摘要：

Poly(p-phenylene terephthalamide) (PPTA or Kevlar[sup TM]) adopts an extended-chain crystal habit in the solid state which derives from the chain parallelism in its liquid crystalline (LC) phase. The objective of this dissertation is to investigate how the deviation from core linearity which is achieved by replacing terephthalic acid, a linear monomer, with 2,5-thiophenedicarboxylic acid, a monomer with a [approximately]148[degrees] bend, affects liquid crystalline properties of aromatic polyamides. The mesogenic properties of the 2,5-thiophene are initially verified for newly synthesized low molar mass diamide model compounds where the characteristic texture of the smectic A phase is observed by polarized optical microscopy. Differential scanning calorimetry (DSC) studies shows that the observed mesophase for the thiophene-based diamide is much narrower and exists at lower temperatures than that observed for the corresponding p-phenylene diamide. Extensions to macromolecules show that if each terephthalamide unit in PPTA is replaced with a 2,5-thiophenylamide unit, high molecular weight polymer can be synthesized and, moreover, such 2,5-thiophene-based aromatic polyamide (PPThA) is stable and exhibits liquid crystalline phases in concentrated sulfuric acid solutions. Preliminary mechanical properties of coarse fibers spun from the liquid crystalline solutions and isotropic solutions in sulfuric acid are compared. PPThA lyotropic solutions are studied using deuterium NMR spectroscopy. LC solutions may be aligned in a magnetic field to yield a [open quotes]single liquid crystal[close quotes]-a macroscopically oriented phase. In such uniaxial solutions, the transition to isotropic state at elevated temperatures may be indirectly monitored by deuterium NMR; quadrupolar interactions of the probe molecule (deuterated sulfuric acid) reflects changes in the macromolecular ordering. Direct monitoring of polymer order is accomplished using deuterium labeled PPThA.

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