Classifications

• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
  • C08F293/00—Macromolecular compounds obtained by polymerisation on to a macromolecule having groups capable of inducing the formation of new polymer chains bound exclusively at one or both ends of the starting macromolecule
  • C08F293/005—Macromolecular compounds obtained by polymerisation on to a macromolecule having groups capable of inducing the formation of new polymer chains bound exclusively at one or both ends of the starting macromolecule using free radical "living" or "controlled" polymerisation, e.g. using a complexing agent
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D1/00—Processes for applying liquids or other fluent materials
  • B05D1/62—Plasma-deposition of organic layers
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
  • C08F2438/00—Living radical polymerisation
  • C08F2438/01—Atom Transfer Radical Polymerization [ATRP] or reverse ATRP

Definitions

• the present invention relates to a method of creating a polymer brush bearing covalently bound polymeric side chains, a polymer brush formed by the method, and use of the polymer brush as a lubricant .
• molecular bottle brushes Well-defined linear polymer brushes bearing a large number of covalently bound polymer side chains are commonly referred to as molecular bottle brushes. These have attracted significant attention in view of their novel properties which include stimuli-responsive action and supersoft rheological behaviour for potential applications such as sensors, nanoscopic
• Atom transfer radical polymerization is widely used for controlled / living polymerization because of the mild reaction conditions involved and applicability to a wide range of monomer functionalities. This technique is frequently adapted for the synthesis of well defined co-polymers, for instance the
• Another variant comprises well-defined linear polymer brushes bearing a large number of covalently bound polymer side chains which are referred to as molecular bottle brushes. ATRP initiated from surface sites is well documented for producing densely grafted polymer /
• Plasma deposition techniques have been quite widely used for the deposition of polymeric coatings onto a range of surfaces. This technique is recognised as being a clean, dry technique that generates little waste compared to conventional wet chemical methods. Using this method, plasmas are generated from organic molecules, which are subjected to an electrical field. When this is done in the presence of a substrate, the radicals of the compound in the plasma polymerize on the substrate.
• a first aspect of the present invention provides a method of creating a polymer brush bearing covalently bound polymeric sid chains, comprising the steps of:
• step (b) surface ATRP growth, from the halogen containing initiator film formed in step (a) , of a polymeric brush backbon incorporating side groups;
• step (c) growth, from the polymeric brush backbone formed in step (b) , of polymeric side chains on the polymeric brush backbone, to form a polymer brush in which the polymeric side chains constitute the bristles of the brush.
• polymer includes homopolymers and co-polymers .
• the polymer brush comprises a polymer bottle brush, which can, for example, be a polymer nano bottle brush.
• the side groups on the polymeric brush backbone formed in step (b) comprise further initiation sites for polymer growth.
• step (c) the growth of polymeric side chains on the polymeric brush backbone occurs via one or more derivatization steps in which the side groups on the polymeric brush backbone are derivatized to form further initiation sites for polymer growth.
• the halogen containing initiator film is formed by polymerization, resulting in a polymeric halogen containing initiator film.
• the halogen containing initiator film is formed by polymerization of a halogen containing molecule.
• the halogen containing molecule is a vinylbenzyl halide.
• the vinylbenzyl halide is vinylbenzyl chloride.
• the deposition of the halogen containing initiator film or the precursor is by a technique selected from the group of plasma polymerization, thermal chemical vapour deposition, initiated chemical vapour deposition (iCVD) , photodeposition, ion-assisted deposition, electron beam
• the halogen containing initiator film is deposited by plasma polymerization.
• one or more of the following conditions may be used: a. a pressure of from 0.01 mbar to 1 bar, for example from 0.01 or 0.1 mbar to 1 mbar or from 0.1 to 0.5 mbar, such as about 0.2 mbar .
