具有规则间隔溴化基团的聚乙烯中量子化片晶增厚的分子模拟
Molecular Simulations of Quantized Lamellar Thickening in Polyethylenes with Regularly Spaced Brominated Groups
AI总结：
本文通过分子动力学模拟研究了规则间隔溴化基团的精密聚乙烯的结晶行为，揭示了二溴基团作为优先折叠位点导致片晶厚度量子化增厚，并证实了竞争性多晶型的形成机制。
摘要：
聚乙烯（PE）链中CH2基团被CBr2基团以规则间隔替换（“精密PE”），Tasaki等人观察到其表现出由量子化折叠长度偏好驱动的竞争性多晶型。受这一近期发现的启发，本文利用分子动力学模拟研究了此类精密PE链的结晶行为，该链长400个碳原子，CBr2基团规则地分布在第21个碳原子上。将PE的联合单体模型扩展以包含二溴基团，溴原子处的空间位阻通过三阱弯曲势反映，证明了其作为优先折叠位点的功能。不同的结晶方案，连续冷却和自接种，揭示了显著不同的晶体。使用自接种时，结晶片晶厚度随温度单调增加，且以二溴单元间距的量子化倍数增加。观察到聚合物链优先在二溴基团处折叠，且这些基团似乎在晶体片晶内被容纳。在淬火时，溴基团组装形成规整层，类似于液晶中观察到的近晶相，这证实了实验中观察到的竞争性Form I和Form I'多晶型。
英文摘要：
Polyethylene (PE) chains, with CH2 groups replaced by CBr2 at regular intervals ("precision PE"), have been observed to exhibit competing polymorphs driven by a preference for quantized fold lengths by Tasaki et al. Motivated by this recent discovery, the crystallisation behaviour of such precision PE chains, 400 carbons long with CBr2 groups placed regularly at every 21st carbon, is investigated using molecular dynamics simulations. The united-monomer model of PE is extended to include dibromo groups, with steric clashes at the bromines reflected in a triple-well bending potential, demonstrating its function as a preferred fold site. Different crystallisation protocols, continuous-cooling and self-seeding, reveal remarkably different crystals. Using self-seeding, the crystalline lamellar thickness increases monotonically with temperature, in quantized multiples of the distance between dibromo units. Polymer chains are observed to fold preferentially at the dibromo groups and such groups appear to be tolerated within the crystal lamellae. On quenching the bromos assemble to form registered layers, not unlike Smectic phases observed in liquid crystals, which confirms the experimental observation of competing Form I and Form I' polymorphs.
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