EP3116936A1

EP3116936A1 - High permittivity polymers based on functionalized silicones

Info

Publication number: EP3116936A1
Application number: EP15711646.8A
Authority: EP
Inventors: Dorina Maria OPRIS; Simon Johannes DÜNKI; Carmen RACLES; Adrian BELE; Maria CAZACU
Original assignee: Eidgenoessische Materialprufungs und Forschungsanstalt EMPA
Current assignee: Eidgenoessische Materialprufungs und Forschungsanstalt EMPA
Priority date: 2014-03-14
Filing date: 2015-03-13
Publication date: 2017-01-18
Also published as: WO2015135086A1

Abstract

The present invention relates to silicon-based dielectric elastomers having repeating units of formula (I) wherein R1 represents alkyl or a group -X-S-Y-Dz; X represents a spacer, S represents sulphur; Y represents a spacer and Dz represents one, two or three polar groups. The invention further relates to methods of manufacturing such elastomers by thiol-ene reactions; and to the use of such elastomers, particularly in dielectric elastomer actuators.

Description

High permittivity polymers based on functional!zed silicones

The present invention relates to high permittivity silicone-based polymers, to methods of manufacturing such silicone-based polymers, methods of manufacturing elastomers containing such polymers and to the use thereof, particularly the use in dielectric elastomer actuators. Dielectric elastomers and their use in actuators (DEA) are known. Known elastomers meet the mechanical requirements for actuators, however, they suffer from a low permittivity

(dielectric constant) {ε' ) . Therefore high driving voltages are needed to induce mechanical work which either hinders applications or makes the electronics complex and expensive. A possible way to improve is to increase ε' of the elastomers, which follows directly from an equation Pelrine introduced for the actuation strain: ϊ^r (equation 1)

where s is the actuation, εο=8.854*10^~12 F rtr¹ the vacuum permittivity, Y the elastic modulus, and E the electric field strength. An ideal dielectric material for dielectric elastomer actuators (DEA) application should have a high ε' and a very low conductivity. The modulus of elasticity Y is also a very important material parameter because it directly influences the achievable strain of the actuator. Thus polymers with a lower modulus of elasticity will show more strain for the same applied electric field, while polymers with high ε' will generate more output stress.

It is further known that the ε' can be increased when dipoles are attached to polymer chain . Racles et al, (Smart Mater. Struct, 2013, 22, 104004) disclose that silicone functionalized by cyanopropyl groups showed an increase in the ε' from 2.3 for the unmodified to about 7 for the modified silicone. Kruger et al ( O2013/038093 and WO2013/113593 ) also describe the use of cyanopropyl groups to increase the ε' as well as their application in DEA. Kruger et al. (US2013/0253146 Al) discloses silicones containing an electronic dipole covalently bonded to the silicone chain.

Known silicone materials, prepared by blending with ceramics or conductive nanoparticles , show an increase in permittivity, but also a stiffening of the material with increasing filler's volume fraction. Additionally, the breakdown field is also sinking with the filler content. However, the materials described in these documents are unsuitable for commercial applications. The main disadvantages of known dielectric elastomers are:

1) The attachment of the polar groups is not quantitative and also quite sensitive towards various impurities, e.g. trace amounts of thiols or amines which can poison the catalyst .

2) Known silicone materials used for functionalization are based on polymers with molecular weight typically below 20' 000 g/mol. This results in insufficient mechanical properties and the need to use these components as fillers in silicone matrix materials with better mechanical properties. As a consequence, however, the then blended silicone materials are non-homogeneous. Non-homogeneity leads to interfaces, defects and therefore higher probability of material failure.

Silicones functionalized via thiol-ene reaction are also known. Fahem et al (Microelectronic Engineering 2013, 74) describes Thiol-ene polymers useful for organic FETs. Boileau et al (FR2708272) describes specific silicon resins useful as adhesives. Boutevin et al (J. of Fluorine Chemistry, 1986, 425) describes the synthesis of fluorinated polysiloxanes, and specifically discloses a cyclic tetramer (p.430, structure 13). Mosch (US5057589) discloses polysiloxanes having C_B+ perfluoralkylgroups as the materials being useful as oil- and water repellents. Thus, it is an objective of the present invention to mitigate at least some of these drawbacks of the state of the art. In particular, it is an aim of the present invention to provide silicon-based dielectric elastomers with increased ε' and good mechanical properties. The invention further aims providing a synthesis of such elastomers that is easy, cheap, and allows up-scaling.

These objectives are achieved by the novel polymer as defined in claim 1 and the novel process as defined in claim 14. Further aspects of the invention are disclosed in the specification and independent claims, preferred embodiments are disclosed in the specification and the dependent claims. The present invention will be described in more detail below. It is understood that the various embodiments, preferences and ranges as provided / disclosed in this specification may be combined at will. Further, depending of the specific embodiment, selected definitions, embodiments or ranges may not apply.

Unless otherwise stated, the following definitions shall apply in this specification: As used herein, the term "a", "an", "the" and similar terms used in the context of the present invention (especially in the context of the claims) are to be construed to cover both the singular and plural unless otherwise indicated herein or clearly contradicted by the context.

As used herein, the terms "including", "containing" and "comprising" are used herein in their open, non-limiting sense .

Percentages are given as mol-%, unless otherwise indicated herein, or clearly contradicted by the context.

The term "silicon based elastomer" is known in the field; it particularly describes a polymer having- [Si-O] n- repeating units and elastic properties.

The term "silicone" is known in the field; it particularly describes an oligomer or a polymer having -[Si-0]n- repeating units and side groups covalently attached, as defined below.

The term "tiol-ene reaction", or simply "thiolen reaction" is known in the field; it particularly describes the addition of a thiol to a carbon-carbon double bond in the presence of a radical initiator. It is done under irradiation such as, for example, UV-irradiation, ionizing radiation (e.g. gamma ray or x-ray irradiation), microwave irradiation and the like. Thermal thiolene reaction is also well-known to a person skilled in the art.

