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The nonisothermal crystallization half-time (t0.5), defined as the time taken for a polymer film to reach half of its equilibrium crystallinity, was estimated from Raman spectroscopic measurements of crystallinity during blown film extrusion of a linear low-density polyethylene (LLDPE) and an isotactic polypropylene (i-PP). The crystalline a- and c-axis orientation of LLDPE and i-PP films, respectively, increased with decreasing crystallization half-time. The transverse direction tensile modulus and tear strengths for LLDPE films also increased with decreasing half-time. However, for i-PP films, only the transverse direction tear strength increased with decreasing t0.5, while the machine direction properties did not show a significant dependence on half-time. Our real-time Raman spectroscopy studies provide experimental evidence to theories proposed in the literature1-3 with regards to the influence of the nonisothermal crystallization process (along the film axis) on the imparted final film structure.
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Cherukupalli et al. (2005) studied this question.
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