WO2014184566A1

WO2014184566A1 - Method of forming polymer nanocomposites in joins between substrates

Info

Publication number: WO2014184566A1
Application number: PCT/GB2014/051497
Authority: WO
Inventors: Jas Pal Singh Badyal; Thomas J. Wood
Original assignee: Surface Innovations Limited
Priority date: 2013-05-17
Filing date: 2014-05-16
Publication date: 2014-11-20
Also published as: GB2516352A; GB201308973D0; GB201408689D0

Abstract

A method of forming a polymer nanocomposite in a join between first and second substrate elements. The method comprises introducing an atomized monomer and a nanofiller component into an excitation medium to activate at least part of the monomer and/or nanofiller component; and exposing the join between the substrate elements to the monomer and nanofiller component to form a polymer nanocomposite in the join.

Description

METHOD OF FORMING POLYMER NANOCOMPOSITES IN JOINS

BETWEEN SUBSTRATES

Field of the invention

The present invention relates to polymer nanocomposites. In particular, though not exclusively, the invention relates to methods of forming polymer nanocomposites in joins between substrate elements, as well as structures and composites formed thereby.

Background to the invention

Polymer nanocomposites, which comprise a polymer or copolymer matrix having nanoparticles dispersed therein, are known. They find application in a range of technological areas.

Generally, owing to the large interface between the nanoparticles and the polymer matrix, such materials possess a high potential with regard to their chemical, physical and mechanical properties, which cannot be achieved by milli- or microscale dispersions of conventional inorganic constituents in polymer matrices.

Many processes are known to date for producing polymer nanocomposites. Such processes may, for example, be based on direct mixing of nanoparticles with a polymer in solution or the melt, the in situ preparation of polymer by polymerizing monomers in the presence of inorganic nanoparticles, sol-gel techniques and combinations of these measures.

The uses of polymer nanocomposites depend on their particular structure, i.e. the type and proportions of polymer or copolymer and nanoparticles. Polymer nanocomposites formed from hydroxyl monomers, i.e. monomers comprising at least one -OH functional group, are of interest in a variety of areas. For example, poly(2-hydroxyethyl methacrylate) coatings are used for a plethora of technological applications including heavy metal ion removal,¹ luminescent materials,^2,3, biomaterials,^4,5,6 nanostructures,^7,8 polymer electrolytes,⁹ bioactivity,¹⁰ tissue culture,^11,12,13 and solar cells.¹⁴ Furthermore, the inherent biocompatibility of poly(2-hydroxyethyl methacrylate)¹⁵ makes it suitable as an adhesive for biomedical applications such as dentistry¹⁶ and bone implants.¹⁷ Nanocomposites can be formed by the addition of inorganic particles (e.g. zinc oxide,² calcium carbonate,¹⁸ or silica^19,20,21) to this polymer, which can be utilized to improve, for example, the luminescent,² water uptake,²¹ or mechanical properties of materials.^18,19,20 Previous methods for preparing poly(2-hydroxyethyl methacrylate) nanocomposite layers have included sol-gel reaction,^2,19,20 free radical polymerization,²² photopolymerization,^23,24 emulsion polymerization,^25,26 controlled radical polymerization,^27,28 in-situ reduction^29,30 and solution intercalation.³¹ Such wet chemical approaches tend to require catalysts,²⁷ high

temperatures,²⁰ multiple steps,² or long reaction times.¹⁹

Plasmachemical deposition of functional thin films is recognized as being a single-step, solventless technique, which provides conformal coatings.³² It has previously been shown that where an electrical discharge is modulated in the presence of precursor vapour high levels of functional retention can be achieved.³³ An alternative approach for achieving such high levels of structural retention is to raise precursor vapour pressure within the reactor (i.e. increase the pressure/flow rate), such that the average plasma power per reactant molecule decreases.^32,34 However, in this case there exist limitations due to high precursor vapour pressures/flow rates leading to plasma instabilities/inhomogeneity and eventually extinction.

Particular difficulties arise in applying polymer nanocomposites in joins between substrate elements, for example in order to hold such substrate elements together.

It is an object of the invention to provide a method of forming polymer nanocomposites which is particularly suited to joins. It is also an object of the invention to provide polymer nanocomposites with improved properties, for example improved adhesion and/or mechanical strength. Statements of the invention

From a first aspect, the invention resides in a method of forming a polymer nanocomposite in a join between first and second substrate elements, the method comprising: introducing an atomized monomer and a nanofiller component into an excitation medium to activate at least part of the monomer and/or nanofiller component; and exposing the join between the substrate elements to the (activated) monomer and nanofiller component to form a polymer nanocomposite in the join . Preferably the excitation medium may comprise a plasma discharge.

