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Y. Maeda, N. Tanigaki, and A. Blumstein
[Mol. Cryst. Liq. Cryst., Vol.237, pp.407-418, 1993]
The phase behavior of a liquid crystalline polyester, poly(4,4'-dioxy-2'-dimethylazoxybenzene dodecanedioyl) (labeled as DDA-9), under hydrostatic pressures up to 300 MPa has been studied by using a wide-angle X-ray scattering apparatus equipped with a high-pressure sample vessel. Generally, the normal crystalline form, form I, is predominantly formed, on cooling from the isotropic liquid phase, via the nematic mesophase. An interesting fact is found under hydrostatic pressure: a crystal polymorph, form II, is concurrently formed by cooling slowly from the nematic phase just above the crystal-nematic point T_{k-n} (form I) at pressures ranging from 50 to 200 MPa. Furthermore, the form II polymorph can be seen at very low pressures as 1 MPa when the sample is annealed at temperatures just above T_{k-n} (form I) for 1-2 hrs under pressure and then cooled slowly. Once formed, the form II polymorph is metastable at room temperature under atmospheric pressure (Fig.1). The polymorph has a melting point T_{k-n} (form II) higher by 4-5°C than the T_{k-n} (form I). It is concluded that the formation of the crystal polymorph is substantially accelerated by hydrostatic pressure and heat treatment.
