Opaque polymers

Abstract

A method for forming an opaque electron-rich polymer comprising the steps: 1) Coating a substrate with an adhesive either having the structure (i) as above ,where F1 is independently selected from, su

Description

Field of the Invention The present invention relates to a method for forming an opaque polymer layer, preferably an opaque parylene layer, on a substrate, and to opaque polymer products that are obtainable using such methods. The opaque polymer may be used in applications where opacity is desirable either for aesthetic or functional purposes. Background to the Invention Parylenes are a class of poly(para-xylene) polymers which have been used extensively as coating materials in a wide range of applications. Parylenes exhibit a number of properties that make them desirable as a coating. In particular, parylene layers are relatively flexible, electrically insulating, effective barriers to moisture and gases, biocompatible and stable to polymer degradation. The properties of parylenes may be fine-tuned by modifying the substituents on the paraxylene. For instance, unsubstituted parylenes may exhibit a low friction coefficient (parylene N), chlorine-substituted parylenes may exhibit excellent barrier properties (parylene C) or temperature stability (parylene D), fluorine-substituted parylenes may exhibit high temperature stability (parylene VT-4) and UV-resistance (parylene AF-4), and alkylsubstituted parylenes may exhibit enhanced solubility (parylene M, E or AM-2). Parylene is often used as a coating for electronic circuits and other equipment to provide electrical insulation, moisture barriers, or protection against corrosion and chemical attack. Parylene coating may also be used to reduce friction, and in medicine to prevent adverse reactions to implanted devices. More recently, parylene layers have been adopted as highly effective adhesive layers between polymers, and between polymers and metals, in multi-layer flexible electronic devices, such as those disclosed in WO 2019 / 102227, WO 2020 / 225565 and WO 2020 / 225566. Typically, parylenes are prepared by vapor deposition of para-xylene monomers to form a polymeric thin film on a substrate. The synthesis method may be carried out without using an initiator or a terminator, at or near room temperature, in the absence of a solvent, and with minimal by-product production. As such, conventional parylene synthesis methods are seen as environmentally friendly. Although other parylene synthesis methods are known, such as the thermal decomposition of the [2.2] paracyclophane dimer at low pressure, but these are rarely adopted on a large scale. When parylene is prepared by known methods, it deposits in the form of a transparent layer. This transparency is a highly desirable property for many applications. For instance, when parylene is used as a coating, it allows the surface and surface markings beneath the coating to remain visible. As well as aesthetic benefits, transparency can also confer functional benefits onto a parylene coating in applications in which light penetration is essential, such as in optical devices, cameras, and LED lights. However, in some applications, the transparent quality of parylene is not desirable. This can be for aesthetic reasons, for instance where a coating to obscure the object it coats while acting as a protective barrier, or for functional reasons, for instance where parylene is used to coat a light-sensitive material. Thus, there is a need in the art for methods for preparing a polymer which is opaque, while also exhibiting the beneficial properties of parylenes. Summary of the Invention The present invention is based on the surprising discovery that an opaque polymer such as parylene may be formed by a method in which an adhesive layer is applied to a substrate, provided that the polymer is then added to the adhesive layer within 30 minutes. Thus, the present invention provides a method of forming an opaque polymer. The method comprises: i) coating a substrate surface with an adhesive to form an adhesive-coated substrate, the adhesive either having the structure: Fl\ ^Fl where: each Fi is independently selected from sulfur-containing, silicon-containing, nitrogen-containing, oxygen-containing, phosphorus-containing and boron-containing groups, and L is selected from linking groups, or being an oxysilane; and ii) applying an electron-rich polymer to the adhesive-coated substrate surface to form an opaque electron-rich polymer layer, the electron-rich polymer having the structure: n where: E is selected from electron-rich groups, and R4 is selected from C1.4 alkanediyl, optionally substituted with one or more halide groups, or is absent; wherein step (ii) is started within 30 minutes of the end of step (i). A product comprising an opaque electron-rich polymer is also provided, wherein the electron-rich polymer has the structure: where: E is selected from electron-rich groups, and R4 is selected from C1-4 alkanediyl, optionally substituted with one or more halide groups, or is absent. Further provided is an opaque electron-rich polymer product obtainable by the method of the present invention, as well as the use of an adhesive for obtaining an opaque electron-rich polymer, the adhesive either having the structure: Fl\ ^Fl r where: each F1 is independently selected from sulfur-containing, silicon-containing, nitrogencontaining, oxygen-containing, phosphorus-containing and boron-containing groups, and L is selected from linking groups; or being an oxysilane. Brief Description of the Drawings Figure 1 shows photographs of an opaque parylene layer that was prepared using a method of the present