It has been found that atomized spray deposition of excited monomer and nanofiller, particularly atomized spray plasma deposition (ASPD) is particularly effective and

advantageous in forming polymer nanocomposites in joins. Atomized spray of monomer (precursor) is known in the art ^{35, 36} and discussed in WO2006092614 in the context of nanocomposites. However, it has now been found that such deposition allows for particularly effective penetration between substrate joins.

Previous approaches for preparing polymer nanocomposites have entailed wet chemical syntheses, which involve multiple steps,² high temperatures,²⁰ and normally require solvent extraction as well as a separate casting step.²⁶ In contrast, atomized spray plasma deposition of the invention may be carried out as a single step direct application. An additional advantage of the atomized spray plasma is that deposition rates can be vastly enhanced compared to conventional vapour-phase plasma polymerization (e.g. by a factor exceeding 250),⁴¹ which is due to the high speed of monomer delivery into the plasma zone.

Precise conditions under which the excitation medium, e.g. plasma discharge, provides for activation of the monomer may vary depending upon factors such as the nature of the monomer and the substrate. In general, the plasma discharge may take any suitable form, and is generated using techniques known in the art. Suitable plasmas for use in the invention include non-equilibrium plasmas such as those generated by radiofrequency (RF), microwave, audiofrequency or direct current (DC). RF plasmas tend to be particularly homogeneous. In one embodiment, the plasma discharge is generated by a radiofrequency power supply. The output impedance of such a supply may be matched to a partially ionized gas load via an L-C matching unit. Plasma discharges may be pulsed or continuous. In one embodiment, the plasma discharge is a continuous-wave plasma. Continuous wave plasma is particularly suited to atomized monomer activation. Without wishing to be bound by theory, it is believed that the plasma initiates monomer activation at the surface of atomized monomer droplets, with a polymerisation reaction then propagating inwards. Continuous wave plasma assists more complete droplet polymerization resulting in improved nanocomposite characteristics.

The power of the plasma discharge may be adjusted to any suitable value. The power will generally depend on the desired rate and type of deposition, the nature of the monomer and nanoparticles, the degree of atomisation (if any) of the monomer, the rate of introduction (or flow rate) of the monomer and nanoparticles, the size of the reactor etc... In general a plasma discharge may, for example, have a power in the range of from 1 to 900 W. In embodiments, the power is in the range of from 20 to 80 W. Where a pulsed plasma is employed, an average power may be specified. In one

embodiment, the average power of the plasma may be in the range of from 0.01 to 500 W, such as in the range of from 1 to 50 W.

The power density of the plasma may also be tailored. The power density of the plasma is defined herein as the plasma power per unit volume of a reactor in which the plasma is contained. In embodiments, the power density of the plasma is in the range of from

O.OOlW/cm³ to 2 W/cm³, such as in the range of from 0.02W/cm³ to 0.1 W/cm³.

In one embodiment, the plasma discharge is operated under sub-atmospheric pressure, e.g. 100 mbar or less, or even 10 mbar or less. Such sub-atmospheric pressures facilitate ignition of the plasma and removal of unreacted monomer or side products. It has been found that the method of the present invention may be carried out advantageously at a sub atmospheric pressure in the range 3 to 8 mbar, e.g at a pressure of approximately 6 mbar. Where a higher pressure is employed, the discharge may advantageously be operated in the presence of air or an inert gas, e.g. helium and/or argon. In one embodiment the plasma discharge is a sub-atmospheric continuous wave plasma generated by a radiofrequency power supply.

The excitation medium, e.g. plasma discharge and/or any ionized gas stream resulting therefrom, are generally contained in a reaction chamber into which the monomer and nanofiller component are introduced. Such reaction chambers are known in the art, for example from WO2006/092614. The volume of the reaction chamber may vary. In one embodiment, the volume of the reaction chamber is in the range of from 0.5 to 3 litres. The reaction chamber may be enclosed in a Faraday cage. In an embodiment, the reaction chamber comprises a precursor inlet through which the monomer and/or nanoparticles are introduced. The precursor inlet may comprise a conductive coil, e.g. a copper coil, for generating a plasma discharge. The coil may define a plasma discharge region of the reactor.

In one embodiment, the monomer and/or nanoparticles may be introduced and travel through a plasma discharge region such that their average residence time in the plasma discharge region is in the range of from 0.01 to 20s, such as in the range of from 0.1 to 2s.

