The drawing behavior of ultrahigh molecular weight polyethylene fibers in supercritical carbon dioxide (scCO(2)) is compared to that in air at different temperatures. The temperature substantially influences the drawing properties in air, whereas in scCO(2), a constant draw stress and tensile strength are observed. Differential scanning calorimetry shows an apparent development of a hexagonal phase along with a significant increase in the crystallinity of air-drawn samples with increasing temperature. The existence of this phase is not confirmed by wide-angle X-ray scattering, which instead shows that air-drawn samples crystallize in an internally constrained manner. In contrast, scCO(2) allows crystals to grow without constraints through a possible crystal-crystal transformation, increasing the processing temperature to 11,0 degreesC. (C) 2003 Wiley Periodicals, Inc.