The term "free radial initiator" is known in the field and includes photoinitiators and thermal initiators. A "photoinitiator" refers to a chemical that initiate free radial reaction by the use of light, such as, for example, from the class of phenones. Specific photoinitiators , suitable in the context of this invention include: 2,2- dimethoxy-2-phenylaceto-phenone , acetophenone , anison, antraquinonem benzyl, benzoin, benzoin ethyl ether, benzoin isobutyl ether, benzoin methyl ether, benzophenone, benzophenone/1-hydroxycyclohexyl phenyl ketone (50/50 blend, 3, 3' , 4, 4' -benzophenonetetracarboxylic dianhydride, benzoylbiphenyl , 2-benzyl-2- ( diumetylamino ) - ' -morpholinobutyrophenone, , ' -bis (diethylamino) benzophenone, 4 , 4 ' -bis (dimethylamino) benzophenone, camphor- quinone, 2-chlorothioxanthen-9-one, dibenzosuberenone, 2 , 2-diethoxyacetophenone, 4 , 4 ' -dihydroxybenzophenone , 4- (dimethylamino) benzophenone, 4 , ' -dimethylbenzyl , 2,5- dimethylbenzophaneone , 3 , 4 -dimethylbenzophenone , diphenyl ( 2 , 4 , 6-trimethylbenzoyl ) phosphine oxide/2-hydroxy-2- methylpropiophenone (50/50 blend), ' -ethoxyacetophenone, 2-ethylanthraquinone , 3, 3' -hydroxy-acetophenone, 4,4'- hydroxyacetophenone, 3-hydroxybenzophenone, 4-hydroxy- benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2- hydroxy-2-methylpropiophenone , 2 -methylbenzophenone , 3- methylbenzophenone, methylbenzoylformate , 2-methyl-4 ' - (methyltio) -2-morpholinopropiophenone , phenanthrene- quinone, ' -phenoxyacetophenone , thioxanthen-9-one .

A "thermal initiator" refers to a chemical that initiates radial reaction by the use of heat energy, such as, for example, from the class of diazo-compounds and from the class of orgainic peroxides. Specific thermal initiators, suitable in the context of this invention, include: 2,2'- azobisisobutyronitrile (AIBN) , cyclohexyl analogs of AIBN, 2,2'-azobis ( 4-methoxy-2 , 4 -dimethyl valeronitrile) (V70) and mixtures thereof, organic peroxides like di-tert-butyl peroxide, benzoyl peroxide, methyl ethyl ketone peroxide, acetone peroxide, cumene peroxide, terz-amyl peroxybenzoate, 4 , 4-azobis ( 4-cyanovaleric acid),

1, 1' azobis (cyclohexanecarbonitrile) and mixtures thereof.

The term "alkyl" is known and includes linear and branched alkyls. Unless otherwise specified, alkyls do not contain functional groups, i.e. they are unsubustituted . In the context of the present invention, the term alkyl denotes both, monovalent radicals (i.e. end-groups, such as - CH2CH3) and divalent radicals (i.e. spacers, such as - CH2CH2-) . Specifically, ^XC1-C20 alkyl" includes methyl, ethyl, n-propyl, iso-propyl, n-butyl, sec. -butyl, iso- butyl and tert. -butyl; particularly methyl.

The term "aryl" is known and includes phenyl and naphtyl. Unless otherwise specified, aryls do not contain functional groups, i.e. they are unsubustituted . Again, the term includes monovalent and divalent radicals.

The present invention will be better understood by reference to the figures ; wherein figures l.X relate to the first embodiment of the invention, figures 2.X relate to the second embodiment of the invention, and figures 3.X relate to the third embodiment of the invention. Figure 1A shows the synthesis of poly (dimethyl- methylvinyl) siloxane whereby, different end groups (R) can be introduced by using functionalized end-blockers (BL) [first step] . These end-blockers are selected such that the thiolene addition is not disturbed or they are introduced after the dipole attachment. The poly (dimethyl- methylvinyl ) siloxane copolymer is reacted with functionalized thiols [second step] . R represents a functional end-group and D represents a group with a large dipole moment, both as outlined herein. Figure IB shows a specific example of the functionalization with mercaptopropanenitrile .

Figure 1C shows ¹H NMR spectra of the poly ( dimethyl- metyhlvinyl ) siloxane (bottom) and of the prepared polymer by thiolene addition of mercaptopropanenitrile (top) . The disappearance of the vinyl protons at about δ = 6 ppm, indicated by an arrow, is a clear proof of quantitative conversion .

Figure ID shows dielectric properties of silicones containing different percentages of methyl ethylthiopropanenitrile siloxy repeat units (19%, 8%, 4.4%, 1.7%, 0%); X-axis: frequency [Hz]. The arrow indicates an increasing amount of CN groups.

Figure IE shows the synthesis of polar silicones by thiolene addition starting from silicones containing thiol groups and a polar vinyl compound; substituents as in fig. 1A.

Figure 2A shows the synthesis of polymethylvinylsiloxane containing different end groups and its functionalization by thiolene addition using different proportions of thiol/dipole functionali zed thiol. R represents an end group, such as OH; R^{" N} represents a C1-C20 alkyl; D represents a group with a large dipole moment as outlined herein . Figure 2B shows a specific example of this embodiment using mercaptopropanenitrile .

Figure 2C shows ¹H NMR spectra of the copolymers prepared starting from polymethylvinylsiloxane and mercapto- propanenitrile / butanethiol containing different percentages of methylethylthiopropanenitrile siloxy repeat units. From bottom to the top: starting material (reference), 0, 10, 20, 30, 40, 50, 100%. Figure 2D shows dielectric properties of functionalized silicones that contain different percentages of methyl ethylthiopropanenitrile siloxy and methyl ethylthiobutane siloxy repeat units. The permittivity is increasing with increasing the proportion of methyl ethylthio- propanenitrile siloxy units. X-axis: frequency [Hz].

Figure 3A shows in-situ thiolene functionalization and cross-linking of polymethylvinylsiloxane or poly (dimethyl- methylvinyl) siloxane in thin films.

Figure 3B briefly describes the inventive procedure leading to silicone-based elastomers with superior properties.

