Time-resolved synchrotron X-ray study of chain-folded crystallization of long paraffins

Abstract

The examination of the crystallization of strictly uniform ultra-long n-alkanes was resumed with the view of exploring the onset of chain-folding, as laid out previously in ref. 1. The present study, largely on C 246H 494, centred on the initial stages of crystallization in the melt, registered in situ by time-resolved small-angle X-ray scattering using a synchrotron X-ray source. The salient new feature was the identification of transient initial fold lengths which were non-integer fractions (NIF) of the chain length. This NIF structure transforms subsequently into forms with integer fraction (IF) fold lengths. In the present study the latter have the extended chain (E) and once-folded (F2) configurations, while NIF has a fold length between the two. The NIF → IF transformation occurs either by lamellar thickening or thinning, or by both. It was found that the NIF state had a more disordered layer surface as compared to the final E and F2 structures, the latter being the states on which the conclusions in ref. 1 had been drawn, which accordingly should apply to the transformed material. Implications of these and several other findings for some central issues in polymer crystallization are briefly discussed. The existence of an initial NIF phase focusses attention to the importance of the fastest kinetic pathway as the determining factor for chain-folded crystal growth with particular attention to the initial chain deposition probability, a line made accessible by the present alkanes.