• a power (or in the case of a pulsed plasma, a peak power) of at least 0.1 W, or at least 1 W, or at least 5 W, or at least 10 W, or at least 20 W; and/or of up to 40 W, or up to 50 W, or up to 60 W, or up to 70 W, or up to 100 W, or up to 500 W; such as for example from 0.1 to 500 W, or from 0.1 to 100 W, or from 5 to 70 W or from 5 or 10 to 50 or 60 W, or from 20 to 40 W, such as about 30 W. d.
• a ratio of duty cycle on- period to off-period of from 1 x 10 ⁇ 5 to 1.0, or from 0.001 to 0.1, for example from 0.001 to 0.05 or from 0.01 to 0.05 or from 0.01 to 0.04, such as about 0.025.
• the plasma polymerization is pulsed plasma polymerization . In an embodiment, the plasma polymerization is continuous wave (CW) plasma polymerization.
• CW continuous wave
• the plasma contains one or more monomeric compounds suitable for forming the halogen containing initiator film. In an embodiment, the plasma contains a single monomeric compound suitable for forming the halogen containing initiator film. In an alternative embodiment, the plasma contains a mixture of different monomeric compounds suitable for forming the halogen containing initiator film.
• the plasma further contains an inert carrier gas.
• the inert carrier gas is helium or argon .
• the deposition of the halogen containing initiator film in step (a) on a substrate can comprise partial or complete coverage of the substrate.
• the halogen containing initiator film is deposited on part of the substrate.
• the halogen containing initiator film is deposited on the substrate in the form of a pattern.
• a pattern can, for example, be formed by using a mask as a template for the pattern.
• the monomer used for the surface ATRP growth of the polymeric brush backbone is selected from the group of styrenes, acrylates, methacrylates , and acrylonitrile .
• the monomer comprises glycidyl methacrylate .
• step (c) the growth of polymeric side chains on the polymeric brush backbone occurs via controlled graft polymerization.
• step (c) the growth of polymeric side chains on the polymeric brush backbone is ATRP growth.
• step (c) the side groups on the polymeric brush backbone are derivatized to form ATRP initiation sites for ATRP growth of the polymeric side chains.
• the side groups on the polymeric brush backbone are derivatized to form halogenated ATRP initiation sites .
• side groups on the polymeric brush backbone are derivatized by reaction with bromoacetic acid.
• the monomer used for the growth of the polymeric side chains is selected from the group of styrenes, acrylates, methacrylates, and acrylonitrile.
• the monomer comprises sodium styrene sulfonate.
• the bristles of the polymer brush formed in step (c) are formed by poly (sodium styrene sulfonate) side chains .
• the polymer brush formed in step (c) is a polyelectrolyte brush.
• the substrate in step (a) is selected from the group of glass, metal, silicon, woven or non-woven fibres, natural fibres, synthetic fibres, ceramics, semiconductors, cellulosic materials, paper, wood, and polymers such as, for example, polytetrafluoroethylene , polyethylene or polystyrene.
• a second aspect of the present invention provides a polymer brush formed by the method of the first aspect.
• step (b) of the method has resulted in an ATRP grafted layer on the substrate which layer has an average thickness of less than 10 microns.
• the polymer brush is formed by the method wherein in step (b) , the monomer used for the surface ATRP growth of the polymeric brush backbone comprises glycidyl methacrylate and the ATRP grafted layer is a poly (glycidyl methacrylate ) layer.
• a third aspect of the present invention provides a use of a polymer brush according to the second aspect as a lubricant.
• surface tethered polymer brushes have been prepared by ATRP grafting of the macroinitiator polymeric brush backbone onto plasmachemically deposited poly (vinylbenzyl chloride) initiator nanofilms followed by ATRP growth of the polymeric side chains (bristles) .
• the surface density of polymer brushes can be precisely tailored by varying the plasmachemical deposition parameters employed for making the poly (vinylbenzyl chloride) initiator nanofilms. Lateral force scanning probe microscopy has shown that polymer brush decorated surfaces lead to an enhancement in nanolubrication .
• a fourth aspect of the present invention provides a method of creating an initiator film for subsequent ATRP growth of a polymer brush, the method comprising deposition of a halogen containing initiator film or a precursor which is derivatized into a halogen containing initiator film onto a substrate by continuous wave plasma polymerization.