Figure 3C shows stress-strain curves of the prepared material according to the third embodiment (radical induced cross-linking) , whereby about 50% of the vinyl groups were reacted with mercaptopropanenitrile . Y-axis stress [N/mm2] ; X-axis Strain [%]. Figure 3D shows dielectric properties of films of modified silicones according to the third embodiment. X-axix: freuquency [Hz]

Figure 3E shows the stress-strain curves for materials A, B1-B3 and CI, C2 as well as of commercially available VHB4905 and Elastosil RT 745 from standard tensile tests at 500 mm min-1. The stress-strain curves were averaged from three independent tests. The strain at break for VHB and Elastosil was 850% and 770%, respectively. Figure 3F shows Permittivity (bottom) , loss factor (middle), and conductivity (top) of material C2, Elastosil RT 745, and VHB 4905 foil as a function of frequency at room temperature.

Figure 3G shows the measured lateral actuation strain of a membrane actuator. The actuators were made by a silicone film prestrained 20% and fixed between two circular frames (25 mm diameter). Circular electrodes (8 mm) of carbon black powder were applied to each side of the film.

Figure 3H shows lateral actuation strain membrane actuators for materials A, C2 and Elastosil RT 745 (each with 28.6% prestrain) as well as of VHB 4905 foil (300% prestrain) as a function of the applied voltage (top) and the lateral actuation strain of materials CI and Elastosil (both without prestrain) (bottom) .

Figure 31 shows long term stability of a film measured at 1000 V. The actuators survived thousands of cycles, x-axis time in seconds.

Fig. 4 outlines synthesis of starting materials, in case z represents 2 or 3.

In more general te ms , in a first aspect, the invention relates to polymers comprising 1 - 100 % of repeating units of fo ula (I)

wherein

represents an alkyl group, preferably a C1-C12 alkyl, or a C6-C10 aryl group, or a -X-S-Y-D_z group; X represents a spacer, preferably selected from the group consisting of C2-C12 alkyl, C2-C12 alkyl interrupted by C6-C10 aryl, C2-C12 alkyl interrupted by C6-C10 aryl whereby said aryl being substituted by 1-4 C1-C4 alkyl; S represents sulphur;

Y represents a spacer, preferably selected from the group consisting of C1-C20 alkyl, C1-C20 alkyl interrupted by C6-C10 aryl, C1-C20 alkyl interrupted by C6-C10 aryl whereby said aryl being substituted by 1-4 C1-C4 alkyl; C1-C20 alkyl interrupted by a carboxyl group, C1-C20 alkyl interrupted by amide group;

D represents a polar group, preferably selected from the group consisting of CN, N0₂, NO, CF₃, F, CI, Br, OCN and SCN;

z represents the integer 1, 2 or 3, preferably 1.

Polymers according to this invention preferably comprise 3 or more repeating units as defined herein, particularly preferably 10 or more repeating units as defined herein.

In an advantageous embodiment, the polymer comprises repeating units where Rl represents C1-C12 alkyl, particularly methyl. In case the polymer comprises 100% of these repeating units , the resulting polymer is a homo- polymer. In case the polymer comprises less than 100% of these repeating units, the resulting polymer is a copolymer .

In a further advantageous embodiment, the polymer comprises repeating units where Rl represents a group -X-S-Y-D_z. In case the polymer comprises 100% of these repeating units, the resulting polymer is a homo-polymer. In case the polymer comprises less than 100% of these repeating units, the resulting polymer is a co-polymer. In a further advantageous embodiment, the polymer comprises repeating units where Rl represents C1-C12 alkyl, particularly methyl and repeating units where Rl represents a group -X-S-Y-D_z. In this embodiment, the polymer is a copolymer, depending on its manufacturing a random co-polymer or block-copolymer .

In a further advantageous embodiment, the polymer comprises repeating units where X represents 1 , 2-ethandiyl and/or 1 , 1-ethandiyl . Polymers according to this embodiment are obtained when thiol-ene reaction is performed using a vinyl-compound as starting material. Typically, the 1,2- ethandiyl is predominantly formed.

In a further advantageous embodiment, the polymer comprises repeating units where X represents 1 , 2-propanediyl and/or 1, 3-propanediyl . Polymers according to this embodiment are obtained when thiol-ene reaction is performed using an allyl-compound as starting material. Typically, the 1,3- propanediyl is predominantly formed.

In a further advantageous embodiment, the polymer comprises repeating units where z represents 1, i.e. repeating units having one polar group D

In a further advantageous embodiment, the polymer comprises repeating units where z represents 2 or 3, i.e. repeating units having two or three polar groups D in its side chain. Exemplary groups Y-D_z are shown below,

wherein D is as defined above, preferably C . If z represents 2 or 3, Y preferably represents branched C1-C20 alkyl. If z represents 2 or 3, the D groups are preferably the same, such as CN .

In a particularly preferred embodiment, the polymer comprises repeating units of formula (I), where D represents CN, R¹ represents methyl, X represents 1,2- ethanediyl and Y represents 1 , 2-ethanediyl .

As outlined in further detail below, these polymers are available by a straightforward synthesis, show superior permittivity and good elastic properties. This aspect of the invention shall be explained in further detail below, with particular reference to

• the first embodiment, particularly relating to thiolene functionalization of poly (dimethyl-methylvinyl ) siloxane- type with a functional dipole that carry a thiol group or relating to functionalization of thiol containing silicones by thiolene addition with functionalized dipoles that carry a double bond;

· the second embodiment, particularly relating to thiolene functionalization of polymethylvinyl-siloxane-type starting materials;

• the third embodiment, particularly relating to thiolene functionalization of polymethylvinylsiloxane or poly (dimethyl-methylvinyl) siloxane - type starting materials followed by crosslinking reaction; and

• the forth embodiment, particularly relating to thiolene functionalization and crosslinking in a single step. In an advantageous embodiment, the invention relates to polymers as described herein having a permittivity above 3, preferably above 4, particularly preferably above 7, very particularly preferable above 10. Even higher values can be reached. Thus, the polymers described herein have outstanding electrical properties.

In an advantageous embodiment, the invention relates to polymers as described herein having moduli of elasticity of 50 kPa to 2 MPa (preferably: 50 kPa to 5 MPa) and strain limits between 50 to 800% preferably 300% and higher. Thus, the polymers described herein have very good mechanical properties .

