Polymorphous Structures and Their Phase Relationships in a Main-Chain/Side-Chain Liquid Crystalline Polyester

A series of specially designed polyesters consisting of aromatic main-chain/side-chain liquid crystalline (LC) polymers for flat-panel display applications have been synthesized via polycondensation of 2,2‘-bis(trifluoromethyl)-4,4‘-biphenyldicarbonyl chloride with 2,2‘-bis{ω-[4-(4-cyanophenyl) phenyoxy]- n -alkoxycarbonyl}-4,4‘-biphenyldiol (PEFBP( n ), where n is the number of methylene units in the side chains). For a PEFBP polyester of which a 4-cyanobiphenyl mesogen in each side chain is coupled with the aromatic backbone via seven methylene units, PEFBP( n =7), complicated polymorphism has been identified during its structural evolutions. On the basis of differential scanning calorimetry, wide-angle X-ray diffraction, electron diffraction (ED), polarized light, and transmission electron microscopy (TEM) experiments, it is found that this polymer possesses three triclinic crystalline (K t1, K t2, and K t3 ) phases having the same symmetry but different unit cell dimensions, one highly ordered LC smectic phase having an orthorhombic (SK O ) structure, and one nematic (N) phase. Both of the triclinic K t1 and K t3 structures are also confirmed by the ED results. Investigations of thermodynamic phase stability relationships indicate that when PEFBP( n =7) is crystallized from the N phase, the K t1 phase forms. During heating, this K t1 phase transfers to the K t3 phase via reorganization and melting/recrystallization before it enters the N phase. However, when the sample is first cooled to below 130 °C to form the SK O phase, the K t2 phase develops during heating. Further heating the K t2 phase leads to a transformation to the K t3 phase at high temperatures via, again, reorganization and melting/recrystallization. Nevertheless, no apparent transformation between the K t1 and K t2 phases is experimentally observed. Polarized infrared spectroscopy experimental results indicate that the aromatic polyester backbones and 4-cyanobiphenyl mesogens are parallel to each other and packed into one crystal unit cell. TEM observations show different morphological characteristics of the K t1 and K t3 phases. It is concluded that the K t3 phase is the thermodynamically most stable phase, and the K t1 and K t2 phases are metastable. These two phases can be experimentally accessed only because the crystallization rate of the K t3 phase is much slower than those of the K t1 and K t2 phases.