Increased electromechanical sensitivity of polysiloxane elastomers by chemical modi ﬁ cation with thioacetic groups

• Novel silicone-based elastomers modi- ﬁ ed with thioacetic groups were prepared. • Surface treated silica particles improve themechanicalpropertiesoftheformed elastomers. • The increased permittivity and the opti- mized elasticity allowed construction of DEA responsive to a low electric ﬁ eld. • The elastomers can be processed in thin ﬁ lms from which actuators operated at low voltages were constructed. Chemicallycross-linkedpolydimethylsiloxane(PDMS)elastomershavegainedimportanceasadielectricinelas- tic capacitor actuators, which elongate when electrically charged. Common PDMS elastomers have a dielectric permittivity of only about 3 and thus elongations interesting for real applications occur only at high voltages. In this work, a new class of silicone-based elastomers with increased permittivity are synthesized starting from polymethylvinylsiloxane and thioacetic acid via a photo-induced thiol-ene reaction, whereby polar thioacetate groups are introduced at every siloxy unit. The silanol end-groups of the formed polymer are subsequently used for cross-linking into thin ﬁ lms using a condensation reaction with poly(methylhydrosiloxane- co - dimethylsiloxane). These ﬁ lms show good mechanical properties and have a low glass transition temperature (-58°C),whichallowsfora widerangeofusetemperatures.Furthertheyhavean increasedpermittivity ( ε ’ = 4.7) and an increased sensitivity to electric ﬁ eld as compared to regular polydimethylsiloxane elastomers. Our new elastomers are easilyaccessibleand can beprocessed into very thin ﬁ lms,whichallows accesstoactuators oper- able way below 1000 V. An actuation strain of 12.8% at an electric ﬁ eld of 21.5 V/ μ m was measured. μ m).Whilethismaterialwaspro- cessible into thin ﬁ lms, the detachment of ﬁ lms from the substrate was tedious because the strain at break was only 80%. The most promising materials in the current work have an average strain at break of about 250% and are easily handled in thin ﬁ lms. Addi-tionally, the lateral actuation strain of 5.6% at 650 V (27 V/ μ m) is attractive for practical use.Therefore,forapplicationswheresinglemembraneactuatorsareneeded,thematerial developed in this work is attractive. Because of the promising actuation at low voltages, furtherresearchwillbecarriedouttoreducethemechanicallosses,toincreasethedielec-tric breakdown ﬁ led, and to construct stacked actuators with these materials.


Introduction
Dielectric elastomer actuators (DEAs) are electromechanical transducers which have properties that remind of those of natural muscles [1]. They are lightweight elastic capacitors that have a

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Materials and Design j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m a t d e s noiseless reversible deformation when charged-discharged [2,3]. Plenty of applications have been proposed, but, their implementation in products is still hindered by the high voltages required for DEA operation, which are typically above 1 kV [4][5][6][7]. There are mainly three screws to turn to reduce the actuation voltage: the elastic modulus [8], which should be low, the thickness of the dielectric film [9], and the dielectric permittivity, which should be high [10,11]. Soft elastomers are prone to electromechanical instability [12] and have low electrostatic pressure [13]. Therefore, increasing the dielectric permittivity of an elastomer is a very promising approach as the driving voltage is reduced and the actuation pressure is increased [14]. While several strategies have been followed to increase the dielectric permittivity of an elastomer [15][16][17][18][19][20][21][22][23][24], as of now, the most promising approach is to chemically modify a low glass transition temperature (T g ) polymer with polar groups [10]. A large range of dipoles like nitrile [14,25,26], nitroaniline [4,27,28], disperse Red 1 [29], chloro [30], trifluoropropyl [31,32], nitrobenzene [33], sulphonyl groups [34], thioether [35], ester [36], etc. have been used. However, often the polar groups unfavorably interfere with the cross-linking reaction thus preventing the formation of cross-linked materials with suitable elastic properties [37]. Therefore, it is not uncommon that modified polymers are used as fillers in an elastic matrix that provides the necessary elasticity [38,39]. However, such materials are not homogenous at a molecular level. This is especially important when very thin films are envisioned [40]. An ideal dielectric elastomer for actuators should have the following set of properties: a low elastic modulus (Y b 1 MPa), low viscous losses, high strain at break, high dielectric permittivity, low dielectric losses, low conductivity, and high dielectric breakdown field [41]. Additionally, the materials should be easily processed into defect-free thin films, with thicknesses ideally below 50 μm [9,42]. Despite intensive research in this field, the replacement of the commercial polydimethylsiloxane elastomer currently used by companies for DEAs production by better performing materials is still challenging [10].