In an advantageous embodiment, the invention relates to polymers as described herein having a molecular weight in the range of Mw 500-500000 g/mol, preferably 500-150000 g/mol more preferably 1000-150000 g/mol and even more preferably 1000-100000 g/mol. Again, the high molecular weight available through the synthesis described below, results in polymers having very good mechanical properties.

In an advantageous embodiment, the invention relates to polymers as described herein, said polymers being cross- linked .

In an advantageous embodiment, the invention relates to polymers as described herein additionally containing fillers, such as nanoparticles . Suitable nanoparticles are known in the field and include silica, titania, ferroelectric ceramics, BaTi0₃. Suitable amounts of fillers may be determined by routine experiments, but are typically in the range of up to 50 vol% (preferably: 1 to 30 vol%) . Such fillers may be added to fine-tune mechanical properties, particularly for reinforcing the inventive polymers . In a first embodiment, the invention provides for polymers comprising 1 - 100 % of repeating units of formula (I) and 99 to 0% of repeating units of formula (II)

wherein

R¹, X, Y, D, z are as defined above and

R² represents, independent from each other, alkyl, such as C1-C12 alkyl, or aryl. Such polymers are available by the methods outlined below and depicted in fig. 1A and IB and IE.

Advantageously, such polymers possess end-groups R⁶, selected from the group consisting of hydroxyl, vinyl, allyl, acryl, methacryl, thiol, C1-C12 alkyl thiol groups, amino- Cl-C12-alkyl groups, halogenated- Cl-C12-alkyl groups, epoxy groups, C1-C12 alkyl oxirane groups, C1-C12 alkyl-carboxy groups, hydrogen. Accordingly, such polymers are of formula (1-1)

wherein R¹, R², R⁶, X, Y, D_z are as defined above;

m represents 0 to 99% repeating units and

n represents 1 to 100% repeating units. Advantageously, R² represents methyl.

Advantageously, R⁶ represents hydroxyl, vinyl, hydrogen, C1-C12 alkyl thiol. In a second embodiment, the invention provides for polymers comprising 1 - 100 % of repeating units of formula (I) and 0-99 % of repeating units of formula (III)

wherein

R¹, X, Y, D, z are as defined above and

R³ represents, independent from each other, C1-C12 alkyl, or C6-C10 aryl, or a X-S-Y-H group.

R³ preferably represents methyl.

H represents hydrogen.

Advantageously, such polymers possess end-groups R⁶ as defined above. Accordingly, such polymers are of formula (1-2)

wherein R¹, R², R⁶, X, Y, D, z are as defined above;

m represents 0-99% repeating units and

n represents 1-100% repeating units.

Certain polymers comprising repeating units similar to those of formula (III) are known and described in CA1331010. This document is silent about the use of polymers having dielectric properties or applications as outlined herein.

In a third embodiment, the invention provides for polymers comprising 1 - 100 % of repeating units of formula (I) and 0-99 % of repeating units of formula (III) and of 1-50 of formula (IV)

wherein

the substituents are as defined above and X" represents either a single bond or a spacer selected from the group of C1-C12 alkanediyl; preferably a single bond.

Advantageously, these polymers are cross-linked. Due to the double bond in formula (IV) such cross-linking is achieved by radical polymerisation, adding a UV or thermally active initiator. Thus, the invention also provides for polymers comprising the above repeating units of formula (I) , (III) and (IV) characterized in that the polymer is cross-linked. Fig. 3 A outlines possible mechanisms and structures of such cross-linked polymers.

In a second aspect, the invention relates to a process for manufacturing of polymers as described herein, comprising the step of reacting a suitable thiol-component with a suitable alkene-component with way of a thiolene reaction. In its broadest sense, the invention thus relates to the use of the thiol-ene reaction for manufacturing silicon- based dielectric elastomers. According to this aspect, the present invention uses the robust thiol-ene addition instead of using a hydrosilylation reaction for the dipole attachment to the silicone chain for this purpose.

Functionalizat ion of silicones by thiolene addition (also termed thiol-ene-reaction) is known per se, but not yet applied to the starting materials as described herein The reaction is reliable, quantitative, and fast. Additionally, there is no need for excess of reagents required and the chemicals used are readily available. The reaction was exemplified using 3-mercaptopropanenitrile, but is not restricted to this specific starting material.

Generally speaking, polysiloxanes that contain methylvinylsiloxy repeat units in their chains and defined functional end groups are prepared first. The vinyl groups are then used to functionalize the silicone with polar groups by thiolene addition chemistry. This synthetic approach has a number of advantages. Most importantly, the polymer synthesis

• is straightforward and allows tuning the dielectrical as well as the mechanical properties of the resulting materials ;

• leads to homogenous materials - in contrast to the commonly applied blending of higher permittivity phases into a silicone matrix. The distribution of the polar groups in the material is controlled on the molecular level throughout the entire bulk;

• uses starting materials which are readily available and cheap;

• is not only simple but also up-scalable ;

· can be used for any kind of thiol-functionalized compound .

This aspect of the invention shall be explained in further detail below, specifically referring to the polymers disclosed above, first aspect 1^st to 4^th embodiment :

Synthesis of polymers according to the first embodiment,

(Method A: thiolene functionalization of poly (dimethyl- methylvinyl ) siloxane ) : In a first step poly (dimethyl-methylvinyl ) siloxane) is prepared by cationic or anionic copolymerization of octamethylcyclotetrasiloxane (D4) with 1,3,5,7- tetramethyl-1, 3, 5, 7-tetravinylcyclotetrasiloxane (V4) (fig. 1A) . The mole fraction of Si-vinyl in the polymer chain can easily be tuned by varying the proportion between D4 and V4. The molecular weight is controlled by the amount of end-blocker and initiator used. The end-blocker is selected such that the introduced end functional group would not interfere with the thiolene reaction. Suitable end-groups are listed above as R6, for example, hydrogen or hydroxyl groups .

In a second step, polar groups D are subsequently attached by thiolene addition with suitable thiols containing small or large dipoles. The dielectric properties of the final material are tuned by changing the amount and type of polar thiol used (depending on the composition of the initial poly (dimethyl-methylvinyl) siloxane) . NMR data show that the thiolene addition was quantitative (fig. 1C) . In case z represents 2 or 3, the corresponding starting materials are available according to methods known per se and outlined in fig. 4 for D = C .

