This dissertation explores the potential of improving the performance of poly(ethylene terephthalate) fibers by incorporating novel thermotropic liquid crystalline polymers. To determine if a system exhibited desirable characteristics, a screening procedure was developed to assess the various blends. Evaluations focused on blend compositions ranging from 2 to 20 wt.% LCP. Fibers were obtained by melt extrusion and the effect of processing conditions, i.e. spinning temperature, stretch ratio, and post treatment evaluated. The fibers were tested for mechanical performance, dimensional instability (shrinkage), and the development of shrinkage stresses. Test results were used to determine the critical parameters necessary for in-situ reinforcement and to develop strategies for improving LCP architecture and processing techniques. The novel TLCP's incorporated into the PET were mesogenic copolymers containing either alternating or random flexible groups within the polymer backbone. The flexible moieties were used to promote compatibility between the PET matrix phase and the TLCPs. Two systems were found to significantly improve fiber stiffness compared to neat PET fibers. A Random Copolymer based on the reaction of oxyethylene substituted hydroquinone, ethylene glycol, and terephthaloyl chloride was found to effectively enhance the performance of PET fibers. Fibers containing only 5% TLCP exhibited a 50% increase in modulus, while maintaining an ultimate strength equivalent to the PET control. The thermal behavior of the 5% blend, as determined by free shrinkage and force-temperature experiments, was similar to the PET control. A segmented block copolymer consisting of rigid-rod, diad, and flexible coil segments was also found to improve the performance of PET fibers. At a concentration of 20 wt. percent, the alternating block copolymer, Triad2 (2:6:7), increased the tensile modulus of the fibers 40% and decreased free shrinkage by 20% compared to neat PET. The mechanism of reinforcement for these systems is unclear, but morphological, thermal and mechanical evidence suggest that the TLCPs are modifying the PET matrix and not providing true mechanical reinforcement.

This thesis describes the synthesis and characterization of segmented, triblock and star block copolymers with mesogenic hard segments and long flexible spacers. This thesis explores two important aspects of structure-property relationships of phase separated block copolymers: (1) the effect of soft segment structure on the polymer thermal phase behavior; (2) the effects of hard segment structure and soft segment topology on polymer properties. The first part tests the hypothesis that the soft segment substituents that are adjacent to the hard segment lower the polymer thermal transitions temperatures. Copolymers with pentad hard segments and soft segments with varying degrees of substitution were synthesized to test this hypothesis. Pentad copolymers with completely substituted soft segments showed the lowest melting transition and exhibited no liquid crystallinity. When the substituents are separated from the hard segment, the polymer exhibits higher melting transitions and liquid crystalline behavior. In the second part, efforts were concentrated to synthesize block co-oligomers that behave as high polymers at ambient temperatures but have low melt viscosities. As part of this study, block co-oligomers with dyad esteramide and bisamide hard segments and amine terminated poly(propylene glycol)s of linear and star topologies were synthesized. The block co-oligomers showed that variations in the hard segment structure have a dramatic effect on both the melting temperature and the stability of the liquid crystalline phase. Topology of the soft segment does not effect the thermal transitions, but does affect hard segment organization. In general, star block co-oligomers showed better organization than their linear counterparts.

The synthesis and physical characterization of various poly(1,4-phenylene vinylene) (PPVs) derivatives with electron withdrawing substituents on the phenyl rings and some additional PPV analogues are described. The synthetic routes used were a variation on the Wessling route to PPV which involves a processible precursor polymer. All polymers were characterized by IR, UV and elemental analysis. The effect of substitution on the phenyl ring and solvent effects on polymerization were also discussed. Para-xylylenes were generated by treatment of various 1,4-bis(dialkylsulfoniomethyl) arene dihalides in different solvents and detected by UV-VIS spectroscopy. The utility of studying the UV-VIS spectral behavior of para-xylylenes and analogues in the Wessling polyelectrolyte precursor process for synthesis of poly(arylene vinylene)s is demonstrated by several examples. Electronic spectral properties such as UV-VIS absorption and photoluminescence spectroscopy of PPV derivatives and analogues were investigated.

