Abstract
A series of polyesters, which are comprised of aromatic main chain backbones and flexible aliphatic side chains with 4-cyanobiphenyl end groups, has been synthesized based on a polycondensation of 2,2′-bis(trifluoromethyl)-4,4′-biphenyldiyldicarbonyl chloride with 2,2′-bis{ω-[(4-(4-cyanophenyl)phenoxy]-n-alkoxy)carbonyl]}-4, 4′-biphenyldiol (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 wide-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 °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 °C, a new ordered low-temperature phase with an orthorhombic lattice (Ko) starts to form at a cooling rate equal to or slower than 10 °C/min. However, the formation temperature of this phase is too close to the glass transition temperature (60 °C) to proceed to completion. Only at very slow heating rates (e.g., 1 °C/min), can the K o phase further develop. This phase melts at around 120 °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 °C. It then transfers to a second triclinic crystalline phase (K T2 ). This K T2 phase melts at around 180 °C and, again, returns to the N phase. At 193 °C, 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.