On the problem of crystallization of polymers from the melt with chain folding

Abstract

It is shown that the “reptation” process proposed by de Gennes allows molecules to be reeled from the melt onto the crystal surface with chain folding by the force associated with crystallization at a rate that is comparable with that demanded by the observed crystallization kinetics for polyethylene fractions n= number of C atoms = 1290–5310. Hence, the rate of transport in the melt is sufficient to permit a considerable amount of chain folding, and an objection due to Flory and Yoon is thereby countered for the range of n noted. The deductions of Yoon and Flory from the neutron scattering data of Schelten and co-workers on PEH + PED mixtures (n_ped≅ 3750) quench-crystallized from the melt are considered next. It is shown that Yoon and Flory's favoured p_es= 0.3 model, which gives a probability of adjacent re-entry p_ar close to zero, is deficient despite the good fit of the scattering data, since it exhibits a large density anomaly in the region between the crystal lamellae. This opposes their own view that the material in the interlamellar region has essentially normal amorphous state properties. A “central core” model is proposed that does not possess a density anomaly, and which predicts the scattering curve, characteristic ratio and crystallinity with fair accuracy. This and certain other models give p_ar≈ 0.65, indicating that the adjacent position is by a considerable margin the most probable site for re-entry, in contrast to the analysis of Yoon and Flory. The core model exhibits a mean throw distance of ≈ 22 Å for the non-adjacent re-entry loops. This is comparable with the mean “niche” distance calculated from nucleation theory. The number of ties between the lamellae is less than one per chain. Hence the connections of this type between the lamellae are less profuse than have sometimes been depicted.