Molecular Weight and Molecular Weight Distribution Effects on the Liquid Crystalline and Biphasic Behavior of a Thermotropic Polyester

Several samples of a liquid crystalline semiflexible polyester 1 with different molecular weights (M_n = 2800-37 400) and molecular weight distributions (M_w/M_n = 1.30-2.35) were prepared. Two polymer blends were also obtained from 1 components with specifically designed M_n and M_w/M_n values. The liquid crystalline properties of the samples and their biphasic behavior at the nematic to isotropic phase transition were investigated. The melting and isotropization temperatures as well as the isotropization entropy increase with increasing number average molecular weight until a saturation value for about M_n = 10 000 is reached. Beyond this value, they remain constant. The biphasic behavior was studied by annealing the samples inside the apparent nematic to isotropic biphasic gap. The onset and extension of the biphase segregation is critically affected by the molecular weight and molecular weight distribution with a clear cut at intermediate values of these parameters. In particular, the position of the biphasic region depends on the number average molecular weight, whereas the width of the biphasic region depends on the width of the molar mass distribution.