polymers). Since the PVCL contains proton-accepting carbonyl groups in its repeating units, the complementary functional groups of the Optional Film-Forming Polymer are preferably proton-donating, hydroxyl, phenol or carboxyl groups. Thus, for use with PVCL and PEG, suitable non-covalent polymer crosslinkers are long chain polymers such as polyvinyl alcohols, polyacrylic acid, polymethacrylic acid, polymaleic acid, poly(vinyl phenol), homo- and co-polymers thereof, as well as polysulfonic acid and alginic acid.

Another illustrative composition uses with the noted above PVCL-PEG thermoswitchable adhesive hydrogel a copolymer of methacrylic acid and methyl methacrylate as the Optional Film- Forming Polymer, performing the functions of non-covalent polymer crosslinkers. This composition is used here to facilitate in understanding the principles of the invention.

The PVCL-PEG complex combines high cohesive toughness (due to PVCL-PEG hydrogen bonding) with a large free volume (resulting from considerable length and flexibility of PEG chains). In order to emphasize enhanced free volume in the PVCL-PEG blend, this type of complex structure is defined as a "carcass-like" structure (see Figure. 1). The carcass-like structure of the complex results from the location of reactive functional groups at both ends of PEG short chains. When the Non-Covalent Polymer Crosslinker contains reactive functional groups in repeating units of the backbone, the resulting interpolymer complex resembles a ladder and has so-called "ladder-like" structure (see Figure 2). The ladder-like type of interpolymeric complex was first described by Kabanov and Zezin [47,48]. While the formation of the carcass- like complex leads to enhanced cohesive strength and free volume (which determines the adhesive properties of PVCL-PEG blends), the formation of the ladder-like complex, shown in Figure 2, is accompanied by the loss of blend solubility and the increase of cohesive strength coupled with the decrease in free volume. For this reason, the structure unplasticized the ladder-like complex provides no adhesion.

Due to the decrease in free volume and the increase in cohesive energy, the PVCL-PEG blend mixed with a long chain polymer giving the ladder-like complex with PVCL, provides no or negligible initial tack. However, as the non-adhesive PVCL-PEG blend with the long chain polymer is plasticized by water, the glass transition temperature of the blend shifts toward lower values, which are typical features of pressure-sensitive adhesives, and adhesion arises.

There are certain preferred combinations of components in the adhesive composition. For example, when the Film-Forming LCST-Polymer is a poly(N-vinyl lactam) such as poly(N-vinyl caprolactam), the ladder-like polymer crosslinker (Optional Film-Forming Polymer) is preferably a poly(dialkyl aminoalkyl acrylate), poly(dialkyl aminoalkyl methacrylate), polyacrylic acid, polymethacrylic acid, polymaleic acid, polyvinyl alcohol, polyvinyl phenol, poly(hydroxyalkyl acrylate), or poly(hydroxyalkyl methacrylate) such as poly(hydroxyethyl methacrylate).

For any of the aforementioned combinations, a preferred carcass-like crosslinker is an oligomeric alkylene glycol comprising about 1-20 alkylene oxide units in its chain such as polyethylene glycol, carboxyl-terminated oligomeric alkylene glycol such as carboxyl-terminated poly(ethylene glycol), or polyhydric alcohols.

Other examples of suitable blends are shown in the following table:

To illustrate the approach used herein, a PVCL-PEG-Polyacid (Eudragit L-100-55) blend was used as a typical example, although the approach is general and can be easily reproduced using other water-soluble, hydrophilic polymers.

The properties of adhesive polymer blends were evaluated and are set forth in the examples. The behavior of these polymer blends was found to be typical of covalently crosslinked polymers. However, in contrast to covalently crosslinked systems, the triple polymer blends combining the carcass-like and the ladder-like non-covalent networks can be easily prepared using a straightforward process, and, furthermore, provide film-forming properties that are unattainable using chemically crosslinked polymers.

EXPERIMENTAL:

The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to manufacture the thermoswitchable adhesive compositions of the invention , and are not intended to limit the scope of that which the inventors regard as the invention. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperatures, etc.) but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in degrees Celsius (°C), and pressure is at or near atmospheric.

The abbreviations used in the examples are as follows:

Eudragit L 100-55: methacrylic acid copolymer (Rohm America Inc., Evonik)

Gantrez ES-425: monobutyl ether of maleic acid - methylvinyl ether copolymer (Ashland)

Gantrez S-97: maleic acid - methylvinyl ether copolymer (Ashland)

HPC: hydroxypropyl cellulose

HPMCP: hydroxypropyl methylcellulose phthalate

PEG 400: polyethylene glycol 400

PolyHEMA: poly(hydroxyethyl methacrylate)

PVA: polyvinyl alcohol

PVCL - polyvinyl caprolactam) MW « 2,500,000 (HMW PVCL, courtesy of Prof. Y.E. Kirsh, L.Y. Karpov Institute of Physical Chemistry, Moscow, Russia), and 100,000 g/mol (LMW PVCL, available from the BASF as a Luviscol-Plus^®)

PVP K90: Kollidon^® 90F polyvinylpyrrolidone (BASF)

PVPh: polyvinyl phenol

EXPERIMENTAL METHODS

Adhesive joints strength of adhesive blends and hydrogels to standard polypropylene - polyester (PP-PE) Multiplex two-layer film and corona-treated polypropylene film, both of 50 μιη in thickness, was evaluated at different temperatures by 80° peel test with an Instron 1221 Tensile Strength Tester at the peeling rate of 10 mm/min. The adhesives were saturated with water by equilibrating in dessicators with controlled pressure of water vapor of 50 % at ambient temperature for 6 - 7 days. The time to attain a maximum strength of adhesive contact with the substrate was 15 - 20 min.