In one embodiment, the method comprises applying a preliminary plasma before introduction of the monomer. Such a preliminary plasma can aid the removal of contaminants from the reactor and may preferably be continuous. The nanofiller component comprises (or may consist entirely of) nanoparticles and/or a nanoparticle precursor. Nanoparticles are defined herein as particles having at least one dimension less than 100 nm. In one embodiment, the nanoparticles are defined according to ASTM E2456-06. The nanofiller component may, for example, consist of a distribution of particles with a volume mean particle size (diameter) of less than 500 nm, preferably less than 100 nm, or less than 50 nm. Typically, the volume mean particle size is at least 1 nm, e.g. at least 5 nm. Particle sizes may be measured by dynamic light scattering, e.g. using a Horiba SZ 100 nanoparticle analyser. The shape of nanoparticles and any other particles in the nanofiller component may vary. For example, the nanoparticles may be generally spherical, branched, stellar, tube, oblate or platelet-like.

The chemical nature of the nanoparticles in the nanofiller may be chosen consistent with achieving desired properties in the polymer nanocomposite. In an embodiment, the nanoparticles are inorganic. The nanoparticles may, for example, comprise a metal or metalloid oxide, carbide or nitride. In an embodiment, the nanoparticles comprise material selected from one or more of: Si0₂, Al₂0₃, Ti0₂, ZnO, CaC0₃, fused silica, borosilicate, quartz, and glass. In one embodiment, the nanoparticles comprise silica, in particular fumed silica, nanoparticles.

The nanofiller component may comprise or consist of surface-modified nanoparticles. In an embodiment, the nanoparticles are inorganic, e.g. silica, and comprise a surface modification that facilitates dispersion of the nanoparticles in the monomer. In an embodiment, the surface modification may comprise a functional group capable of reacting with the monomer. In an embodiment, the surface modification may comprise a silanol or a silane. The silane may, for example, comprise 3-methacryloxypropyltrialkoxysilane or hexadecyltrialkoxysilane.

In one embodiment the nanofiller component comprises methacryloyl modified inorganic particles and the monomer is an acrylate or methacrylate bearing monomer. In another embodiment, the nanofiller component comprises alkyl modified inorganic particles (e.g. silica, 805/R816 Aerosil, Evonik) and the monomer is an alkyl bearing monomer. In yet another embodiment, the nanofiller component comprises lH,lH,2H,2H-Perfluorooctyldimethylchloro- silane modified silica and the monomer is a perfluoroalkyl bearing monomer. The term "monomer" is used herein to embrace any molecule which can be bonded to another molecule to form a polymer or copolymer. Thus this term may also embrace oligomeric molecules. In an embodiment, the monomer has a (number average) molecular mass of less than 500 Da, preferably less than 200 Da, or less than 150 Da.

The monomer may be chosen consistent with achieving desired properties in the polymer nanocomposite. The monomer may optionally be used in combination with one or more comonomers to form a polymeric nanocomposite comprising a copolymer matrix. In an embodiment, the comonomer is present in an amount of at most 50 % wt, such as at most 20% by weight of total monomer and comonomer.

The monomer comprises at least one polymerizable functional group. In one embodiment, the monomer is an organic, ethylenically unsaturated monomer. In an embodiment the monomer is a hydroxyl monomer, i.e. comprises at least one -OH functional group. The selection of a hydroxyl monomer results in nanocomposites in which a high level of functional retention of the hydroxyl monomers acts in synergy with the nanofiller to offer uniquely advantageous potential with regard to chemical, physical and mechanical nanocomposite properties, particularly in the context of joins. The at least one -OH (hydroxyl) group of the monomer enhances adhesion of the nanocomposite, particularly to surfaces such as glass. The hydroxyl group may constitute or form part of a pendant group.

Indeed, from a second aspect, the invention resides in a method of forming a polymer nanocomposite on a substrate, the method comprising: introducing an atomized hydroxyl monomer and a nanofiller component into an excitation medium, e.g. a plasma discharge, to activate at least part of the hydroxyl monomer and/or the nanofiller component; and exposing the substrate to the hydroxyl monomer and nanofiller component to form a polymer nanocomposite thereon. One particular class of useful monomers are acrylic hydroxyl monomers, for example acrylic or methacrylic acid esters or amides comprising one or more hydroxyl groups. Acrylates, in particular methacrylates, are preferred.

In an embodiment, the monomer is a compound of formula (I)

in which formula R¹, R² and R³ are independently selected from hydrogen, alkyl or aryl and R⁴ is a group X-OH where X is an amide, alkyl and/or aryl bridging group or a group of formula -C(0)0(CH₂)_n- where n is an integer of from 1 to 10.

Suitably R¹, R² and R³ may be independently selected from hydrogen and alkyl groups having from 1 to 6 carbon atoms. At least one, preferably two, of R¹, R² and R³ may be hydrogen. In an embodiment, R¹ and R² are hydrogen and R³ is an alkyl group such as methyl, ethyl or propyl.

