Synthesis and characterization of thermotropic liquid crystalline polyesters for use in molecular composites.

摘要

A molecular composite can be defined as a system where one polymer reinforces another at a molecular level to give enhanced properties. This concept could be applied to the enhancement of mechanical properties of flexible coil polymers like PET. Molecular level reinforcement of PET with rigid polymers could be achieved by increasing the flexibility of the latter in an effort to increase the entropic mixing and by selection of the rod-like and coil molecule, with specific interactions to increase enthalpy of mixing. Thermotropic liquid crystalline polyesters (LCP) were chosen for the rigid counterpart in our research as these materials exhibit orientation in the direction of draw resulting in unusually high strengths and moduli of the fibers in the draw direction. Also they exhibit lower melt-viscosity values at higher temperatures consequently lowering the processing temperatures of their blends compared to PET. Thus they function as processing-aids in addition to being reinforcing agents.; This dissertation work has involved the synthesis of thermotropic liquid crystalline polyesters containing various oxyethylene substituted hydroquinones. The hydroquinone monomers were synthesized, which were then polymerized with terephthaloyl chloride by a solution state polycondensation method. Random and block copolymers containing the mesogenic segments and flexible PET/PBT segments were also synthesized and studied as these LCPs are expected to be miscible with PET by the interaction of the isotropic segments in the melt state. The systems investigated include poly (2-butoxyethoxy-1,4-phenylene terephthalate) (PBEHT), poly (2-ethoxy-1,4-phenylene terephthalate) (PEHT) and the random and block copolymers containing PEHT and PET/PBT segments. Some copolymers containing poly (1,4-phenylene terephthalate) and PET/PBT segments were also investigated.; The polymers were mainly characterized by viscometry, thermogravimetry, DSC, NMR and optical microscopy. Especially in the case of the copolymer systems, extensive NMR analysis was done to get both the composition and sequence distribution. Structure-property relationships were drawn from copolymers with varying ratios of the rigid and flexible block sizes. Optimization of reaction conditions like solvent, acid-acceptor, temperature and time, to get high molecular weight polymers formed an important aspect of the research work. Rheological studies were done to study the role of the LCPs as processing-aids in the blends.; The presence of the flexible pendant group brings down the transition temperatures of the LCP to the processing temperature range of PET. The PBEHT system was found to be a good processing-aid for PET. The high molecular weight PEHT/BT copolymers obtained by conducting polymerization at high temperatures yielded thin films of good strength and mechanical integrity. The processing parameters of the blends of these polyesters with PET have to be optimized to draw conclusions about the reinforcing ability of the LCPs.