PROCESSING AND CHARACTERIZATION OF THERMALLY CROSS-LINKABLE POLY[P-PHENYLENETEREPHTHALAMIDE-CO-P-1,2-DIHYDROCYCLOBUTAPHENYLENETEREPHTHALAMIDE] (PPTA-CO-XTA) COPOLYMER FIBERS

A scheme was developed to cross-link poly(p-phenyleneterephthalamide) (PPTA or Kevlar) in order to modify its macroscopic properties. The method is based on incorporating XTA, a benzocyclobutene-modified derivative of terephthalic acid, into the polymer backbone and then inducing cross-linking by heat treatment after the fiber is formed. PPTA-co-XTA copolymers with various XTA contents exhibited lyotropic nematic liquid crystalline behavior and could be spun into fibers by dry-jet wet spinning techniques. As-spun fibers were heat-treated at intermediate temperatures (200-300 degrees C) to increase crystallinity and orientation and at higher temperatures (above 320 degrees C) to trigger cross-linking. Wide angle X-ray diffraction confirmed high molecular orientation in the fibers before and after cross-linking. The mechanical properties of these fibers were studied as a function of XTA content and conditions of heat treatment. Cross-linked copolymer fibers generally showed an improvement in tensile modulus over as-spun fibers. For the PPXTA homopolymer, however, the tensile strength and toughness tended to decrease with increasing length and temperature of the heat treatment. FTIR and ESR spectroscopic studies suggested this resulted from a degradative chain scission process. Compressive properties of these fibers were investigated through elastica and recoil tests, and through measurement of the fiber critical strain to kinking in a beam bending geometry. The strain to induce kinking in cross-linked PPXTA fibers is approximately twice that of the un-cross-linked material. The copolymer fibers also exhibited increased resistance to creep and lateral deformation after heat treatment.