invention. For comparison, transparent parylene is also shown. Detailed Description of the Invention Method of forming an opaque polymer The present invention provides a method for forming an opaque polymer. The method comprises the following steps: iii) coating a substrate surface with an adhesive to form an adhesive-coated substrate; and iv) applying an electron-rich polymer to the adhesive-coated substrate surface to form an opaque electron-rich polymer layer; wherein step (ii) is started within 30 minutes of the end of step (i). Step (i) In step (i) of the method of the present invention, a substrate a surface is coated with an adhesive to form an adhesive-coated substrate. For instance, step (i) of the method may involve coating a substrate surface with the adhesive layer by exposing the substrate surface to an atmosphere of the adhesive, e.g. in a coating chamber. The coating chamber may be any chamber capable of providing an atmosphere comprising the adhesive, such as a desiccator. The adhesive may be added to the coating chamber in a gaseous state to provide the atmosphere of the adhesive. However, the method preferably comprises adding the adhesive into the chamber in a liquid state, and lowering the pressure to evaporate the adhesive so as to provide the atmosphere of the adhesive. It will be appreciated that both the substrate and the adhesive are preferably present inside the coating chamber before the pressure is lowered Preferably, the adhesive is added to the substrate surface using a self-assembly process. Typically, the self-assembly process will comprise placing the adhesive and the substrate under vacuum. Under low pressure conditions, the adhesive vaporises, e.g. from an open container in which it is held, and deposits on the substrate surface, typically as a selfassembled monolayer. The head group of the adhesive will generally bond to the substrate surface, leaving the end group free to bond to the electron-rich polymer in step (ii). Step (i) may be carried out at a pressure of up to 50,000 Pa, preferably up to 25,000 Pa, and more preferably up to 15,000 Pa. Step (i) may be carried out at a pressure of at least 100 Pa, preferably at least 1,000 Pa, and more preferably at least 5,000 Pa. Thus, step (i) may be carried out at a pressure of from 100 to 50,000 Pa, preferably from 1,000 Pa to 25,000 Pa, and more preferably from 5,000 to 15,000 Pa. The partial pressure of the adhesive in step (i) may be at least 50 %, preferably at least 70 %, and more preferably at least 90 % of the total pressure in step (i). Step (i) may be carried out at a temperature of at least 5 °C, preferably at least 10 °C, and more preferably at least 15 °C. Step (i) may be carried out at a temperature of up to 100 °C, preferably up to 60 °C, and more preferably up to 40 °C. Thus, step (i) may be carried out at a temperature of from 5 to 100 °C, preferably from 10 to 60 °C, and more preferably from 15 to 40 °C. Step (i) may be carried out for a period of at least 1.5 hours, preferably at least 2 hours, and more preferably at least 2.5 hours. Step (i) may be carried out for a period of up to 12 hours, preferably up to 6 hours, and more preferably up to 4 hours. Thus, step (i) may be carried out for a period of from 1.5 to 12 hours, preferably from 2 to 6 hours, and more preferably from 2.5 to 4 hours. These periods provide thorough coating of the substrate surface with adhesive. Step (ii) Once the adhesive-coated substrate formed has been formed, an electron-rich polymer is applied to the surface in step (ii) to form an electron-rich polymer layer. It has been surprisingly found that, by starting step (ii) within 30 minutes of the end of step (i), the resulting electron-rich polymer layer may be provided in an opaque form. Preferably, step (ii) is started within 20 minutes, and more preferably within 15 minutes, even more preferably within 10 minutes, even more preferably still within 5 minutes, most preferably within 1 minute of the end of step (i). It will be appreciated that the end of step (i) is the point at which the substrate is no longer exposed to an adhesive atmosphere, e.g. because the adhesive-coated substrate has been removed from the coating chamber in which the adhesive was applied or because the adhesive atmosphere has been vented from the chamber. The adhesive coating chamber and an electron-rich polymer coating chamber may form part of an apparatus configured for a continuous process in which the adhesive-coated substrate is directly, and preferably automatically, transferred from the adhesive coating chamber to the electron-rich polymer coating chamber. Alternatively, one chamber may fulfil both roles. Preferably, the electron-rich polymer layer is applied to the adhesive-coated substrate using a chemical vapour deposition process. By using chemical vapour deposition polymerisation, a thin film of the electron-rich polymer may be applied to the adhesive-coated substrate surface. Typically, the chemical vapour deposition process will involve vaporising the precursors (e.g. monomers or dimers) from which the electron-rich polymer is obtained, and depositing the electron-rich polymer layer on the adhesive layer. In the case of poly(xylylenes), the precursors may be xylylene monomers or [2.2] paracyclophane dimers. Typically, the chemical vapour deposition will take place under vacuum. The vaporisation step may be carried out at a pressure of up to 10,000 Pa, preferably up to 1,000 Pa, and more preferably up to 200 Pa. The vaporisation step may be carried out at a pressure of at least 10 Pa, preferably at least 20 Pa, and more preferably at least 50 Pa. Thus, the vaporisation step may be carried out at a