Based on our interest in developing materials for actuators that respond to low voltages, we here aimed to prepare silicone-based elastomers modified with polar thioacetate groups, to investigate how this group influences dielectric, mechanical, and electromechanical properties. This group was recently grafted to cyclic siloxane [43] and polysiloxanes [44] and subsequently deprotected to give polysiloxanes modified with thiol groups. However, the dielectric properties of the polysiloxanes modified with thioester groups has never been reported before. Sulfur has interesting dielectric properties as it has a large atomic volume and thus a strong electron polarization [45] and we were curious to see whether elastic materials can be prepared at all and how the mechanical, dielectric and electromechanical properties of the cross-linked films will be.

1
H and 13 C NMR spectra were recorded on a Bruker Avance III 400 NMR spectrometer using a 5 mm BBO Prodigy™ CryoProbe at 400.18 and 100.63 MHz, respectively. Chemical shifts (δ) in ppm are calibrated to residual solvent peaks (CDCl 3 : 7.26 and 77.16). Size-exclusion chromatograms were recorded with an Agilent 1100 Series HPLC (Columns: serial coupled PSS SDV 5 μm, 100 Å and PSS SDV 5 μm, 1000 Å, Detector: DAD, 235 nm and 360 nm; refractive index). THF was used as mobile phase, polydimethylsiloxane (PDMS) standards were used for the calibration, and toluene as internal standard. Differential scanning calorimetry (DSC) investigations were undertaken on a Pyris Diamond DSC (PerkinElmer USA) instrument under nitrogen flow (50 ml × min -1 ), in aluminum crucibles shut with pierced lids and using about 10 mg sample mass. Tensile tests were performed on a Zwick Z010 tensile test machine with a crosshead speed of 50 mm/min. Tensile test specimens with a gauge width of 2 mm and a gauge length of 18 mm were prepared by die-cutting. The strain was determined over the traverse moving sensor. The curves were averaged from 3 replicates per material (see ESI), and the values reported are averaged. The Young modulus Y 10% was determined from the slope of the stress-strain curves using a linear fit to the data points from 0 to 10%. The Young modulus Y 50% and Y 100% was determined from the slope of the stress-strain curves using a linear fit to the data points within ±10% strain. For the cyclic hysteresis tests, one sample for each material was subjected to cyclic loading (up to 50% strain) at a ramp rate of 20 mm/min and unloading for 5 cycles. Dynamic mechanical analysis was carried out on a RSA 3 DMA from TA Instruments. Stripes of 10 mm width and 2 cm long were measured under a dynamic load of 2.5 g, at 2% strain in the frequency range of 0.05-10 Hz at 25°C and 65% humidity. Dielectric permittivity measurements at room temperature were done in the frequency range of 1 Hz to 1 MHz, using a Novocontrol Dielectric Spectrometer equipped with an Alpha-A Frequency Analyzer. The samples were prepared by sputtering gold electrodes with a thickness of 20 mm. The V RMS (root mean square voltage) of the probing AC electric signal applied to the samples was 1 V.
Electromechanical tests were performed using circular membrane actuators at ambient temperature and humidity. The films were biaxial prestrained by 30% and fixed between two circular rigid frames that have an inner diameter of 25 mm. Circular electrodes (8 mm diameter) of carbon black powder were applied to each side of the film. A FUG HCL-35-12′500 high voltage source served as a power supply for actuator tests. The voltage was increased by 100 V or 50 V steps every 2 s up to maximum 5.6 kV. The actuation strain was measured optically as the extension of the diameter of the electrode area via a digital camera, using an edge detection tool of a LabView program to detect the boundary between the black electrode area and the transparent silicone film.
Photoreactions were conducted with a UVAHAND 250 GS H1 mercury vapor UV lamp from Dr. Hoenle AG.

Preparation of thin films
Material A n : A solution of P2 (1 g), CL and Sn catalyst in toluene (4 ml) was processed in thin films on a Teflon substrate by doctor blade technique. For the amounts of reagents used please see Table 1. The films were left at room temperature for 24 h to cross-link, followed by 2 days at 70°C to remove any residual solvent. Different elastomers named A n were prepared for which the amount of reagents used is listed in Table 1.
Material B n% : A solution of P2 (1 g), CL, Sn catalyst and different amount of hexamethyldisilasane treated silica particles in toluene (4 ml) was processed into thin films on a Teflon substrate by doctor blade technique. The reagents and amounts used for the synthesis of materials B n are listed in Table 1. The films were left to cross-link at room temperature for 24 h and at 70°C for 2 days.
