The structure of crystals formed from dilute polymer solutions†

The structures of crystallites formed from a dilute solution of linear polyethylene and of an ethylene-butene copolymer (hydrogenated polybutadiene) have been studied using several different methods. In addition to the conventional thermodynamic methods, small-angle x-ray scattering measurements and analysis of the low-frequency Raman longitudinal acoustic mode (LAM) were also carried out. The major objectives were to establish the level of crystallinity and to determine the thickness of the crystallite core. The results of these studies were also applied to the analysis of the literature data for isotactic polystyrene crystallites formed from dilute solution. For the linear chains and the isothermally crystallized copolymer the different methods gave essentially the same results. The thickness of the amorphous overlayer for the lamellas formed by the homopolymers was found to be about 25 Å, in agreement with previous conclusions. It represents about 20–30% of the lamellar thickness. For the isothermally crystallized copolymers, the overlayer thickness increases to as much as 60 Å and the level of crystallinity is reduced to about 50%. For the rapidly crystallized, small crystallite size copolymer there is a discrepancy in the sizes obtained by the different methods. Possible reasons for the differences are pointed out However, a rather large overlayer is still deduced. When examined in perspective, it becomes clear that the lamellar-like crystallites, typical of crystallization from dilute solution, contain a significant disordered overlayer. The relative extent of this overlayer depends on the chain structure.