pressure of from 10 to 10,000 Pa, preferably from 20 Pa to 1,000 Pa, and more preferably from 50 to 200 Pa. The vaporisation step is preferably carried out at elevated temperatures. The vaporisation step may be carried out at a temperature of at least 50 °C, preferably at least 100 °C, and more preferably at least 120 °C. The vaporisation step may be carried out at a temperature of up to 1,000 °C, preferably up to 800 °C, and more preferably up to 700 °C. Thus, the vaporisation step may be carried out at a temperature of from 50 to 1,000 °C, preferably from 100 to 800 °C, and more preferably from 120 to 700 °C. In some instances, e.g. where the electron-rich polymer is a parylene, the vaporisation process may comprise sub-steps in which a para-xylylene dimer is vaporised (e.g. at a temperature of from 120 to 180 °C) then cleaved (e.g. at a temperature of from 630 to 750 °C) to its monomeric form. The deposition step may be carried out under vacuum. The deposition step may be carried out at a pressure of up to 5,000 Pa, preferably up to 500 Pa, and more preferably up to 100 Pa. The deposition step may be carried out at a pressure of at least 0.5 Pa, preferably at least 5 Pa, and more preferably at least 10 Pa. Thus, the deposition step may be carried out at a pressure of from 0.5 to 5,000 Pa, preferably from 5 to 500 Pa, and more preferably from 10 to 100 Pa. The deposition step may be carried out at a temperature of at least 5 °C, preferably at least 10 °C, and more preferably at least 15 °C. The deposition step may be carried out at a temperature of up to 100 °C, preferably up to 60 °C, and more preferably up to 40 °C. Thus, the deposition step may be carried out at a temperature of from 5 to 100 °C, preferably from 10 to 60 °C, and more preferably from 15 to 40 °C. Step (ii) may be carried out for a period of at least 0.5 hours, preferably at least 1 hour, and more preferably at least 1.5 hours. Step (ii) may be carried out for a period of up to 4 hours, preferably up to 3 hours, and more preferably up to 2.5 hours. Thus, step (i) may be carried out for a period of from 0.5 to 4 hours, preferably from 1 to 3 hours, and more preferably from 1.5 to 2.5 hours. Other method steps In some embodiments, the method of the present invention comprises one or more further steps. Though the substrate will generally be preformed, in some embodiments, the method may comprise preparing the substrate. For instance, where the substrate comprises a base polymer, the method may comprise a step of preparing the base polymer. This step will be carried out before step (i). The base polymer may be prepared by mixing the base polymer monomers with any other components that are present in the base polymer, such as a dye and / or a cross-linker, then curing the mixture. Curing may be carried out at a temperature of at least 20 °C, preferably at least 50 °C, and more preferably at least 70 °C. Curing may be carried out at a temperature of up to 150 °C, preferably up to 120 °C, and more preferably up to 100 °C. Thus, curing may be carried out at a temperature of from 20 to 150 °C, preferably from 50 to 120 °C, and more preferably from 70 to 100 °C. Curing will typically be carried out for 1 to 5 hours, preferably from 2 to 3 hours, though it will be appreciated that longer and shorted curing times may be required when lower and higher curing temperatures, respectively, are used. The mixture is preferably shaped before it is cured, e.g. by pouring it into a mould or preferably by pouring it onto a surface and allowing it to spread under gravity. The mixture, preferably in its shaped form, may be placed in a vacuum oven for degassing before it is cured. The method of the present invention may comprise a step in which the opaque electron-rich polymer is separated from the substrate, e.g. by delamination. Typically, at least some of the adhesive will detach with the opaque polymer. The detached polymer may then be applied to a wide range of different substrates. Other steps that may be present in the method of the present invention include depositing of further layers before the substrate and / or on top of the electron-rich polymer layer and / or shaping of the multi-layered structure. Preferred materials for use in the present invention will now be described. Substrate A wide range of substrates may be used in the present invention, and the preferred substrate will typically depend on the desired end product. It will be appreciated that the substrate surface is one that is suitable for coating with an adhesive as defined herein. Suitable substrates include ceramics, metals and metal-containing materials such as oxides, glass, quartz and polymers. The substrate preferably comprises a base polymer, which is preferably an elastomer. Elastomers are well-known in the art as polymers that display rubber-like elasticity. The base polymer may be selected from polysiloxanes, polyimides (PI), polybutyrates (PBAT), polymethyl methacrylates (PMMA), polyacrylic acids, polyethylenes (PE) e.g. high density polyethylenes (HDPE) and low density polyethylenes (LDPE), polyethylene terephthalates (PET), polyurethanes (PU) including thermoplastic polyurethanes (TPU), polyvinyl chlorides (PVC), polyethylenimines (PEI), polyethylene naphthalates (PEN), polypropylenes (PP), polystyrenes (PS), polyamides including aliphatic or semi-aromatic polyamides, polytetrafluoroethylenes (PTFE), and polyvinylidene difluorides (PVDF). Preferred base polymers include polysiloxanes, polyimides, polybutyrates, polymethyl methacrylates, polyethylenes e.g. high density polyethylene and low density polyethylene, polyethylene terephthalates, polyvinyl chlorides, polyethylenimines, polyethylene