Where X is a group -C(0)0(CH₂)_n-, n is an integer which provides a suitable spacer group. In particular, n may be from 1 to 5, preferably 1 to 3. In one embodiment the monomer is 2- hydroxyethyl methacrylate.

To facilitate introduction of the monomer, the monomer may preferably be liquid at standard temperature and pressure. It may have a relatively low kinematic viscosity that facilitates atomization. Particularly to accommodate higher viscosity monomers or monomers with high melting points, the method may comprise solubilizing the monomer and introducing it as part of a monomer component. Such a monomer component may comprise or consist of one or more monomers, solvent and any other suitable additives. In a preferred embodiment the precursor is in the form of a liquid as opposed to in the form of a solution so that it is not necessary to remove the solvent from the area in which the reaction is carried out. In one embodiment, the monomer is introduced at ambient temperature. Alternatively, the monomer may be heated, e.g. to a temperature of at least 100 °C such as at least 200 °C prior to its introduction.

The monomer may be atomized in any suitable manner. It has been found that droplet size has an advantageous effect on the method of the present invention. Too large a droplet size may lead to an incomplete reaction taking place, whereas too small a droplet size may affect penetration ability. In an embodiment, the monomer is atomized to a spray of droplets having a Sauter mean diameter in the range of from 10 to 100 pm, preferably 10 to 50 pm. Droplet sizes may be measured directly from a high speed photograph or calculated based on atomization conditions. Suitable means for forming the atomized spray will be well known to the skilled person. They will include gas atomized spray formation, in which a high pressure gas jet impinges on the monomer, which is either in the liquid phase or in solution, so as to cause atomisation into droplets. In one embodiment, the monomer is atomized by a spray nozzle, particularly an ultrasonic spray nozzle. Suitable nozzles for use in the present invention are ultrasonic nozzles from Sono-Tek Corporation. Such nozzles use high frequency vibration, e.g. in the range 15 kHz to 200 kHz to produce very narrow drop size distribution and low velocity spray from the precursor. A plurality of spray nozzles may be utilised where appropriate, for example to introduce a plurality of immiscible monomers, though use of a single nozzle is preferred for simplicity. The atomisation of the monomer may be controlled so as to produce the required drop size, e.g. by varying the frequency of vibration, flow rate through the nozzle and the form in which the precursor is provided. The rate of introduction (or flow rate) of monomer and nanoparticles may be adjusted according to the size and nature of a reaction chamber being used and/or the frequency at which the atomiser is being operated. Suitable flow rates are in the range of 1 pL s^"1 to 1 mL s^"1 and preferably within the range 0.01 to 0.05 mL s ¹. For an atomizer operating at 120 kHz the flow rate is suitably in the range of approximately 0.01 to 0.05 mL s ¹. A flow rate of approximately 0.02 mLs^"1 has been found particularly suitable for liquid passing through an ultrasonic nozzle operating at around 120 kHz.

The monomer and nanofiller component may be introduced separately or in combination. In an embodiment, the monomer and nanofiller component are introduced simultaneously. For example, the monomer and nanofiller component may be combined to form a precursor slurry, which is atomized, e.g. as described above in respect of the monomer alone. It has been found that advantageous polymer nanocomposites with excellent properties may be readily formed according to the invention using such an atomized precursor slurry. In embodiments the slurry comprises in the range of from 0.1 to 5 wt% of nanoparticles or a nanofiller component, with the balance being made up by one or more monomers and optionally any additives. In embodiments, a slurry comprising in the range of from 0.2 to 1 wt%, preferably 0.4 to 0.8 wt% of nanoparticles has been found to be particularly effective.

The substrate or substrate elements may take any desired form. They may for example comprise glass, metal, ceramic, plastics, siloxane, woven or non-woven fibres, natural fibres, synthetic fibres, cellulosic material or the like. In an embodiment, the substrate or substrate elements and the join between them are positioned to be clear of the plasma discharge during exposure to the monomer and nanofiller component. This helps to minimise damage of growing nanocomposite by plasma-excited species such as ions.

The nanocomposites may, for example, have utility in providing: adhesion, protection against corrosion, a barrier to oxidation, liquid repellence, biomedical compatibility, antibacterial activity, or protein resistance. The potential uses of the polymer nanocomposites are of course dependent on their properties. A wide variety of properties is achievable within the ambit of the invention.

Nanocomposites may be applied any suitable type of join, particularly e.g. overlapping join between substrate panels. The join may comprise or define a gap between substrate elements, e.g. of about 1 pm to 1 mm, such as 10 pm to 100 pm.

