NEUTRON-SCATTERING STUDIES OF SEMI-CRYSTALLINE POLYMERS IN THE SOLID-STATE - STRUCTURE OF ISOTROPIC POLYOLEFINS

It has been shown that on solidification from the melt, the radius of gyration (Rw) of organic polymers does not change significantly. This shows that any conformational changes occurring involve only fragments of the polymer chain, and these are distributed throughout several lamellae in space. It is not always possible to differentiate between a coil or an array of sets of stems since at high molecular weight, for both structures, Rw shows an M 1 2 w dependence. At low molecular weight, the length of the stems which constitute the array can be recovered. In polypropylene and quenched polyethylene this is shown to be approximately equal to twice the X-ray lamellar thickness (d) showing that rods which constitute the array contain two stems d and 2d in length. The latter act as tie molecules between lamellae. The structure is supported by chemical etching experiments which can differentiate between the tie molecules and folds in the amorphous phase (Figure 6b) and show the presence of two stems d and 2d in length in polypropylene and polyethylene. Finally polypropylene, which is crystallized by 'seeding', folds fully in such a way as to preserve the tie molecules and the chain occupies two lamellae. Polyethylene crystallized under high pressure on the other hand, in which d {reversed tilde equals} 2000 A ̊, is fully folded and each lamella contains complete polymer molecules. No evidence has been obtained for the nature of the re-entry in any of these structures. © 1979.