Poly(ethylene terephthalate) (PET) is an important commercial polyester and widely used as fibers, packagings, containers and engineering materials. It is believed that the incorporation of a low level of ionic groups into PETs dramatically improves the mechanical performance and compatibility with other substrates. However, polymers containing ionic groups always exhibit complicated behavior due to the presence of ionic aggregates in the organic matrix, and this thesis investigates the effect of backbone architectures on the properties of PET ionomers in detail. Three series of random and telechelic PET ionomers with equivalent molecular weights and ionic contents were synthesized using conventional melt polymerization. Solid state sodium NMR spectroscopy and melt rheological analysis demonstrated that the stability of ionic aggregates of telechelic ionomers decreased dramatically with an increase in temperature. A slightly branched structure resulted in high molecular weight ionomers bearing more than two ionic end groups. However, when the level of the branching reagent was lower than 3 mol%, the ionomers with flexible backbone (poly (ethylene terephthalate-co-ethylene isophthalate)) tended to form a high fraction of intramolecular aggregates at high temperatures. When the level of branching agent was higher than 3 mol%, the compact structures led to strong intermolecular aggregates. PEG endcapped PET and PET random ionomers were synthesized to investigate the effect of PEG end groups on the morphology and rheology of PET and PET ionomers. A small fraction of incorporated PEG end groups increased PET crystallization rate dramatically. Moreover, the PEG endgroups tended to aggregate on the surface of PET to result in a PEG rich layer, which improved the biocompatibility and decreased protein adhesion. The PEG end groups also plasticed the ionic clusters of PET ionomers to decrease melt viscosity, and resulted in a water soluble polyester. Hyperbranched polymers contain a well-defined plurality of peripheral functionalities. These functionalities subsequently serve as sites for further chemical modification or as templates for noncovalent intermolecular association. In most cases, hyperbranched polymers are prepared using a one-step polymerization process involving ABn monomers. A novel AB2 monomer, 4-(fluorophenyl)-4',4"-(bishydroxyphenyl) phosphine oxide, was synthesized. The monomer was successfully polymerized to a modest molecular weight with various catalysts, including K₂CO₃ and Cs₂CO₃/Mg(OH)₂. Moreover, an efficient approach to hyperbranched polyarylates via the polymerization of A2 and B3 monomers without gelation was also developed. A dilute bisphenol A (A2) solution was added slowly to a dilute 1,3,5-benzenetricarbonyl trichloride (B3) solution at 25 °C to prepare hyperbranched polyarylates in the absence of gelation. / Ph. D.

published_or_final_version / Dentistry / Master / Master of Dental Surgery

Thesis (M.D.S.)--University of Hong Kong, 1996. / Includes bibliographical references (leaves 163-180) Also available in print.

In this work an attempt is made to correlate polymer miscibility with diffusion and with molecular interactions. A system with lower critical solubility temperature has been selected, namely polystyrene (PS) and polyvinyl methyl ether (PVME). Most of the published work has been done on polymers in solutions or on solvent cast specimens and therefore on ternary systems. The role of solvent has not yet been fully evaluated and it was of interest to compare the results on solvent cast samples with those prepared by mechanical blending and by diffusion. Molecular interaction is dependent on functional groups present and for this reason experiments have been performed on PVME blends with PS of different levels of sulfonation (SPS). Selective deuteration (d-PS) was used to identify and assign some absorption peaks in the infrared spectra. DSC measurements have shown that only one Tg is present for all blends prepared by solvent casting. It was necessary to use an extreme quenching rate down to liquid nitrogen in order to preserve the high temperature (above 150°C) phase separated structure, represented by two Tg of homopolymers. The mechanically blended system, on the other hand, did not show a single T g of the blend, unless annealed for one day at 110°C. This confirms the results obtained by diffusion studies using light microscopy and neutron reflectivity, that the diffusion rates are extremely slow and therefore do not control the phase formation and separation processes. These experiments also indicate that the microstructures of solvent cast samples and samples prepared by mechanical blending are different. The Tg of mechanical blended polymers indicate, that the composition of diffusion swollen PS does not correspond to the phase diagram measured in solutions, confirming thus the above result. The FTIR studies at different temperatures have shown that changes in spectra of polymer blends, as reported in the literature can be explained by temperature changes in pure homopolymers. This indicates that molecular interactions, which are responsible for miscibility, are not detectable by infrared absorptions and are therefore of unspecified strength and location. The FTIR of SPSIPVME blends show that sulfonate groups on PS affect polymer miscibility through changes in configuration of molecules, rather than through direct interaction with the PVME, as suggested in the literature. An attempt has been made to study diffusion of SPS and polycarbonate (PC) system by neutron reflectivity. Preliminary results indicate that surface relaxation effects make the data interpretation difficult.