A series of para-linked aromatic polyamides synthesized with the aim of making optically uniaxial, transparent films and fibers for optical applications, are found to have anomalous properties. Stretched films of these polyamides are highly birefringent and non-crystalline at the same time. These rod-like polyamides do not form lyotropic solutions and are soluble in common solvents like THF, unlike other rod-like polymers. With the goal of understanding this behavior from a molecular standpoint we have quantitatively characterized the geometric, optical and hydrodynamic properties of one of these polyamides. With angle light scattering measurements on polyamide in THF were used in conjunction with electric birefringence measurements to determine the weight average molecular weight, M$\sb{\rm w}$, the root mean square z-averaged radius of gyration, R$\sb{\rm gz}$, the apparent second virial coefficient, A$\sb{\rm 2app}$ and the monomer molecular anisotropy ratio $\varepsilon$. The polydispersity correction was applied theoretically by assuming the most probable distribution. Hydrodynamic and optical properties were determined with viscometry and differential refractometry respectively. The aromatic polyamide studied can be satisfactorily modeled as a Kratky-Porod wormlike chain with a persistence length of 220 $\pm$ 50 A and a monomer optical anisotropy ratio of 2.3 $\pm$ 0.3. The excluded volume effect is found to be negligible in THF at 25$\sp\circ$C. The small axial ratio of 30 may be partly responsible for the non-lyotropic behavior. The refractive index of 1.67 is in good agreement with that of similar polyamides. The repeat unit has a high optical anisotropy leading to highly birefringent films. It is also conclusively established that there is no aggregation due to H-bonding in the absence of moisture. The light scattering theory of Nagai and the hydrodynamic theory adopted for semiflexible chains is found to hold very well for the polyamide studied. Based on the agreement between experiment and theory we infer that the molecular weight distribution is of the most probable type. Our depolarized light scattering data indicate that the straight line behavior observed in Zimm plots even for $\rm R\sb{g}\sp2q\sp2>1$, upto $\rm R\sb{g}\sp2q\sp2$ of 3.5 is due to the combined effect of polydispersity, large size and optical anisotropy of the molecule.

Cross-laminates composed of unidirectionally-reinforced composites are usually used to prepare polymeric materials with enhanced properties in a plane. In the present investigation, solid-state forging is considered as an alternate method to increase planar properties in semicrystalline flexible chain polymers by imparting equibiaxial orientation and extension of molecules in the deformation plane. Polyundecanamide (nylon 11) and High Density Polyethylene (HDPE) are chosen in this study to illustrate several aspects of forging on Polyamides and Polyolefins. They represent two families of polymers with already a large impact in the fiber technology. First, novel aspects of the nature and origin of polymorphism in melt crystallized nylon 11 observed both prior and after forging are presented. Two distinct crystalline species (semidisordered smectic $\delta\sp\prime$-form and 3-dimensional crystal $\alpha$-form) have been identified. Their respective content is highly dependent upon thermal history. The smectic $\delta\sp\prime$-form is kinetically favored but does progressively transform into the thermodynamically preferred crystal form on heat treatment. Thermal analysis provides data on the thermodynamics and kinetics of melting and crystallization for each form. Also, new enthalpic measurements were obtained which correspond to the change of symmetry in the crystalline phase from the low-temperature $\alpha$-form to the high-temperature $\delta$-form. It exhibits a characteristic broad transition typical of crystal-condis crystal transition (introduced by Wunderlich). Differences in the mode of hydrogen-bonding are given as an explanation of polymorphism. Both nylon 11 and HDPE exhibit alternate behaviors when forged to their maximum compression ratios over temperature ranges above and below about 100$\sp\circ$C. Modifying the forging rate slightly shifts this temperature. For nylon 11, low temperature deformation favors the formation and/or stability of the smectic which is found to significantly inhibit large deformation without mechanical failure. Optimal forging conditions however are found over the high temperature range where the crystal forms ($\alpha$- and $\delta$-forms) are stable during the process. Accordingly, the in-plane modulus increases by an amount comparable to the one predicted by the composites theory. Inversely, for HDPE, optimal forging conditions are found at low temperature where the stress-induced orthorhombic to monoclinic transformation is observed. Results are analysed in terms of differences in slip (or shear) mechanisms at the molecular scale with forging conditions.

