Use of SAXS and linear correlation functions for the determination of the crystallinity and morphology of semi-crystalline polymers. Application to linear polyethylene

The use of correlation functions to obtain the morphological parameters of crystalline-amorphous two-phase lamellar systems is critically reviewed and extended. If is shown that processing of the experimental SAXS-patterns on ly significantly affects the curvature of the autocorrelation triangle and that the parameters of the corresponding ideal two-phase structure can be d etermined independently of the data processing procedure. The methods to be used depend on the normalization of the correlation function. The validity of the formulation is illustrated for a sample of linear polyethylene, coo led and heated at 10 degrees C per min. Crystallite thickening during cryst allization and surface melting during heating are observed. The overall cry stallinity and the fraction of semi-crystalline stacks during crystallizati on and melting are determined quantitatively as a function of temperature u sing the total scattering power of the corresponding ideal two-phase struct ure, correlation functions, and a scaling procedure. Absolute intensities a re not required. The SAXS results are confirmed by independent techniques ( DSC, WAXD, and SALLS). During crystallization, amorphous regions are presen t outside the semi-crystalline regions because growing spherulites do not f ill space completely. During melting;, larger amorphous regions develop in the spherulites because of the complete melting of stacks. (C) 1999 John Wi ley & Sons, Inc.