Adhesive properties were also studied with probe tack tests using TA.XT.plus texture analyzer from Stable Micro Systems (UK) equipped with thermal chamber for measurements at temperature above and below ambient. The probe tack test can be divided in two stages. The first stage is compression where the flat stainless steel probe of 4 mm in diameter penetrates into adhesive film with constant rate of 0.1 mm/s and stops, when compressive bonding stress achieves a value of 0.8 MPa. After 1 s of a contact, the probe is detached from the adhesive layer at a constant rate of 0.1 mm/s. The probe used in this test was a standard, cylindrical, polished stainless steel probe obtained from Stable Micro Systems. The probe was cleaned with acetone after each test. Such a cleaning procedure was adequate to obtain meaningful and reproducible results. Force vs. time and displacement vs. time curves were thus directly obtained from this test. For each experimental condition, we carried out three to five probe tests. The specific stress-strain curves shown herein are representative of one of these individual tests while the mechanical parameters such as the maximum stress o_max, the maximum extension ε _max are average values.

Rheological properties of the adhesives in the linear viscoelastic regime were measured on a parallel plate Dynamical Mechanical Analyzer DMA 861 from Mettler Toledo, Switzerland. The amplitude of shear deformation was chosen to be in the linear regime of the elastic modulus G ' over the whole range of temperatures. In dependence on PSA properties and temperature, this zone corresponded to a deformation less than 3 μπι. All DMA measurements were performed at the temperatures ranged from -80 to 100 °C and at 1 Hz frequency. This deformation frequency was used because according generally accepted statement of the Dahlquist criterion for tack the storage modulus G ' of any tacky PSA is less than 0.1 MPa when measured at a frequency of 1 Hz. The heating rate was 3 ⁰ C/min.

Phase behavior of the hydrophilic polymers blends with PEG-400 over an entire composition range was investigated by Differential Scanning Calorimetry (DSC) using a Mettler TA 4000 / DSC-30 differential scanning calorimeter calibrated with indium and gallium. In the DSC apparatus the samples were first quench cooled with liquid nitrogen from ambient temperature to -100° C over 2 - 3 min and then heated up at a rate of 20° C min ¹ to 220° C. The glass transition temperatures, T_g, were recorded at half-height of the relevant heat capacity jumps in DSC heating thermograms. All reported values are the average of replicate experiments varying less than 1-2 %. Samples of 5-15 mg in weight were sealed in standard aluminum pans supplied with pierced lids so that absorbed moisture could evaporate upon heating. An argon purge (50 mL min^"1) was used to avoid moisture condensation at the sensor. The content of absorbed water in the blends was determined by weighing the samples before and after DSC scans using a Mettler Analytical Balance, AE 240, with an accuracy of ± 0.01 mg. Weight loss of the sample after scanning was compared to the amount of desorbed water evaluated from the enthalpy change associated with water evaporation from the sample by DSC.

Water vapor sorption isotherms: Adhesive films were equilibrated at room temperature in desiccators over aqueous H₂S0₄ solutions of controlled density which maintained the required relative humidity ranged from 10 to 90 %. Equilibrium water sorption was measured gravimetrically and confirmed with a vacuum assembly containing a MacBen quartz-spring microbalance.

EXAMPLE 1

Preparation of thermoswitchable PSA films by blending PVCL with PEG-400: Effects of blend composition on adhesion

Adhesive films of 250 - 300 μ ι in thickness were prepared by dissolving hydrophilic polymers in a common solvent (ethyl alcohol premixed with liquid PEG-400) followed by casting the solution on a backing film and drying. Unsupported PVCL-PEG hydrogels were obtained by casting the relevant solutions onto release liner followed by drying at ambient temperature.

From this point on the PVCL means high molecular weight PVCL (HMW PVCL), unless otherwise indicated.

Probe tack curves of PVCL blends with various amounts of PEG-400 are presented in Figure 8. The quantitative measure of adhesive bond strength is an area under probe tack curve, outlining the total amount of mechanical energy dissipated during adhesive debonding process and known as a practical work of adhesion.

The shape of the probe tack curves in Figure 8 is informative on the mechanisms of adhesive debonding. All PSAs demonstrate dualistic behavior, combining the properties of liquids and solid, elastic materials. Liquid like properties of the PSAs are necessary to wet the substrate surface as external bonding pressure is applied and to form good adhesive contact. At the same time, solid-like behavior of the PSAs is required to make a strong adhesive bond and dissipate large amount of mechanical energy as detaching force is applied.

Probe Tack Test is a most informative and highly illustrative tool that enables not only characterizing an adhesive joint strength, but also gaining an insight into relative contributions of solid-like and liquid-like behaviors to adhesion. As the elastic contribution dominates that of liquid-like fluidity, Probe Tack curve has a shape illustrated in Figure 8 by the curves for PVCL blends with 30 and 40 % of PEG-400, which are typical for debonding of solid-like PSAs. These curves are characterized by a sharp maximum at rather low strains and a usually comparatively small area under the stress - strain curve. Adhesive joint failure in this case proceeds predominantly through interfacial crack propagation between the probe and adhesive film surface and is called "adhesive debonding".

