Timescales of polyamide 6 melt-shaping technologies, relative to the dynamics of conformational rearrangements upon crystallization, challenge the formation of the most thermodynamically favorable chain packing and thus optimum performance. In this publication, we make use of the mediation of hydrogen bonding by water molecules in the superheated state of water, i.e., above 100 °C in a closed environment, in the structural refinement of polyamide 6 for enhanced thermomechanical performance. The paper addresses dissolution and (re)crystallization of different polyamide 6 polymorphs in the superheated state of water by time-resolved simultaneous small- and wide-angle X-ray scattering and solid-state ¹H NMR spectroscopy and the effect on mechanical properties. The experiments reveal that upon heating in the superheated state of water, the pseudo-hexagonal phase dissolves at relatively low temperature and instantly crystallizes in a defected monoclinic phase that successively refines to a perfected monoclinic structure. The dissolution temperature of the pseudo-hexagonal phase of polyamide 6 is found to be dependent on the degree of crystal perfection originating from conformational disorder and misalignment of hydrogen bonding in the lattice, retrospectively, to the Brill transition temperature. The perfected monoclinic phase below the dissolution temperature can be preserved upon cooling but is plasticized by hydration of the amide moieties in the crystalline phase. The removal of water from the hydrated crystals, in the proximity of Brill transition temperature, strengthening the hydrogen bonding, occurs. Retrospectively, the most thermodynamically stable crystallographic phase is preserved and renders an increase in mechanical properties and dimensional stability of the product. The insight obtained on the influence of superheated water on the structural refinement of imperfected crystallographic states assists in polyamide 6 postprocessing strategies for enhanced performance.

中文翻译：

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聚酰胺 6 多晶型物中结晶相的水合、细化和溶解以获得最终的热机械性能

聚酰胺 6 熔体成型技术的时间尺度，相对于结晶时构象重排的动力学，挑战了热力学上最有利的链堆积的形成，从而形成了最佳性能。在本出版物中，我们利用水分子在水的过热状态（即，在封闭环境中高于 100°C）中介导氢键键合来改进聚酰胺 6 的结构，以提高热机械性能。本文通过时间分辨同时小角和广角 X 射线散射和固态^{1解决了不同聚酰胺 6 多晶型物在水的过热状态下的溶解和（重）结晶问题}H NMR光谱和对机械性能的影响。实验表明，在水的过热状态下加热时，拟六方相在相对较低的温度下溶解，并立即结晶成缺陷单斜相，随后逐渐细化为完美的单斜结构。发现聚酰胺 6 的假六角相的溶解温度取决于晶格的构象无序和晶格中氢键的错位导致的晶体完美程度，追溯至布里尔转变温度。低于溶解温度的完美单斜晶相可以在冷却后保存，但会通过结晶相中酰胺部分的水合而增塑。从水合晶体中除去水，在接近布里尔转变温度时，会发生加强氢键。回顾过去，最热力学稳定的结晶相得以保留，并提高了产品的机械性能和尺寸稳定性。关于过热水对不完美结晶状态的结构细化影响的见解有助于聚酰胺 6 后处理策略以提高性能。