Optionally, in a third step, the hydroxyl/hydro end group of the functionalized silicone can be replaced by other functional end groups by using reactions that are known in the art (In the context of this invention, such functional end groups include vinyl, allyl, acryl , methacryl , thiol, C1-C12 alkyl thiol, amino- Cl-C12-alkyl groups, halogenated- Cl-C12-alkyl groups , epoxy groups , hydrogen) . These end groups are used as described below. The molecular weight of the functionalized silicone has a direct impact on the mechanical properties o the material , polymers with high Mw are preferred to ensure good elastic properties, whereas low molecular weight polymers are preferred in combination with chain extenders. In the context of this invention, "Chain extenders" are molecules that carry two reactive end groups and allow formation of a copolymer with the low molecular weight polymer.

When low molecular weight polymers are used suitable chain extenders and cross-linkers are added, whereby chains growth and crosslinking occur in a single step.

In conclusion, this polymer synthesis is straightforward and allows introducing polar groups D on the silicones in a very easy, fast and controlled way. Poly (dimethyl- metyhlvinyl) siloxanes (M_w = 500 to 500000 g/mol) with defined functional end groups are obtained by this method. Random copolymers are generally obtained by this method. This is beneficial, to ensure homogenous dipole dispersion in the material and prevent phase separation The permittivity of the copolymers that contain different mol% of polar groups was investigated in uncross-linked state and found to increase from ε = 2.3 for an unmodified silicone to about ε = 6.4 for a copolymer containing 13 mol% of polar groups (fig. ID). By further increasing the amount of polar groups in the silicone, an even higher increase in the permittivity is expected.

Optionally, particularly for reinforcing the obtained polymers, silica or other nanoparticles may be added.

Synthesis of polymers according to the first embodiment,

(Method B: Functionalization of thiol containing silicones by thiolene addition with functionalized dipoles that carry a double bond . )

As an alternative approach, it is also possible to use a silicone containing thiol group in a thiolene addition with a polar vinyl compound (fig. IE) . In this approach, the thiol-ene reaction is reversed: In the second step, a vinyl group is attached to the polymer containing thiol- functionalities .

Synthesis of polymers according to the second embodiment

(thiolene functionalization of polymethylvinylsiloxane) First step: In this synthetic approach, V4 is first polymerized to a low or high molecular weight polymetyhlvinylsiloxane (Mw = 500 to 150000 g/mol) by using cationic or anionic polymerization (fig. 2A) . Molecular weight and end functionality are controlled by using different amount and type of suitable end-blockers as well as by the amount of the initiator used.

Second step: These polymers were subsequently used in a thiolene reaction using a mixture of thiol that carry a polar group and an alkyl thiol. To avoid uncontrolled cross-linking, slightly larger than stoichiometric amounts of thiols to vinyl groups were used. Figure 2C shows the ^XH NMR spectrum of the prepared polymers. The proportion between the two thiols was changed which allows fine tuning the dielectric properties of the resulting polymers (fig. 2D) .

Third step (optional) : The hydroxyl end group of the functionalized silicone can be replaced by other functional end groups by using reactions that are well known in the art (e.g. vinyl, allyl, acryl, methacryl, thiol, alkyl thiol, amino alkyl groups, epoxy groups, etc). These end groups are used directly for crosslinking high molecular weight polymers. When low molecular weight polymers are used suitable chain extenders and cross-linkers are added, whereby chains growth and crosslinking occur in a single step . Forth step: These polymers were cross-linked in thin films, whereby a large variety of chemical reactions can potentially be used for cross-linking. Optionally, particularly for reinforcing the obtained polymers, silica or other nanoparticles may be added.

Synthesis of polymers according to the third embodiment

(Thiolene functionalization of polymethylvinylsiloxane or poly (dimethyl-methylvinyl) siloxane and crosslinking in thin films)

When the stoichiometry between the thiol and the vinyl groups is adapted in that the vinyl groups are in excess, an uncontrolled cross-linking occurred with formation of a soft gel (fig. 3A) . This offers a powerful opportunity to prepare thin films with good mechanical properties.

First step: vinyl functionalized polymers or copolymers were mixed together with the thiols carrying polar groups and an UV initiator (e.g. 2 , 2-dimethoxy-2- phenylacetophenone, DMPA) .

Second step: The resulting mixture is poured into a Teflon mold and irradiated, whereby the dipole attachment and the crosslinking occur at the same time. To improve the mechanical properties of the resulting films, silica and/or cross-linkers with two or more thiol groups were used. This led to a substantial improvement in that soft elastomers with high permittivity and good elastic properties were obtained (fig. 3B) .

Analysis : The mechanical properties of the prepared materials were investigated by tensile tests. The stress- strain curves, averaged from several independent tests are shown in fig.3C and fig. 3E. The dielectric properties of the prepared films were also investigated (fig. 3D and 3F) . Elastomers obtained according to this synthetic approach show permittivity up to ε = 14. The mechanical and dielectric properties of the resulting elastomers are tuned by the molecular weight of the polymer, amount and type of thiol, and amount and type of cross-linker used.

In a third aspect, the invention relates to the use of the polymers as described herein and to devices comprising such polymers. In its broadest sense, the invention provides for the use of polymers as described herein as a dielectric material and to devices comprising such polymers.

This aspect of the invention shall be explained in further detail below: In one embodiment, the invention provides for the use of polymers as described herein in dielectric elastomer actuators (DEA) and to DEAs comprising such polymers, particularly in the form of a cross-linked film. DEAs are known per se; they consist of a thin elastomeric film sandwiched between two compliant electrodes. When a voltage is applied, an electrostatic pressure is acting on the film which is compressed. Since elastomers conserve their volume upon deformation, the film is elongated perpendicular to the applied electric field. This process is reversible; after removal of the field the polymer relaxes back to its original form. Due to DEA simple working principle and their excellent properties that include lightweight, quiet muscle like actuation, high actuation strain and electromechanical efficiency, their application potential is immense. Such DEAs may be useful in a number of technological fields and a wide variety of end-products. The invention therefore provides for devices selected from the group consisting of actuators, robotics, prosthetic and rehabilitation devices (such as implantable prosthetic and rehabilitation devices) , vehicles and aeroflight capable devices, energy harvesting devices, sensors, optical devices, comprising the polymers as described herein .

