252
dielectric elastomers. The self-assembling of such metal complexes in aggregates
with hydrophobic shell ensures a good compatibility and dispersion of the filler
within the hydrophobic environment created by the polymer matrix, without requiring laborious operations (Stiubianu et al. 2016).
In general, different simultaneous or sequential crosslinking pathways to form
silicone based IPNs have been used. IPNs are expected to have an improved combination of properties from both networks, such as better mechanical properties or
degradation stability. There are reports in the literature about silicon-based IPNs
(Mark and Ning 1985; Hamurcu and Baysal 1993; Shah 2004; Brochu et al. 2013;
Madsen et al. 2014a; Yu et al. 2014), many of which are made by combining kits
commercially available (Shah 2004; Brochu et al. 2013; Yu et al. 2014). With respect
to electromechanical applications, most studies include networks formed of PDMS
with different Mws or crosslinking mechanism (sometimes based on reaction of
functional groups), and only a few of them refer to functional silicones in IPNs.
Brochu et al. (2013) chose hydroxyl-terminated PDMS crosslinked at RT with tetrapropylorthosilicate (TPOS) to form the first network, and two low Mw polysiloxanes (vinyl terminated PDMS and PMHS) crosslinked together via hydrosilylation
at 160 °C forming the second network. The partially crosslinked RT network was
pre-strained, then heated to allow full curing while retaining some previously
applied strain. This strategy led to high area strains (45% at 60 V/μm in diaphragm
mode and 15% in linear mode). Tugui et al. (2015a) applied the same crosslinking
systems to combinations of PDMS and chemically modified silicones carrying
cyanopropyl, phenyl or trifluoropropyl moieties, which were used in IPNs without
preserved pre-strains. In all cases, dielectric E BD decreased drastically; the IPN with
cyanopropyl moieties were the most promising (E BD = 29 V/μm, ε’ = 9.27 at 5 Hz).
Tugui et al. (2015b) also used two PDMS of different Mw in several ratios, sequentially crosslinked, to obtain full silicone bimodal IPNs, thus highlighting the beneficial role that pre-straining applied to the first network had on the electromechanical
properties.
In another approach, ionic crosslinking between amine-functional PDMS
(AMS-162) and carboxydecyl-ended PDMS in different ratios were used by Yu
et al. (2015) for the first network, while a two-component commercial mixture was
crosslinked by hydrosilylation, forming the second network. Overall, the presence
of amine-functional PDMS increased the σ m and decreased the S max , but an optimized ratio of 50:50 wt.% between networks gave the highest dielectric permittivity
(1.7 × 10
2
at 0.1 Hz and 11.7 at 10
6
Hz) and moderate to high dielectric E BD (78 V/
μm). A very important feature was that ionic IPNs presented self-healing properties.
Tugui et al. (2016, 2017) combined PDMS with poly(siloxane-urethanes) and
poly(siloxane-imides) as polar components in interpenetrated networks to obtain
dielectric elastomers with high dielectric permittivity. The organic components in
the siloxane-organic copolymers act as enhancers of the dielectric permittivity,
while the siloxane segment was thought to ensure compatibility and good dispersion of the copolymer within the silicone matrix. Although phase separation has not
been completely avoided, with moderate polar component contents it did not have
C. Racles et al.
dielectric elastomers. The self-assembling of such metal complexes in aggregates
with hydrophobic shell ensures a good compatibility and dispersion of the filler
within the hydrophobic environment created by the polymer matrix, without requiring laborious operations (Stiubianu et al. 2016).
In general, different simultaneous or sequential crosslinking pathways to form
silicone based IPNs have been used. IPNs are expected to have an improved combination of properties from both networks, such as better mechanical properties or
degradation stability. There are reports in the literature about silicon-based IPNs
(Mark and Ning 1985; Hamurcu and Baysal 1993; Shah 2004; Brochu et al. 2013;
Madsen et al. 2014a; Yu et al. 2014), many of which are made by combining kits
commercially available (Shah 2004; Brochu et al. 2013; Yu et al. 2014). With respect
to electromechanical applications, most studies include networks formed of PDMS
with different Mws or crosslinking mechanism (sometimes based on reaction of
functional groups), and only a few of them refer to functional silicones in IPNs.
Brochu et al. (2013) chose hydroxyl-terminated PDMS crosslinked at RT with tetrapropylorthosilicate (TPOS) to form the first network, and two low Mw polysiloxanes (vinyl terminated PDMS and PMHS) crosslinked together via hydrosilylation
at 160 °C forming the second network. The partially crosslinked RT network was
pre-strained, then heated to allow full curing while retaining some previously
applied strain. This strategy led to high area strains (45% at 60 V/μm in diaphragm
mode and 15% in linear mode). Tugui et al. (2015a) applied the same crosslinking
systems to combinations of PDMS and chemically modified silicones carrying
cyanopropyl, phenyl or trifluoropropyl moieties, which were used in IPNs without
preserved pre-strains. In all cases, dielectric E BD decreased drastically; the IPN with
cyanopropyl moieties were the most promising (E BD = 29 V/μm, ε’ = 9.27 at 5 Hz).
Tugui et al. (2015b) also used two PDMS of different Mw in several ratios, sequentially crosslinked, to obtain full silicone bimodal IPNs, thus highlighting the beneficial role that pre-straining applied to the first network had on the electromechanical
properties.
In another approach, ionic crosslinking between amine-functional PDMS
(AMS-162) and carboxydecyl-ended PDMS in different ratios were used by Yu
et al. (2015) for the first network, while a two-component commercial mixture was
crosslinked by hydrosilylation, forming the second network. Overall, the presence
of amine-functional PDMS increased the σ m and decreased the S max , but an optimized ratio of 50:50 wt.% between networks gave the highest dielectric permittivity
(1.7 × 10
2
at 0.1 Hz and 11.7 at 10
6
Hz) and moderate to high dielectric E BD (78 V/
μm). A very important feature was that ionic IPNs presented self-healing properties.
Tugui et al. (2016, 2017) combined PDMS with poly(siloxane-urethanes) and
poly(siloxane-imides) as polar components in interpenetrated networks to obtain
dielectric elastomers with high dielectric permittivity. The organic components in
the siloxane-organic copolymers act as enhancers of the dielectric permittivity,
while the siloxane segment was thought to ensure compatibility and good dispersion of the copolymer within the silicone matrix. Although phase separation has not
been completely avoided, with moderate polar component contents it did not have
C. Racles et al.