naphthalates, and polypropylenes. These polymers are particularly suitable for forming a substrate which is highly flexible. Particularly preferred are polysiloxanes and polyimides. Polysiloxanes are polymers with a backbone of alternating silicon and oxygen atoms. Preferred polysiloxanes have the structure: where: each R1 is independently selected from hydrogen, alkyl and phenyl groups, preferably from hydrogen and C1.6 alkyl groups, more preferably from hydrogen, methyl (i.e. -CH3), ethyl ( / .e. -CH2CH3) and propyl ( / .e. -CH2CH2CH3 or -CH(CH3)CH3), still more preferably from hydrogen and methyl. It will be appreciated that a variety of siloxane monomers may be used in the polysiloxane or, in other words, the Ri groups may differ between the monomers making up the polysiloxane. Preferred polysiloxanes comprise at least 50 molar %, preferably at least 80 molar %, and more preferably at least 85 molar % of monomers in which each Ri is methyl. Preferably, the polysiloxane is a polydimethylsiloxane. Polyimides may take a range of structures provided that the imide monomers from which the polymer is prepared include the imide ( / .e. -C(O)-NR-C(O)-) functional group. Preferred polyimides include poly(4,4'-oxydiphenylene-pyromellitimide) (e.g. Kapton®): or, more preferably, a structure selected from: where: R2 is an alkyl group, preferably a C1-6 alkyl groups, more preferably is methyl (i.e. -CH3), ethyl ( / .e. -CH2CH3) or propyl (i.e. -CH2CH2CH3 or -CH(CH3)CH3), and most preferably is methyl, and R3 is an alkanediyl group, preferably a C1.6 alkanediyl (i.e. alkanediyl) group, more preferably is methanediyl (i.e. -CH2-), ethanediyl (i.e. -CH2CH2-) or propanediyl (i.e. -CH2CH2CH2- or-CH(CH3)CH2-), and most preferably is methanediyl. Polysiloxanes are preferably used as the base polymer in the substrate, as these materials are flexible, stretchable and bendable making them highly robust in a wide range of applications. The substrate may be made up of a blend of base polymers, for instance a blend of the polymers listed above, though preferably it will consist of a single polymer such as a polysiloxane. The base polymer may have a Young’s modulus of up to 5 MPa, preferably up to 3 MPa, and more preferably up to 2 MPa. The base polymer may have a Young’s modulus of at least 200 kPa, preferably at least 350 kPa and more preferably at least 500 kPa. Thus, the base polymer may have a Young’s modulus of from 200 kPa to 5 MPa, preferably from 350 kPa to 3 MPa, and more preferably from 500 kPa to 2 MPa. The base polymer may have a number average molecular weight of at least 7,500 Da, preferably at least 15,000 Da, and more preferably at least 20,000 Da. The base polymer may have a number average molecular weight of up to 200,000 Da, preferably up to 150,000 Da, and more preferably up to 100,000 Da. Thus, the base polymer may have a number average molecular weight of from 7,500 to 200,000 Da, preferably from 15,000 to 150,000 Da, and more preferably from 20,000 to 100,000 Da. Polymers having these molecular weights will typically exhibit preferred levels of elasticity. Where the opaque polymer is to be used in electronic application, the base polymer is preferably an electrical insulator, e.g. it will meet the requirements of ASTM D5213-12. The substrate may comprise the base polymer in an amount of at least 75 %, preferably at least 80 %, and more preferably at least 85 %, by weight. Preferably, the substrate comprises a cross-linking agent. A wide range of cross-linking agents may be used, with siloxanes such as methyl hydrogen siloxane preferred. The crosslinking agent may be present in the base polymer in an amount of up to 15 %, and preferably up to 10 %, by weight. Other components that may be present in the substrate include dyes. The substrate will typically be in the form of a layer. The substrate may have a thickness of less than 5 mm. The substrate may have a thickness of up to 2 mm, preferably up to 1.5 mm, and more preferably up to 1 mm. The substrate may have a thickness of at least 500 pm. Thus, the substrate may have a thickness of from 500 pm to 2 mm, preferably from 500 pm to 1.5 mm, and more preferably from 500 pm to 1 mm. In some embodiments, the substrate will have a thickness of about 500 pm. Substrates of this thickness exhibit excellent flexibility, while still being robust. In some instances where a more robust substrate is preferred, e.g. where use against rough surfaces such as the ground is anticipated, the substrate may have, in its most preferred form, a thickness of from 750 pm to 1 mm. It will be appreciated that that the thicknesses referred to above represent the minimum and maximum thicknesses observed in the substrate, i.e. a thickness of from A to B means that the minimum thickness of the layer is at least A and the maximum thickness of the layer is up to B. The thickness of the substrate may be measured using scanning electron microscopy. Specifically, scanning electron microscopy may be used to generate images of the substrate, with the thickness preferably determined using a computer program. Adhesive The adhesive is used to coat a surface of the substrate. It will be appreciated that the term ‘adhesive layer’ does not encompass a junction at which two materials are directly bonded together (e.g. by a bonding technique like plasma bonding), but rather requires an adhesive to be present so that the two materials are indirectly bonded together and separated by the adhesive. The adhesive preferably has the structure: Fl\ ^Fl r where: each Fi is independently selected from sulfur-containing, silicon-containing, nitrogencontaining, oxygen-containing, phosphorus-containing