• the halogen containing initiator film is formed by
• the halogen containing initiator film is formed by polymerization of a halogen containing molecule.
• the halogen containing molecule is a vinylbenzyl halide.
• the vinylbenzyl halide is vinylbenzyl chloride .
• Plasma polymers are typically generated by subjecting a coating forming precursor to an ionising electric field under low pressure conditions. Deposition occurs when excited species generated by the action of the electric field upon the precursor (radicals, ions, excited molecules etc) polymerize in the gas phase and react with the substrate surface to form a growing polymer film.
• Precise conditions under which either pulsed or continuous wave plasma deposition of the initiator films takes place in an effective manner will vary depending upon factors such as the nature of the monomer, the substrate, the size and architecture of the plasma deposition chamber etc and will be determined using routine methods and/or the techniques.
• Suitable plasmas for use in the methods described herein include non-equilibrium plasmas such as those generated by
• radiofrequencies RF
• microwaves microwaves or direct current (DC) .
• DC direct current
• RF radiofrequencies
• Various forms of equipment may be used to generate gaseous plasmas. Generally these comprise containers or plasma chambers in which plasmas may be generated. Particular examples of such equipment are described for instance in WO 2005/089961 and WO02/28548, but many other conventional plasma generating apparatus are available.
• the item to be treated is placed within a plasma chamber together with the material to be deposited in gaseous state, a glow discharge is ignited within the chamber and a suitable voltage is applied.
• the gas used within the plasma may comprise a vapour of the monomeric compound alone, but it may be combined with a carrier gas, in particular an inert gas such as helium or argon.
• a carrier gas in particular an inert gas such as helium or argon.
• helium is a preferred carrier gas as this can minimises fragmentation of the monomer.
• the relative amount of the monomer vapour to carrier gas is suitably determined in accordance with procedures which are conventional in the art.
• the amount of monomer added will depend to some extent on the nature of the particular monomer being used, the nature of the substrate being treated, the size of the plasma chamber etc.
• monomer is delivered in an amount of from 50-250 mg/min, for example at a rate of from 100-150 mg/min.
• Carrier gas such as helium is suitably administered at a constant rate for example at a rate of from 5-90, for example from 15-30 seem.
• the ratio of monomer to carrier gas will be in the range of from 100:1 to 1:100, for instance in the range of from 10:1 to 1:100, and in particular about 1:1 to 1:10. The precise ratio selected will be so as to ensure that the flow rate required by the process is achieved.
• a preliminary continuous power plasma may be struck, for example for from 2-10 minutes such as for about 4 minutes, within the chamber.
• This may act as a surface pre- treatment step, ensuring that the monomer attaches itself readily to the surface, so that as polymerization occurs, the coating "grows" on the surface.
• the pre-treatment step may be conducted before monomer is introduced into the chamber, in the presence of only the inert gas.
• a glow discharge is suitably ignited by applying a high
• the gas, vapour or gas mixture is supplied at a rate of at least 1 standard cubic centimetre per minute (seem) and preferably in the range of from 1 to 100 seem.
• the monomer vapour this is suitably supplied at a rate of from 80-300 mg/minute, for example at about 120 mg per minute depending upon the nature of the monomer, whilst the voltage is applied.
• Gases or vapours may be drawn or pumped into the plasma region.
• gases or vapours may be drawn into the chamber as a result of a reduction in the pressure within the chamber, caused by use of an evacuating pump, or they may be pumped or injected into the chamber as is common in liquid handling.
• Polymerization is suitably effected using vapours of initiator film precursor, which are maintained at pressures of from 0.1 to 200 mtorr, suitably at about 80-100 mtorr.