Material C: A solution of P1 (1 g), surface functionalized silica particles (0 wt%, 5 wt%, and 10 wt% for C 0% , C 5% , and C 10% respectively), TAA (0.85 ml), 2,2′-(ethylenedioxy)diethanethiol (17 μl), and DMPA (5 mg) in toluene (2 ml) was placed on a glass plate coated with a Teflon film and sandwiched between a second glass plate. The thickness of the film was adjusted by two spacers with a thickness of 1 mm. The film was irradiated with a UV light for 5 min. Thereafter, the top glass plate was removed and the films were detached from the Teflon coated glass substrate. They were left at room temperature for 24 h and then dried at 70°C for 2 days prior further characterizations.

Results and discussion
To prepare silicone elastomers modified with polar thioacetate groups, two synthetic paths were followed (Fig. 1). The first started with a polymethylvinylsiloxane (P1) which was modified first with thioacetate groups and then cross-linked into thin films [44]. The second approach used a one-step process in which the modification of P1 with thioacetic acid (TAA) and the cross-linking occurred simultaneously within the thin films. The starting polymer, P1, was prepared from 2,4,6,8,-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (V 4 ) monomer by anionic ring-opening polymerization in presence of tetramethyammonium hydroxide. P1 carries a vinyl group at every siloxy unit which we used to introduce thioacetate groups by thiol-ene addition of TAA in presence of 2,2dimethoxy-2-phenylacetophenone (DMPA) triggered by UV light. The structure and molecular weight of the synthesized polymers were analyzed by 1 H NMR and GPC. The complete disappearance of the vinyl and thiol groups proton signals of the starting reagents and the appearance of new signals in the aliphatic region of the 1 H NMR spectra confirmed the reaction to be complete and thus the formation of thioacetate functionalized polymer (P2) (Fig. 2). The thiol-ene addition reaction gave predominantly the anti-Markovnikov product. After functionalization, the M n increased from 78 kg mol -1 for P1 to 122 kg mol -1 for P2 (Fig. 3).
P2 is a highly viscous polymer with T g = -58°C (Fig. S1). Because of this low T g it has to be cross-linked to obtain materials with good elastic properties. P2 carries hydroxyl end-groups which we subsequently used for cross-linking into thin films using a condensation reaction with poly(methylhydrosiloxane-co-dimethylsiloxane) cross-linker (CL) in the presence of dibutyltin dilaurate (Sn-cat). For that purpose, a mixture of P2, CL, Sn catalyst, and eventually silica particles in toluene was placed on a Teflon substrate and processed into thin films by doctor blade technique. This allowed the formation of materials A n , for which the amount of cross-linker was fine-tuned, and of materials B n% that contain different amount of silica particles, where n% refers to the wt% of silica used. Hexamethyldisilazane treated silica particles were used to optimize the mechanical properties of the resulting materials. After cross-linking, the films were carefully peeled from the substrate and stored between two polypropylene films.
For the synthesis of materials C n% , where n% again refers to the wt% of silica used, a one-step process to thin elastic films was used. Shortly, a solution of P1, thioacetic acid, 2,2′-(ethylenedioxy)diethanethiol cross-linker, and UV initiator in THF was placed on a glass plate coated with Teflon. This solution was sandwiched by a second glass, while the thickness of the film was controlled by two precision spaces (1 mm or 0.5 mm). After cross-linking by exposing the mixture to the UV light for 5 min, the formed films were detached from the Teflon and glass substrates and dried. Both, the functionalization of P1 with thioacetate groups and the cross-linking occurred simultaneously in thin films. To reinforce the materials, 5 wt% and 10 wt% trimethylsilyl surface-functionalized silica particles were used. Although all materials prepared were elastic, they ruptured relatively easily. Even for materials where silica reinforcing filler was used, the strain at break was not improved and the materials were stiffer.
Each material was subjected to tensile tests. Fig. 4 and Figs. S3-S12 show the stressstrain curves of the prepared materials, while Table 2 gives an overview of Young's moduli at different strain levels of 10%, 50%, and 100% and of elongation at break. The Young modulus at 10% strain was determined from the slope of the stress-strain curve using a linear a A solution of dibutyltin dilaurate (50 vol%) in toluene was used. b a dispersion of hexamethyldisilazane treated silica particles (0.5 g) in toluene (10 ml) was used. Fig. 1. Synthesis of polymethylvinylsiloxane (P1) from monomer V 4 , functionalization of P1 by thiol-ene addition with thioacetic acid to polymer P2, which is cross-linked with poly (methylhydrosiloxane-co-dimethylsiloxane) CL to form materials A n (a) and a one-step process to materials C n in which the functionalization and cross-linking of P1 occur simultaneously in thin films (b).
