Kutlwano Gabana, Gillian A. Gehring, Hendrik Meyer, Goran Ungar, Xiangbing Zeng, William S. Fall
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引用次数: 0
摘要
在 Tasaki 等人的研究中,观察到 CH2 基团被 CBr2 以规则间隔取代的聚乙烯(PE)链("精密 PE")在偏好量化折叠长度的驱动下表现出相互竞争的多态性。聚乙烯的单体模型扩展到包括二溴基团,溴的立体碰撞反映在三孔弯曲势中,证明了其作为首选折叠位点的功能。连续冷却和自播种这两种不同的结晶方案显示出明显不同的晶体。使用自播种方法时,结晶薄片厚度随温度的升高而单调增加,与二溴单位之间的距离成量化倍数关系。据观察,聚合物链优先在二溴基团处折叠,而这些基团似乎在晶体内部是可以容忍的。在淬火时,溴基团聚集形成注册层,这与液晶中观察到的 Smectic 相并不相同,从而证实了实验中观察到的相互竞争的 Form I 和 Form I' 多晶体。
Molecular Simulations of Quantized Lamellar Thickening in Polyethylenes with Regularly Spaced Brominated Groups
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.