No description available.

The structure-property behavior of two types of elastomeric telechelic ionomers has been investigated. Sulfonated polyisobutylene telechelic ionomers were studied in both bulk and solution. The bulk mechanical properties of carboxylated polyisoprene telechelic ionomers were also investigated. The sulfonated polyisobutylene telechelic ionomers are available in three different molecular architectures - linear monofunctional, linear difunctional, and three-arm star trifunctional. These materials, which had previously been neutralized only with monovalent and divalent cations, were neutralized with cerium(III) and lanthanum(III). These trivalent cations resulted in a material with improved network stability over those materials previously studied. Long term stress relaxation and creep experiments revealed that at short times the networks are stable while the materials flow after about one week of relaxation. Studies of the solution behavior of the sulfonated polyisobutylene telechelic ionomers concerned dilute solution viscometric behavior in nonpolar solvents and nonpolar-polar solvent mixtures as well as the viscoelastic behavior of more concentrated solutions in nonpolar solvents. Gelation was observed in nonpolar solvents at concentrations as low as 1-2 g/dl. The gelation concentration is dependent upon molecular architecture, molecular weight, neutralizing cation type, and excess neutralizing agent. The viscosity of these ionomers in nonpolar-polar solvent mixtures may increase with increasing temperature due to the temperature -dependent equilibrium between the ionic groups, the polar cosolvent, and the nonpolar solvent. More concentrated solutions or gels in nonpolar solvents display a unique viscoelastic behavior which is very dependent upon architecture, molecular weight, and neutralizing cation. Solutions of ionomers (5 g/dl) neutralized with zinc display a low-frequency viscous region and a high-frequency elastic region at room temperature. Solutions of ionomers neutralized with more ionic cations display an elastic response over essentially the entire frequency range. Storage and loss modulus curves obtained at different temperatures do not superpose, and these solutions are thus thermorheologically complex. The bulk mechanical properties of carboxylated polyisoprene telechelic (linear) ionomers were studied. For materials neutralized with Group IA or IIA metals, resistance to flow was found to increase with decreasing ionic radius. The zinc-neutralized material flowed with little resistance, while the nickel-neutralized material exhibited the best elastomeric characteristics. Materials neutralized with titanium(IV) required a fourfold stoichiometric excess to reach maximum strength due to the difference in neutralization pathway. An amine-terminated polybutadiene crosslinked with copper(II) chloride displayed greater strength and network stability than any of the carboxylated polyisoprene telechelic ionomers. / Ph. D.

It is observed for the first time that telechelic ionomers based on sulfonated polyisobutylene exhibit primary and secondary SAXS peaks when the molecular weight distribution is narrow (Mw/Mn ca. 1.15). These SAXS peaks are interpreted as due to non-global lamellar or cylindrical ionic domain order. The non-global ordering strongly affects the tensile and dynamic mechanical properties of the ionomer. This effect is attributed to local ionic phase continuity and the concept of restriction of chain motion by the ionic domain. Triblock copolymer ionomers from t-butyl methacrylate (tBMA) end blocks and polybutadiene or tapered poly(styrenelbutadienel styrene) (tSBS) mid blocks exhibit complex SAXS patterns indicating dispersed spheroidal ionic end block domains in the matrix. TEM shows that ionization of the 9 mol % PtBMA-PBD triblock produces a morphological transformation from unoriented rodlike domains to ionic spheroids. / Ph. D.

published_or_final_version / Dentistry / Master / Master of Dental Surgery

No description available.

Thesis (M.D.S)--University of Hong Kong, 1996. / Includes bibliographical references (leaves 207-231) Also available in print.