Regime III crystallization in melt-crystallized polymers: The variable cluster model of chain folding

The kinetic nucleation theory of chain folding, including the effects of reptation, is extended to predict the increase in crystal growth rate G that is implied by measurements on PE and POM at moderately large undercoolings. The increased growth rate denotes the rather abrupt transition from Regime II where $\text{G∝i}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ to Regime III where G∝ i (i = surface nucleation rate). The distance between the niches on the growth front in Regime II diminishes rapidly with falling crystallization temperature T_x, and approaches the molecular width at a specified undercooling where Regime III begins. In PE, the Regime I → Regime II transition occurs at $\text{ΔT ? 16°}\text{C}$, and the Regime II → Regime III transition is predicted to occur at ΔT ～ 23°C (ΔT based on T°_m(∞) = 145°C). Growth rate data on PE and POM crystallized from the melt suggest conformity with the theoretical predictions. The implications of Regime III crystallization to chain morphology are discussed. The kinetic theory, which predicts narrowly spaced niches on the growth front, taken together with the restrictions on the degree of non-adjacent re-entry imposed by the ‘Gambler's Ruin’ treatment, leads directly to the ‘variable cluster’ model as the relevant morphology in Regime III. Here runs of adjacently chain-folded stems of varying size (averaging about three or so stems) are laid down interspersed with non-adjacent re-entries, leading to a lamellar surface that is about two-thirds ‘regular’ or ‘tight’ folds, most or all of these representing strictly adjacent re-entries. The steady-state reptation process operative in Regimes I and II in PE is impaired at temperatures just below the inception of Regime III, and it is suggested that at lower temperatures the ‘slack’ portions of the chains engage in forming the small clusters of adjacent stems. The variable cluster model leads in a natural way to the amorphous component found in quench-crystallized polymers, and is consistent with neutron scattering data on quench-crystallized PEH-PED.