Probing the melting behavior of a homogeneous ethylene/1-hexene copolymer by small-angle light scattering

  • Li Y
  • Akpalu Y
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The melting behavior of a homogeneous ethylene/1-hexene copolymer (Mw ) 70 000 g/mol; F ) 0.90 g/cm3; 6.4 mol % hexene) is studied by the simultaneous measurement of small-angle light scattering (SALS) under cross-polarized (HV) or parallel-polarized (VV) optical alignments and transmitted light. The temperature variation of HV and VV patterns and integrated intensities during melting is consistent with the predictions of a generalized SALS model presented. The data show that HV and VV SALS can be used to determine the number of crystal populations, the melting temperature of each crystal population, and the spatial organization of crystalline aggregates. For a given crystallization condition, the final melting temperature (Tm f ) 114 °C) obtained from HV and VV SALS is in good agreement with values obtained from differential scanning calorimetry and small-angle X-ray scattering (SAXS) measurements on thicker samples with the same thermal history. The thickness of the largest crystals that can form (lf ) 11 nm) is determined from Guinier plots of the SAXS profiles obtained during the final stage of melting. The equilibrium melting temperature (Tm c ) 136 ( 2 °C) calculated from the modified Gibbs-Thomson relation with the Tm f and lf values obtained is consistent with the value (Tm c ) 134 °C) predicted from the Flory’s equilibrium theory of melting for a random copolymer with 6.4 mol % comonomer. Our results demonstrate that SALS can be used to understand and provide a quantitative description of how crystallization conditions affect the supermolecular structure organization in copolymers of ethylene and R-olefins.

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  • Ying Li

  • Yvonne A. Akpalu

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