Influence of molecular weight and polydispersity on phase behaviour of polyesters with laterally fixed or cross-shaped mesogens

To supplement previous studies of polyesters with laterally attached and cross-shaped mesogens the influence of molecular weight and molecular weight distribution on the phase behaviour has been investigated. For that purpose two polyesters have been fractionated by preparative gel permeation chromatography under high pressure and observed by polarizing microscopy and DSC measurements. A monotropic nematic polyester with laterally attached mesogens shows changing phase transitions up to a molecular weight of 10 000 (M_w); at higher molecular weight only the clearing transition is still slightly influenced. The molecular weight distribution at an average molecular weight of 15 000 (M_w) has no influence on the melting and clearing temperatures, but does effect recrystallization. The tendency to recrystallize decreases with increasing polydispersity, with increasing aberration from a monomodal molecular weight distribution. The recrystallization and the melting enthalpy are most distinguished at molecular weights around 12 000 (M_w) and crystallization disappears at molecular weights under about 5000 (M_w). In this way, fractions with stable nematic phases are obtained. Additionally, the broadness of the biphasic region shows a distinct dependence on molecular weight. Clearing temperatures show the most significant dependence on the molecular weight of an enantiotropic polyester with crossshaped mesogens dropping significantly below a molecular weight of about 20 000 (M_w). Oligomers with molecular weights below 10 000 (M_w) do not exhibit a mesophase. Polyesters with laterally attached mesogens as well as with crossshaped mesogens show no new liquid-crystalline phases by varying the molecular weight or the molecular weight distribution.