Abstract
Many polymers exhibit bimodal temperature-dependent crystallization kinetics, with two distinct crystallization time minima at low and high temperatures. These regimes are typically governed by different nucleation mechanisms and are often associated with formation of different crystal polymorphs. In this work, we identify a strategy to systematically modulate the crystallization kinetics of polyamide 66 (PA 66) across both regimes by incorporating a random noncrystallizable poly-(hexamethylene isophthalamide-co-terephthalamide) (PA 6I/6T). The addition of PA 6I/6T systematically slows down crystallization of PA 66, and an apparent transition from a bimodal to unimodal crystallization kinetics is observed when the PA 6I/6T content exceeds 30 wt %. Although this change of the kinetics suggests α-phase dominance, fast scanning calorimetry (FSC), combined with other analytical techniques, collectively provide undisputable evidence that the persistence of mesophase formation at low temperature is not governed by overall crystallization kinetics. Instead, our results indicate that the selection between mesophase and α-phase is dictated by a fundamental, temperature-dependent switch in nucleation mechanism from heterogeneous to homogeneous nucleation. This finding rules out purely kinetic control as the primary cause of polymorph selection under deep undercooling conditions. Thermal analysis also shows a single glass transition temperature (T (g)) across all blends, in both amorphous and semicrystalline states, confirming molecular-level miscibility between PA 66 and PA 6I/6T. The upward shift in T (g) after crystallization indicates enrichment of PA 6I/6T in the amorphous matrix, consistent with selective exclusion during PA 66 crystallization. The mechanisms governing the slowing down of the crystallization process are extensively discussed. Together, these findings provide fundamental insights into how an amorphous component affects the crystallization kinetics, polymorphic selection, and phase behavior in polymer blends across a wide crystallization temperature range.