and boron-containing groups, and L is selected from linking groups. Thus, the adhesive contains two functional groups: a head group (one of the Fi groups) which is capable of bonding, preferably via a covalent bond, to the substrate surface and an end group (the other Fi group) which is capable of bonding, preferably via a covalent bond, to the electron-rich polymer layer. This enables the adhesive to bond the substrate surface and electron-rich polymer layers together. It will be appreciated that further functional groups (i.e. groups other than those consisting of hydrogen atoms and saturated carbon atoms) may be present in the adhesive, though generally the adhesive will contain only two functional groups namely the head group and the end group. The preferred functional groups will in part depend on the composition of the substrate surface. Preferably, each Fi is independently selected from thiol (-SH), alkoxysilanes (-Si(Ra)x(0Ra)3-x, where x is 0-2, preferably 0), silanes (-Si(H)y(Ra)3-y, where y is 1-3, preferably 1), phosphonates (-P(O)(ORa)2), hydroxy (-OH), carboxy (-C(O)OH), amino (-NH2), amide (-C(O)NH2), where each Ra is independently selected from hydrogen and C1.10 alkyl groups, preferably C1.3 alkyl groups, and more preferably is methyl or ethyl. More preferably, each Fi is independently selected from thiol and alkoxysilanes. In particularly preferred embodiments, one Fi (preferably the head group) is -Si(OMe)3 or -Si(OEt)3 and the other Fi (preferably the end group) is -SH. The linking group, L, is preferably an alkyl group. For instance, the linking group may be an unsubstituted alkyl group, such as a C1.10 alkyl group, more preferably a Ci-e alkyl group, and still more preferably a C2-4 alkyl group. Preferably, the linking group is a straight-chain alkyl group. Though less preferred, other linking groups such as ethers, thioethers or polyethers may also be used. (3-Mercaptopropyl)trimethoxysilane or (3-mercaptopropyl)triethoxysilane are preferred for use in the present invention, with (3-mercaptopropyl)trimethoxysilane particularly preferred. These adhesives are very effective at binding to siloxane substrate surfaces and to electronrich polymers. Oxysilanes may also be used as the adhesive. It will be appreciated that oxysilanes are compounds in which a Si atom is bound to four atoms, at least one of which is an O atom. Preferably, the oxysilane is an alkoxysilane, i.e. the Si atom is bound to at least one O atom which is, in turn, bound to an alkyl group. The oxysilane may have the structure: Si(Rb)z(ORc)4-z where: z is 1-3, and each Rb and Rc is independently selected from hydrogen and C1.10 alkyl groups. In the adhesive, z is preferably 1. Each Rb is preferably independently selected from hydrogen and C1-3 alkyl groups, and more preferably is hydrogen. Each Rc is preferably independently selected from C1-3 alkyl groups, and more preferably is methyl or ethyl. Trimethoxysilane or triethoxysilane are preferred oxysilane adhesives for use in the present invention, with trimethoxysilane particularly preferred. The adhesive may comprise a mixture of adhesives, such as a mixture of one or more of the adhesives described above, though preferably it will contain just a single adhesive. The adhesive preferably forms a layer on the substrate surface. The adhesive layer preferably comprises the adhesive in an amount of at least 90 %, preferably at least 95 %, and more preferably at least 99 % by weight. The adhesive will preferably be in the form of a monolayer, and more preferably a selfassembled monolayer. (3-Mercaptopropyl)trimethoxysilane and (3-mercaptopropyl)-triethoxysilane are particularly suitable for forming a self-assembled monolayer. The adhesive may have a thickness of up to 10 nm, preferably up to 5 nm, and more preferably up to 1 nm. The adhesive may have a thickness of at least 0.1 nm, preferably at least 0.25 nm and more preferably at least 0.5 nm. Thus, the adhesive may have a thickness of from 0.1 to 10 nm, preferably from 0.25 to 5 nm, and more preferably from 0.5 to 1 nm. It will be appreciated that that the thicknesses referred to above represent the minimum and maximum thicknesses observed in the adhesive layer. Thus, a thickness of from A to B means that the minimum thickness of the layer is at least A and the maximum thickness of the layer is up to B. The thickness of the adhesive layer may be measured using atomic force microscopy. Specifically, atomic force microscopy may be used to generate images of the adhesive layer, with the thickness preferably determined using a computer program. Electron-rich polymer The electron-rich polymer layer sits directly on top of the adhesive. The electron-rich polymer used in the electron-rich polymer layer comprises electron-rich groups linked directly to one another or by optionally substituted C1.4 alkanediyl groups. Thus, the electron-rich polymer has the structure: where: E is selected from electron-rich groups, and R4 is selected from C1.4 alkanediyl, optionally substituted with one or more halide groups, or is absent. A group will be considered electron rich provided that all of the atoms which form part of the polymer backbone form part of a double or triple bond, or have a lone pair of electrons. Electron-rich groups must have at least 2, but preferably at least 3, and more preferably at least 4, atoms in the polymer backbone. Electron-rich groups may have up to 10, preferably up to 7, and more preferably up to 5 atoms in the polymer backbone. Thus, electron-rich groups may have from 2 to 10, preferably from 3 to 7, and more preferably from 4 to 5 atoms in the polymer backbone. Where multiple electron-rich paths may be taken along