Part I of this dissertation deals with the surface modification of poly(tetrafluoroethylene-co-hexafluoro-propylene) (FEP). The reduction of FEP using sodium naphthalide was studied with regard to the kinetics of reaction and the product structure. The thickness of the resulting modified surface layer could be controlled, using reaction time and temperature, in the ranges of 45-90 and 250-800 A. The air-sensitive reduction product contained carbon-carbon double and triple bonds, aliphatic C-H bonds, alcohols, carbonyls and very little fluorine. Hydroboration/oxidation introduced hydroxyl groups in high yield to the surface. The alcohols exhibited low reactivity in esterification reactions in the absence acylation catalysts. Treatment of this surface with ethylene oxide in the presence of LDA rendered a more reactive, primary alcohol-containing, surface. Reaction of the hydroxyl surface produced by hydroboration and oxidation with 1,4-toluene diisocyanate or isophorone diisocyanate resulted in surface-bound isocyanate groups. The reactivity of these surfaces towards nucleophiles was investigated. Part II deals with graft polymerization from surface-confined initiator species. Halogenated surfaces were prepared by treating reduced FEP with chlorine or bromine. Treatment of these surfaces with silver trifluoromethane-sulfonate in the presence of tetrahydrofuran (THF) resulted in the polymerization of THF from the surface. Initiation at temperatures less than $-$10$\sp\circ$C, followed by polymerization at $-$10$\sp\circ$C, resulted in a relatively thick, homogeneous overlayer of the graft polymer. Neopentyl and n-propyl alcohol groups were introduced to the surface of poly(chlorotrifluoroethylene) (PCTFE) by reaction with the appropriate protected alcohol-containing lithium reagent. The corresponding tosylate surfaces were prepared and the reaction of these surfaces with 2-methyloxazoline was studied. Acrylamide monomers were polymerized from hydroxyl surfaces derived from FEP using ceric ion redox initiation. The introduction of trimethylsilyl ketene acetal groups to polymer surfaces was also investigated. Part III deals with the surface chemistry of fibrillar carbon. Oxidation and reactions with carbenes were studied as a means of introducing functional groups to the surface. X-ray photoelectron spectroscopy was used to characterize the products; chemical derivatization was used to determine the functional group composition of the oxidized surface.

Thermotropic polyesters are a new class of high strength materials of significant commercial importance because of their ease of melt processing and superior properties. Several series of homopolyesters and copolyesters containing 3,3,3$\sp\prime$,5$\sp\prime$-tetrakis(sec-butyl)-4,4$\sp\prime$-biphenol, 3,3$\sp\prime$-bis(tert-butyl)-4,4$\sp\prime$-biphenol, 3-phenyl-4,4$\sp\prime$-biphenol, 3,3$\sp\prime$-bis(phenyl)-4,4$\sp\prime$-biphenol and 1,1$\sp\prime$-binaphthyl-4,4$\sp\prime$-diol were synthesized by the melt polycondensation method and characterized for their thermotropic behavior by conventional techniques. Many of these aromatic polyesters exhibited an anisotropic melt with either relatively low melting transitions or flow temperatures well below 300$\sp\circ$C, which is much lower than the thermal transitions of currently available thermotropic polyesters. Another aspect of this research program was concerned with the application of WAXD techniques to study the effect of molecular packing in the solid state of these wholly aromatic polyesters for three different series of polymers, which contained either symmetrically or unsymmetrically substituted biphenol residues in their mesogenic groups. The unsymmetrically substituted polymers showed considerably reduced crystalline order or perfection as expected, compared to the symmetrically substituted polymers which had very high degree of crystalline order or perfection. Copolymerization of both types of monomers reduced the degree of crystallization order or perfection and the melting or flow temperatures of the resulting thermotropic polyesters, as expected.

