Properties of Liquid Crystalline Polymers
LCPs show excellent mechanical and physicomechanical properties due to the
combination of liquid and crystalline nature. They have potential in
photoresponse-related applications, haptic displays, flow control, catalysis, optics/
photonics, etc. LCNs exhibit negative thermal expansion and multiple phase transformation in the same material. The photoresponsive behavior in LCNs is due to the
anisotropic organization of mesogenic moieties. Azobenzene-containing LCNs are
recently utilized as shape memory polymer and adaptive material (Harris et al. 2005;
Lee et al. 2011, 2012). McConney et al. have reported a photoresponsive LCN,
where all the surface features have been initiated by the application of photons
(McConney et al. 2013). One of the striking features of liquid crystal materials is that
they have the capability to arrange direct profiles into intricate patterns. The
direction-oriented patterning is possible by the use of photo-aligned surfaces based
on azobenzene material. In an interesting study, Broer and group have fabricated
freestanding films composed of three-dimensional molecular ordering via photoalignment of polymerizable liquid crystals (de Haan et al. 2012). The film so
obtained upon heat treatment deforms to cone and saddle forms.
Similar to low molecular weight liquid crystals, the liquid crystal monomers are
quite birefringent and have a high refractive index when the measurement is carried
out under light polarization in a direction parallel to the director. When the measurement is conducted in a direction orthogonal to the director, the refractive index
becomes much lower. The monomer behaves similarly as that of low molecular
weight nematic liquid crystals and possesses a huge dependence on temperature
when transition proceeds from nematic to isotropic phases. Upon heating of the
polymer to a temperature near thermal degradation, no more new isotropic phase is
formed, and the birefringence is only nominally affected. This remains a common
behavior for all the LCPs based on diacrylates. The temperature dependence on the
optical properties of the LCPs is much dependent on the spacer chain length and
crosslinking density of the polymer.
LCPs of aramid types exhibit good elastic modulus and tensile strength as
compared to that of networks formed from LCPs of the same kind. The liquid crystal
networks exhibit modulus in the range of GPa, and the tensile strength remains in the
range of 10–100 MPa (Liu and Broer 2014). The modulus remains anisotropic in
nature; however, it is only three times higher when the measurement is conducted in
a direction along the director as compared to that perpendicular to it. Anisotropic
thermal expansion of LCN remains much interesting compared to its strength or
modulus parameters (Broer and Mol 1991). The majority of the covalent bonds
remain in the direction of the director, which leads to lower linear thermal expansion
in the direction parallel to the director. Again, during thermal treatment, as the
material passes above the glass transition temperature, the order parameter is
decreased further leading to a contraction in the material upon further increase in
the temperature. The linear coefficient of thermal expansion remains negative in the
direction parallel to the orientation direction. The thermal expansion becomes much
large when the measurement is conducted in the perpendicular direction and shows
16
S. Banerjee and K. K. Kar
LCPs show excellent mechanical and physicomechanical properties due to the
combination of liquid and crystalline nature. They have potential in
photoresponse-related applications, haptic displays, flow control, catalysis, optics/
photonics, etc. LCNs exhibit negative thermal expansion and multiple phase transformation in the same material. The photoresponsive behavior in LCNs is due to the
anisotropic organization of mesogenic moieties. Azobenzene-containing LCNs are
recently utilized as shape memory polymer and adaptive material (Harris et al. 2005;
Lee et al. 2011, 2012). McConney et al. have reported a photoresponsive LCN,
where all the surface features have been initiated by the application of photons
(McConney et al. 2013). One of the striking features of liquid crystal materials is that
they have the capability to arrange direct profiles into intricate patterns. The
direction-oriented patterning is possible by the use of photo-aligned surfaces based
on azobenzene material. In an interesting study, Broer and group have fabricated
freestanding films composed of three-dimensional molecular ordering via photoalignment of polymerizable liquid crystals (de Haan et al. 2012). The film so
obtained upon heat treatment deforms to cone and saddle forms.
Similar to low molecular weight liquid crystals, the liquid crystal monomers are
quite birefringent and have a high refractive index when the measurement is carried
out under light polarization in a direction parallel to the director. When the measurement is conducted in a direction orthogonal to the director, the refractive index
becomes much lower. The monomer behaves similarly as that of low molecular
weight nematic liquid crystals and possesses a huge dependence on temperature
when transition proceeds from nematic to isotropic phases. Upon heating of the
polymer to a temperature near thermal degradation, no more new isotropic phase is
formed, and the birefringence is only nominally affected. This remains a common
behavior for all the LCPs based on diacrylates. The temperature dependence on the
optical properties of the LCPs is much dependent on the spacer chain length and
crosslinking density of the polymer.
LCPs of aramid types exhibit good elastic modulus and tensile strength as
compared to that of networks formed from LCPs of the same kind. The liquid crystal
networks exhibit modulus in the range of GPa, and the tensile strength remains in the
range of 10–100 MPa (Liu and Broer 2014). The modulus remains anisotropic in
nature; however, it is only three times higher when the measurement is conducted in
a direction along the director as compared to that perpendicular to it. Anisotropic
thermal expansion of LCN remains much interesting compared to its strength or
modulus parameters (Broer and Mol 1991). The majority of the covalent bonds
remain in the direction of the director, which leads to lower linear thermal expansion
in the direction parallel to the director. Again, during thermal treatment, as the
material passes above the glass transition temperature, the order parameter is
decreased further leading to a contraction in the material upon further increase in
the temperature. The linear coefficient of thermal expansion remains negative in the
direction parallel to the orientation direction. The thermal expansion becomes much
large when the measurement is conducted in the perpendicular direction and shows
16
S. Banerjee and K. K. Kar