In a further embodiment, the invention provides for the use of polymers as described herein in generators and to generators comprising such polymers, particularly in the form of a film. Devices that convert mechanical energy into electrical energy ("generators") are known per se. In this embodiment, the inventive dielectric elastomers are used to generate electrical energy when mechanical work is done against the electric field. An external mechanical force is used to stretch the dielectric elastomer, followed by charging of the stretched film. It is thereafter allowed to contract by using the elastic forces. Thus mechanical forces are working against the electric field pressure and the electrical energy increases. In a further embodiment, the invention provides for the use of polymers as described herein in capacitors and transistors and to capacitors and transistors comprising such polymers, particularly in the form of a film. Due to its high permittivity, the polymers as described herein are also suited for such applications.

In a further embodiment, the invention provides for the use of polymers as described herein in optical devices and to optical devices comprising such polymers, particularly in the form of a film. Due to its high permittivity, the polymers as described herein are also suited for such applications To further illustrate the invention, the following examples are provided. These examples are provided with no intend to limit the scope of the invention.

3-Thiopropionitril

To a degassed solution of NaSH (7.54 g, 135 mmol) in H2O (40 ml), acrylonitril (5.66 g, 107 mmol) was added under argon. The reaction mixture was heated to 50°C for 45 min. It was then cooled to RT . The pH value was set to 8 by adding cone. aq. hydrochloric acid. The solution was extracted with CH₂CI₂ (5 x 25 ml) , the combined org. layers were dried over MgSC>4 and the solvent removed under reduced pressure. The reaction mixture was purified by column chromatography on silicagel using hexane/ethylacetate (8/1) as solvent to give colorless oil (4.64 g, 64 mmol, 60%) .

¾ NMR (CDCI3) : 2.82 - 2.76 (m, 2H, =CH₂) ; 2.71 - 2.67 (m, 2H, CH₂-S); 1.80 (t, 1H, J = 8.6, SH) ; ¹³C NMR (CDCI3) : 118.0 (s, CN), 23.0 (t, CH2-CN) ; 20.7 (t, CH₂-SH) .

Functionalizing of Poly (dimethyl-me hyl inyl) siloxane

(General Procedure)

Poly (dimethyl-methylvinyl) siloxane (15 g) with a methylvinyl content between 1.55 and 18 % were dissolved in THF (20 ml). To this mercaptopropionitrile (1.2 Eq to vinyl) and DMPA (0.1 Eq to vinyl) were added. The reaction mixture was degassed and set under argon. After reaction mixture was exposed to UV-light for 15 min. The mixture was precipitated with MeOH . The solution was decanted; the product was dissolved in toluene and precipitated again with MeOH (this procedure was repeated 4 times ) . The resulting polymers were dried at elevated temperature under HV to afford a yellowish highly viscous liquid . Thiolene addition of poly (dimethyl -methyl inyl) siloxane

To a degassed solution of poly (dimethyl-methyl inyl ) - siloxane (5.01 g, 6.32 mmol vinyl) in benzene (10 ml), thiopropionitril (0.690 g, 7.93 mmol, 1.25 eq) and DMPA (328 mg, 1.28 mmol, 0.20 eq) were added. The flask was irradiated 2 x 5 min with an UV-Lamp under vigorous stirring at R . The reaction mixture was precipitated with MeOH and the supernatant was removed. The polymer was dissolved in toluene and precipitated again with MeOH. The product was dried under HV to obtain yellowish oil (ca. 3.34 g, ca . 67 %) . The purity of the polymer was confirmed by ^XH NMR.

Polymerization of V

From a solution of TMAH x 5 H₂0 (0.5 g, 2.75 mmol) in MeOH (5.5 g) , 0.53 g (0.24 mmol TMAH) was given in a schlenk flask. The solvent was evaporated under HV, the residue was twice dissolved in dry benzene (7 ml) and evaporated to dryness to remove water via azeotropic distillation. Then V₄ (dry, 70.70 g, 0.205 mmol) was added, stirred for 1 h at RT and then heated to 70 °C for 70 h. The highly viscous polymer was then heated to 140 °C for 4 h and distilled under HV at 140 °C for additional 13 h to remove the cycles. A transparent polymer (66 g, 93%) was obtained. ¾ NMR (CDCI3, 400 MHz): 6.07 - 5.88 (m, 2H, =CH₂) ; 5.84 - 5.74 (m, 1H, Si-CH=) ; 0.20 - 0.11 (m, 3H, Si-CH₃) .

GPC: Mn = 68 kDa, D = 2.5

Thiolene addition of 3-mercaptopropionitrile to poly (methyl inyl) siloxane

To polymethylvinylsiloxane (20.01 g, 232.2 mmol, 1 eq) dissolved in distilled THF (200 ml), 3- mercaptopropionitrile (40.48 g, 464.5 mmol, 2.0 eq) and DMPA (560 mg, 2.18 mmol, 0.009 eq) were added. The solution was then irradiated with a UV light for 20 min. The reaction mixture was concentrated under vacuum and precipitated in methanol (50 ml) . The precipitation step was repeated several times. The product was dried in high vacuum at elevated temperatures to give yellowish highly viscous liquid. 1H NMR (400 MHz, CDC1₃, δ) : 2.85 - 2.78 (m, 2H, CH2-CH2-CN) , 2.72 - 2.64 (m, 4H, CH2-S-CH2-CH2-CN ) , 0.95 - 0.88 (m, 2H, Si-CH2-) , 0.18 (s, 3H, Si-CH3) ; 13C NMR (CDC13, δ) : 118.8, 27.5, 26.6, 19.0, 18. 0.0; 29Si NMR (CDC13, δ) : 24.2; EA: Calcd. : C 41.58, H 6.40, N 8.08, S 18.5; found C 39.47, H 6.49, N 8.04, S 17.63.