a polymer backbone in the electron-rich group, the number of atoms in the shorter path are considered, so for instance the electron-rich groups in poly(meta-xylylene) would be considered to have 3, rather than 5, atoms in the polymer backbone. The electron-rich group, E, is preferably a conjugated group. As is standard in the art, a conjugated group can be represented as alternating single and multiple (e.g. double or triple) bonds, such that conjugation occurs across the group. Since conjugation is the interaction of a p-orbital from one double bond with a p-orbital from another double bond across an intervening single bond, the conjugated group must contain at least two multiple bonds and it must start and end with a multiple bond, i.e. the bridging alkanediyl group, R4, must be bonded to an atom which forms part of a multiple bond. Suitable conjugated groups may be selected from aromatic groups, and more preferably from Cb-io aromatic hydrocarbon and C5-10 heteroaromatic groups. Where the conjugated group is heteroaromatic, one or more of the ring members is a heteroatom, e.g. selected from O, N and S, though preferably no more than two and more preferably no more than one of the ring atoms is a heteroatom. Particularly preferred conjugated groups are selected from optionally substituted benzene groups. The conjugated group may be unsubstituted or substituted with one or more groups selected from -RbX, where Rb is selected from C1.3 alkanediyl groups or is absent, and X is selected from -Cl, -Br, -I, -F, -CF3, -C=CH, -CN, -NH2, -OH and H, provided that Rb is present where X is H. Rb is preferably selected from ethanediyl, methanediyl or is absent, and more preferably is absent (unless X is H). Preferably, the conjugated group is substituted with one or more groups selected from -Cl, -Br, -F, -C=CH, -NH2 and -CH2NH2 or is unsubstituted. Other electron-rich groups that may be used in the electron-rich polymer include -C(O)-NR- and -C(O)-NR-C(O)-. The electron-rich groups in the electron-rich polymer polymer are directly linked or preferably bridged by R4, a C1.4 alkanediyl group optionally substituted with one or more halide groups. By using a relatively short alkanediyl bridge between the electron-rich groups, the electronrich polymer is afforded enough flexibility to contour to the surface of the substrate. Preferably, R4 is selected from optionally substituted C2-3 alkanediyl groups, more preferably from ethanediyl optionally substituted with two to four groups selected from -Cl, -Br, -I, -F, -CF3 and C1.3 alkyl, and preferably optionally substituted with two to four groups selected from -F, -Cl and methyl. More preferably, R4 is selected from ethanediyl, -CHF-CHF-, -CF2-CF2-and -CH(CH3)-CH(CH3)-. More preferably, the electron-rich groups are bridged by ethanediyl. Preferred electron-rich polymers are selected from poly(xylylenes) in which the phenyl ring and methylene bridge are optionally substituted, e.g. as described above. Particularly preferred are poly(para-xylenes). Specific examples of para-xylene repeating units are provided in the following table along with names by which the resulting parylene polymers are commonly known. Substitution Common name Repeating unit Parylene N 2-chloro Parylene C 2,5-dichloro Parylene D 2,3,5,6-tetrafluoro Parylene F(VT-4) bis(benzyldifluoro) Parylene F(AF-4) 2-ethynyl Parylene X J n 2-amino Parylene A 2-methylamino Parylene AM 2-methyl Parylene M 2-ethyl Parylene E bis(benzyl methyl) Parylene AM-2 Particularly preferred are poly(para-xylenes) in which the phenyl ring is preferably substituted with 1 or 2, and more preferably 1, chloride groups. These polymers have the structure: It will be appreciated that the position of the chlorine atom on the phenyl ring may vary between repeating units. Preferably, the electron-rich polymer is selected from parylenes, i.e. optionally substituted poly(para-xylenes) which have been vapour deposited. Preferably, the electron-rich polymer is selected from parylene N, parylene C, parylene D, parylene F(VT-4), parylene F(AF-4), parylene A, parylene AM, parylene X, parylene M, parylene E, parylene AM-2, or copolymers thereof. Particularly preferred is parylene C, which has a single chlorine atom on each phenyl ring, as shown above. The electron-rich polymer will typically be a homopolymer. Though less preferred, the electron-rich polymer may also be a copolymer, e.g. containing more than one of the repeating units listed in the table above. The electron-rich polymer may be made up of a blend of polymers, for instance a blend of the electron-rich polymer polymers described above, though preferably it will consist of a single electron-rich polymer. The electron-rich polymer preferably has a Young’s modulus which is greater than that of the base polymer. It is believed that the higher Young’s modulus contributes to the smoothing effect of the electron-rich polymer on the substrate. The electron-rich polymer may have a Young’s modulus which is at least 100 kPa, preferably at least 250 kPa and more preferably at least 500 kPa higher than that of the base polymer. The electron-rich polymer may have a Young’s modulus of at least 1 MPa, preferably at least 2 MPa and more preferably at least 3 MPa. The electron-rich polymer may have a Young’s modulus of up to 5 GPa, preferably up to 4 GPa, and more preferably up to 3 GPa. Thus, the electron-rich polymer may have a Young’s modulus of from 1 MPa to 5 GPa, preferably from 2 MPa to 4 GPa, and more preferably from 3 MPa to 3 GPa. It will be appreciated that, as with the base polymers, the electron-rich polymers may be electrical insulators, e.g. they will meet the requirements of ASTM D5213-12. The