The invention has been found to have particular utility in providing adhesive polymer nanocomposites. In an embodiment, the method may comprise exposing a join (e.g. an overlapping join) between first and second substrate elements to the monomer and nanofiller material to form a polymer nanocomposite that bridges the join. Such a bridging

nanocomposite may advantageously hold together the first and second substrates.

The present invention also provides a method of producing a multi-layered nanocomposite by the above described processes. In this case the nanocomposite may be applied by a plurality of repeat passes.

The invention also embraces, from a third aspect, a structure comprising first and second substrate elements joined by a polymer nanocomposite, obtainable by any of the methods described herein. From a fourth aspect, the invention embraces a polymer nanocomposite obtainable by any of the methods described herein.

The substrate elements or substrate may be exposed such that polymer nanocomposites of any desired form and thickness are deposited. In one embodiment, the polymer

nanocomposite is in the form of a layer having a thickness in the range of from 0.05 pm to 1000 pm, such as in the range of from 1 to 100 pm. In one embodiment, the polymer nanocomposite may have a shear bond strength of at least 50 MPa, preferably at least 60 MPa or even at least 70 MPa. Additionally or alternatively, the polymer nanocomposite may have a shear modulus of at least 1 GPa, preferably at least 2 GPa. Shear bond strength and shear modulus may be measured as described below. Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", mean "including but not limited to", and do not exclude other moieties, additives, components, integers or steps. Moreover the singular encompasses the plural unless the context otherwise requires: in particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Preferred features of each aspect of the invention may be as described in connection with any of the other aspects. Generally speaking the invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims and drawings). Thus features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. Moreover unless stated otherwise, any feature disclosed herein may be replaced by an alternative feature serving the same or a similar purpose.

Particular utility has been found to arise from certain combinations of optional and preferred features of the invention. In the context of providing a superior adhesive polymer nanocomposite, in one exemplary embodiment, the invention envisages a method of forming a polymer nanocomposite on a substrate, the method comprising: introducing a precursor slurry of atomized monomer of Formula I and silica nanoparticles that are surface-modified to comprise a functional group capable of reacting with the monomer into a continuous plasma discharge to activate at least part of the monomer; and exposing the substrate to the monomer and nanofiller material to form a polymer nanocomposite thereon.

Other preferred and advantageous features of the invention will become apparent from the following examples.

Where upper and lower limits are quoted for a property, then a range of values defined by a combination of any of the upper limits with any of the lower limits may also be implied. In this specification, references to compound properties are - unless stated otherwise - to properties measured under ambient conditions, i.e. at atmospheric pressure and at a temperature of from 16 to 22 or 25 °C, or from 18 to 22 or 25 °C, for example about 20 °C or about 25 °C. Specific description

The present invention will now be further described with reference to the following non- limiting examples, and the accompanying illustrative drawings, of which:

Figure 1 shows a schematic view of an atomized spray plasma deposition chamber set-up for performing exemplary embodiments of the invention; Figure 1A shows a schematic view of atomized spray plasma deposition of nanocomposite poly(2-hydroxyethyl methacrylate)-silica layers;

Figure 2 shows X-ray photoelectron C(ls) spectra of: (a) theoretical poly(2- ydroxyethyl methacrylate), and (b) atomized spray plasma deposited poly(2-hydroxyethyl methacrylate) on a glass substrate comprising 1 % wt methacrylsilane treated silica nanoparticles;

Figure 3 shows Fourier transform infrared spectra of: (a) 2-hydroxyethyl methacrylate monomer; and (b) atomized spray plasma deposited poly(2-hydroxyethyl methacrylate) film on a glass substrate comprising 1 % wt methacrylsilane treated silica nanoparticles (* Denotes absorbances due to polymerisable C=C double bond present in monomer); Figure 4 shows transmission electron microscopy images for atomized spray plasma deposited poly(2-hydroxyethyl methacrylate) on a polypropylene substrate comprising 1 wt % methacrylsilane treated silica nanoparticles at (a) x25,000 and (b) xl30,000 magnification;

Figure 5 shows Raman intensity relative to background of 900-950 cm^"1 C-C skeletal stretch peaks versus the penetration distance of atomized spray plasma deposited poly(2- hydroxyethyl methacrylate)-1 wt % methacrylsilane treated silica nanoparticle coating for overlapping glass substrates;

Figure 6 shows lap shear bond strengths of atomized spray plasma deposited poly(2- hydro xyethyl methacrylate) bonded glass-glass overlap joints as a function of methacrylsilane treated silica nanoparticle loading (solid line denotes cohesive failure and dashed line denotes adhesive failure); and

Figure 7 shows lap shear moduli of atomized spray plasma deposited poly(2- hydroxyethylmethacrylate) onto glass-glass overlap joints as a function of methacrylsilane treated silica nanoparticle loading.