General procedure for the synthesis of polar silicones with different content of polar CN group

To a solution of polymethylvinylsiloxane (8 g) in THF (200 ml), different x:y ratios of thiols 1-butanethiol (or any alkyl thiol) : 3-mercaptopropionitrile (1) and DMPA (0.01 eq to vinyl) were added. Table 1 includes the amount of substances used for the synthesis of different polymers. The reaction mixture was set under argon and irradiated with UV-light for 20 min. It was then concentrated under vacuum to about 50 ml. To this concentrated solution, methanol was added whereupon the polymer precipitates. The polymer was dissolved in toluene and again precipitated with methanol. The dissolution/precipitation process was repeated three times. The polymers were then dried under HV at 60 °C to give yellowish highly viscous liquids .

Sample polymethylvi 1- l^b CN^C CN^d Mw PDI nylsiloxane⁸ butanethiol

[g] [g] [g] mol% mol% [kDa]

P2 7.98 13 - 0 0 97 2.2

P2₉P3i 8.08 15 1.62 10 11 99 2.6

P2₈P3₂ 8.07 13.3 3.22 20 20.6 100 2.7

P2₇P3₃ 8.01 11.73 4.83 30 30.6 98 2.9 P2₆P3₄ 8.01 10.05 6.50 40 42.7 93 2.8

P2iP3i 7.99 8.42 8.15 50 48 97 3.0

P2₄P3₆ 8.04 6.69 9.8 60 58 95 3.1

P2₃P37 7.99 5.02 11.32 70 70 86 2.8

P3 20.0 - 40.5 100 100 52 2.4 ^a0.01 eq DMPA to vinyl were used; ^bMercaptopropionitril (1) ^ctheoretical mol% CN; ^dmol% CN as calculated from the 1H NMR spectra.

Table 1. The amounts of reagents and reaction conditions used for the synthesis of different polymers contacting different amount of polar CN groups and their composition, molecular weights and distributions.

General synthesis for the end-groups modification

To a solution of hydroxyl end-functionalized polymer I, or copolymer of I and II, or copolymer of I and III in dry THF a slight excess of 1 , 3-divinyltetramethyldisilazane or vinyldimethylchlorosilane was added and stirred for two days. The polymer was precipitated with methanol. In-situ thiolene functionalization and crosslinking in thin films of polymethylvinylsiloxane

A mixture of poly (dimethyl-methylvinyl ) siloxane-a, ω-οΐ (0.5 g) , toluene (1 ml), mercapropropionitril (0.25 g) and 2 , 2-dimethoxy-2-phenylacetophenone (14 mg) was casted to a film and subsequently the crosslinking and dipole grafting was initiated by UV-irradiation for few minutes. The cross- linked film was dried at elevated temperature for several hours . In-situ thiolene functionalization and crosslinking in thin films of polymethylvinylsiloxane with dithiol as crosslinker :

A mixture of poly (dimethyl-methylvinyl) siloxane-a, ω-οΐ (0.5 g) , toluene ( 2 ml), mercapropropionitril (0.48 g) , 2 , 2 ' - (Ethylenedioxy) diethanethiol (5 μΐ) and 2,2- dimethoxy-2-phenylacetophenone (5 mg) was casted to a film and subsequently the crosslinking and dipole grafting was initiated by UV-irradiation for few minutes. The cross- linked film was dried at elevated temperature for several hours .

In-situ thiolene functionalization and crosslinking in thin films of polymethylvinylsiloxane with tetrathiol as crosslinker :

A mixture of poly (dimethyl-methylvinyl) siloxane-ot, ω-οΐ (0.5 g) , toluene (1.8 ml), mercapropropionitril (0.48 g) , pentaerythritol tetrakis ( 3-mercaptopropionate) (2.5 μΐ) and 2 , 2-dimethoxy-2-phenylacetophenone (5 mg) was casted to a film and subsequently the crosslinking and dipole grafting was initiated by UV-irradiation for few minutes. The cross-linked film was dried at elevated temperature for several hours.

General synthesis of polysiloxane containing ethylthiopropanenitrile elastomers

A solution of polymethylvinylsiloxane, 3-mercapto-propio- nitrile (1), 2,2'- (ethylenedioxy) diethanethiol (2), sur^¬ face functionalized silica particles and 2 , 2-dimethoxy-2- phenylacetophenone in toluene was casted to a film and initiated with UV light for few minutes. During this time the dipole grafting to the silicone chains and the cross- linking occurred simultaneously. The films were dried at elevated temperature to constant mass before further testing. Amounts of reagents are given in Table 1.

Synthesis of material C2

A solution of polymethylvinylsiloxane (2 g, 23 mmol vinyl groups), 3-mercaptopropionitrile (1) (21.8 mmol), 2,2'- (ethylenedioxy) diethanethiol (2) (34 μΐ), surface functionalized silica particles (0.2 g dispersed in 4 ml toluene) and 2 , 2-dimethoxy-2-phenylacetophenone (11 mg 0.04 mmol) in toluene (2 ml) was casted to a film using a doctor blade with 1200 μιτι gap and irradiated with a UV light for 2 minutes. During this time the dipole grafting to the silicone chains and the cross-linking occurred simultaneously. The film was let drying at RT for 18 h and at 60 °C until constant mass. Table 1. The characteristics of the starting polymethylvinylsiloxane and the amount of reagents used for the synthesis of materials A, Bl, B2 , B3, CI, and C2.

aFor all samples the amount of starting polymer (2 g) , Mercaptopropionitril 1 (1.9 g) , DMPA (25 mg) , and toluene (2 ml) was kept constant.

Claims

Claims :

A polymer comprising 1 - 100 % of repeating units of formula (I)

R¹ represents a C1-C12 alkyl, or C6-C10 aryl, or a group -X-S-Y-D_z;

X represents a spacer, selected from the group consisting of C2-C12 alkyl, C2-C12 alkyl interrupted by C6-C10 aryl, C2-C12 alkyl interrupted by C6-C10 aryl whereby said aryl being substituted by 1-4 Ci- C4 alkyl;

S represents sulphur;

Y represents a spacer, selected from the group consisting of C1-C20 alkyl, C1-C20 alkyl interrupted by C6-C10 aryl, C1-C20 alkyl interrupted by C6-C10 aryl whereby said aryl being substituted by 1-4 Ci- C alkyl, C1-C20 alkyl interrupted by ester group; C1-C20 alkyl interrupted by a amide group;

D represents a polar group, selected from the group consisting of CN, N0₂, NO, CF₃, F, Cl, Br, OCN and SCN

z represents 1, 2 or 3, preferably 1.

