Transitions in thermotropic liquid crystal polymers

For polymers which, on heating, yield Mesophases (liquid crystal melts), the so-called mesogenic polymers or liquid crystal polymers (LCPs), the situation of phase transitions is much more complex. In this case the simple Volume-Temperature diagram, given in Fig. 4.2 is not valid anymore and has to be substituted by a more complicated one, which is shown in Fig. 6.12. 

The liquid crystal melt, which comes into being at the glass-rubber transition or at the crystal-melt transition, may have several phase states (Mesophases) one or more smectic melt phases, a nematic phase and sometimes a chiral or cholesteric phase the final phase will be the isotropic liquid phase, if no previous decomposition takes place. All mesophase transitions are thermodynamically real first order effects, in contradistinction to the glass-rubber transition. A schematic representation of some characteristic liquid crystal phase structures is shown in Fig. 6.13, where also so-called columnar phases formed from disclike molecules is given. 

The designation of the different phase transitions in the literature is confusing. We prefer -as proposed by Wunderlich and Grebowicz (1984) - to reserve the symbol Tm for the normal polymers, which do not give mesophases on heating, but a direct transition into the isotropic 

Polybutadiene (trans) Polyisoprene (guta-percha) (trans) 

Poly(vinyl fluoride) Poly(vinyl chloride) Poly(trifluorochloroethylene) Poly(vinylidene fluoride) 

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