Full silicone interpenetrating bi-networks with different organic groups attached to the silicon atoms

Abstract Mixed silicone interpenetrating networks have been prepared using two groups of siloxane polymers and copolymers as precursors in different combinations. Each combination chosen, after a good mixing in solution with cross-linking agents was processed into film, which has been stabilized through sequential crosslinking using separate chemical pathways: condensation with tetraethyl orthosilicate at room temperature for the first network and addition (hydrosilylation) at higher temperatures in the case of the second one. The morphology of the resulted materials was studied by scanning electron microscopy (SEM). The mixing degrees of the two networks were estimated on the basis of the differential scanning calorimetry (DSC) traces as well as by small angle X-ray scattering (SAXS). The obtained films were characterized from point of view of the properties of interest for electromechanical applications by mechanical testing, dielectric spectroscopy and electrical breakdown measurements. The influence of the nature and content of the polar groups attached on the silicon on these characteristics was discussed.

Abstract: Mixed silicone interpenetrating networks have been prepared using two groups of siloxane polymers and copolymers as precursors in different combinations. Each combination chosen, after a good mixing in solution with cross-linking agents was processed into film, which has been stabilized through sequential crosslinking using separate chemical pathways: condensation with tetraethyl orthosilicate at room temperature for the first network and addition (hydrosilylation) at higher temperatures in the case of the second one. The morphology of the resulted materials was studied by scanning electron microscopy (SEM). The mixing degrees of the two networks were estimated on the basis of the differential scanning calorimetry (DSC) traces as well as by small angle X-ray scattering (SAXS). The obtained films were characterized from point of view of the properties of interest for electromechanical applications by mechanical testing, dielectric spectroscopy and electrical breakdown measurements. The influence of the nature and content of the polar groups attached on the silicon on these characteristics was discussed.
angle, accounted either to (p→d)π back-bonding or to ionic character. 3 These lead to high intramolecular bond energy and flexible polymer chain. The presence of the organic nonpolar groups attached to the silicon atoms, as in PDMS for example, is the reason for weak intermolecular forces. 4 Silicones show a unique flexibility, with shear modulus G values between 100 kPa and 3 MPa and loss tangent values, tan σ<<0.001. [5][6][7] On the other hand, the tensile strength is higher than for most organic elastomers at elevated temperatures. In addition, silicone elastomers can operate in a wide range of temperature, without being affected by the moisture, are resistant to oxygen, ozone and sunlight irradiation, have a good dielectric strength and no toxicity, which make them suitable for a large number of applications. [7][8][9][10][11] However, as concerning the electromechanical applications, silicones have the disadvantage of a low dielectric permittivity. Therefore, a significant research effort was focused to improve the dielectric permittivity by incorporating various fillers in the polymer matrix or by attaching polar groups to the polymer network. [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] However, by these approaches, the mechanical properties of these materials could be damaged. 27 As for most electroelastomers, mechanical prestrain is generally required to obtain high electromechanical strain and high elastic energy density although the prestrain causes a reduction in certain parameters of the actuators. An interesting approach is to create and maintain prestrain by interpenetrating polymer networks (IPNs). 28 In the literature there are a number of polysiloxane-organic polymers interpenetrated networks, those reported before 2000 and excellently reviewed in ref. 29 It is highlighted the difficulty of obtaining homogeneous networks due to well-known incompatibility of the silicones with almost any organic component. These silicone-containing networks generally have been found useful as pervaporation membrane, in gears, for medical aims (silicone rubber-nylon or PU with trade name Rimplast), in drug release (poly(vinyl alcohol) -polydimethylsiloxane hydrogels), for high temperature damping (polydimethylsiloxane -polyacrylates -polymethacrylates), or sound and vibration damping (polysiloxanes -poly(methyl methacrylate)). [30][31][32][33][34] The use of interpenetrating polymer networks (IPN) as dielectric elastomers is a less addressed attempt to improve both dielectric and mechanical properties. 35,36 Only recently the electroactive interpenetrating networks that to deform easily under the influence of an electric field become of interest. In general, these are constructed by generating a network in the presence of a basic, already formed one. 35 For enhanced deformation of actuation, the primary network is prestrained. 33 As electroactive elastomers, IPNs based on VHB and poly(1,6-hexanediol diacrylate) or trimethylolpropane trimethacrylate (TMPTMA) as well as IPNs of silicone and 3M VHB 4910 have been prepared. [35][36][37] The interpenetrating polymer networks are formed in the highly prestrained VHB acrylic elastomer network. 36 Brochu et al. used a commercial kit consisting in soft room temperature vulcanizing (RTV) silicone as the host elastomer and a more rigid high temperature vulcanizing (HTV) silicone to prepare IPN. Prestrain was applied to the crosslinked RTV silicone host material, after that the HTV silicone was cured locking the host material in this state. [38][39][40] Prompted by the Brochu works, our approach consists in the preparation of siloxane-siloxane networks on the basis of several available, in house prepared polymers as network precursors. 38 The two networks differ by the nature of the substituents at the silicon atom, by the molecular masses of the precursors and also by the crosslinking path. Polar groups (phenyl, trifluoropropyl, 3-cyanopropyl) were attached to the silicon belonging to one of the network in order to increase the dielectric permittivity. In this case, a big challenge is to find a good compatibility between networks, mainly when one network is more polar then the other, because a phase separation may occur. Further, the interpenetration of two networks with high value of dipole moment can lead to a significant decrease of the breakdown strength.
