Synthesis, randomization, and characterization of liquid crystalline copolyesters containing substituted phenylene terephthalate and ethylene terephthalate units for blending studies with poly(ethylene terephthalate) (PET)

The main objective of this dissertation was the synthesis and modification of thermotropic liquid crystalline copolyesters to be blended with isotropic engineering thermoplastics such as PET. There has been a lot of interest in the last several years in the blending of thermotropic LCPs with engineering thermoplastics to form in situ composites. Yet, due to the typically high melt transitions of highly aromatic thermotropic LCPs, several methods have been studied in this dissertation to reduce the melt transitions of LCPs to within the processing window of engineering thermoplastics.; Three series of thermotropic, aromatic copolyesters derived from EHQ, PHQ, HQ, EG, and TA were synthesized, and characterized by PLM, DSC, NMR, TGA, and solution viscometry. It was shown that the melt transition was effectively reduced through the copolymerization of the monomers. For melt blending with engineering thermoplastics, such as PET, the transition temperatures for the Series III samples were too high, while some of the Series I and II copolymers with low amounts of PT units had thermal transitions in the range which would make them more favorable for blending.; Several different liquid crystalline copolyesters were thermally post-treated successfully to increase their degree of randomness. Both poly(ethoxyphenylene terephthalate-co-ethylene terephthalate)s and poly(phenylphenylene terephthalate-co-ethylene terephthalate)s were thermally randomized. It was found that increased randomness numbers caused decreased melt transition temperatures and crystallization temperatures. The more random sequence distributions also resulted in a decreased crystallinity of the copolyesters as observed by reduced enthalpies of fusion and crystallization.; Two different LC copolyesters, poly(EPT-co-ET) and poly(PPT-co-ET), were solution blended with PET using a mixture solvent of TFAA/Chloroform. Four different samples of each LC copolyester, with varying degrees of randomness, were used in the blends. The blends were characterized by DSC, polarized light microscopy, and rheological testing. It was observed that the melt viscosity of the blend had a strong dependence on the degree of randomness of the LC copolymer used. The copolyesters with high degrees of randomness caused a reduction of the melt viscosity.