255
cholesteryl mesogenic groups, compared to the corresponding LCs that have normal
alkyl chains spacers. The nature of the LC phases was associated with the nanosegregation. It was found that all compounds showed a smectic A phase in a wide
temperature range, and only the dimeric LC with the shortest siloxane was found to
exhibit a chiral nematic phase and a blue phase in addition to the smectic A phase.
11.4.1 Low Mw and Polymeric Siloxane-Containing LCs
Li et al. (2007) reported that trisiloxane-terminated LCs with 2-phenylpyrimidine
cores formed partially bilayered SmA and SmC phases, exhibiting a more ‘de Vrieslike’ character (minimal layer shrinkage after SmA-SmC transition) compared to a
non-siloxane analogue. This particular behavior is explained in correlation with
nanophase segregation. In ferroelectric SmC∗ phase, the consequence of de Vries’
behavior is the minimization of chevrons and zigzag defects that severely degrade
the quality of electro-optical devices. Siloxane-based ferroelectric LCs have
revealed chevron-free long-term bistable behavior and an easy regeneration of
bistable performance after damage (Xu et al. 2011).
Trisiloxane-containing polymers with pendant azobenzene moieties were
obtained by Pandey et al. (2012) via click chemical reaction and showed LC behavior and photoactuation. The order of the mesophase depended on the nature of the
substituent at the para- position of the azobenzene mesogen, i.e. nematic for -H,
smectic for -CN and no mesophase for –COOCH 3 .
LC cyclosiloxanes have exhibited properties intermediate between low Mw LCs
and polymeric ones, combining the low melt viscosity of the former with the glassforming capacity of the latter, and providing the advantage of molecular monodispersity (Soltysiak et al. 2004).
Supramolecular polymers are based on chemical recognition, e.g. between
bipyridine and carboxylic acids (Racles and Cazacu 2008). Such new materials may
exhibit an LC behavior, due to the H-bonds, while depending on the structure of the
siloxane diacids (chain mobility, free volume and molecular polarizability), the
mesophase range can be tailored. One of them, based on 1,3-bis(carboxypropyl)tetramethyldisiloxane (CX) and 4,4′-bipyridine (BPy) was subsequently analyzed
by Racles et al. (2013b) using single crystal X-ray diffraction, which confirmed the
strong H-bonding between the carboxyl and pyridyl groups with the formation of
the supramolecular structure.
Recently, Nickmans et al. (2018a, b) reported a very nice development in the
supramolecular approach of LC materials. These authors synthesized liquid crystalline H-bonding hepta(dimethylsiloxane) carboxylic acid dimers, which formed
sub-5 nm lamellar characteristics in the bulk and at the liquid solid interface or
hexagonal columnar phase, depending on the coil-to-rod volume fraction. These
compounds were mixed with a homopolymer or a block copolymer containing H
bond-accepting moieties (poly(4-vinylpyridine) or poly(styrene)-b-poly(4vinylpyridine)), thus obtaining nanostructured hierarchical materials with
11 Reactive and Functional Silicones for Special Applications
cholesteryl mesogenic groups, compared to the corresponding LCs that have normal
alkyl chains spacers. The nature of the LC phases was associated with the nanosegregation. It was found that all compounds showed a smectic A phase in a wide
temperature range, and only the dimeric LC with the shortest siloxane was found to
exhibit a chiral nematic phase and a blue phase in addition to the smectic A phase.
11.4.1 Low Mw and Polymeric Siloxane-Containing LCs
Li et al. (2007) reported that trisiloxane-terminated LCs with 2-phenylpyrimidine
cores formed partially bilayered SmA and SmC phases, exhibiting a more ‘de Vrieslike’ character (minimal layer shrinkage after SmA-SmC transition) compared to a
non-siloxane analogue. This particular behavior is explained in correlation with
nanophase segregation. In ferroelectric SmC∗ phase, the consequence of de Vries’
behavior is the minimization of chevrons and zigzag defects that severely degrade
the quality of electro-optical devices. Siloxane-based ferroelectric LCs have
revealed chevron-free long-term bistable behavior and an easy regeneration of
bistable performance after damage (Xu et al. 2011).
Trisiloxane-containing polymers with pendant azobenzene moieties were
obtained by Pandey et al. (2012) via click chemical reaction and showed LC behavior and photoactuation. The order of the mesophase depended on the nature of the
substituent at the para- position of the azobenzene mesogen, i.e. nematic for -H,
smectic for -CN and no mesophase for –COOCH 3 .
LC cyclosiloxanes have exhibited properties intermediate between low Mw LCs
and polymeric ones, combining the low melt viscosity of the former with the glassforming capacity of the latter, and providing the advantage of molecular monodispersity (Soltysiak et al. 2004).
Supramolecular polymers are based on chemical recognition, e.g. between
bipyridine and carboxylic acids (Racles and Cazacu 2008). Such new materials may
exhibit an LC behavior, due to the H-bonds, while depending on the structure of the
siloxane diacids (chain mobility, free volume and molecular polarizability), the
mesophase range can be tailored. One of them, based on 1,3-bis(carboxypropyl)tetramethyldisiloxane (CX) and 4,4′-bipyridine (BPy) was subsequently analyzed
by Racles et al. (2013b) using single crystal X-ray diffraction, which confirmed the
strong H-bonding between the carboxyl and pyridyl groups with the formation of
the supramolecular structure.
Recently, Nickmans et al. (2018a, b) reported a very nice development in the
supramolecular approach of LC materials. These authors synthesized liquid crystalline H-bonding hepta(dimethylsiloxane) carboxylic acid dimers, which formed
sub-5 nm lamellar characteristics in the bulk and at the liquid solid interface or
hexagonal columnar phase, depending on the coil-to-rod volume fraction. These
compounds were mixed with a homopolymer or a block copolymer containing H
bond-accepting moieties (poly(4-vinylpyridine) or poly(styrene)-b-poly(4vinylpyridine)), thus obtaining nanostructured hierarchical materials with
11 Reactive and Functional Silicones for Special Applications