fit to the data points from 0 to 10% strain, while for the Young moduli at 50% and 100% a linear fit to the data points within ±10% strain was used. For the synthesis of materials A n the amount of catalyst was kept constant, while the amount of CL used was gradually increased from 20, 40, 60, to 160 μl per 0.5 g P2. With increasing the amount of cross-linker used, the materials A n became stiffer, except for A 4 which was softer as compared to A 3 . This indicates that for A 4 an excess of CL was used which acts as a plasticizer. Material A 1 showed a strain at break of 198% and a Y 10% = 68 kPa, material A 3 had a strain at break of 124% and a Y 10% = 110 kPa, and material A 4 showed a strain at break of 141% and a Y 10% = 80 kPa. All films prepared from materials A n were rather sticky and difficult to work with. In an attempt to optimize the properties of A n , surface treated silica particles were used and the corresponding materials B 0% , B 2% , B 5% , and B 10% with 0 wt%, 2 wt%, 5 wt%, and 10 wt% were prepared, respectively. As expected, with increasing silica amount in the elastomers, the materials become stiffer (Fig. 4). Thus, the elastic modulus at 10% strain increased from 79 kPa for B 0% , to 119 kPa for B 2% , and reached a maximum value of 210 kPa for B 10% . The strain at break increased with silica content from 175% to 265%, for materials B 0% and B 10% , respectively. These materials were not sticky and therefore handling thin films of B n was easier as compared to A n .
Materials C 0% , C 5% , and C 10% showed a strain at break of 125%, 64%, and 110%, and a tensile strength of 135 kPa, 385 kPa, and 362 kPa, respectively. Because the properties of materials C n% were inferior to those of B n% , these materials were discarded and no further investigations were conducted with them.
Because of the good processability into thin films and promising mechanical properties of materials B n% , further investigations were conducted. Cyclic relaxation tests showed no hysteresis for B 2% and B 5% , and a very small hysteresis for B 10% , but all materials recovered immediately the initial shape after the stress was removed (Fig. 4). The very good elastic properties of materials B n% modified with silica are further supported by the dynamic mechanical analysis (DMA) tests conducted at room temperature and at frequencies between 0.01 to 10 Hz and at a stress value of 30 Pa. With increasing the amount of silica in materials B n% , an increase in the storage modulus can be observed from 64 kPa, 136 kPa-242 kPa for B 2% , B 5% , and B 10% , respectively (Fig. 5). The storage modulus remained almost constant from 0.01 Hz to 1 Hz and then increased slowly at higher frequencies. The loss factor at low frequency was below 0.1, but it increased at higher   frequencies, which may indicate that these materials are not suitable for actuators operated at high frequencies.
The dielectric properties of B n% were investigated by dielectric spectroscopy at room temperature between 1 Hz to 10 6 Hz. Fig. 6 shows the variation in permittivity (ε′), dielectric loss (ε"), loss factor (tan δ) and conductivity (σ) for B n% . Table 3 summarizes the dielectric properties measured at 10 3 Hz. Compared to conventional polydimethylsiloxanes elastomers, for which ε' is around 3, the newly developed materials show an increased permittivity of up to 4.7. The permittivity of the materials slightly decreased with the addition of silica. The conductivity values at low frequencies varied between 1.3 × 10 -12 S cm −1 for B 2% and to 1.7 × 10 -11 S cm −1 for B 10% . An increase in permittivity at low frequencies was observed for all materials due to electrode polarization, which is increasing with the silica addition.
Materials B n% (except B 0 which was too sticky to allow handling) as well as one commercial reference material, silicone Elastosil, were evaluated regarding their actuation strain at different electric fields using circular actuators having 8 mm carbon black electrodes. A biaxial prestrain of 30% was applied to all films to avoid wrinkling of the electrode during actuation. The prestrained films were placed between two rigid plastic frames that have an inner diameter of 25 mm. The voltage applied was increased gradually   '' Tan S/cm Fig. 6. Dielectric permittivity (ε′), dielectric loss (ε"), loss factor (tan δ) and conductivity (σ) of the B n% materials at room temperature and different frequencies.