Examples

A. Atomized spray plasma deposition (A5PD) of 2-hydroxyethyl methacrylate-silica nanoparticle slurry mixtures to form highly adhesive nanocomposite layers. With reference to Figure 1, atomized spray plasma deposition was carried out in an electrodeless, cylindrical, T-shape, glass reactor (volume 820 cm³, base pressure of 3 x 10^~3 mbar, and with a leak rate better than 2 x 10^"9 mol s^"1), enclosed in a Faraday cage.

A precursor inlet of the reactor was surrounded by a copper coil (4 mm diameter, 7 turns). The chamber was pumped down using a 30 L min ¹ rotary pump attached to a liquid nitrogen cold trap, and a Pirani gauge was used to monitor system pressure. The output impedance of a 13.56 MHz radio frequency (rf) power supply was matched to the partially ionized gas load via an L-C matching unit connected to the copper coil.

The nanocomposites layers were deposited/coated onto the following substrates in turn: silicon (100) wafer pieces (Silicon Valley Microelectronics Inc.); borosilicate glass microscrope slides (Smith Scientific Ltd.); and polypropylene pieces (capacitor grade, Lawson-Mardon Ltd.). To test adhesion properties, the nanocomposite layers were deposited/coated onto overlapping joints of adjacent glass slides, illustrated schematically in Figure 1 A.

Prior to each deposition, the reactor was scrubbed using detergent, rinsed in propan-2-ol, and dried in an oven. A continuous-wave air plasma was then run at 0.2 mbar pressure and 50 W power for 30 min in order to clean any remaining trace contaminants from the chamber walls. Substrates were placed downstream in line-of-sight from an atomizer nozzle (Model no. 8700- 120, Sono Tek Corp.). Mixtures of 2-hydroxyethyl methacrylate (+97% Aldrich Ltd.) and methacrylsilane treated fumed silica particles (Aerosil R71 1 , Evonik Industries AG) were loaded into a sealable glass delivery tube and degassed using several freeze-pump-thaw cycles to form precursors. The silica content of the precursors was varied from 0.25 to 2.40 wt %. For comparison, deposition was also performed with only 2-hydroxyethyl methacrylate as precursor, i.e. no silica particles.

In each case, the precursor was introduced into the reactor at a flow rate of 0.02 ml_ s^"1 through the ultrasonic nozzle operating at 120 kHz. Deposition entailed running a continuous- wave plasma at 50 W for 150 s in the presence of precursor atomization. Upon plasma extinction, the system was evacuated to base pressure before venting to atmosphere.

Nanocomposite film layers deposited on the substrate were analysed.

B. Film Characterisation Surface elemental compositions were determined by X-ray photoelectron spectroscopy (XPS) using a VG ESCALAB II electron spectrometer equipped with a non-monochromated Mg Ka X-ray source (1253.6 eV) and a concentric hemispherical analyser. Photoemitted electrons were collected at a take-off angle of 20° from the substrate normal, with electron detection in the constant analyser energy mode (CAE, pass energy = 20 eV). Experimental instrument sensitivity (multiplication) factors were C(1 s): 0(1 s) equals 1 .00: 0.36. All binding energies were referenced to the C(1 s) hydrocarbon peak at 285.0 eV. A linear background was subtracted from core level spectra and then fitted using Gaussian peak shapes with a constant full-width-half-maximum (fwhm).^37,38

The absence of any XPS Si(2p) signal confirmed pinhole-free surface coverage of the glass substrate following ASPD of poly(2-hydroxyethyl methacrylate), Table 1 .

With reference to Figure 2, the C(1 s) spectra can be fitted to three components corresponding to: hydrocarbon C_xH_y (285.0 eV), singly bonded carbon-oxygen C-O (286.6 eV), and the carbonyl ester 0-C=0 (288.9 eV). There were no discernible differences in the C(1 s) XPS spectra regardless of percentage silica content in the precursor (up to the maximum loading of 2.4 wt %). Infrared spectra were acquired using a FTIR spectrometer (Perkin-Elmer Spectrum One) fitted with a liquid nitrogen cooled MCT detector operating at 4 cm^"1 resolution across the 700-4000 cm^"1 range. Attenuated-total-reflection spectra were obtained using a Golden Gate accessory (Specac Ltd.). Transmission electron microscopy images were obtained using a Phillips CM100 microscope. This entailed embedding plasma coated polypropylene squares into an epoxy resin , and then cross-sectioning using a cryogenic microtome. The cross-sections were mounted onto copper grids prior to electron microscopy analysis.

