KINETICS OF CRYSTALLIZATION FROM THE MELT AND CHAIN FOLDING IN POLYETHYLENE FRACTIONS REVISITED - THEORY AND EXPERIMENT

The rate of growth of chain-folded lamellar crystals from the subcoole d melt of polyethylene fractions is treated in terms of surface nuclea tion theory with the objective of illuminating the origin of the chain folding phenomenon and associated kinetic effects in molecular terms. An updated version of flux-based nucleation theory in readily usable form is outlined that deals with the nature of polymer chains in more detail than previous treatments. The subjects covered include: (i) the origin of regimes I, II, III, and III-A and the associated crystal gr owth rates, including the effect of forced steady-state reptation and reptation of 'slack' in the subcooled melt; (ii) the variation of the initial lamellar thickness with undercooling; (iii) the origin of the fold surface free energy sigma(e) and the lateral surface free energy sigma; (iv) the generation and effect of nonadjacent events (such as t ie chains) on the crystallinity and growth rates; and (v) 'quantized' chain folding at low molecular weight. The topological limitation on n onadjacent re-entry and the value of the apportionment factor psi an d iscussed. Key experimental data are analysed in terms of the theory an d essential parameters determined, including the size of the substrate length L involved in regime I growth. The degree of adjacent and/or ' tight' folding that obtains in the kinetically-induced lamellar struct ures is treated as being a function of molecular weight and undercooli ng. New evidence based on the quantization effect indicates a high deg ree of adjacent re-entry in regime I for the lower molecular weight fr actions. The quality of the chain folding at higher molecular weights in the various regimes is discussed in terms of kinetic, neutron scatt ering, i.r., and other evidence. Application of the theory to other po lymers is discussed briefly. (C) 1997 Elsevier Science Ltd.