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, DeltaT, 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 DeltaT(I-II) = 15.8 degreesC and the II-III transition at DeltaT(II-III) = 23.8 degreesC. (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) congruent to K-g(I) = 2K(g(II)). Theoretical relationships for the preexponential factors were employed using the observed Go'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 congruent to 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(0). for the stem separation between primary surface nuclei in regime II was employed to calculate DeltaT(I-II) and DeltaT(II-III). This was successful for both M congruent to 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.