The rheological behavior of polymers at the gel point (critical gels) is characterized by a power law distribution of relaxation times, large normal stress differences, and physical rupture of molecular network strands. This is demonstrated in a series of well-defined experiments conducted on a model network polymer system consisting of a linear, telechelic, vinyl-terminated polydimethylsiloxane (PDMS) and a four-functional siloxane crosslinker. Stable samples of this model polymer can be prepared at different extents of reaction in the vicinity of the gel point (GP), thereby separating the effects of reaction and flow. The linear viscoelastic constitutive equation (Gel Equation) has been extended to include the effect of finite strains, and has been found to describe the rheological behavior of critical gels extremely well. Large strains have been found to disrupt the network structure of the crosslinking polymer, and introduce a mechanical delay to gelation. As a result, a stable sample can be reduced from a viscoelastic solid to a critical gel, or even to a viscoelastic liquid, depending on the magnitude of the shear strain. This mechanical effect also shifts the value of the relaxation exponent, n, at the gel point towards lower values. The material properties can therefore be controlled by both the chemistry and the processing, to suit the application.

Disclinations and inversion walls in the director field of nematic thermotropic liquid crystal polymers (TLCP's) are imaged at high resolution using the lamellar decoration technique of Wood and Thomas. The utility of the lamellar decoration technique has also been greatly extended by using an etch and replication technique to image the director in sections of a bulk sample. The interaction of disclinations in the presence of an applied magnetic or extensional flow field which tends to align the director is studied. At intermediate field strengths, where the disclination separation is comparable to the characteristic length for director field distortions, pairwise interaction no longer dominates, but clusters of disclinations are observed to form. The applied field suppresses fluctuations of the disclination positions which increase the topological dipole moment of a cluster while enhancing those that preserve zero dipole. The applied field tends to minimize the long range distortional energy which is proportional to the square of the dipole moment. As a result, quadrupolar (Lehmann) clusters of disclinations are commonly observed in either magnetically or extensionally flow aligned samples. Many-bodied interaction of disclinations in the absence of an applied field is studied via computer simulation, and the results agree favorably with recent experimental work. The structure of the cores of disclinations has been observed. The core size is on the order of a few molecular lengths, and its structure depends upon polymer architecture. A rigid polymer splays more within the core of a $\pi$ wedge disclination, and an excess of chain ends aggregates. A semi-flexible polymer, in contrast, bends more within the core. Hairpins are not thought to be abundant in the semi-flexible TLCP. The morphology of the TLCP semi-crystalline state has also been examined. It is observed to depend upon the relative primary crystal nucleation and crystal growth rates. The lamellar decoration morphology, where the crystalline lamellae are everywhere perpendicular to the director of the precursor nematic, is favored by rapid nucleation and slow growth. This morphology is unique to TLCP's, but other morphologies are possible. Spherulites have also been grown from the nematic melt. Their growth is favored by slow nucleation and rapid growth.

Various important aspects of discontinuous fiber composites have been studied in detail both experimentally and theoretically through a model composite containing polydiacetylene (PDA) single crystal as the reinforcement fiber and epoxy resin as the matrix. The aspects encompass varying degrees of fiber behavior as well as fiber/matrix interaction, such as compressive behavior of the reinforcement fiber, geometry effects of the fiber on the stress distribution along the fiber, fiber orientation, residual thermal stress, and the role of fiber/matrix interfacial properties on the performance of composites. The compressive failure mode of the PDA fiber used in this study is the formation of readily observable kink bands. The critical compressive strain of the fiber was found to be 0.3%. By monitoring the behavior of the C$\equiv$C bond frequency in PDA in response to applied compressive loads, the stress along the fiber can be determined leading to the establishment of a calibration curve describing the relationship between the frequency change of the bond and the compressive strain due to residual thermal stress caused by differences in thermal expansion between fiber and matrix. This thermal stress will cause fiber compressive failure if the critical compressive strain is achieved. A quantitative analysis of the data reveals that no slippage occurs at the fiber/matrix interface during the build-up of thermal stress. The geometry of the fiber was found to play a significant role. Both experimental and finite element analysis demonstrate the advantage of tapered end fiber over the ordinary blunt end fiber, i.e. fiber with a uniform diameter. The experimental apparatus constructed in this study allows the measurement of tensile strain distribution of the fibers orientated at any angle with respect to the draw direction to be determined accurately. For a thin layer coating of the fiber, it was found that the tensile strain distribution along the fiber was unaffected, and so by extension neither would the modulus of a composite in fiber axial direction be affected. This observation indicates that for most composite systems perfect bonding is easily formed at the fiber/matrix interface as far as tensile stress transfer is concerned. (Abstract shortened with permission of author.)