electron-rich polymer layer will typically contain the electron-rich polymer in an amount of at least 90 %, preferably at least 95 %, and more preferably at least 99 %, by weight. The electron-rich polymer layer is preferably in the form of a thin film. For instance, the electron-rich polymer layer may have a thickness of up to 10 pm, preferably up to 1.5 pm, and more preferably up to 1 pm. The electron-rich polymer layer may have a thickness of at least 10 nm, preferably at least 250 nm, and more preferably at least 500 nm. Thus, the electron-rich polymer layer may have a thickness of from 10 nm to 10 pm, preferably from 250 nm to 1.5 pm, and more preferably from 500 nm to 1 pm. However, for some applications, a thicker electron-rich polymer layer is preferred, i.e. a thickness of at least 10 pm. The electron-rich polymer layer may have a thickness of up to 100 pm, preferably up to 70 pm, more preferably up to 40 pm. It will be appreciated that that the thicknesses referred to above represent the minimum and maximum thicknesses observed in the electron-rich polymer layer, i.e. a thickness of from A to B means that the minimum thickness of the layer is at least A and the maximum thickness of the layer is up to B. The thickness of the electron-rich polymer layer may be measured using atomic force microscopy. Specifically, atomic force microscopy may be used to generate images of the electron-rich polymer layer, with the thickness preferably determined using a computer program. Opaque electron-rich polymer products and other aspects The methods of the present invention are used to prepare opaque polymers. Thus, also provided is a product comprising an opaque electron-rich polymer. A polymer is considered to be opaque if it is substantially impenetrable by light, for instance exhibiting less than 1% transmittance of visible light. Transmittance can be defined by the percentage of visible light (electromagnetic radiation having a wavelength of about 380 nm to about 750 nm) which travels through a medium. Therefore, % transmittance = (transmitted light (Im) / incident light (Im)). Transmittance may be reduced by reflection, scattering, or absorption of visible light by the medium. Transmittance of light through the electron-rich polymer may be measured using a transparency meter (also known as a clarity meter). Transmittance may be measure through an electron-rich polymer thickness of 500nm. Transmittance may be measured following the procedure set out in ASTM D1746 -15. In some embodiments, the product comprising an opaque electron-rich polymer is present on the substrate on which it was prepared, e.g. a substrate as described herein. Thus, the product may comprise: a substrate, e.g. as described herein; and an opaque electron rich polymer attached to the substrate by an adhesive, e.g. as described herein. In other embodiments, the opaque electron-rich polymer may be separated from the substrate on which it was prepared, for instance by delamination. In these embodiments, at least some of the adhesive which joined the opaque electron-rich polymer to the substrate will be removed with the opaque polymer. Although this adhesive could be removed, e.g. by chemical means, this is generally not necessary. Thus, in some embodiments, the product further comprises adhesive, e.g. as described herein. The opaque electron-rich polymer may be used in a very wide variety of products. In some embodiments, the product comprising the opaque electron-rich polymer is a flexible electronic device. The opaque electron-rich polymer product may be obtainable, and is preferably obtained, by a method as defined herein. Also provided is the use of an adhesive layer as described herein in obtaining an opaque electron-rich polymer layer on a substrate, wherein the adhesive layer, electron-rich polymer and substrate are as described herein. The invention will now be illustrated with the following non-limiting examples. Examples Example 1: preparation of opaque parylene An opaque parylene layer was prepared by a method which comprises the following steps: (i) exposing a polydimethylsiloxane substrate surface to an atmosphere of (3-mercaptopropyl)trimethoxysilane in an adhesive coating chamber to form an adhesive-coated substrate surface, and (ii) within 30 minutes, depositing a parylene C layer onto the (3-mercaptopropyl)trimethoxysilane layer in a parylene coating chamber to form an opaque parylene C layer. A polydimethylsiloxane substrate was spin coated on a glass tile. The glass tile was placed in a tray and cured at 70 °C for 1 hour in a fan assisted oven to provide a glass base substrate coated with a polydimethylsiloxane substrate surface to a thickness of 700 pm. Step (i) The glass base substrate coated with the polydimethylsiloxane substrate surface was placed in a desiccator beside a cup filed with 1 mL of (3-mercaptopropyl)trimethoxysilane. The desiccator was sealed an adhesive atmosphere provided by reducing the pressure to 10,000 Pa . Step (i) was carried out for 150 minutes. Step (ii) The adhesive-coated substrate was removed from the desiccator and was placed in the parylene deposition chamber within 30 minutes. Parylene C was immediately deposited onto the (3-mercaptopropyl)trimethoxysilane coated substrate, to a thickness of 800 nm. When the deposition was complete the parylene C on silicone elastomer was opaque. Example 2: comparative preparation A similar experiment to Example 1 was conducted. Unlike Example 1, parylene was deposited on the adhesive-coated substrate more than 30 minutes after application of the (3-mercaptopropyl)trimethoxysilane adhesive. The parylene that formed was conventional transparent parylene.

