An intensity superposition model to fit the small angle X-ray scattering of semicrystalline polymers and its application to the monitoring of non-isothermal crystallization

摘要

Small angle X-ray scattering (SAXS) of Ethylene-1-Octene copolymer cooled from the melt at a constant rate is recorded and modeled. The theoretical SAXS curves based on the paracrystalline layer stacking model of Hermans cannot be well fitted to the experimental ones. Therefore it is assumed that lamellar stacks filling the copolymer volume may differ in their crystallinity and the curves are fitted by a model that combines intensities from all stacks under control of a crystallinity distribution function P(phi). It is shown that a symmetric Gauss function is not suitable to approximate this distribution while the best fit is obtained for an asymmetric, positively skewed split Gauss function. The model provides good agreement of the morphological parameters of stacks with the values obtained from the one-dimensional correlation function. Additional determined parameters are: average crystallinity of the copolymer phi(s), invariant Q as well as standard deviation and skewness coefficient of P(phi). During cooling the P(phi) distribution becomes less asymmetric but simultaneously it clearly broadens. The skewness decreases first slowly but much faster for temperatures lower than 40 degrees C. Based on the transformations of the shape of P(phi) distribution the changes in the structure of stacks and homogeneity of copolymer are discussed and related to the crystallization conditions. (C) 2015 Elsevier Ltd. All rights reserved.