With reference to Figure 3, the following infrared assignments can be made for the 2- hydroxyethyl methacrylate monomer: antisymmetric CH₃ stretch (2953 cm^"1), antisymmetric CH₂ stretch (2928 cm^"1), symmetric CH₃ stretch (2881 cm^"1), carbonyl C=0 stretch (1713 cm^" ¹) , vinyl C=C stretch (1635 cm^"1), =CH₂ wag (941 cm^"1) and =CH₂ twist (814 cm^"1). Atomized spray plasma deposited poly(2-hydroxyethyl methacrylate) layers show similar absorbances except for the absence of peaks due to C=C double bonds (C=C stretch, =CH₂ wag and =CH₂ twist) which are replaced by a peak at 747 cm^"1 attributed to -CH₂- twist. These changes are consistent with conventional polymerization taking place at the C=C double bond. As noted for XPS, there were no discernible differences in the infrared spectra for varying silica contents. The excellent bulk structural retention illustrated by the infrared spectra (which analyses the entire coating thickness) is consistent with residual plasma induced modification/damage of the deposited film being limited to the surface (since XPS only probes the outermost 5 nm³⁹). With reference to Figure 4, transmission electron microscopy of the atomized spray plasma deposited 2- hydroxyethyl methacrylate/1 wt % silica nanocomposite clearly shows clusters of silica nanoparticles (average diameter 15 nm) embedded within the poly(2-hydroxyethyl methacrylate) host matrix.

Film thicknesses were measured by freezing coated silicon samples in liquid nitrogen followed by fracture to reveal a cross-section. These were then imaged using an optical microscope (Olympus BX40) fitted with a x20 magnification lens.

Deposition rates for the atomized spray plasma deposited poly(2-hydroxyethylmethacrylate)- silica nanocomposite layers were 3.7±0.4 μιτι min^"1 and measured to be independent of silica loading. Precursor mixtures exceeding 2.4 wt % silica content were found to be too viscous to atomize, and therefore unable to be deposited.

Penetration of deposited coatings between two overlapping pieces of flat glass was examined using a Raman microscope (LABRAM, Jobin Yvon Ltd.). A He-Ne laser was employed as the excitation source (632.8 nm line, operating at 20 mW). The unattenuated laser beam was focused onto the sample using a x10 microscope objective, and the corresponding Raman signals were collected by the same microscope objective in a backscattering configuration in combination with a cooled CCD detector system. The spectrometer diffraction grating (300 g/mm) was calibrated against neon light emission lines in the 600-700 nm range. The depth of penetration was measured by monitoring the relative intensity of the polymer C-C skeletal stretch peaks at 900-950 cm^"1 with distance.

With reference to Figure 5, Raman spectroscopy showed that the atomized spray plasma deposited poly(2-hydroxyethyl methacrylate)-silica coatings are able to penetrate between two overlapping glass substrates to a depth of 743±53 μιτι. This phenomenon can be attributed to the liquid precursor droplets hitting the surface and wetting into the joint. Given that initiation of polymerization happens during the flight of the droplets through the plasma, then conventional polymerization mechanisms will continue to take place at the surface/joint interface. In other words, the plasma provides initiation of the polymerization reaction at the droplet surface. The polymerization carries after droplet impact onto the substrates, initially enabling penetration and then binding the substrates.

C. Adhesion of overlapping joints

Adhesion testing of deposited coatings comprised depositing directly onto two overlapping borosilicate glass microscope slide pieces. Subsequently, lap shear adhesion tests

(attributable to penetration of deposited material at the joint) were carried out using an Instron 5543 tensilometer operating at a crosshead speed of 1 mm min^"1.

With reference to Figure 6, the adhesive bond strength of overlapping glass slides subjected to the atomized spray plasma deposition of poly(2-hydroxyethyl methacrylate) with no silica content was 5.1 MPa, which rose rapidly with increasing silica content to reach a maximum value of approximately 84 MPa at 0.5 wt % silica loading for which the adherent (bulk glass) failed. At lower silica loadings the weaker bond failure occurs due to cohesive failure (i.e. the coating itself breaking), whilst at higher silica content, the bond strength drops reaching 9.8 MPa at 2.4 wt % silica content, which is due to adhesive bond failure (i.e. the coating coming away from the glass-coating interface). This trend would be consistent with the methacryloyl modified silica particles acting as crosslinkers, which enhance the coating strength (i.e. a move from cohesive fracture of the adhesive to adhesive failure— the coating coming away from the glass).