At the other extreme, as the liquid-like behavior prevails, Probe Tack curve has a view shown by the curve for 60 % PEG in Figure 8. This type of adhesive joint failure is characterized by comparatively low cohesion strength, indicated by lower peak of debonding stress, σ, coupled with relatively high value of elongation, ε. In this case, the adhesive joint breaks by cohesive fracture within the bulk of adhesive layer and the debonding process is governed by viscous flow. This type of debonding is also called "cohesive debonding", where some residues of adhesive are left on the probe at the end of the test.

In between these two cases, when high elasticity is properly equilibrated with large fluidity of adhesive material, the area under Probe Tack curve, defined as the practical work of adhesion, achieves its maximum value. Debonding proceeds via cavitation and fibrillation of adhesive layer, which are typical for the PSAs with optimized adhesion (curves relating to PVCL blends with 45 and 50 wt. % of PEG-400). The curves show the peaks of debonding stress followed by a more or less pronounced plateau. The curve finally ends up by a gradual or sharp decrease of detaching force to zero. Detachment in that case occurs at the interface between the probe and the adhesive layer. No macroscopic residue occurs on the probe at the end of the test. When material strain- hardening in fibrils is observed just before the final detachment, the Probe Tack stress - strain curve can also demonstrate a slight increase in the stress or a second peak.

The impact of PEG concentration on the practical work of adhesion, W, and the maximum debonding stress of the PVCL-PEG PSAs, a_max, is shown in Figure 9. Both curves go through a maximum at 40 wt. % of PEG in blend. Because this blend demonstrates transitional type of deformation from solid-like to more ductile, in following description we use PVCL blend with 45 wt. % of PEG-400 as a model platform for development of thermoswitchable PSA.

EXAMPLE 2

Relation of adhesion to stoichiometry of PVCL complex with PEG - 400 oligomer

As has been earlier established, mixing PVP with oligomeric PEG-400, accompanied by stoichiometric complex formation, is a three-stage process [35]. At the first stage (0 - 20 wt. %

PEG-400 in blends), stoichiometric PVP-PEG non-covalent network complex forms. In this complex approximately 20 % of PVP recurring units are crosslinked through terminal hydroxyl groups of PEG-400 short chains. The second stage of mixing represents gradual swelling of the stoichiometric complex in excess PEG-400, whose chains are bonded to the carbonyls in PVP repeat units only through single terminal OH - group. Throughout this stage the polymer-oligomer network complex composition is kept constant. Finally, as the total concentration of PEG-400 in blend becomes 80 wt % and higher, the amount of absorbed unbound PEG exceeds a swell ratio, and the non-covalent network complex collapses. This final stage of PVP mixing with PEG-400 is polymer dissolution in liquid oligomer.

Composition dependence of glass transition temperature in PVCL blends with PEG-400 is illustrated in Figure 10. The line relates to theoretical relationship calculated using the Fox equation (1), whereas the points represent the data of DSC measurements. As has been earlier shown, observed negative deviations of the blend T_g from the relationship predicted with the Fox equation (1), w*_ncc , relate to the weight fraction of PEG-400 molecules, forming with polymer recurring units two hydrogen bonds through both terminal hydroxyl groups (Equation 2) [33]. Dependence of the content of cross-linked PVCL recurring units, calculated with equation (2), on the blend composition expressed in terms of the amount of PEG terminal hydroxyl groups accounted for each PVCL unit in blends, is presented in Figure 1 1.

Independence of the amount of PVCL recurring units, non-covalently crosslinked by H- bonding through short PEG chains, on the blend composition is a direct evidence of non-equimolar stoichiometry of LMW PVCL complex with PEG-400. In this complex approximately 60 % of the PVCL monomer units form H-bonded network junctions with PEG-400 Non-Covalent Oligomer Crosslinker. It is notable that in PVP blends with PEG-400 the amount of crosslinked polymer recurring units is about 20 %, that is three times lower than in PVCL-PEG blends (see Figure 4).

It is also insightful to compare the composition behavior of adhesion with the stoichiometry and mechanism of stoichiometric polymer - oligomer complex formation (Figure 12). As is mentioned above, in PVP blends with PEG-400 the stoichiometric complex is fully formed at 20 % PEG in blends, while the maximum of adhesion relates to 36 wt. % PEG-400 in blends. Correspondingly, in PVCL blends with the same Non-Covalent Crosslinker, the complex is formed at PEG-400 content below 30 wt.%, and the H-bonded network is three times denser. In this connection it is of no wonder that maximum adhesion is achieved at greater PEG-400 concentration (40 - 45 wt. % PEG). In this way, qualitative correlation exists between the stoichiometric composition of PVP - PEG and PVCL - PEG polymer - oligomer complexes and their adhesive properties.

As follows from comparison of Figures 11 and 12, the molecular weight of Film-Forming LCST-Polymer affects very negligibly the stoichiometry of PVCL - PEG complex. Indeed, the data in Figures 1 1 and 12 relate to LMW and HMW PVCL, respectively.

