Date of Award
Doctor of Engineering Science in Mechanical Engineering
R. S. Sodhi
Rajesh N. Dave
A series of thermotropic liquid crystalline polyesters were synthesized, based on Bis-hydroxyethyl terephthalate (BHET), para-acetoxybenzoic acid (PAB) and terephthalic acid (TA). Structural studies of these aromatic polyesters using Nuclear Magnetic Resonance (NMR), Infrared Spectroscopy (IR) and X-ray diffraction techniques indicated the existence of substantial amounts of rigid -PAB-PAB- linkages within the mesogenic backbone. Optical study of their melts using Polarizing Microscopy indicated a "threaded" microstructure typical of a nematic mesophase. Morphological rearrangements were found to occur at higher temperatures as was evident from a two-phase structure, one phase speculated to be rich in flexible Poly(ethylene Terephthalate) (PET) segments and the other rich in -PAB-PAB- segments. At elevated temperatures, thermal degradation occured before these polyesters could become completely isotropic. Thermal analysis using Differential Scanning Calorimetry (DSC) showed that the melting and crystallization transitions of these polyesters were dependent on composition and thermal history. Most of these polyesters showed multiple endotherms on melting to the nematic phase.
A detailed study was also conducted on the depolymerization of PET into its monomer, BHET, by glycolysis in the presence of metal acetate catalysts. A model describing an equilibrium reaction between BHET, dimer and ethylene glycol (EG) has been proposed. The relative effectiveness of the four catalysts used, in terms of the initial rate of depolymerization, was as follows :
Zn2+ > Pb2+ > Mn2+ > Co2+
The PET used in this study was reclaimed from post-consumer soft drink bottles, in an effort to find an economically viable end-use for this material. The BHET monomer thus obtained was used as a building block in the synthesis of the liquid crystalline polyesters.
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