T. Jiang et al., PROCESSING AND CHARACTERIZATION OF THERMALLY CROSS-LINKABLE CO-P-1,2-DIHYDROCYCLOBUTAPHENYLENETEREPHTHALAMIDE] (PPTA-CO-XTA) COPOLYMER FIBERS, Macromolecules, 28(9), 1995, pp. 3301-3312
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.