• the applied fields are suitably of a power (or in the case of a pulsed plasma, a peak power) of at least 0.1 W, or at least 1 W, or at least 5 W, or at least 10 W, or at least 20 W; and/or of up to 40 W, or up to 50 W, or up to 60 W, or up to 70 W, or up to 100 W, or up to 500 W; such as for example from 0.1 to 500 W, or from 0.1 to 100 W, or from 5 to 70 W or from 5 or 10 to 50 or 60 W, or from 20 to 40 W, such as about 30 W.
• the fields are suitably applied from 30 seconds to 90 minutes, preferably from 5 to 60 minutes, depending upon the nature of the precursor and the item being treated etc.
• a plasma chamber used is of sufficient volume to accommodate multiple items .
• These conditions are particularly suitable for depositing good quality uniform coatings, in large chambers, for example in chambers where the plasma zone has a volume of greater than 500 cm 3 , for instance 0.5 m 3 or more, such as from 0.5 m 3 - 10 m 3 and suitably at about 1 m 3 .
• the layers formed in this way have good mechanical strength.
• the dimensions of the chamber will be selected so as to be selected so as to be selected so as to be selected.
• elongate or rectangular chambers may be constructed or indeed cylindrical, or of any other suitable shape.
• the chamber may be a sealable container, to allow for batch processes, or it may comprise inlets and outlets for the items, material or yarn, to allow it to be utilised in a continuous process.
• the pressure conditions necessary for creating a plasma discharge within the chamber are maintained using high volume pumps, as is
• the substrate may be selected from the group of glass, metal, silicon, woven or non-woven fibres, natural fibres, synthetic fibres, ceramics, semiconductors, cellulosic materials, paper, wood, and polymers such as, for example, polytetrafluoroethylene, polyethylene or polystyrene.
• any feature disclosed herein may be replaced by an alternative feature serving the same or a similar purpose.
• references to compound properties are - unless stated otherwise - to properties measured under ambient conditions, i.e. at atmospheric pressure and at a temperature from 16 to 22 or 25 °C, or from 18 to 22 or 25 °C, for exampl about 20 °C or about 25 °C.
• Figure 1 shows infrared spectra of: (a) vinylbenzyl chloride monomer; (b) pulsed plasma deposited poly (vinylbenzyl chloride) film; and (c) continuous wave plasma deposited poly (vinylbenzyl chloride) film;
• Figure 2 shows infrared spectra of: (a) glycidyl
• Figure 3 shows variation of poly (glycidyl methacrylate) film thickness as a function of ATRP grafting time onto pulsed plasma deposited poly (vinylbenzyl chloride) initiator film;
• Figure 4 shows poly (sodium styrene sulfonate) film
• Figure 5 shows infrared spectra of: (a) pulsed plasma deposited poly (vinylbenzyl chloride) ATRP initiator film; (b) ATRP grafted poly (sodium styrene sulfonate) onto pulsed plasma deposited poly (vinylbenzyl chloride) initiator film; and (c) sodium styrene sulfonate monomer;
• Figure 6 shows infrared spectra of poly (glycidyl)
• methacrylate brushes grafted by ATRP onto continuous wave plasma deposited poly (vinylbenzyl chloride) ATRP initiator film, followed by derivatization with bromoacetic acid to yield macroinitiator layer. These were then employed for ATRP grafting of sodium styrene sulfonate side chains (bristles) for: (a) 0 min; (b) 30 min; and (c) 60 min;
• Figure 7 shows change in polymer film thickness as a function of poly (sodium styrene sulfonate) ATRP grafting time for macroinitiator layers produced by bromoacetic acid
• Figure 8 shows friction signals obtained by lateral force microscopy as a function of normal load for: continuous wave plasma deposited poly (vinylbenzyl chloride) film; ATRP grafted poly (glycidyl methacrylate) on pulsed plasma deposited
• Figure 9 is a scheme illustrating the main steps in formation the polymer brushes of the present invention.
• controlled ATRP surface grafting of poly (glycidyl methacrylate) brush layers is achieved using plasma deposited poly (vinylbenzyl chloride) nanofilms. These are then derivatized with bromoacetic acid to introduce ATRP initiation sites along the polymer brush backbone needed for the subsequent ATRP grafting of poly (sodium styrene sulfonate) side chains (bristles), as illustrated in Figure 9.
