Shawn E. Jenkins

The effect of hydrogen bonding on the physical and mechanical properties of rigid-rod polymers

The idea of competing effects between intramolecular and intermolecular hydrogen bonding was investigated. Results indicate that the formation of one type of hydrogen bond does not preclude the formation of the other. The strength of the intermolecular association was measured by abinitio calculations for several polymer systems, including methyl pendant poly(p-phenylene benzobisimidazole) and poly-{2,6-diimidazo[4,5-b:4′5′-e]pyridinylene-1,4(2,5-dihydroxy)phenylene} (PIPD). Fibers with strong intermolecular association have high compressive strength and torsional modulus. The influence of intermolecular hydrogen bonding on torsional modulus is discussed in light of the transverse texture present in poly(p-phenylene terephthalamide) and some other high-performance fibers. Enhanced intermolecular interaction not only influences the aforementioned properties but also results in higher fiber density.

Synthesis and spinning of a thermotropic liquid crystal copolyester containing a semirigid cycloaliphatic spacer

A thermotropic, liquid crystalline copolyester, based on 2-chlorohydroquinone, 1,4-cyclohexanedimethanol and terephthaloyl chloride, has been synthesized and melt spun. The cyclohexanedimethylene moiety acts as a semirigid spacer, introducing flexibility while preserving the thermotropic nature of the polymer. Melt-spun fibers were observed to have a high degree of molecular alignment owing to the nematic nature of the melt. Both polymer and fiber properties have been characterized. Characterization techniques used to this end include elemental analysis, hot-stage polarized light microscopy, scanning electron microscopy, dilute solution viscometry, Fourier transform infrared spectroscopy, nuclear magnetic resonance, differential scanning calorimetry, and thermogravimetric analysis.

Modeling the effect of crosslinking in methyl-pendant poly(p-phenylene benzobisthiazole)

Molecular mechanics and dynamics simulations have been performed on methyl-pendant PBZT to study the effects of intermolecular crosslinking. Several possible crosslinked structures were investigated. The effect of crosslinking was found to be strongly dependent upon crosslink type and, in some instances, crosslink density. A significant axial stress is predicted to occur upon the formation of phenyl-to-phenyl type crosslinks. This provides a reasonable explanation for the experimental observation of transverse cracks in the skin of crosslinked, MePBZT fiber.

Prediction of crosslinking in polyamides containing the diacetylene functionality

Abstract The potential for crosslinking in various polyamides containing the diacetylene moiety has been investigated via computer simulation. Results of simulations performed on an aliphatic, diacetylene-modified polyamide concurred with earlier experimental work, in that the reaction of adjacent diacetylenes was predicted to occur only between mutually hydrogen bonded chains. Hydrogen bonding was predicted to remain intact subsequent to this reaction, also in agreement with experiment. Conversely, other structural arrangements consequent to crosslinking were not predicted accurately. Possible reasons for this disparity have been discussed. A similar study on a diacetylene-modified PPTA system, for which no experimental work exists, has also been conducted. Molecular mechanics simulations indicate crosslinking in this system would be most probable between mutually hydrogen-bonded chains. However, taking into account room temperature thermal vibrations, a three-dimensional network of covalent bonds is predicted.

Effects of electron radiation on mono- and di-methyl pendant poly(p-phenylene benzobisthiazole) fibers

The effect of electron radiation on mono- and di-methyl pendant poly(p-phenylene benzobisthiazole) polymers (MePBZT and DiMePBZT, respectively) has been investigated. MePBZT is chemically stable (as measured by 13C NMR) upon exposure to 1 Grad electron radiation at room temperature. The irradiation of DiMePBZT at 225°C was also carried out, which was found to have no effect up to a dosage of 850 Mrad. Thus, the additional molecular mobility brought about by heating (225°C) did not enhance the reactivity of DiMePBZT. The compressive and tensile properties of DiMePBZT fiber remained unchanged at lower radiation dosages. At a dosage of 850 Mrad, the tensile properties were found to decrease substantially, which may be attributed to defects observed in these fibers. The defects can be attributed to the effects of electron radiation, rather than prolonged heating at 225°C.