EXAMPLE 3

Water-absorbing properties of the PSAs based on PVCL complex with PEG - 400 oligomer

Figure 13 illustrates isotherms of water vapor sorption by the PVCL blends with various amounts of PEG-400. In spite of the fact that PVCL is much more hydrophobic polymer than PVP (Figure 7), in comparison with earlier reported data on water absorption isotherms by PVP - PEG- 400 blends [43,49], the PVCL - PEG blends demonstrate similar hydrophilicity and water- absorbing capacity. Nonetheless, while in PVP - PEG systems the water absorption capacity increases with the PEG content in blends, the PVCL - PEG mixtures demonstrate somewhat different behavior. At comparable values of relative humidity the water vapor absorption grows with PEG content in the range from 10 to 40 wt. % of PEG-400 in blends, and than declines as PEG concentration achieves 45 and 50 %. In this way, the maximum of water absorption capacity coincides with the peak of practical work of adhesion (see Figure 9).

According to TGA data, at ambient conditions (room temperature and RH), equilibrium content of absorbed water in parent PVCL varies between 2.3 and 5.1 wt. %, whereas in PEG-400 the amount of absorbed water does not exceeds 1.5 wt. %. This implies that PEG-400 is less hydrophilic polymer that PVCL. By this means, the implication of obtained data is that stoichiometric network complex formation of PEG-400 with PVCL, accompanied by the decrease of T_g and the increase in free volume, leads to the decrease of water absorption. As soon as all the PEG is bound with polymer (at 40 % PEG-400), following swelling of network complex in free PEG leads to the decrease of water-absorbing capacity.

EXAMPLE 4

Effect of absorbed water on adhesion of PVCL -PEG and PV -PEG hydrogel PSAs

Both PVP and PVCL blends with PEG-400 display rather low adhesion in dry state, however, the adhesion increases in the process of blend hydration (Figure 14). Compared to PVP blend with 36 wt. % of PEG-400, the PVCL plasticized with the same amount of PEG-400 exhibits much higher adhesion. The reason for higher adhesion of the PVCL blends with PEG-400 is still elusive and needs to be investigated. However, there are strong grounds for believing that ultimate reason is in a difference of polymer - oligomer complexation mechanisms and the complex stoichiometry.

The left branches of the curves in Figure 14 relate to adhesive type of debonding, while right branches correspond to miscellaneous and cohesive types of adhesive bond failure. With this respect, the shift of maximum adhesion toward lower magnitudes of relative humidity for PVCL- PEG hydrogels is a result of a decreased cohesive toughness. This finding is explicable, because the LMW PVCL has molecular weight of 100,000 g mol, whereas the PVP molecular weight in Figure 14 is 10 times higher.

EXAMPLE 5

LCST and cloud point temperature behavior of a PSA hydrogel, based on PVCL - PEG complex

Temperature behavior of the PVCL / water system has been studied in [45]. Experiments and theoretical calculations indicate a typical Flory-Huggins (Type I) demixing behavior with an LCST. The critical concentration and LCST shift to lower values with increasing molar mass of the polymer. The classical Type I demixing results in a temperature induced continuous network swelling-deswelling. The LCST behavior shows one minimum in the demixing curve, situated close to the pure solvent axis in the temperature-concentration diagram. The demixing behavior of aqueous solutions of linear PNIPAM can be classified as Type II demixing, whereas the behavior of linear PVME in water corresponds to Type III demixing.

Effects of temperature and solution composition on cloud point behavior in PVCL-PEG- water system are demonstrated in Figure 15. The shape of curve for binary PVCL - water system is in good agreement with literature data by Meeussen et al. [45]. The LCST is observed at 36 ° C and its position is shifted toward dilute solutions ( 10 wt. % of polymer). The LCST magnitude is slightly higher than the value reported by Meeussen et al for closely approximating molecular weight polymer (-30 °C), but correlates ideally with the data by Kirsh, measured for the same PVCL sample [44]. With the rise of PVCL concentration the cloud point temperature climbs smoothly, achieving the value of 60 °C at 26 wt. % of polymer in water.

In general, the behavior of ternary PVCL - PEG (45 wt. %) - water system follows the pattern shown by the PVCL solution in water. At high water concentrations (80 - 90 %) the curves are practically superimposed. This behavior is explainable, because at such high concentrations of water, forming H-bonds with both Film-Forming LCST -Polymer (PVCL) and Non-Covalent Oligomer Crosslinker (PEG), the stoichiometric PVCL - PEG complex formation can hardly be conceived. At higher PVCL concentrations the curves run parallel to each other, the polymer - oligomer complex curve running 3 - 5 ⁰ C below that of pure PVCL.

EXAMPLE 6

Behavior of PVCL - PEG adhesive hydrogel detaching temperature

Effects of temperature and the amount of absorbed water on 180° Peel Adhesion of the HMW PVCL with 45 wt. % PEG-400 are demonstrated in Figure 16. As follows from these data, in temperature range from 20 to 90 °C, the PVCL - PEG hydrogels containing 10 and 20 wt. % of water exhibit gradual reduction of adhesion with the increase in temperature. In contrast to this behavior, the hydrogels containing 30 wt. % of water and more, demonstrate the loss of adhesion in rather narrow temperature ranges. Thus, the PVCL blend with 45 wt. % of PEG-400, containing 30 wt. % of absorbed water loses adhesion sharply between 55 and 70⁰ C. The higher the content of absorbed water, the lower the temperature of spontaneous detaching of adhesive film. Thus the temperature behavior of adhesion in the PVCL - PEG hydrogels correlates fairly reasonably with the temperature relationship of cloud point, shown in Figure 15.

Temperature transitions of mixing-demixing behavior and the change of adhesion, presented in Figures 15 and 16, are fully reversible. As opaque detached adhesive film is removed from warm aqueous solution, it becomes transparent and tacky within 1 - 1.5 minutes as the result of both cooling and partial evaporation of absorbed water.

