an increasing trend with temperature. Above the glass transition temperature, the
anisotropic property of thermal expansion becomes larger. The influence of temperature of polymerization and the presence of substituents play a minor role in this
respect. A high polymerization temperature in general leads to the formation of less
ordered network structure and, hence, the anisotropy in the material decreases.
In general, LCPs are of high mechanical strength at high temperatures, superior
chemical resistance, good weatherability, and flame-retardant properties. They may
create several forms via sintering and molding. LCPs possess a high coefficient of
thermal expansion along the Z-axis. LCPs are of exceptional chemical resistance at
elevated temperature and in presence of strong acids, bases, and aromatic and
halogenated hydrocarbons. The dimensional stability of LCP is quite good even in
boiling water. Polar and bowlic LCPs are by nature ferroelectrics with a very low
reaction time compared to conventional liquid crystals making them a material of
choice for ultrafast switches. Bowlic polymers are columnar in shape and can be
used to form ultrahigh superconductors. Table 1 shows some important properties of
few industrial-grade LCPs.
LCPs are known for high modulus thermoplastic matrix. It is also reported that
the addition of LCPs to a traditional thermoplastic matrix reduces the viscosity of the
compounding process, and hence the processability could be improved to certain
extent. The LCP/thermoplastic blends also have other advantages over the standard
glass-fiber-reinforced composites in addition to reduced melt viscosity and slashed
energy consumption in processing. Few important LCP-based composites along
with properties are mentioned in Table 2.
Processing of Liquid Crystalline Polymers
LCPs are generally processed and molded by five ways: melt spinning, pressing,
injection molding, extrusion, and coating (Kar and Hodzic 2011; Kar 2011).
Aromatic LCPs composed of copolyesters exhibit exceptional orientational behavior
and properties in its solid state. In an interesting study, the orientation of molecules is
examined under shear and elongation flow/deformation. The study exhibits that high
molecular orientation can be observed under elongation, but no such observation can
be seen in case of shear force. Rod-shaped molecules from isotropic solutions are
oriented in the flow direction, whereas, in the case of shear stress, the molecules will
revolve intermittently. Again, if the molecules are under an anisotropic state in the
melt or solution phase, where the molecular domains are already oriented, the
application of shear stress will not affect the orientation of stabilized molecules.
However, in the case of elongational flow, the molecular domain will preferably
orient and stretched in the direction of flow yielding monodomain state formation
(Ide and Ophir 1983).
During the injection molding of the LCPs, a distinct core and skin morphology
has been reported. This can be explained by the quenching process at the walls and
elongational flow history of the material. The molten polymer when being injected
into the cavity touches the flow front and experiences a strong elongational flow and
1 Introduction to Liquid Crystalline Polymers
17
anisotropic property of thermal expansion becomes larger. The influence of temperature of polymerization and the presence of substituents play a minor role in this
respect. A high polymerization temperature in general leads to the formation of less
ordered network structure and, hence, the anisotropy in the material decreases.
In general, LCPs are of high mechanical strength at high temperatures, superior
chemical resistance, good weatherability, and flame-retardant properties. They may
create several forms via sintering and molding. LCPs possess a high coefficient of
thermal expansion along the Z-axis. LCPs are of exceptional chemical resistance at
elevated temperature and in presence of strong acids, bases, and aromatic and
halogenated hydrocarbons. The dimensional stability of LCP is quite good even in
boiling water. Polar and bowlic LCPs are by nature ferroelectrics with a very low
reaction time compared to conventional liquid crystals making them a material of
choice for ultrafast switches. Bowlic polymers are columnar in shape and can be
used to form ultrahigh superconductors. Table 1 shows some important properties of
few industrial-grade LCPs.
LCPs are known for high modulus thermoplastic matrix. It is also reported that
the addition of LCPs to a traditional thermoplastic matrix reduces the viscosity of the
compounding process, and hence the processability could be improved to certain
extent. The LCP/thermoplastic blends also have other advantages over the standard
glass-fiber-reinforced composites in addition to reduced melt viscosity and slashed
energy consumption in processing. Few important LCP-based composites along
with properties are mentioned in Table 2.
Processing of Liquid Crystalline Polymers
LCPs are generally processed and molded by five ways: melt spinning, pressing,
injection molding, extrusion, and coating (Kar and Hodzic 2011; Kar 2011).
Aromatic LCPs composed of copolyesters exhibit exceptional orientational behavior
and properties in its solid state. In an interesting study, the orientation of molecules is
examined under shear and elongation flow/deformation. The study exhibits that high
molecular orientation can be observed under elongation, but no such observation can
be seen in case of shear force. Rod-shaped molecules from isotropic solutions are
oriented in the flow direction, whereas, in the case of shear stress, the molecules will
revolve intermittently. Again, if the molecules are under an anisotropic state in the
melt or solution phase, where the molecular domains are already oriented, the
application of shear stress will not affect the orientation of stabilized molecules.
However, in the case of elongational flow, the molecular domain will preferably
orient and stretched in the direction of flow yielding monodomain state formation
(Ide and Ophir 1983).
During the injection molding of the LCPs, a distinct core and skin morphology
has been reported. This can be explained by the quenching process at the walls and
elongational flow history of the material. The molten polymer when being injected
into the cavity touches the flow front and experiences a strong elongational flow and
1 Introduction to Liquid Crystalline Polymers
17