• Figure 9 is a scheme showing the main steps of the invention which comprise poly (glycidyl methacrylate) brushes grafted by ATRP onto plasma deposited poly (vinylbenzyl chloride) initiator films, followed by esterification of poly (glycidyl methacrylate) with
• bromoacetic acid to form tethered macroinitiation sites for the subsequent ATRP of poly (sodium styrene sulfonate) side chain ⁇ bristles' .
• Plasma depositions were performed inside a cylindrical glass reactor (5.5 cm diameter, 475 cm 3 volume) located within a
• Substrate preparation (glass cover slips or silicon wafer pieces) comprised successive sonication in propan-2-ol and cyclohexane for 15 min prior to insertion into the centre of the chamber. Further cleaning entailed running a 50 W continuous wave air plasma at 0.2 mbar for 30 min prior to film deposition.
• the vinylbenzyl chloride (+99.9%, Aldrich) precursor was loaded into a sealable glass tube, degassed via several freeze-pump thaw cycles, and then attached to the reactor. Monomer vapour was then allowed to purge the apparatus at a pressure of 0.2 mbar for 3 min prior to electrical discharge ignition.
• Pulsed plasma deposition was performed using a duty cycle on-period of 100 and a duty cycle off-period of 4 ms in conjunction with a peak power of 30 W. Continuous wave plasma deposition was carried out at 30 W. Upon plasma extinction, the precursor vapour continued to pass through the system for a further 3 min, and then the chamber was evacuated back down to base pressure.
• the mixture was thoroughly degassed using freeze-pump-thaw cycles and then immersed into an oil bath maintained at 80 °C for a range of grafting times (1.0 - 3.5 hr) .
• Final cleaning and removal of any physisorbed polymer was achieved by Soxhlet extraction using hot toluene for 16 hr.
• Bromine-containing macroinitiator films were derived from surface tethered ATRP grafted poly (glycidyl methacrylate) brushes via esterification with bromoacetic acid (+99.9%
• Bromoacetic acid was loaded into a sealable glass tube, degassed via several freeze-pump-thaw cycles, and then attached to the reactor.
• the system was evacuated to 4 x 10 ⁇ 3 mbar and heated to 75 °C.
• bromoacetic acid vapour was purged through for 5 min, and then the reaction chamber isolated from the pump for 4 hr to allow reaction, followed by cooling to room temperature and evacuation to base pressure.
• the substrates were thoroughly rinsed in high purity water and iV / iV-dimethylformamide ( +99.9%, Fisher) .
• reaction vessel was then immersed into an oil bath set to 50 °C for a predetermined grafting time.
• the substrate was then thoroughly rinsed in high purity water to remove any physisorbed polymer and allowed to dry in air.
• the charge fraction was 100% for grafted poly (sodium styrene sulfonate) .
• XPS photoelectron spectroscopy
• Plasma deposited Theoretical 90 0 10 0 0 0 poly (vinylbenzyl
• Table 1 XPS elemental compositions of plasma deposited
• Infrared spectroscopy of the ATRP grafted poly (sodium styrene sulfonate) layers revealed fingerprint features matching those associated with the monomer, Figure 5. These include absorbances at 1140 cm -1 , 1188 cm -1 and 1234 cm -1 (antisymmetric S0 2 stretches) and 1058 cm -1 (symmetric S0 2 stretch) .
• the monomer vinyl C C stretch absorption at 1638 cm -1 disappeared following ATRP polymerization.
• macroinitiators consisting of bromoacetic acid derivatized poly (glycidyl methacrylate) brushes grafted onto pulsed plasma deposited poly (vinylbenzyl chloride) initiator films, which yielded identical bromine content by XPS analysis, Table 1.
• ATRP grafting of poly (sodium styrene sulfonate) for 120 min resulted in an attenuated increase in film thickness (5 nm vs 24 nm) and a lower amount of sulphur, when compared to macroinitiators based on continuous wave plasma deposited poly (vinylbenzyl chloride) nanofilms, Table 1.
