PROCESSING AND CHARACTERIZATION OF THERMALLY CROSS-LINKABLE 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 me thod is based on incorporating XTA, a benzocyclobutene-modified deriva tive of terephthalic acid, into the polymer backbone and then inducing cross-linking by heat treatment after the fiber is formed. PPTA-co-XT A copolymers with various XTA contents exhibited lyotropic nematic liq uid 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 orienta tion and at higher temperatures (above 320 degrees C) to trigger cross -linking. Wide angle X-ray diffraction confirmed high molecular orient ation in the fibers before and after cross-linking. The mechanical pro perties 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 treatme nt. FTIR and ESR spectroscopic studies suggested this resulted from a degradative chain scission process. Compressive properties of these fi bers 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 copolym er fibers also exhibited increased resistance to creep and lateral def ormation after heat treatment.