Melt Memory Effect on Biaxially Oriented Random Copolyamides: Direct Evidence for Stretch-Induced Crystal Retention above the Equilibrium Melting Point

In the processing of semicrystalline polymers, the oriented crystal through flow-induced crystallization (FIC) represents a significant phenomenon that can profoundly influence the final properties of materials. This study investigates the melting and crystallization behavior of biaxial stretch-induced crystal structures in BOPA56/512 (biaxially oriented PA56/512 films) with varying copolymerized compositions at temperatures above T m, utilizing DSC, XRD, FTIR, and in situ POM. Results reveal that BOPA56/512 with relatively high PA512 mol % in the random copolymer PA56/512 develops distinct and long-range shish-kebab structures. These structures exhibit remarkable stability, persisting even above the equilibrium melting point ( T m 0 ). Notably, the thermal history effect from biaxial stretching, which elevates the crystallization temperature, persists even upon heating to the molten state. Additionally, increasing the heat treatment temperature induces a γ-α form transition in BOPA56/512. Conversely, with decreasing PA512 mol % content, BOPA56/512 forms only irregular and shorter secondary crystals, and the increasing temperature induces merely changes in the d -spacing of the γ form rather than a phase transition. CP/MAS NMR and crystallization Gibbs free energy ( G c ) calculations suggest that shish-kebab formation and γ-α transition in high PA512 mol % BOPA56/512 result from better chain mobility and lower G c of PA512. Remarkably, even when heated to 90 °C above T m, these shish kebab/secondary crystal structures are partially preserved. Upon cooling of the molten state, these preserved structures serve as initial nucleation sites for recrystallization. This unprecedented observation of stable shish-kebab structures persisting at temperatures significantly above ( T m 0 ) may represent a novel finding in the field. Results demonstrate that in PA56/512 random copolymers, the biaxially stretch-induced oriented structures do not dissolve completely into relaxed random coils under the influence of a single temperature. Instead, they undergo partial relaxation into crystalline precursors, maintaining some degree of structural order even at elevated temperatures. The results of this study provide a theoretical basis for designing and regulating the aggregation structure and macroscopic properties of biaxially oriented random copolyamide films.