PHASE STRUCTURES, TRANSITION BEHAVIORS, AND SURFACE ALIGNMENT IN POLYMERS CONTAINING RIGID-RODLIKE BACKBONES WITH FLEXIBLE SIDE CHAINS .1. MONOTROPIC PHASE BEHAVIOR IN A MAIN-CHAIN/SIDE-CHAIN LIQUID CRYSTALLINE POLYESTER

摘要

A series of polyesters, which are comprised of aromatic main chain backbones and flexile aliphatic side chains with 4-cyanobiphenyl end groups, has been synthesized based on a polycondensation of 2,2'-bis(trifluoromethyl)-4,4'-biphenyldicarbonyl chloride with 2,2'-bis{omega-[(4-(4-cyanophenyl)phenoxyl]-n-alkoxy)carbonyl]}-4,4'-biphe nyldiol (PEFBP). For a PEFBP polyester containing eleven methylene units in the side chains, PEFBP(n = 11), multiple phase transitions can be found via differential scanning calorimetry during cooling and heating at varying rates. Different phase structures are identified by Ride-angle X-ray diffraction and electron diffraction experiments, while morphologies of these ordered states are observed by polarized light and transmission electron microscopy, During cooling, a high temperature nematic (N) phase is formed at 193 degrees C independent of the cooling rate due to the combined orientational order of the cyanobiphenyl groups in the side chains and the aromatic polyester backbones. At a temperature of 90 degrees C, a new ordered low-temperature phase with an orthorhombic lattice (K-o) starts to form at a cooling rate equal to or slower than 10 degrees C/min. However, the formation temperature of this phase is too close to the glass transition temperature (60 degrees C) to proceed to completion. Only at very slow heating rates (e.g., 1 degrees C/min), can the K-o phase further develop. This phase melts at around 120 degrees C during heating and returns to the N phase. A new crystalline phase with a triclinic lattice at high temperatures (K-T1) appears at 130 degrees C. It then transfers to a second triclinic crystalline phase (K-T2) This KTP phase melts at around 180 degrees C and, again, returns to the N phase. At 193 OC, isotropization occurs. This complicated phase behavior can be explained by the monotropic origin of the K-o and K-T1 phases with respect to the K-T2 phase, which are metastable in the whole temperature region. [References: 26]