The fact that the PVCL - PEG -400 blends with 10 and 20 wt. % of water demonstrate only smooth decrease of adhesion with temperature increase implies most likely that the amount of absorbed water in these hydrogels is too low in order to inhibit adhesion. Indeed, the PVCL is hydrophilic water-absorbing polymer (Figure 13) and the fraction of absorbed water, most toughly associated with polymer recurring units via H-bonding within the first hydrate shell, can hardly induce the demixing and the process of detaching.

As follows from Figure 16, in thermoswitchable PVCL-PEG PSA hydrogels the Peel adhesion passes through a maximum at 20 wt. % of water and is arranged in a row: PVCL- PEG+20 % water > P VCL-PEG+ 10 % water > PVCL-PEG+30 % water. Absolute values of peel force are extremely high, ranging from 2280 to 570 N/m at 20 ⁰ C. For painless skin application the value of peel adhesion is generally regarded to be maximum 350 N/m. These data, obtained with HMW PVCL, are in good accordance with the results shown in Figure 14, relating to LMW PVCL - PEG complexes.

It is worthy of note, that the PVCL-PEG PSA composition has been in first time described in US Patent 6,576,712 by Feldstein et al. (2003) [32]and later in WO 2004/108854 by R.A. Asmus, B. Etzold and J. A. Packard (2004) [50]. In both cases adhesive behavior of the PVCL- PEG PSA composition was not addressed to its LCST value and to the thermoswitchable adhesion.

EXAMPLE 7

Tuning of adhesion switching temperature

As is evident from above presented data, the adhesion switching temperature is controlled by PVCL : PEG ratio and the amount of absorbed water. While the former value is easily controllable, the adjustment of the latter is a challenge due to hydrophilicity and high water- absorbing capability of PSA material. The more the water is captured by the PVCL - PEG PSA, the lower the switching temperature (Figure 15). In order to govern the amount of absorbed water, present invention proposes using the controlling elements, which are typically backing films of different permeability for liquid water. Two kinds of backing films of different permeability for liquid water illustrate herein the procedure method of detaching temperature tuning: a PP-PET film of comparatively low permeability and a corona-treated PP film of higher permeability.

As is seen from Figure 17, the kinetics of liquid water penetration across one-layer corona- treated PP membrane has linear character. Treatment of hydrophobic PP film with corona discharge leads to the appearance of hydrophilic propylene glycol functional groups as is ascertained with FTIR spectroscopy. As a result, the treated membrane displays a high permeability for liquid water. Linear character of water transport kinetics is a feature of one-layer, homogeneous polymer membranes. For multilayer inhomogeneous membranes the relationship between the amount of water penetrated across polymer film on time is often described by more complex functions.

With the rise of temperature the permeability of liquid water across the polymer films increases, as is evidenced by the data presented in Figure 18 for the corona-treated PP film.

Application of polymer membranes controlling water permeability, which serve as backing films of adhesive patches or tacky labels, allows fine tuning of detaching temperature. As follows from the data shown in Figures 15 and 16, detaching temperature is a function of the content of absorbed water in thermoswitchable PSA polymer. The time / required to achieve the patch or label detaching is a function of water permeability and can be evaluated using Equation (3):

JCST H-)0 pol JCST

where m is adhesive polymer mass (g), S is the area of adhesive sample covered by a water-permeable backing film (cm²), V_H is a rate of liquid water penetration across backing film (mg/cm² min), w_H > and w_poi are weight fractions of absorbed water and adhesive polymer, relating to particular value of Critical Solution Temperature (CST) on phase diagram presented in Figure 15, p is adhesive polymer density (g/cm ), and / is a thickness of adhesive layer. The density of adhesive polymer material can be evaluated using Equation (4):

w pol w

+^■ PEG

(4)

P Ppv . PPEG

where p and w are the densities and weight fractions of PVCL and PEG-400 in adhesive blend.

According to Equation 3, the time to the moment of adhesive patch detachment at particular CST value is directly proportional to the mass and thickness of adhesive material layer, and inversely related to the flux of water across a permeability controlling backing film. Under our experimental conditions, the thickness of adhesive layer was in the range of 150 - 200 μηι.

Necessary permeability levels of liquid water across the backing films result from a character of specific applications of the products based on thermoswitchable hydrophilic PSA compositions. In following description a range of specific applications serves to illustrate the method of tuning the switching temperature.

EXAMPLE 7.1

Adhesion switching temperature 37 - 45 ⁰ C. Painless removal of skin applications.

Occlusive backings

In order to enhance transdermal drug delivery rate, many transdermal patches contain water-impermeable, occlusive backing films which provide accumulation of moisture at the application site and promote percutaneous penetration of therapeutic agents via hydrated epidermis. Similarly, single-use surgical drapes are designed to establish and maintain a sterile field during and after thoracoabdominal operations. Due to large application areas of the surgical drapes, their painless removal from skin surface after operation is a problem of great difficulty and importance. In both instances, thermoswitchable PSA compositions detaching at temperature slightly above body temperature can be useful.

