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. [Macromolecules, 2014, 47, 4738–4749]. Motivated by this recent discovery, the crystallization behavior 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 crystallization 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.

中文翻译：

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具有规则间距溴化基团的聚乙烯中量子层状增稠的分子模拟


Tasaki 等人观察到聚乙烯 （PE） 链，其中 CH₂ 基团以规则的间隔被 CBr₂ 取代（“精密 PE”），表现出由对量化折叠长度的偏好驱动的竞争性多晶型物 [大分子， 2014， 47， 4738–4749]。受这一最新发现的启发，使用分子动力学模拟研究了这种精密 PE 链的结晶行为，长 400 个碳，CBr₂ 基团定期放置在每 21 个碳上。PE 的联合单体模型扩展到包括双溴基团，溴的空间冲突反映在三孔弯曲电位中，证明了其作为优选折叠位点的功能。不同的结晶方案，连续冷却和自晶种，揭示了截然不同的晶体。使用自晶种，结晶层层厚度随温度单调增加，以双溴单元之间距离的量化倍数增加。观察到聚合物链优先在二溴基团处折叠，并且此类基团似乎在晶体薄片内被容忍。在淬灭时，溴组装形成注册层，这与在液晶中观察到的 Smectic 相没有什么不同，这证实了对竞争性 I 型和 I 型多晶型物的实验观察。