Above 0.5 wt % silica content, the bond strength falls due to it becoming more difficult to form Si-O-C bonds between the hydroxyl groups present on the glass surface and those contained in the poly(2-hydroxyethyl methacrylate) coating via condensation reactions because the inherent bulk crosslinking causes a drop in polymer chain mobility.⁴⁰

With reference to Figure 7, shear moduli obtained from lap shear tests gave 0.35 GPa for atomized spray plasma deposited poly(2-hydroxyethyl methacrylate) coatings containing no silica, and the measured value rose linearly with silica content before levelling off at around 6 GPa for silica loading exceeding 1 wt % . This trend is also consistent with the methacryloyl modified silica particles inducing greater crosslinking within the films and therefore greater stiffness (shear modulus).

The shear bond strength (84 MPa) of the optimum poly(2-hydroxyethyl methacrylate)-silica nanocomposite prepared in the present study by the atomized spray plasma deposition method exceeds those of conventional poly(2-hydroxyethyl methacrylate) based adhesives (10-45 MPa).⁴²'⁴³'⁴⁴'^45,46'^47,48 These high bond strengths for the ASPD nancomposite coating can be attributed to the poly(2-hydroxyethyl methacrylate) hydroxyl groups undergoing condensation reactions with glass surface hydroxyl groups to create Si-O-C bonds at the glass-coating interface.⁴⁰ In addition, the methacryloyl groups present on the silica particles help to enhance the adhesive bond strength by acting as crosslinkers within the bulk polymer thus raising its stiffness, which is confirmed by the increase in shear modulus of the coatings from 0.35 GPa to 6 GPa, Figure 7. These stiffness values are comparable to those reported previously for conventional poly(2-hydroxyethyl methacrylate) grafted from silica nanocomposites.⁴⁹ Finally, the outlined atomized spray plasma deposition approach is capable of performing in-situ bonding at room temperature via penetration between overlapping substrates. This is far more simplistic and straightforward compared to existing methods for bonding glass or silicon (such as anodic bonding⁵⁰ requiring high substrate temperatures,⁵¹ or the requirement for metallic interlayers⁵²). In summary, poly(2-hydroxyethyl methacrylate)-silica nanocomposite layers have been prepared by a single-step, solventless atomized spray plasma deposition process. Excellent adhesion and mechanical strength have been measured following in-situ application to overlapping joints.

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Claims

1. A method of forming a polymer nanocomposite in a join between first and second substrate elements, the method comprising: introducing an atomized monomer and a nanofiller component into an excitation medium to activate at least part of the monomer and/or nanofiller component; and exposing the join between the substrate elements to the monomer and nanofiller component to form a polymer nanocomposite in the join.

2. The method of claim 1, wherein the excitation medium is a plasma discharge.

3. The method of claim 2, wherein the plasma discharge is a sub-atmospheric continuous wave plasma generated by a radiofrequency power supply.

4. The method of any preceding claim, wherein the nanofiller component includes nanoparticles comprising material selected from one or more of: Si0₂, Al₂0₃, Ti0₂, ZnO,

CaC0₃, fused silica, borosilicate, quartz, and glass.

5. The method of claim 4 wherein the nanoparticles comprise silica nanoparticles.

6. The method of any preceding claim wherein the nanofiller component comprises surface-modified nanoparticles.

7. The method of claim 6 wherein the nanoparticles are surface modified with a silanol or a silane.

8. The method of claim 6 or claim 7 wherein the nanoparticles are surface modified to comprise a functional group capable of reacting with the monomer.

9. The method of any preceding claim, wherein the monomer is a hydroxyl monomer.

10. The method of any preceding claim wherein the monomer is an organic, ethylenically unsaturated monomer.

11. The method of any preceding claim, wherein the monomer is an acrylic monomer. The method of any preceding claim, wherein the monomer is a compound of formula

13. The method of claim 12 wherein R¹ and R² are hydrogen and R³ is an alkyl group.

14. The method of any preceding claim wherein the monomer is 2-hydroxyethyl methacrylate.

15. The method of any preceding claim wherein the monomer and nanofiller component are introduced simultaneously as an atomized precursor slurry.

16. The method of claim 15, wherein the slurry comprises in the range of from 0.2 to 1 wt% nanoparticles.

17. The method of any preceding claim wherein the substrate elements are positioned to be clear of the excitation medium during exposure to the monomer and nanofiller component.

18. The method of any preceding claim comprising exposing the join to the monomer and nanofiller component to form a polymer nanocomposite that bridges the join.

19. A structure comprising first and second substrate elements joined by a polymer nanocomposite, obtainable by the method of any preceding claim.

20. The structure of claim 19, wherein the polymer nanocomposite provides a shear bond strength of at least 50 MPa and/or a shear modulus of at least 1 GPa.