generally alkoxyl or alkyl types leading to the formation of branched LCPs commonly known as comb-shaped LCPs.
LCPs with mesogens in the main chain already find applications in numerous fields,
development of strong fibers based on lyotropic liquid crystalline polyamides, such as
Kevlar, Twaron, etc. The strength and modulus of these LCP-based fibers are about
2–2.5 and 10–20 times better as compared to those made of yarns of aliphatic polyamides and about 2–4 times greater compared to steel and glass fibers (Shibaev and
Bobrovsky 2017). This sudden improvement in mechanical properties is due to the
packing of mesogenic units in the liquid crystal state leading to the generation of
systematic regions with parallelly oriented polymer chains during yarn formation or
spinning of the fiber. Among the liquid crystal-based plastics, Xydar and Vectra are of
great technological importance due to the extremely beneficial mechanical properties
used as self-reinforced plastic materials (Kar and Otaigbe 2004). The structural arrangement of the mesogenic units takes place at the time of melt spinning during extrusion of
the melt through the aperture of the spinneret. New-generation plastics based on liquid
crystalline polyester are also prepared by the melt extrusion process. These superplastics are self-reinforcing in nature as described earlier and are of high modulus and
strength of about 60–70 GPa and 700 MPa, respectively, with a very low elongation at
break of 1.5–2% (Shibaev and Bobrovsky 2017). Liquid crystal-based polyesters are
also of low coefficient of thermal expansion ( 10
À6 K
À1
) that are comparatively lower
than that of non-crystalline of the same type (10
À4 K
À1
) (Shibaev and Bobrovsky 2017).
As mentioned earlier, LCPs can be broadly classified as thermotropic LCPs and
lyotropic LCPs depending on the presence of liquid crystalline phases either in the melt
or in the solution form. Lyotropic LCPs are of good liquid crystalline nature, and the
extent of crystallinity is dependent on the nature and temperature of the solvent and
concentration of the polymer. These materials are incapable of exhibiting liquid crystallinity in the melt state since they degrade before melting and exhibit transition of
phases through addition or removal of solvent molecules (Gray et al. 2009). For
thermotropic LCP materials, the transition among the phases is governed by a thermal
process and controlled by the thermal history of the material and melt temperature. The
thermally triggered liquid crystal mesogens form liquid crystal phases from the crystalline melting point to the isotropic temperature of the material (Gray et al. 2009).
Fig. 4 Macromolecules of different chain rigidity: (a) flexible polymer chain, (b) rigid polymer
chain, (c) LCPs containing mesogens in the main chain, and (d) LCPs containing mesogens in the
side-chain (comb-shaped LCPs)
6
S. Banerjee and K. K. Kar
LCPs with mesogens in the main chain already find applications in numerous fields,
development of strong fibers based on lyotropic liquid crystalline polyamides, such as
Kevlar, Twaron, etc. The strength and modulus of these LCP-based fibers are about
2–2.5 and 10–20 times better as compared to those made of yarns of aliphatic polyamides and about 2–4 times greater compared to steel and glass fibers (Shibaev and
Bobrovsky 2017). This sudden improvement in mechanical properties is due to the
packing of mesogenic units in the liquid crystal state leading to the generation of
systematic regions with parallelly oriented polymer chains during yarn formation or
spinning of the fiber. Among the liquid crystal-based plastics, Xydar and Vectra are of
great technological importance due to the extremely beneficial mechanical properties
used as self-reinforced plastic materials (Kar and Otaigbe 2004). The structural arrangement of the mesogenic units takes place at the time of melt spinning during extrusion of
the melt through the aperture of the spinneret. New-generation plastics based on liquid
crystalline polyester are also prepared by the melt extrusion process. These superplastics are self-reinforcing in nature as described earlier and are of high modulus and
strength of about 60–70 GPa and 700 MPa, respectively, with a very low elongation at
break of 1.5–2% (Shibaev and Bobrovsky 2017). Liquid crystal-based polyesters are
also of low coefficient of thermal expansion ( 10
À6 K
À1
) that are comparatively lower
than that of non-crystalline of the same type (10
À4 K
À1
) (Shibaev and Bobrovsky 2017).
As mentioned earlier, LCPs can be broadly classified as thermotropic LCPs and
lyotropic LCPs depending on the presence of liquid crystalline phases either in the melt
or in the solution form. Lyotropic LCPs are of good liquid crystalline nature, and the
extent of crystallinity is dependent on the nature and temperature of the solvent and
concentration of the polymer. These materials are incapable of exhibiting liquid crystallinity in the melt state since they degrade before melting and exhibit transition of
phases through addition or removal of solvent molecules (Gray et al. 2009). For
thermotropic LCP materials, the transition among the phases is governed by a thermal
process and controlled by the thermal history of the material and melt temperature. The
thermally triggered liquid crystal mesogens form liquid crystal phases from the crystalline melting point to the isotropic temperature of the material (Gray et al. 2009).
Fig. 4 Macromolecules of different chain rigidity: (a) flexible polymer chain, (b) rigid polymer
chain, (c) LCPs containing mesogens in the main chain, and (d) LCPs containing mesogens in the
side-chain (comb-shaped LCPs)
6
S. Banerjee and K. K. Kar