High-Pressure DTA Study on Liquid Crystalline Polyesters with Biphenyl As Mesogen

Y. Maeda, T. Mabuchi, and J. Watanabe
[Thermochimica Acta., Vol. 266, pp. 189-202, 1995]

The phase transition of a homologous series of liquid crystalline polyesters was studied under hydrostatic pressures up to 300 MPa by using a high-pressure DTA apparatus. The thermotropic polyesters, abbreviated as PB-n (where n is an even number of methylene units), were prepared from 4,4'-dihydroxybiphenyl and aliphatic dibasic acids containing an even number of methylene groups from 8 to 22 units. The T vs. phase diagrams of the PB-10, PB-12, PB-14 and PB-18 polyesters were constructed. PB-10 shows a simple phase transition of crystal (K)-smectic-H(S_H)-isotropic melt (I) over the whole pressure region. PB-12 shows a similar K-S_H-I transition in the low-pressure region below about 100 MPa. At high pressures above 100-120 MPa, however, PB-12 exhibits a T vs. P relation that is different from that in the low-pressure region. The phase diagram suppports the existence of the pressure-induced smectic phase which was assigned to be the smectic-B (SB) phase (Fig. 1). The PB-14, PB-16, and PB-18 samples show an additional first-order transition just below the K-S_H transition at atmospheric pressure. The new transition peak increases with increasing spacer n in the PB-n samples, in contrast to the decrease in the K-S_H transition peak. The transition is a kind of crystal transition between the low- and high-temperature phases, denoted here as K₁ and K₂. The thermal behavior of these samples can be understood as enantiotropic transitions through the K₁-K₂-S_H-I route. They show the same type of T vs. P phase diagram, indicating the K₁-K₂, K₂-S_H and S_H-I transition lines. The phase diagrams indicate that the stable region of the S_H phase broadens with increasing pressure. However, the K₂-S_H transition peak becomes too small to be detected under pressures of 300-400 MPa.