The samples were processed as thin films, which have been stabilized through sequential cross-linking using two separate chemical pathways, without prestrain: one is condensation of OH-terminated siloxane copolymers containing various percents of polar groups (phenyl or trifluoropropyl) along the chain and cured by condensation (used as main network) and, the second one is addition of α,ω-bis(vinyl)polydimethylsiloxane to α,ω-bis(trimethylsiloxy)poly(dimethylmethyl-H-siloxane) or α,ω-bis(trimethylsiloxy)poly(methylcyanopropylmethylhexyl-methylhydro)siloxanes. The mechanical and dielectric properties, morphology and thermal behaviour of the resulting IPNs were studied. Polydimethylsiloxane-α,ω-diol (PDMS) with M n =377300 g mol -1 , as determinated by gel permeation chromatography (GPC), was prepared by cationic ring-opening polymerization of octamethylcyclotetrasiloxane (D 4 ) catalysed by sulphuric acid, according to an already reported procedure. 39 Polydimethyldiphenylsiloxane-α,ω-diol, PDMDPhS, with M n =104500 g mol -1 was prepared by bulk anionic ring opening copolymerization of the octamethylcyclotetrasiloxane (D 4 ) and octaphenylcyclotetrasiloxane (Ph 4 ), using tetramethylammonium hydroxide as catalyst and a Lewis base (DMF) as promoter, according to procedure described in ref. 40 The content in phenyl groups estimated on the basis of 1 H NMR was 18.4 mol% (Figure 1).
The α,ω-bis(trimethylsiloxy)poly(dimethylmethyl-H-siloxane), PDMMHS, was synthetized by equilibrium copolymerisation of D 4 with α,ω-bis(trimethylsiloxy)poly(methyl-H-siloxane) homopolymer in the presence of Purolite CT-175. 39 The molecular mass of the resulted copolymer determined by GPC was M n =17800g mol -1 . Based on 1 H NMR spectrum it has been estimated a content of 19.2 mol% Si-H groups ( Figure 1). This copolymer was used as a precursor to generate Net B, or in general the second networks, by hydrosilylation with another one containing vinyl group (see below Vi 2 PDMS). The α,ωbis(trimethylsiloxy)poly(methylcyanopropyl-methylhexyl-methylhydro)siloxanes, PMCyMHS 1 and PMCyMHS 2 , with 3-cyanopropyl/hexyl/hydro contents 9.2/82.9/8.9 and 62.1/26.9/11 and Mn=4600 and 4200, respectively were prepared by co-hydrosilylation reactions of the commercial α,ω-bis(trimethylsiloxy)poly(methyl-H-siloxane) with found molecular weight Mn of 2200 g mol -1 , with allylcyanide and n-hexene. 24 α,ω-Bis(vinyl)polydimethylsiloxane, Vi 2 PDMS, was synthesized by an equilibrium cationic ring-opening polymerisation of D 4 in presence of 1,3-bis(vinyl)tetramethyldisiloxane used to block the chains ends. 41 The resulted polymer had M n =40200 g mol -1 as determined by GPC and was used in the addition reaction to achieving the second network of IPN. The percentages of the polar units contained in copolymers were calculated from 1 H NMR spectra ( Figure 1).

