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

Abstract

The melting behavior of a homogeneous ethylene/1-hexene copolymer (M w = 70 000 g/mol; ρ = 0.90 g/cm 3; 6.4 mol % hexene) is studied by the simultaneous measurement of small-angle light scattering (SALS) under cross-polarized (H V) or parallel-polarized (V V) optical alignments and transmitted light. The temperature variation of H V and V V patterns and integrated intensities during melting is consistent with the predictions of a generalized SALS model presented. The data show that H V and V V 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 (T mf = 114°C) obtained from H V and V V 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 (l f = 11 nm) is determined from Guinier plots of the SAXS profiles obtained during the final stage of melting. The equilibrium melting temperature (T mc = 136 ± 2°C) calculated from the modified Gibbs-Thomson relation with the T mf and l f values obtained is consistent with the value (T mc = 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 α-olefins.