Thermal-Induced Structural Evolution of Melt-Stretched PA11: Direct Evidence for the Preservation of Hydrogen-Bonded Sheets Above the Brill Transition Temperature

Despite extensive research into the thermally induced structural transitions of polyamides (PAs), the stability of hydrogen-bonded (H-bonded) sheets above the Brill transition temperature (TB) remains a contentious issue. Herein, we investigated the structural development of melt-stretched PA11 during heating and cooling cycles, utilizing in situ synchrotron wide-angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR) spectroscopy. By leveraging the unique twisted and oriented lamellar morphology created during melt stretching, we successfully identified and monitored, for the first time by WAXD, the H-bonded sheets across a broad temperature range up to the melting point (Tm). Our findings demonstrate that the H-bonded sheets are well maintained above TB until the sample fully melts, exhibiting distinct evolutionary trends in interplanar spacing, diffraction azimuth and orientation in response to temperature and melt-stretch ratio, compared to other crystallographic planes. The preserved H-bonded sheets have stronger interchain interactions, which impart a high anisotropy of thermal expansion to the high-temperature δ-phase. Further analysis of the FTIR data indicates that lattice variations below TB are driven by significant conformational twisting around the amide groups in the α′-phase, while thermal expansion predominantly dictates the variations in the δ-phase above TB. Additionally, the absence of notable alterations in H-bond interactions supports the continued stability of H-bonded sheets below Tm. This study enhances our understanding of the molecular mechanisms underlying thermally induced crystalline structural evolution in polyamide systems.