Painless removal of skin applications is illustrated by following example. When patient takes a bath at temperature ranged from 39 to 45 °C, spontaneous detaching of adhesive skin application occurs as the amount of absorbed water achieves 45 % and more (Figure 15). Because backing film is impermeable for liquid water or possesses very low liquid water transmittance rate, the time required for spontaneous detaching of adhesive patch or surgical drape from skin surface is expected to be extremely long. In this way, the flux of water across impermeable baking film is unable to provide spontaneous patch detaching. However, in addition to the the water transfer across backing film, there is another moisture transfer process, transdermal moisture loss, directed from deep layers of live derma through stratum corneum to the surface of epidermis. Under conditions of warm bath, when skin epidermis is highly hydrated, such outward-directed percutaneous water loss represents basic component of thermoswitchable PSA hydration process.

When the patient plunges into a bath with warm water for 25 minutes, edges of an adhesive film start to come unstuck and rise. Under action of grinding with sponge, this process leads to gradual adhesive film peeling off. Detaching occurs without the application of significant external efforts and it is not accompanied by painful feelings.

EXAMPLE 7.2

Switching temperature 45 - 90 ⁰ C. Removal of adhesive labels. Non-occlusive backings

For water-permeable, non-occlusive backing films, thermoswitchable PSA layer is capable to capture suffitient amount of water over comparatively short time that can be evaluated using Equation 3 and the data shown in phase diagram (Figure 15).

Wash away of the glass or plastic containers from adhesive labels can be performed at the temperatures exceeding pain threshold of human skin (50 °C and higher).

Method of tuning the detachment temperature based on phase diagram data for thermoswitchable PSA - water system and Equation (3) is illustrated by the information presented in Table 2. Table 2. Water permeability across backing films and detaching time estimated with Equation 3 for 30 % hydration of adhesive layer.

The higher the v/ater transmittance rate through backing film used as rate-controlling membrane, the shorter time is required to detach adhesive film from substrate.

Correspondingly, if detaching is desirably to achieve at 45 °C and at the amount of absorbed water 45 wt. % (see Figure 15), for the CTPP backing film, demonstrating water transmittance rate of 7.85 mg/cm² min at this temperature (Figure 18), according to Equation (3) the estimated time to adhesive patch detachment should be 3 - 4 min. However, if the thickness of adhesive layer is decreased from 150 to 50 μηι, the estimated time to detachment will be 1 - 1.5 min.

EXAMPLE 8

Mechanical properties of thermoswitchable PVCL-PEG PSA

End-use adhesive properties of materials depend on those molecular parameters which also dictate their viscoelastic behavior. It is, therefore, not surprising that adhesive properties and viscoelastic behavior of polymer materials are closely related [39]. The relationship between adhesion and mechanical properties of the PSAs has been described in terms of generally accepted the Dahlquist's criterion of tack. The Dahlquist criterion, which defines the value of the storage modulus, G', below 0.1 MPa, featured for all the PSAs demonstrating maximum work of debonding, has been found to have a universal character and holds at corresponding temperatures for all the PSAs examined, including both typical and innovative adhesives [39]. As has been shown in [39], the temperature relationship of debonding energy (W) for the PVP - PEG PSA shows a smooth maximum between 30 and 40 °C. Temperature sweep curves of the storage modulus, G' , loss modulus, G' ' , and the G" : G' ratio defined as the loss tangent (tan δ) reveal a glass transition at 10 °C (G' ' peak), and the tan δ maximum (tan δ = 2.66) at 24 °C, where a well pronounced viscoelasticity plateau begins. The point of maximum adhesion of the PVP - PEG PSA at 40 °C relates to G'= 0.05 MPa and tan δ = 0.92.

Following DMA data, shown in Figure 19, thermoswitchable PVCL - PEG PSA demonstrates a glass transition (G" peak) in a range from - 8 to +2 °C. The loss tangent has a maximum (tan δ « 1.56) at 30 °C. The well pronounced viscoelasticity plateau begins at ~ 40 ⁰ C, that is at slightly higher temperature than for the PVP - PEG PSA. The area of the G' values outlined by the Dahlquist criterion of tack (G' < 0.1 MPa) lies at temperature above 30 ⁰ C. In this way, viscoelastic properties of the PVCL - PEG PSA in linear elastic region are in good accordance with the Dahlquist criterion of tack, resemble fairly reasonably the behavior of the PVP - PEG PSA and are typical of others PSAs.

EXAMPLE 9

Effect of an Optional Film-Forming Polymer on the properties of PVCL-PEG PSA The Film-Forming LCST-Polymer, PVCL, is a polybase, containing proton-accepting functional groups in the recurring units of its backbone. Respectively, Optional Film-Forming Polymers, behaving as ladder-like noncovalent crosslinkers of the PVCL, belong to the class of polyacids, bearing proton-donating functional groups in their monomer units. Suitable Optional Film-Forming Polymers include homo- and copolymers of acrylic, methacrylic and maleic acid, carboxyl and hydroxyl derivatives of cellulose, polyvinyl phenol and polyvinyl alcohol. Examples of commercially available Optional Film-Forming Polymers include a copolymer of methacrylic acid with methyl- and butyl methacrylate, known as Eudragit L- 100-55, copolymers of maleic acid with methyl ether of polyvinyl alcohol (Gantrez ES-425 : Gantrez S-97), carboxymethyl cellulose (CMC), hydroxypropyl cellulose (HPC), hydroxypropyl methylcellulose phthalate (HPMCP), polyvinyl phenol (PVPh), and polyvinyl alcohol (PVA):

One embodiment of a PSA composition including both Film-Forming LCST -Polymer (PVCL) and Optional Film-Forming Polymer (Eudragit L- 100-55) was prepared from the following ingredients using a casting - drying process. Preparation of the films: 50 g of PEG 400 was dissolved in 200 g of ethanol. Under vigorous stirring, the HMW PVCL powder was added in amounts as indicated below followed by addition of the Eudragit L 100-55 powder. The mixture was stirred over 2 hours to obtain a homogeneous solution. The solution was stored over 2-5 hours to let air bubbles dissipate. Polymer films were prepared by solution casting onto a PET backing, followed by drying at ambient temperature over 3 days. Films of 0.20 ± 0.03 mm were obtained. The water content in the obtained films was measured gravimetrically by weight loss at 120°C. The water content in the films was found to be in the range 4 ± 1.5 wt%.

