An x-ray diffraction study of nonlinear polyethylene. II. Small-angle scattering observations near the melting point

TLDR: In this article, a method was presented for obtaining from the zeroth-order peak in small-angle x-ray scattering (SAXS) patterns the weight average of the thickness C of crystalline lamellae in polymers close to the melting point.

Abstract: A method is presented for obtaining from the zeroth-order peak in small-angle x-ray scattering (SAXS) patterns the weight average of the thickness C of crystalline lamellae in polymers close to the melting point. The method fails if a first-order peak can be discerned to be superimposed on the zeroth-order peak. The method was applied, together with the one described in Part I of this series for obtaining the number average of C, to three polyethylene samples of different side-group content which were heated to temperatures close to the melting point. While heating always causes an increasing number of lamellae to melt, a linear sample also shows pronounced thickening of the remaining lamellae. This effect, however, decreases with increasing side-group content, and is virtually absent in a vinyl acetate copolymer containing 53 side groups per 1000 C atoms. A qualitative explanation for the role of the side groups in limiting the crystal thickening processes is put forward. By comparing results obtained after isothermal annealing and during isothermal crystallization from the melt it is concluded that at temperatures close to the melting point the polymers approach a final state which no longer depends on the thermal history of the sample.