This thesis describes the synthesis, polymerization, and characterization of poly(butylene terephthalate) (PBT) polymers as produced from cyclic oligomers of PBT using a stannoxane cyclic initiator. The properties of the resulting macrocyclic PBT polymer are compared to the properties of four commercial linear PBTs, covering the range of molecular weights for commercial polymers. The macrocyclic polymers are found to have unique properties in both the melt and the solid state. In the melt, the macrocyclic polymers are found to have significantly lower viscosities vs. the linear resins at equivalent molecular weight. The results reported herein show differences in the zero shear viscosity of macrocyclic and linear PBT melts at equivalent molecular weight that are much larger than the differences reported in the literature between macrocyclic and linear polystyrene melts. It is believed that the unique ring expansion polymerization used in this research has resulted in producing simpler cyclic molecules, having no catenation or knotting, than have been reported elsewhere. When crystallized, these cyclic molecules produce a semi-crystalline spherulitic structure. The spherulite formed is unique in that it is highly nucleated, rapidly crystallized, and found to be the same spherulitic form only previously reported as being produced by slow cooling or solvent crystallization processes. The crystalline structure from these macrocyclic molecules is believed to have a lower intercrystalline tie chain density than found in melt cooled linear PBTs. This macrocyclic polymer, produced from cyclic oligomers, is proposed as being capable of being used as a thermoplastic composite resin, with sufficiently low viscosity in the oligomeric state, and sufficiently high polymerization and crystallization rates, that a melt process is feasible. While being very brittle, the fracture toughness of cyclic PBT is found to increase when the macrocyclic structure is degraded, increasing by three times within ten minutes of thermal degradation in the melt. A new initiator is proposed in the thesis, capable of producing linear PBT from cyclic oligomers with the same reaction kinetics as the stannoxane initiator. Such a system is expected to be as tough as high molecular linear PBT, and have the chemical and heat resistant characteristics necessary for a new thermoplastic composite resin.

Supercritical carbon dioxide (scCO2) has been widely studied as an environmentally friendly alternative to organic solvents in many applications. This thesis will describe specific routes for both melt and solid-state processing of polymers in scCO2-mediated environments. The primary goal is to analyze the influence of scCO2 on the final properties of polymers as well as to design novel processing routes using scCO2 that could allow access to well-defined structures and novel materials, and processing of “intractable” polymers. Most of the fiber-drawing studies of polymers in scCO2 have focused on permeable conditions, where the plasticization process of scCO 2 dominates the interaction and the effect of the imposed hydrostatic pressure is negligible.1–3 In this thesis, the interactions of scCO2 with solid state polymers under non-permeable conditions are investigated through the drawing behavior of highly crystalline, highly oriented, polymorphic fibers (UHMWPE) within this environment. The high-pressure environment appears to stabilize the crystal structure of the fiber, which in this case is the major component. As a consequence, scCO2-treated samples display a constant mechanical and thermal response compared to air-drawn samples that show significant temperature dependence in their behavior. In addition, the interactions of scCO2 with polymers in the melt are analyzed by designing a modified processing system that allows to process polymers in scCO2. Using this system, the foaming process of polyethylene in scCO2 is studied, and scCO2-assisted polymer processing is then applied to high molecular weight polymers, including fluoropolymers and high molecular weight polyolefins, as a novel processing-route. The success of this method is based on the effect of scCO2 on the melting behavior of semicrystalline polymers, along with its large plasticizing properties observed primarily in fluoropolymers. Finally, the feasibility of preparing polymer-clay nanocomposites by this route using a variety of approaches is also studied. The use of polymers with controlled hydrophilicity, as well as the introduction of chemically designed hydrophilic polymers or compatibilizers that enhance the interaction between the polymer and the clay have been used to understand the effect of scCO2 on the melt intercalation process as well as on the final structure and morphology of these systems.

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.

This dissertation discusses studies on the physical properties of poly(ether ether ketone) (PEEK). Several investigations involving the crystallization and melting behavior of PEEK, crystallization of PEEK on carbon fibers, and uniaxial draw of PEEK are presented. The double-melting behavior of isothermally crystallized PEEK was investigated using differential scanning calorimetry (DSC) and wide and small-angle X-ray scattering. The double-melting was found to be due to a crystal reorganization on heating. The low and high-melting endotherms are the sum of four contributions: Melting of the original crystals, their recrystallization, remelting of recrystallized crystals and melting of core crystals. Material parameters such as the thermodynamic melting point (384, 389$\sp\circ$C) and surface free energy (39 erg/cm$\sp2$) of the PEEK crystal were measured. The isothermal and non-isothermal crystallization of PEEK was found to depend on the previous thermal history. This was explained by a persistence of small residual crystalline regions up to the thermodynamic melting point, at which the infinitely large and perfect crystals melt. The crystallization of PEEK on carbon fibers was studied by DSC, electron and optical microscopy. The control, characterization, and effect of the crystalline interface between PEEK and carbon fiber were investigated. The carbon fiber surface was found to compete with nuclei in the PEEK matrix for crystallization growth. Reducing the number of nuclei in the matrix by long preheating favored PEEK crystallization on the carbon fiber, resulting in about 2 times stronger interfacial bond as indicated by transverse tensile tests. PEEK films and rods were solid-state extruded at 154 and 310$\sp\circ$C. The tensile mechanical properties were improved by drawing. The modulus and strength were increased up to 6.5 GPa and 600 MPa, respectively. The structural evolution of PEEK on drawing was studied using wide-angle X-ray diffraction and birefringence. The c axis crystal orientation function (up to 0.67) and birefringence (up to 0.30) were increased with draw ratio.