Melting and heat capacity of gel-spun, ultra-high molar mass polyethylene fibers

The melting behaviors of gel-spun, ultra-high molar mass polyethylene fiber s have been investigated using standard differential scanning calorimetry a nd wide-angle X-ray diffraction. For laterally constrained fibers the stand ard DSC exhibits the usually reported two melting peaks (one at about 415 K and one at about 423 K), a glass transition (at about 275 K) and an unusua lly large and reversible heat capacity as one approaches the melting peaks (starting at about 350-370 K). Earlier observations that sometimes only the lower melting endotherm is retained, are proven to be of fibers free of lo ngitudinal and lateral constraints. Up to the beginning of major melting, a ll fibers are mainly orthorhombic. The orthorhombic-hexagonal transition is proven to occur only for the constrained fibers, implying that the hexagon al phase is a non-equilibrium phase, usually not present in the initial fib ers. An oriented, intermediate phase, discovered by solid state NMR and ful l-pattern X-ray analysis, gives rise to a substantially broadened glass tra nsition that reaches to about 300 K, compared to the glass transition of th e amorphous polyethylene with a midpoint at 237 K and end at 250 K. Publish ed by Elsevier Science Ltd.