253
significant effects on the properties of interest. For example, Tugui et al. (2016)
obtained a 7.1% lateral strain at an applied field of 20 V/μm for a network containing 10 wt.% PU. The PDMS interpenetration with polyimides resulted in an increase
of dielectric permittivity and breakdown strength, thus, despite the unwanted growth
of the Y, actuation values up to 8.7% were recorded for a polyimide content of
10 wt.%. In addition, Tugui et al. (2017) reported the piezoelectric coefficient d 33 of
26 pm/V, attributed to the presence of the polyimide component. This dual response
recommends these materials as active elements for both actuators and sensors.
11.3.2 Polar Crosslinking Centers
Another way to increase the polarization of the silicone elastomers is by using polar
crosslinking centers. Kussmaul et al. (2012) and Risse et al. (2012b) used a hydridefunctional polysiloxane as a crosslinking agent, as well as substrate for the chemical
binding of a dipolar molecule (N-allyl-N-methyl-p-nitroaniline) by hydrosilylation.
The chemical modification and crosslinking of a PDMS matrix was produced by a
one-step process. An increase in dielectric permittivity was observed (ε’ = 3.7 for
13 wt.% dipole content) along with a decrease in dielectric breakdown strength. In
addition, competitive reactions resulting in network imperfections, dangling substructures and large sol-fractions can be produced. The optimization of the materials
led to a low Y (0.142 MPa), greater permittivity (ε’ = 5.56) and a maximum actuation strain of 2% at 10 V/μm for 10.7 wt.% polar content (Kussmaul et al. 2011). A
similar strategy was used by Zhang et al. (2015) from a dihydroxyl-functional polar
molecule (DR19) together with TEOS and DBTDL to cross-link a hydroxyl-ended
PDMS. Although DR19 participates in the condensation cure, it can act as a polar
molecule in crosslinking centers or as a chain extender, thus the control of dipole
content in the matrix cannot be easily managed. The dielectric permittivity (a maximum of 4.9 at 10
3
Hz), Y and the S max increase linearly with the dipole molecule
content, while the E BD decreases linearly.
Madsen et al. (2013a, b, 2014a) obtained an azido-functional cross-linker which
was compatible with silicones and allowed high dipole moments at the crosslinking
centers. The curing reactions were made using azide-alkyne 1,3-dipolar cycloaddition, whereby the polar entities (nitrobenzene and nitroazobenzene) were well distributed within the matrix. For a limited number of polar groups present only at
crosslinking centers, a modest increase of dielectric permittivity was registered by
20%. Functional trialkoxysilanes have been used as cross-linkers to obtain silicone
elastomers via Sn-catalyzed condensation route of hydroxyl-ended PDMS chains
(Bele et al. 2015; Dünki et al. 2016a). Due to the low content of polar groups, rather
modest dielectric permittivity values have been obtained when chloride (ε’ = 3.7) or
nitrile (ε’ = 3.9) were used as crosslinking centers. However, in some cases due to
the low Y, quite high lateral actuations were achieved at a very low voltage (1.68%
at 0.83 V/μm).
11 Reactive and Functional Silicones for Special Applications
significant effects on the properties of interest. For example, Tugui et al. (2016)
obtained a 7.1% lateral strain at an applied field of 20 V/μm for a network containing 10 wt.% PU. The PDMS interpenetration with polyimides resulted in an increase
of dielectric permittivity and breakdown strength, thus, despite the unwanted growth
of the Y, actuation values up to 8.7% were recorded for a polyimide content of
10 wt.%. In addition, Tugui et al. (2017) reported the piezoelectric coefficient d 33 of
26 pm/V, attributed to the presence of the polyimide component. This dual response
recommends these materials as active elements for both actuators and sensors.
11.3.2 Polar Crosslinking Centers
Another way to increase the polarization of the silicone elastomers is by using polar
crosslinking centers. Kussmaul et al. (2012) and Risse et al. (2012b) used a hydridefunctional polysiloxane as a crosslinking agent, as well as substrate for the chemical
binding of a dipolar molecule (N-allyl-N-methyl-p-nitroaniline) by hydrosilylation.
The chemical modification and crosslinking of a PDMS matrix was produced by a
one-step process. An increase in dielectric permittivity was observed (ε’ = 3.7 for
13 wt.% dipole content) along with a decrease in dielectric breakdown strength. In
addition, competitive reactions resulting in network imperfections, dangling substructures and large sol-fractions can be produced. The optimization of the materials
led to a low Y (0.142 MPa), greater permittivity (ε’ = 5.56) and a maximum actuation strain of 2% at 10 V/μm for 10.7 wt.% polar content (Kussmaul et al. 2011). A
similar strategy was used by Zhang et al. (2015) from a dihydroxyl-functional polar
molecule (DR19) together with TEOS and DBTDL to cross-link a hydroxyl-ended
PDMS. Although DR19 participates in the condensation cure, it can act as a polar
molecule in crosslinking centers or as a chain extender, thus the control of dipole
content in the matrix cannot be easily managed. The dielectric permittivity (a maximum of 4.9 at 10
3
Hz), Y and the S max increase linearly with the dipole molecule
content, while the E BD decreases linearly.
Madsen et al. (2013a, b, 2014a) obtained an azido-functional cross-linker which
was compatible with silicones and allowed high dipole moments at the crosslinking
centers. The curing reactions were made using azide-alkyne 1,3-dipolar cycloaddition, whereby the polar entities (nitrobenzene and nitroazobenzene) were well distributed within the matrix. For a limited number of polar groups present only at
crosslinking centers, a modest increase of dielectric permittivity was registered by
20%. Functional trialkoxysilanes have been used as cross-linkers to obtain silicone
elastomers via Sn-catalyzed condensation route of hydroxyl-ended PDMS chains
(Bele et al. 2015; Dünki et al. 2016a). Due to the low content of polar groups, rather
modest dielectric permittivity values have been obtained when chloride (ε’ = 3.7) or
nitrile (ε’ = 3.9) were used as crosslinking centers. However, in some cases due to
the low Y, quite high lateral actuations were achieved at a very low voltage (1.68%
at 0.83 V/μm).
11 Reactive and Functional Silicones for Special Applications