Molecular Mechanism of Surface Melting of Polyamide12 as Elucidated by Synchrotron Radiation X-ray Scattering and ¹H Spin Diffusion NMR

Abstract

The surface melting of Polyamide12 (PA12) at elevated temperatures was directly elucidated by 1 H spin diffusion (SD) NMR in combination with synchrotron radiation X-ray scattering, specifically wide-/small-angle X-ray scattering (WAXS/ SAXS). Two significant structure characteristics, namely, the Brill transition and surface melting, were revisited by WAXS/SAXS, where the former happens in the crystalline domain and the latter one refers to the interphase. The surface melting is evidenced by the decreasing lamellar thickness after Brill transition (T > 120 °C). To further unveil the molecular origin of interfacial surface melting, a numerical modeling program named SD NMR-WEBFIT was developed and implemented to interpret the morphology of the interphase by 1 H SD NMR. In the three-phase morphology scenario of the semicrystalline polymer, the rigid crystallite, rigid amorphous fraction (RAF), and mobile amorphous fraction (MAF) coexist. Modeling results from 1 H SD NMR conducted at various temperatures reveal the existence of dynamic heterogeneity within the interphase in low-temperature regions (T < 120 °C), proposing an island structure model. Here, the RAF is distinguished as the hydrogen bonding dominant phase. As the temperature is further elevated, typically above the Brill transition temperature (∼120 °C), the interphase is characterized by greater dynamic homogeneity with an increased long period (the sandwich model). From the aspect of the thermodynamics viewpoint, the interphase is proposed to be transformed from the enthalpy dominant phase to the entropy-dominant one, where the interchain hydrogen bonding and surface crowding play central roles.