369
Novel oligomers with promesogenic groups at both ends were obtained by the
hydrosilylation reaction of well-defined telechelic oligocarbosilazanes terminated
with vinylsilyl groups and Si-H functional promesogenic compounds. The correlations between the structure of the resulting mesogen end-capped oligomers and
their phase behavior were studied by Grande et al. (2009) using differential scanning calorimetry (DSC) and polarizing optical microscopy (POM).
14.6.2.1 Synthesis of Liquid Crystalline Elastomers and Thermosets
Liquid-crystalline silicone elastomers (LCEs) with a polysiloxane backbone can be
covalently crosslinked into three dimensional (3D) networks (Finkelmann et al.
1981; Küpfer and Finkelmann 1991). New side chain LC polysiloxanes were synthesized by substitution of low MW PMHS of average composition MD
H 7 M
(M n = 700–800 g/mol) with different cholesteric and smectic monomers in the presence of Pt catalyst. Some crosslinked LCs showed very wide mesophase temperature ranges and a high thermal stability (He et al. 2005a; Wang et al. 2009). A first
mono-domain polysiloxane elastomer was prepared by Küpfer and Finkelmann
(1991). Other monodomain LC polysiloxane elastomers were synthesized with one
or two crosslinking agents from PMHS and different mesogens in toluene, in the
presence of Pt catalysts and a mechanical field (Lacey et al. 1998).
New network LC polymers were synthesized by graft copolymerization onto
PMHS of the difunctional mesogenic difunctional monomer (4-allyloxy)
benzoyloxy-4′-allyloxy-biphenyl, which served as a mesogenic unit and a crosslinker. The crosslinked polymers rarely exhibited thermal behavior of the elastomer, while densely cured samples behaved like LC thermoset polymers (Lacey
et al. 1998). The T g of LCEs increased with the increasing concentration of the
crosslinker, but the T i and LC range decreased slightly (Jia et al. 2002). The newer
side chain smectic LCEs were synthesized from PMHS (M n = 700–800 g/mol)
and monomer with nematic properties (4,4′-allyloxy-biphenyl-4-yl
4- hep tyloxybenzoate) and the chiral crosslinker (isosorbide-undecanoyloxybisate), in the presence of Pt catalyst at 60–70 °C for three days. The LCEs were
precipitated with methanol and dried under vacuum at RT. These elastomers were
insoluble in chloroform, DMF, toluene and xylene, but could be swollen in these
solvents, thus exhibiting very wide mesophase temperature ranges and high thermal stability. The LC phase of the homopolymer and elastomers changed from
smectic B (SmB) phase to SmA phase and then to SmC∗ phase with an increasing
content of the chiral crosslinker. The T g s first increased, then decreased and finally
increased again with increasing content of the crosslinker, meanwhile the isotropization temperatures and mesophase temperature ranges decreased. The side
chain LCEs containing a rigid mesogenic crosslinker and a nematic monomer,
prepared by the H-silylation reaction and containing <15 mol.% of the bridge units
showed elasticity, reversible phase transitions and a threaded texture (Zhang et al.
2004; He et al. 2005a, b).
14 Hydrosilyl-Functional Polysiloxanes: Synthesis Reactions and Applications
Novel oligomers with promesogenic groups at both ends were obtained by the
hydrosilylation reaction of well-defined telechelic oligocarbosilazanes terminated
with vinylsilyl groups and Si-H functional promesogenic compounds. The correlations between the structure of the resulting mesogen end-capped oligomers and
their phase behavior were studied by Grande et al. (2009) using differential scanning calorimetry (DSC) and polarizing optical microscopy (POM).
14.6.2.1 Synthesis of Liquid Crystalline Elastomers and Thermosets
Liquid-crystalline silicone elastomers (LCEs) with a polysiloxane backbone can be
covalently crosslinked into three dimensional (3D) networks (Finkelmann et al.
1981; Küpfer and Finkelmann 1991). New side chain LC polysiloxanes were synthesized by substitution of low MW PMHS of average composition MD
H 7 M
(M n = 700–800 g/mol) with different cholesteric and smectic monomers in the presence of Pt catalyst. Some crosslinked LCs showed very wide mesophase temperature ranges and a high thermal stability (He et al. 2005a; Wang et al. 2009). A first
mono-domain polysiloxane elastomer was prepared by Küpfer and Finkelmann
(1991). Other monodomain LC polysiloxane elastomers were synthesized with one
or two crosslinking agents from PMHS and different mesogens in toluene, in the
presence of Pt catalysts and a mechanical field (Lacey et al. 1998).
New network LC polymers were synthesized by graft copolymerization onto
PMHS of the difunctional mesogenic difunctional monomer (4-allyloxy)
benzoyloxy-4′-allyloxy-biphenyl, which served as a mesogenic unit and a crosslinker. The crosslinked polymers rarely exhibited thermal behavior of the elastomer, while densely cured samples behaved like LC thermoset polymers (Lacey
et al. 1998). The T g of LCEs increased with the increasing concentration of the
crosslinker, but the T i and LC range decreased slightly (Jia et al. 2002). The newer
side chain smectic LCEs were synthesized from PMHS (M n = 700–800 g/mol)
and monomer with nematic properties (4,4′-allyloxy-biphenyl-4-yl
4- hep tyloxybenzoate) and the chiral crosslinker (isosorbide-undecanoyloxybisate), in the presence of Pt catalyst at 60–70 °C for three days. The LCEs were
precipitated with methanol and dried under vacuum at RT. These elastomers were
insoluble in chloroform, DMF, toluene and xylene, but could be swollen in these
solvents, thus exhibiting very wide mesophase temperature ranges and high thermal stability. The LC phase of the homopolymer and elastomers changed from
smectic B (SmB) phase to SmA phase and then to SmC∗ phase with an increasing
content of the chiral crosslinker. The T g s first increased, then decreased and finally
increased again with increasing content of the crosslinker, meanwhile the isotropization temperatures and mesophase temperature ranges decreased. The side
chain LCEs containing a rigid mesogenic crosslinker and a nematic monomer,
prepared by the H-silylation reaction and containing <15 mol.% of the bridge units
showed elasticity, reversible phase transitions and a threaded texture (Zhang et al.
2004; He et al. 2005a, b).
14 Hydrosilyl-Functional Polysiloxanes: Synthesis Reactions and Applications