using a step of 100 V. Fig. 7 shows the lateral actuation strain as a function of nominal electric field for B n% and Elastosil, where the nominal electric field is the applied voltage divided by the initial actuator thickness. From all materials investigated B 2% showed the poorest actuation, and the breakdown field was only 16.5 V/μm. Material B 5% showed the best performance, a lateral actuation strain of 11.4% at an electric field of 20 V/μm. This material showed a maximum actuation strain of 12.8% when the breakdown was reached at 21.5 V/μm. As expected, the actuation of B 10% occurred at higher electric fields as compared to B 5% , which is due to the fact that this material was stiffer as compared to B 5% . A maximum actuation of 14.5% at the maximum voltage of 29 V/μm was measured for B 10% . For the actuation strain of a different actuators made from B 10% , see Fig. S13. Because of the reduced elastic modulus and increased dielectric permittivity, the actuation of B n% occurred at significantly lower electric fields as compared to the regular silicone film, which has an elastic modulus in the range of 1.3 MPa (Fig. 7). The dielectric breakdown of our materials is not very high, likely because the actuators entered the electromechanical instability, an effect often observed in soft elastomers. Electromechanical instability can be avoided with elastomers that stiffen above a certain strain [46]. Unfortunately, our elastomers do not show strain-stiffening. One possible approach to alter the stressstrain curve and thus to increase the dielectric breakdown is by synthesizing prestrainlocked silicone interpenetrated networks [47,48]. Such networks showed better performance as compared to standard silicone films. Although interpenetrated networks can be in principle synthesized using the materials developed here, more synthetic effort has to be invested to achieve such them. Cyclic actuation tests were also conducted for materials B 5% and B 10% at different frequencies of 0.33 Hz (for 50 cycles), 3.33 Hz (for 100 cycles), and 6.66 Hz (for 100 cycles) (Fig. 8) and show some hysteresis in actuation which increases with the frequency. DMA tests show for all materials B n an increase in the mechanical losses with increasing the frequency. These mechanical losses are likely the reason behind the hysteresis observed in the cyclic actuation tests. Material B 5% seems to have slightly lower hysteresis as compared to B 10% which is in agreement with the DMA which shows slightly more mechanical losses for B 10% .
An actuator constructed from thin films of B 10% (∼24 μm) responded to rather low voltages (Fig. 8). For example at 600 V, which represents an electric field of 25 V/μm in actuator, a lateral actuation strain of 4.5% was measured. We gradually increase the voltage of this actuator to 800 V (Fig. S14). The dielectric breakdown of this actuator was reached at 31 V/μm, at 750 V, where a lateral actuation strain of 7.2% was measured.
Actuators operable at low voltages were reported before by Shea et al. [9] and Sheima et al. [42] The first used a 3 μm thin polydimethylsiloxane based elastomer as dielectric, while the latter was 35 μm thick. A lateral actuation strain of 7.5% was observed at a voltage as low as 245 V (82 V/μm). Sheima used a polysiloxane elastomer modified with nitrile groups which has a ε' = 18 and an elastic modulus in the range of 350 kPa. A 35 μm thick actuator gave 7% lateral actuation strain at 300 V (8.6 V/μm). While this material was processible into thin films, the detachment of films from the substrate was tedious because the strain at break was only 80%. The most promising materials in the current work have an average strain at break of about 250% and are easily handled in thin films. Additionally, the lateral actuation strain of 5.6% at 650 V (27 V/μm) is attractive for practical use. Therefore, for applications where single membrane actuators are needed, the material developed in this work is attractive. Because of the promising actuation at low voltages, further research will be carried out to reduce the mechanical losses, to increase the dielectric breakdown filed, and to construct stacked actuators with these materials.

Conclusions
We have developed a synthetic strategy to polysiloxane elastomers modified with thioacetic groups which showed promising properties for actuation. They were prepared in two steps starting from polymethylvinylsiloxane which was chemically modified with thioacetic acid and subsequently cross-linked via a condensation reaction in presence of Sn catalyst. The mechanical properties of the obtained elastomers were tuned by blending with different amounts of surface treated silica. The glass transition temperature of the formed elastomers of -58°C suggests an attractively broad temperature range in which these elastomers can be used. Because of the increased dielectric permittivity and the rather low elastic moduli, the actuators constructed with these elastomers responded to low electric fields. By reducing the thickness of the dielectric films, it was possible to construct devices that gave about 5.6% lateral actuation strain at 650 V, which is significantly lower as compared to most dielectric elastomers reported to date. Thus, the materials developed here are promising candidates as dielectric in actuators. Future efforts will be directed towards mastering the remaining challenges such as hysteresis between actuation cycles and lifetime evaluation.

Conflicts of interest
There are no conflicts to declare.  Table 3 Permittivity (ε') at 1 kHz, tan δ, conductivity, lateral actuation strain at certain electric fields, maximum actuation at breakdown, the dielectric breakdown of actuators, and the thickness of the film used in actuator measurements.