249
groups or by inserting polar segments in the main chain followed by crosslinking,
alone or in combination (blends, interpenetrating networks) with unmodified PDMS
and (b) crosslinking with polar trifunctional silanes and reinforcing with modified
silica fillers.
11.3.1 Polysiloxanes With Polar Groups
in Dielectric Elastomers
Attaching polar groups to the polysiloxane backbone is one of the many strategies
studied, which mainly focuses on improving dielectric permittivity and energy densities by intrinsically increasing the polarizability of silicone elastomers. For example, polar silicones containing cyanopropyl groups were synthesized by Racles et al.
(2013a) via ROP followed by crosslinking with alkyltriacethoxysilane and tetra(2ethylhexoxide) as a catalyst, finding that the dielectric permittivity increased twice,
but the conductivity also increased along with the poor mechanical properties.
Under another approach, Racles et al. (2014a, 2015a) attached cyanopropyl groups
to the main backbone by hydrosilylation together with non-polar hexyl pendant
groups and mixed with a PDMS polymer in order to obtain elastomers. These
authors reported that together with a higher permittivity, good E BD values were
obtained (~56 V/μm) and actuation strains were achieved 13-times higher compared
to pure PDMS. Hydrosilylation has also been used by Yang and Wnek (1992) for
attaching silyl ketene acetal onto the main backbone followed by reaction with electrophile moieties such as 4-nitrobenzenesulphenyl chloride. The resulting functional polymer with 70% p-nitrothiophenoxy groups led to a dielectric permittivity
of 8.96, compared to 4.83 for the silyl ketene acetal functional polymer. Stepp et al.
(2015) obtained polysiloxanes having chloromethyl groups via polycondensation of
dichloro(chloromethyl)methylsilane and dimethyldichlorosilane, with vinyldimethylchlorosilane as end-blocker. The copolymers were crosslinked via hydrosilylation
between the vinyl end-groups and a multifunctional hydrosilane and for about 60%
mol of chloromethyl side-groups, the dielectric permittivity increased up to around
6–6.8 at 10
5
Hz and the values of the dielectric E BD were from 50 to 60 V/μm (Stepp
et al. 2015).
Madsen et al. (2014b, 2015b) synthesized polar copolymers via Piers-Rubinsztajn
reaction between 3-chloropropyl-methyldimethoxysilane and several hydrideterminated dimethylsiloxane pre-polymers with different Mws, thus creating copolymers with well-distributed azido groups which also offered the possibility to add
dipole molecules (e.g. 1-ethynyl-4-nitrobenzene) via alkyne-azide cycloaddition.
The resulting elastomers had increased breakdown strengths. At high concentrations of nitrobenzene polar groups, high dielectric losses and increased conductivities were registered, while chloropropyl-functional siloxane copolymers (Madsen
et al. 2015a, c) gave elastomers with low modulus of elasticity (0.4–1 MPa, approx.
75–90% lower than the reference silicone elastomer), moderate dielectric permittivity (4.7 at 10
6
Hz), high dielectric breakdown strengths (94.4 V/μm) and seven- to
ten-times higher achievable actuation values.
11 Reactive and Functional Silicones for Special Applications
groups or by inserting polar segments in the main chain followed by crosslinking,
alone or in combination (blends, interpenetrating networks) with unmodified PDMS
and (b) crosslinking with polar trifunctional silanes and reinforcing with modified
silica fillers.
11.3.1 Polysiloxanes With Polar Groups
in Dielectric Elastomers
Attaching polar groups to the polysiloxane backbone is one of the many strategies
studied, which mainly focuses on improving dielectric permittivity and energy densities by intrinsically increasing the polarizability of silicone elastomers. For example, polar silicones containing cyanopropyl groups were synthesized by Racles et al.
(2013a) via ROP followed by crosslinking with alkyltriacethoxysilane and tetra(2ethylhexoxide) as a catalyst, finding that the dielectric permittivity increased twice,
but the conductivity also increased along with the poor mechanical properties.
Under another approach, Racles et al. (2014a, 2015a) attached cyanopropyl groups
to the main backbone by hydrosilylation together with non-polar hexyl pendant
groups and mixed with a PDMS polymer in order to obtain elastomers. These
authors reported that together with a higher permittivity, good E BD values were
obtained (~56 V/μm) and actuation strains were achieved 13-times higher compared
to pure PDMS. Hydrosilylation has also been used by Yang and Wnek (1992) for
attaching silyl ketene acetal onto the main backbone followed by reaction with electrophile moieties such as 4-nitrobenzenesulphenyl chloride. The resulting functional polymer with 70% p-nitrothiophenoxy groups led to a dielectric permittivity
of 8.96, compared to 4.83 for the silyl ketene acetal functional polymer. Stepp et al.
(2015) obtained polysiloxanes having chloromethyl groups via polycondensation of
dichloro(chloromethyl)methylsilane and dimethyldichlorosilane, with vinyldimethylchlorosilane as end-blocker. The copolymers were crosslinked via hydrosilylation
between the vinyl end-groups and a multifunctional hydrosilane and for about 60%
mol of chloromethyl side-groups, the dielectric permittivity increased up to around
6–6.8 at 10
5
Hz and the values of the dielectric E BD were from 50 to 60 V/μm (Stepp
et al. 2015).
Madsen et al. (2014b, 2015b) synthesized polar copolymers via Piers-Rubinsztajn
reaction between 3-chloropropyl-methyldimethoxysilane and several hydrideterminated dimethylsiloxane pre-polymers with different Mws, thus creating copolymers with well-distributed azido groups which also offered the possibility to add
dipole molecules (e.g. 1-ethynyl-4-nitrobenzene) via alkyne-azide cycloaddition.
The resulting elastomers had increased breakdown strengths. At high concentrations of nitrobenzene polar groups, high dielectric losses and increased conductivities were registered, while chloropropyl-functional siloxane copolymers (Madsen
et al. 2015a, c) gave elastomers with low modulus of elasticity (0.4–1 MPa, approx.
75–90% lower than the reference silicone elastomer), moderate dielectric permittivity (4.7 at 10
6
Hz), high dielectric breakdown strengths (94.4 V/μm) and seven- to
ten-times higher achievable actuation values.
11 Reactive and Functional Silicones for Special Applications