2. The polymer according to claim 1 comprising

(a) 1 - 100 % of repeating units of formula (I) and

(b) 0 - 99 % of repeating units of formula (II)

wherein

R¹, X, Y, D, z are as defined in claim 1 and

R² represents, independent from each other, C1-C12 alkyl, or an Ce-Cio aryl. The polymer according to claim 1 , comprising

(a) 1 - 10 0 % of repeating units of formula (I) and - 99 % of repeating units of formula (III)

wherein

R¹, X, Y, D, z are as defined in claim 1 and

R³ represents, independent from each other, C1-C12 alkyl, or an C6-C10 aryl, or a group -X-S-Y-H and H represents hydrogen.

The polymer according to claim 1 , comprising

(a) 1 - 100 % of repeating units of formula (I) and

(b) 0 - 99 % of repeating units of formula (II) and

(c) 0 - 99 % of repeating units of formula III

wherein

R¹, X, S, Y, D, z are as defined in claim 1 ;

R² are as defined in claim 2 and

R³ , H are as defined in claim 3 . The polymer according to claim 1, comprising

(a) 1 - 100 % of repeating units of formula (I) and

(b) 0 - 99 % of repeating units of formula (III) and - 50 of formula (IV)

wherein

the substituents are as defined in claim 1 or 2 and X^x represents either a single bond or a spacer selected from the group of C1-C4 alkandiyl; preferably a single bond;

and wherein said polymer is optionally cross-linked by reaction of the double bonds of unit (IV) .

The polymer according to claim 1, comprising

(a) 1 - 100 % of repeating units of formula (I) and

(b) 0 - 99 % of repeating units of formula (II) and

(c) 0 - 50 % of formula (IV)

wherein

the substituents R¹, X, S, Y , D, z are as defined in claim 1;

R² are as defined in claim 2;

R³ are as defined in claim 3 and

X^" is as defined in claim 5; and

wherein said polymer is optionally cross-linked by reaction of the double bonds of unit (IV) . The polymer according to claim 1, comprising

(a) 1 - 100 % of repeating units of formula (I) and

(b) 0 - 99 % of repeating units of formula (III) and

(c) 0 - 50 % of formula (V)

wherein

R¹, X, S, Y, D, z are as defined in claim 1;

R³ is defined in claim 3, and

X~ is as defined in claim 5;

and wherein said polymer is optionally cross-linked by using the thiol unit (V) .

The polymer according to claim 1, comprising

(a) 1 - 100 % of repeating units of formula (I) and

(b) 0 - 99 % of repeating units of formula (II) and

(c) 0 - 99 % of repeating units of formula (III) and

(d) 0 - 50 % of formula (IV)

wherein

R¹, X, S, Y, D, z, are as defined in claim 1;

R² are as defined in claim 2 ;

R³ and H are as defined in claim 3, and

X ~ is as defined in claim 5;

and wherein said polymer is optionally cross-linked by reaction of the double bonds of unit (IV) . The polymer according to claim 1, comprising

(a) 1 - 100 % of repeating units of formula (I) and

(b) 0 - 99 % of repeating units of formula (II) and

(c) 0 - 99 % of repeating units of formula (III)

(d) 0 50 % of formula (V)

(I) (III) (V) (ii);

wherein

R¹, X, S, Y, D, z are as defined in claim 1;

R² are as defined in claim 2;

R³ and H are as defined in claim 3, and

X is as defined in claim 5;

and wherein said polymer is optionally cross-linked by reaction of the thiol unit (V) .

10. The polymer according to any of the preceding claims, characterized in that

(a) it is cross-linked to an elastomer; and/or

(b) it comprises fillers in an amount of up to 50

vol%, said fillers preferably being selected from the group of nano-particulate silica,

ferroelectric ceramics and metal oxides. 11. The polymer according to claim 10, characterized in that

(a) it has permittivity above 3; and /or

(b) when cross-linked it has an elastic modus in the range of 50 kPa- 5 MPa; and / or

(c) it has a molecular weight in the range of 500-

500000 g/mol.

12. The polymer according to any of the preceding claims wherein

D represents CN

R¹ represents methyl; and / or

X represents 1 , 2-ethandiyl, 1 , 1-ethandiyl , 1,2- propandiyl, 1, 3-propandiyl; and / or

Y represents 1 , 2-ethandiyl or 1 , 1-ethandiyl , 1,2- propandiyl, 1 , 1-propandiyl , 1 , 3-propandiyl .

13. The polymer according to any of the preceding claims wherein

D represents CF3

R¹ represents methyl ; and / or

X represents 1 , 2-ethandiyl, 1 , 1-ethandiyl , 1,2- propandiyl, 1 , 3-propandiyl ; and / or

Y represents 1, 2-ethandiyl or 1 , 1-ethandiyl , 1,2- propandiyl, 1 , l-propandiyl , 1 , 3-propandiyl .

14. A method for manufacturing a polymer according to any of the preceding claims, comprising the step of reacting a thiol-component with an alkene-component with way of a thiolene reaction.

15. An elastomer comprising a polymer according to any of claims 1 - 1 .

16. A device comprising a polymer according to any of claims 1 - 13, preferably in the form of a film, said device being selected from the group consisting of

( a ) DEAs ;

(b) Generators;

(c) Transistors, capacitors, sensors; (d) artificial muscle;

(e) dielectric for flexible electronic components; and

(f) optical devices.

Use of the thiol-ene reaction for manufacturing silicon-based dielectric elastomers.

Use of a polymer as described in any of claims 1-13 as a dielectricum.

A device comprising polymer of formula III as defined in claim 3,

said polymer being crosslinked to an elastomer, said cross-linked polymer preferably being in the form of a film,

said device being selected from the group consisting of

( a ) DEAs ;

(b) Generators;

(c) Transistors, capacitors, sensors;

(d) artificial muscle ;

(e) dielectric for flexible electronic components; and

(f) optical devices.

Use of a polymer of formula III as defined in claim 3 as a dielectricum.