• Friction between a sliding SPM probe tip and the polymer brush layers was measured in an aqueous environment as a function of normal load, Figure 8. Friction data and height images were recorded simultaneously, and homogeneous 1 ⁇ x 1 ⁇ regions (rms roughness ⁇ 2 nm) were selected for data collection.
• the plasma deposited poly (vinylbenzyl chloride) layers exhibit a sharp rise in friction around 130 nN normal load; which is indicative of polymer chain displacement and wear.
• the bottle brush layers consistently gave lower friction readings compared to poly (glycidyl methacrylate) and poly (sodium styrene sulfonate) brush layers (grafted from the pulsed plasma deposited poly (vinylbenzyl chloride) initiator film) .
• This enhancement can be attributed to steric repulsion and water solvation of the bottle brushes leading to a resistance towards penetration (and hence improved lubrication) .
• These tribologica experiments were devised in such a manner so as to utilize an internal reference, namely, the ATRP-grafted poly(NaStS) brushe on the pulsed-plasma-deposited poly (VBC) layer.
• Pulsed plasma deposited poly (vinylbenzyl chloride) has
• electric discharge modulation comprises a short plasma duty cycle on-period (microseconds) to generate active sites in the gas phase as well as at the growing film surface via VUV irradiation, ion and electron bombardment, followed by conventional carbon-carbon double bond
• poly (glycidyl methacrylate ) layers measured for the continuous wave deposited poly (vinylbenzyl chloride) films is considerably lowered in comparison to those prepared using pulsed plasma conditions.
• the grafted poly (glycidyl methacrylate) brushes can be considered to be comparable in length between the pulsed and continuous wave plasma deposited initiator nanofilms, and therefore the lower film thickness in the latter case can be attributed to the collapsed conformation of poly (glycidyl methacrylate) chains, stemming from the lower surface initiation site density.
• macroinitiator brush syntheses as precursors for bottle brushes based on the esterification of pendant hydroxyl groups of poly (hydroxyethyl methacrylate) backbone polymers.
• the epoxide functionalities contained within poly (glycidyl methacrylate) brushes provide a more reactive handle for esterification with bromoacetic acid vapour, Figure 9, Table 1, and Figure 2.
• This overall ATRP approach is key to the formation of well-defined macroinitiator brushes and the subsequent synthesis of the bottle brush bristles (in contrast, the lack of control associated with conventional polymerization initiators leads to ill defined macroinitiator brushes and side chains ) .
• the sulfonate groups of poly (sodium styrene sulfonate) side chains (bristles) provide strong characteristic infrared absorbances which follow the increase in film
• surface tethered well-defined bottle brush polymers prepared by ATRP are an attractive prospect for the development of novel surface properties, such as lubricity, in particular nanolubricity .
• Tailoring of both the backbone and bristle segments can be achieved using the ⁇ grafting from' approach; furthermore the surface density of backbone grafts can also be independently controlled using plasmachemical deposition of initiation sites in order to allow predetermined side chain (bristle) growth.
• polyelectrolyte brushes exhibit a high osmotic pressure (charge repulsion) in aqueous environments, which enhances their lubricity.
• Surface- grafter polyelectrolyte brushes are also reported to display such behavior.
• the surface-tethered bottle brushes investigated are found to display even lower friction when compared to their constituent linear polymer brush
• XPS analysis shows a smaller amount of sulfur at the surface for the poly (sodium styrene sulfonate) bottle brush compared to that for the brush layers (3 ⁇ lvs 7+1%, respectively, Table 1), which indicates a lower counterion density, thereby negating the idea that the reduced friction is due to an increased number of counterions per unit area.
• the decreased friction is related to the molecular geometry (linear vs bottle brush) .
• Grafting side chains from the polymer backbone to form bottlebrush structures can be expected to increase the macromolecular rigidity.
• biological lubrication such as proteoglycans and epithelial-tethered mucins

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