Direct Evidence of Regimes I, II, and III in Linear Polyethylene Fractions As Revealed by Spherulite Growth Rates

Isothermal spherulite growth rates were measured over a sufficient range of undercoolings, ΔT, for a narrow linear polyethylene fraction M = 70 300 (70.3K), polydispersity 1.12, such that the fraction exhibited all three growth regimes as crystallized from the subcooled melt. The I−II transition occurred at ΔT I - II = 15.8 °C and the II−III transition at ΔT II - III = 23.8 °C. (Neither transition was fully abrupt.) The nucleation constants K g and preexponential factors G 0 that described the absolute growth rates for each regime were determined, thus quantifying key parameters for all three regimes for a single specimen measured in the same apparatus. The K g's for 70.3K conformed to the predicted relationship K g(III) ≅ K g(I) = 2K g(II). Theoretical relationships for the preexponential factors were employed using the observed G 0's to investigate the nature of the transport of chain segments to the growth front. It was reconfirmed that this process was forced “near-ideal” reptation for an M ≅ 30K fraction. For M = 70.3K, it was found that the reptational transport mechanism in regimes II and III was perturbed and thereby slowed beyond that attributable to “near-ideal” forced reptation; the additional retardation was taken to be the result of labile chain attachments on a surface some distance from the site where the dangling chain was being drawn onto the substrate. In another test, the expression S k/a o for the stem separation between primary surface nuclei in regime II was employed to calculate ΔT I - II and ΔT II - III. This was successful for both M ≅ 30K (near-ideal reptation) and M = 70.3K (perturbed reptation). In this test, earlier estimates of quantities of importance to nucleation theory, such as C 0, n III, and the substrate length L, were found to be either identical or only slightly modified. The treatment leads to satisfactory numerical estimates of the absolute substrate completion rate g and the nucleation rate i, and is consistent with the crystal morphology present in melt-crystallized PE, including the lenticular crystal → truncated lozenge transformation associated with the I → II regime transition. In general, this work provides significant additional support for the “three regime” concept in narrow PE fractions crystallized from the melt through a consideration of nucleation, regime, and reptation concepts.