Procedure for preparing IPNs
All networks were prepared according to the general procedure described below. Thus, 2 g polymer precursor for the first network was dissolved in 20 mL chloroform, and then different amounts of precursors for the second network (Vi 2 PDMS and PDMMHS, PMCyMHS 1 or PMCyMHS 2 ) were added according to     (Table 2).      To further investigate the phase separation, DSC curves were registered and are presented in Figure 4. Practically, the glass transition temperature and enthalpy depend on the level of cross-linking, which is affected by the degree of mixing and/or the level of interpenetrating polymer chains. In a phase-separated system, several glass transition temperatures may be registered corresponding to each component. In a homogeneous system, there is a single glass transition, generally ranging between transitions of the components that make up the system. 49 The DSC scans (Figure 4) revealed for IPN-R, as expected, a single Tg around -123 o C. IPN-P1 also exhibited a single glass transition at -87 o C as a result of the good mixing (presumed by interpenetration) of PDMS with more rigid dimethyldiphenylsiloxane random copolymeric chain.  (Table 1S). For example, IPN-F1 exhibited a polymorphic melting, while IPN-CN2 showed a very pronounced melting endotherm, in agreement with the most pronounced phase separation, as a consequence of the largest difference in polarity between the two components.
The IPN films in which there are premises for phase separation, as SEM and DSC analyses already emphasized, were analysed by SAXS. Figure 5 shows the obtained SAXS curves (1) The coefficient a c represents the correlation distance defined as the size of an inhomogeneity existent in the sample ( Figure 6). (2) -the second network; a c -short range correlation length.
Since the corresponding scattering intensity could be defined as: (2) 14 where A is a constant, then, a c is easily calculated by plotting I(q) -1/2 versus q 2 , a graphical approach (Figure 7) which is known as Debye-Bueche plot: 55 (3) Stress-strain curves presented in Figure 8a,b and the data centralized in Table 4 show that the blank IPN-R has an intermediary strain value (484 %) as compared with blank samples Net A and Net B, which have elongations of 772 % and 262 %, respectively. It is presumed that the second network limits the extension of the first network. 58      Figure 10c). When polar groups are attached to one of the networks, as can be observed in Figure 10b and Table 4, dielectric permittivity at low frequencies is significantly higher than for blank IPN, depending on the polarity of the organic groups. The highest value at 5 Hz was recorded for sample IPN-CN1 (ɛ = 9.27) followed by IPN-F1 (ɛ = 8.11). As the frequency increases, the permittivity decreases significantly reaching a plateau around 10 3 Hz in all cases, as observed in the case of precursors of the IPN-CNs. 24 In high frequency region, the  Table 4). value as the polarity of the material increases is an expected, well-known behaviour due mainly to large dielectric losses. Failure to entirely dissipate heat generated by these losses, as well as increased amount of moisture absorbed that promotes electrolytic processes could be an explanation for this shortcoming. 25 The values found for IPN-CNs are comparable with those reported by Risse et al. 47 for blends and much lower than those found for interconnected networks with cyanopropyl polar groups 45 . However, the measuring protocol in this case differs from the ones used by other groups 47 , i.e. we used a 400 V/s ramp and 22 mm planar electrodes, versus 50 or 100 V/s and spherical electrodes and the measurements were carried out in actuation mode, as it allowed our equipment. Thus, the film thickness when the breakdown occurs is lower than initial, at that the breakdown voltage was reported.
While a larger electrode area and a greater ramp could give more accurate results in terms of overall material behaviour, the presence of small voids or other defects in the tested materials would clearly limit the breakdown values. Thus, the comparison with similar materials in the literature is rather imprecise.
The mechanical and dielectric characteristics can be used to estimate a parameter of interest for future potential application of such materials in electromechanical devices (e.g., actuation or harvesting). Thereby, the electromechanical sensitivity, β, was calculated using the dielectric permittivity (ɛ') at 5 Hz and the Young's modulus (Y) at 10% strain. 60 Figure 11 reveals that the highest values, 74 and 195 MPa -1 , were obtained for IPN-F1 and IPN-F2 containing high percents (10.5 and 12.5 mol%, respectively) of highly polar trifluoropropyl groups. This means an increase in this parameter with 573 and 1673 %, respectively, compared to the reference sample IPN-R (Figure 11b). These samples showed also the lowest values for Young modulus and highest values for dielectric permittivity (8.11 and 5.85, respectively) at 5 Hz.