In another embodiment a melt extrusion process was used to prepare the adhesive composition including Film-Forming LCST-Polymer, Optional Film-Forming Polymer and Oligomer Non-Covalent Crosslinker of the LCST polymer. The blend composition was as follows:

Eudragit L 100-55 9 wt%

PVCL 44 wt%

PEG 47wt%

The ingredients were melt processed in a Brabender single screw extruder as follows. The ingredients were melt processed in a Brabender single screw extruder as follows: The Eudragit

LI 00-55 was added to the extruder first, followed by PVCL and PEG at a temperature of 100 to 150 °C. The composition was extruded to a thickness of 0.35 mm between two polyethylene terephthalate release liners.

The swelling properties of the films were tested gravimetrically. The samples were placed into a 0.1 M buffer solution, at least 200-fold amount of solution was taken with respect to the sample weight. The samples were stored over 3 days at 25°C. The swollen samples were then accurately removed and dried at 110°C. The Swell Ratio and Sol Fraction were calculated as follows: Swell Ratio = ma /m_s ; Sol Fraction, % = 100 · (m₀ - m )/ mo, where m₀ is initial sample weight, m_s is the weight of the swollen sample and ma is the weight of the sample after drying. According to the data shown above, the higher the pH in water, the greater the swell ratio, whereas the fraction of soluble blend was only slightly affected by pH. The higher the pH, the greater the degree of ionization of Eudragit carboxyl groups and the higher the swelling of the ladder-like complex in water. This data implies that the solubility of the blend in water (expressed in terms of sol fraction) is controlled by non-covalent crosslinking with the Eudragit and depends on the crosslinker content. Actually, with the increase of Eudragit concentration, the sol fraction decreased correspondingly. The value of sol fraction was close to the content of PEG 400 in blends, while the PVCL was mainly in an insoluble state due to the ladder-like crosslinking with Eudragit.

Swell ratio is a measure of the degree of non-covalent crosslinking of the Film-Forming

LCST-Polymer (PVCL). The higher the concentration of the ladder-like crosslinker, Eudragit L 100-55, the lower the swell ratio and the denser the network of PVCL-Eudragit hydrogen bonds. The Oligomer Non-Covalent Crosslinker, PEG, caused the increase of both swell ratio and sol fraction. By this way, the swelling and dissolution of PVCL-PEG-Eudragit triple blends can be readily changed by the change in blend composition.

Varying the ratio of Film-Forming LCST-Polymer (PVCL) to the ladder-like crosslinker (Eudragit L 100-55) and the content of the Oligomer Non-Covalent Crosslinker (PEG 400), was found to be a feasible tool to control the mechanical properties of adhesive hydrogels. Tensile properties of the PVCL-PEG-Eudragit hydrogels were typical of those for cured rubbers. Adding the ladder-like non-covalent crosslinker, Optional Film-Forming Polymer (Eudragit), to the PVCL- PEG adhesives caused a sharp gain in mechanical strength and the loss of ductility. The ultimate tensile stress came through a maximum at 8% Eudragit content, while the maximum elongation at break decreased smoothly with the rise of the Eudragit concentration for single-phase blends. Two-phase compositions exemplified by 36% Eudragit blend exhibited a slight increase of ductility, accompanied with the loss of cohesive strength. The Oligomer Non-Covalent Crosslinker, PEG, was a good plasticizer for the PVCL- Eudragit blends. The rise in PEG content promoted the ductility of hydrogel films. In this way, the behavior of triple the PVCL - Eudragit L- 100-55 - PEG - 400 blends was found to be similar to the properties of the PVP - Eudragit L- 100-55 - PEG-400 blends described in U.S. Patent 6,576,712 to Feldstein et al. References

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Claims

1. A hydrophilic thermoswitchable pressure-sensitive adhesive composition reversibly detaching in aqueous media under temperature elevation, based on a stoichiometric hydrogen-bonded complex of at least one hydrophilic film-forming polymer selected from polymers possessing Lower Critical Solution Temperature (LCST) in water with an Oligomeric Non-Covalent Crosslinker of the film-forming polymer that contains complementary reactive functional groups at both ends of its short chains.

2. The composition of claim 1, wherein the content of the Film-Forming LCST- Polymer is 30 - 60 wt.%.

3. The composition of claim 1 , wherein the Oligomer Non-Covalent Crosslinker is selected from short-chain polymers which are capable of crosslinking the hydrophilic film-forming LSCT-polymer by hydrogen bonds formed between both terminal complementary functional groups of oligomer chain and recurring functional groups in the backbone of the Film-Forming LCST-Polymer.

4. The composition of claim 1, wherein the hydrophilic Film-Forming LCST- Polymer and the Oligomer Non-Covalent Crosslinker are selected from the polymers capable of forming the stoichiometric network complex.