Claims

1. A method for forming an opaque electron-rich polymer, said method comprising:(i) coating a substrate surface with an adhesive to form an adhesive-coated substrate, the adhesive either having the structure:Fl\ / FlLTwhere: each Fi is independently selected from thiol and alkoxysilanes, and L is an alkyl group,or being an oxysilane; and(ii) applying an electron-rich polymer to the adhesive-coated substrate surface to form an opaque electron-rich polymer layer, the electron-rich polymer having the structure:where: E is an aromatic group, andR4 is selected from C2-3 alkanediyl, optionally substituted with one or more halide groups;wherein step (ii) is started within 30 minutes of the end of step (i).

2. The method of Claim 1, one F1 is -Si(OMe)3 or -Si(OEt)3 and the other F1 is -SH.

3. The method of Claim 1 or Claim 2, wherein the linking group, L, is selected from C-mo alkyl groups, preferably C1-6 alkyl groups, and more preferably C2-4 alkyl groups.

4. The method of any preceding claim, wherein the adhesive is (3-mercaptopropyl)trimethoxysilane or (3-mercaptopropyl)triethoxysilane, and preferably is (3-mercaptopropyl)trimethoxysilane.

5. The method of any preceding claim, wherein the adhesive coating has a thickness of from 0.1 to 10 nm, preferably from 0.25 to 5 nm, and more preferably from 0.5 to 1nm.09 12 246. The method of any preceding claim, wherein R4 is selected from ethanediyl optionally substituted with two to four groups selected from -F and -Cl, preferably from ethanediyl, -CHF-CHF- and -CF2-CF2-, and more preferably is ethanediyl.

7. The method of any preceding claim, wherein the aromatic group, E, group is selected from C6-10 aromatic hydrocarbon and C5-10 heteroaromatic groups, and preferably is a benzene group, wherein the aromatic group is unsubstituted or substituted with one or more groups preferably selected from -RbX, where Rb is selected from C1.3 alkanediyl groups or is absent, and X is selected from -Cl, -Br, -I, -F, -CF3, -CnCH, -CN, -NH2 and -OH, and preferably the aromatic group is unsubstituted or is substituted with one or more groups selected from -Cl, -Br, -F, -CECH, -NH2 and -CH2NH2.

8. The method of Claim 7, wherein the electron-rich polymer is selected from poly(xylylenes) in which the phenyl ring and methylene bridge are optionally substituted, and preferably from polyQoara-xylenes) in which the phenyl ring is preferably substituted with 1 or 2, and more preferably 1, chloride groups.

9. The method of any preceding claim, wherein the electron-rich polymer layer has:a thickness of from 10 nm to 10 pm, more preferably from 250 nm to 1.5 pm, and most preferably from 500 nm to 1 pm; ora thickness of at least 10 pm, and preferably up to 100 pm, more preferably up to 70 pm, for instance up to 40 pm.

10. The method of any preceding claim, wherein the substrate surface comprises a polymer which is an elastomer, preferably selected from polysiloxanes, polyimides, polybutyrates, polymethyl methacrylates, polyacrylic acids, polyethylenes, polyethylene terephthalates, polyurethanes, polyvinyl chlorides, polyethylenimines, polyethylene naphthalates, polypropylenes, polystyrenes, polyamides, polytetrafluoroethylenes, and polyvinylidene difluorides; more preferably from polysiloxanes, polyimides, polybutyrates, polymethyl methacrylates, polyethylenes, polyethylene terephthalates, polyvinyl chlorides, polyethylenimines, polyethylene naphthalates, and polypropylenes; and still more preferably from polysiloxanes and polyimides.

11. The method of any preceding claim, wherein the substrate surface comprises a polysiloxane, preferably having the structure:09 12 24where: each R1 is independently selected from hydrogen, alkyl and phenyl groups, preferably from hydrogen and C1-6 alkyl groups, more preferably from hydrogen, methyl, ethyl and propyl, still more preferably from hydrogen and methyl,and wherein the polysiloxane comprise at least 50 molar %, preferably at least 80 molar %, and more preferably at least 85 molar % of monomers in which each R1 is methyl.

12. The method of any preceding claim, wherein step (i) is carried out:under vacuum, for instance at a pressure of from 100 to 50,000 Pa, preferably from 1,000 Pa to 25,000 Pa, and more preferably from 5,000 to 15,000 Pa;at a temperature of from 5 to 100 °C, preferably from 10 to 60 °C, and more preferably from 15 to 40 °C; and / orfor a period of at least 1.5 hours, preferably at least 2 hours, and more preferably at least 2.5 hours.

13. The method of any preceding claim, wherein the partial pressure of the adhesive in step (i) is at least 50 %, preferably at least 70 %, and more preferably at least 90 % of the total pressure in step (i).

14. The method of any preceding claim, wherein step (ii) is carried out by chemical vapour deposition, in which precursors from which the electron-rich polymer is obtained are vaporised, and deposited on the adhesive-coated surface as an opaque electron-rich polymer layer.

15. The method of any preceding claim, wherein step (ii) is started within 20 minutes, and preferably within 15 minutes of step (i).

16. A product comprising an opaque electron-rich polymer, wherein the electron-rich polymer has the structure:_i nwhere: E is an aromatic group, andR4 is selected from C2-3 alkanediyl, optionally substituted with one or more halide groups,wherein the product further comprises an adhesive layer, and the electron-rich polymer forms a layer on said adhesive layer, the adhesive layer comprising an adhesive either having the structure:Fl\ / Fl rwhere: each F1 is independently selected from thiol and alkoxysilanes, andL is an alkyl group, or being an oxysilane.09 12 2417.Use of an adhesive for obtaining an opaque electron-rich polymer, wherein the adhesive either has the structure:where: each F1 is independently selected from thiol and alkoxysilanes, and L is an alkyl group,or is an oxysilane; andthe electron-rich polymer has the structure:where: E is an aromatic group, andR4 is selected from C2.3 alkanediyl, optionally substituted with one or more halide groups.

18. The use of Claim 17, wherein the electron-rich polymer and / or the adhesive layer are as defined in any of Claims 2 to 9.

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