RELATIONSHIP BETWEEN THE LATERAL SURFACE FREE ENERGY-SIGMA AND THE CHAIN STRUCTURE OF MELT-CRYSTALLIZED POLYMERS

A theory is presented for the lateral surface free energy parameter-sigma in the nucleation constant K(g) in the relation G is-proprotational-to exp[-K(g)/T(DELTA-T)f] that describes the growth rate of polymer crystals from the melt at low-to-moderate undercoolings DELTA-T. The theory forms a connection between nucleation theory and the statistics of polymer chain dimensions and provides a new approach to the determination of the characteristic ratio C(infinity). It is predicted that sigma-varies as const x C(infinity)-1, where the constant involves known quantities. The effect of chain structure resides principally in C(infinity. The theory was tested for polyethylene, isotactic polystyrene, and poly(L-lactic acid) by employing known values of sigma obtained from melt crystallization studies to calculate C(infinity) and then comparing these with the C(infinity) values cited in the literature as determined from chain dimensions in dilute THETA-solvents. The agreement is satisfactory, showing the theory for sigma to be valid in these cases, with the further implication that these polymers possessed a close approximation to unperturbed chain dimensions in the melt. Support was provided by data for isotactic polypropylene, poly(pivalolactone), and poly(epsilon-caprolactone), some of this deriving from a method of finding C(infinity) for the melt state without direct reference to sigma. The C(infinity)-dependent "segmental" character induced by the crystal surface in the section of chain comprising the activated complex is discussed. The theory for sigma provides strong support for polymer nucleation theory as there is now a predictive molecular picture for all the nucleation parameters in K(g). The treatment casts new light on the empirical formulas commonly employed to estimate sigma.