5. The composition of claim 1, wherein the hydrophilic Film-Forming LCST- Polymer is a poly(N-vinyl lactam)..

6. The composition of claim 1 , wherein the Film-Forming LCST-Polymer is poly(N- vinyl caprolactam) homopolymer, PVCL.

7. The composition of claim 1 , wherein the Film-Forming LCST-Polymer is N-vinyl caprolactam copolymer.

8. The composition of claim 1, wherein the hydrophilic Film-Forming LCST-

Polymer has a number average molecular weight in the range of approximately 10,000 to 5,000,000 g/mol.

9. The composition of claim 1, wherein the hydrophilic Film-Forming LCST- Polymer has a number average molecular weight in the range of approximately 100,000 to 3,000,000 g/mol.

10. The composition of claim 1 , wherein the Oligomer Non-Covalent Crosslinker is selected from the group consisting of polyalcohols, monomeric and oligomeric alkylene glycols, polyalkylene glycols, carboxyl-teminated polyalkylene glycols, amino-terminated polyalkylene glycols, ether alcohols, alkane diols, hydroquinone, diphenyl, bisphenol A, and carbonic diacids.

1 1. The composition of claim 1 , wherein the Oligomer Non-Covalent Crosslinker is selected from the group consisting of polyalkylene glycols and carboxyl-terminated polyalkylene glycols.

12. The composition of claim 10, wherein the polyalcohols are glycerol, sorbitol, xylitol and others similar polyalcohols.

13. The composition of claim 1 , wherein the Oligomer Non-Covalent Crosslinker is polyethylene glycol having molecular weight from 200 to 600 g/mol.

14. The composition of claim 1 that contains at least one optional film-forming polymer with recurring functional groups capable of forming hydrogen bonds with complementary functional groups in the recurring units of the Film-Forming LCST-Polymer.

15. The composition of claim 14, wherein the Optional Film-Forming Polymer is selected from polyacrylic acid, polymethacrylic acid, polymaleic acid, polysulfonic acid, polyalkylene oxides, polyvinyl alcohols, polyvinyl phenols, poly(hydroxyalkyl acrylates), poly(hydroxyalkyl methacrylates), poly(N-vinyl acrylamides), poly(N-alkylacrylamides), polar derivatives of cellulose containing hydroxyl and carboxyl groups, alginic acid, combinations and copolymers thereof.

16. The composition of claim 15, wherein the cellulose derivative is a cellulose ester polymer containing unesterified cellulose monomer units, cellulose acetate monomer units, and either cellulose butyrate monomer units or cellulose propionate monomer units.

17. The composition of claim 15, wherein the cellulose derivative is a polymer containing hydroxyalkyl cellulose monomer units or carboxyalkyl cellulose monomer units.

18. The composition of claim 15 , wherein the acrylate-based polymer or copolymer is selected from polymers and copolymers of acrylic acid, methacrylic acid, methyl acrylate, ethyl acrylate, methyl methacrylate, and ethyl methacrylate.

19. The composition of claim 14, wherein the Optional Film-Forming Polymer is selected from water-soluble cellulose derived polymers, homopolymer and copolymers of vinyl alcohols, homopolymer and copolymers of vinyl phenols, homopolymer and copolymers of alkylene oxides, homopolymer and copolymers of maleic acid, alginates, starches, naturally occurring polysaccharides, and combinations thereof.

20. The composition of claim 14, wherein the molecular weight of the Optional Film- Forming Polymer is less than the molecular weight of the Film-Forming LCST-Polymer.

21. The composition of claim 1 , further comprising at least one additive selected from absorbent fillers, preservatives, pH regulators, plasticizers, softeners, thickeners, antioxidants, pigments, dyes, conductive species, refractive particles, stabilizers, toughening agents, tackifiers or adhesive agents, detackifiers, flavorants and sweeteners, therapeutic agents and skin permeation enhancers.

22. A thermoswitchable pressure sensitive adhesive material for application onto various surfaces that capable of reversibly disadhering in aqueous media under temperature elevation and includes a backing film with the surface coated by the layer of the adhesive composition of claim 1.

23. The adhesive material of claim 22 wherein the backing film is a polymer membrane with predetermined permeability for water.

24. The adhesive material of claim 22 representing a plaster, patch, label or adhesive tape.

25. A method of manufacturing the thermoswitchable pressure-sensitive adhesive material of claim 22 with predetermined detaching temperature from contact surface of a substrate in aqueous medium, comprising:

(a) preparation of the adhesive composition of claim 1 ;

(b) determining the amount of absorbed water required for a phase separation in the prepared adhesive composition under appropriate temperature;

(c) selection of a backing film with predetermined rate of water transport, defining the amount of the adhesive composition necessary for coating the backing film, determination of the backing film area, and the calculation of expected time (t) necessary to detach the adhesive material from a substrate at the set tem erature using the following Equation:

where m is adhesive polymer mass (g), S is the area of adhesive patch, label or tape covered by a water-permeable backing film (cm²), VH¾D is a rate of liquid water penetration across the backing film (mg/cm min), W_H¾D and w_poi are weight fractions of absorbed water and adhesive polymer, relating to particular value of Critical Solution Temperature (CST) on phase diagram of adhesive composition - water system.

26. The method of claim 25 characterized by measuring the amount of absorbed water necessary for phase separation in the prepared adhesive composition under temperature of interest from a temperature diagram of the phase separation in the mixture of the prepared adhesive composition with water.