Thermal-Induced Structural Evolution of Melt-Stretched PA11: Direct Evidence for the Preservation of Hydrogen-Bonded Sheets Above the Brill Transition Temperature
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引用次数: 0
Abstract
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.
期刊介绍:
Macromolecules publishes original, fundamental, and impactful research on all aspects of polymer science. Topics of interest include synthesis (e.g., controlled polymerizations, polymerization catalysis, post polymerization modification, new monomer structures and polymer architectures, and polymerization mechanisms/kinetics analysis); phase behavior, thermodynamics, dynamic, and ordering/disordering phenomena (e.g., self-assembly, gelation, crystallization, solution/melt/solid-state characteristics); structure and properties (e.g., mechanical and rheological properties, surface/interfacial characteristics, electronic and transport properties); new state of the art characterization (e.g., spectroscopy, scattering, microscopy, rheology), simulation (e.g., Monte Carlo, molecular dynamics, multi-scale/coarse-grained modeling), and theoretical methods. Renewable/sustainable polymers, polymer networks, responsive polymers, electro-, magneto- and opto-active macromolecules, inorganic polymers, charge-transporting polymers (ion-containing, semiconducting, and conducting), nanostructured polymers, and polymer composites are also of interest. Typical papers published in Macromolecules showcase important and innovative concepts, experimental methods/observations, and theoretical/computational approaches that demonstrate a fundamental advance in the understanding of polymers.