Department of Physics and Informatics, Yamaguchi University, Yamaguchi 753-8512, Japan.
J Chem Phys. 2013 Aug 7;139(5):054903. doi: 10.1063/1.4816707.
Molecular mechanisms of the steady-state growth of the chain folded lamella and the cold crystallization across the glass transition temperature Tg are investigated by molecular dynamics simulation for a system of long polyethylene (PE)-like polymers made of 512 united atoms C512. The present paper aims to reconsider results of our previous simulations for short PE-like polymers C100 by carrying out very long simulations up to 1 μs for more realistic systems of much longer chains, thereby to establish the firm molecular image of chain-folded crystallization and clarify the specific molecular process of cold crystallization. We observe that the chain-folded lamella shows fast thickening-growth keeping marked tapered growth front. Despite the fast growth in much longer chains, the fold-surface is found to be predominantly of adjacent-reentry. Detailed inspections of the molecular pathway give an insightful image that can explain the apparently contradicting results. In addition, the fold-structure with specific spatial heterogeneity is found to give rise to heterogeneous mobility within the crystalline region. On the other hand, investigations of the cold crystallization during slow heating of the glassy film across Tg is found to give a granular texture made of small crystallites. The crystallites are found to nucleate preferentially near the free surfaces having lower Tg, and to be dominantly edge-on showing a definite tendency to orient their chain axes parallel to the free surface.
通过分子动力学模拟，研究了长链折叠层和玻璃化转变温度 Tg 以上冷结晶的稳态生长的分子机制，该系统由 512 个统一原子 C512 组成的长聚乙烯（PE）样聚合物组成。本文旨在通过对更长链的更真实系统进行非常长的模拟（长达 1 μs），重新考虑我们之前对短 PE 样聚合物 C100 的模拟结果，从而建立链折叠结晶的可靠分子图像，并阐明冷结晶的具体分子过程。我们观察到，链折叠层表现出快速增厚生长，保持明显的锥形生长前沿。尽管在更长的链中生长速度更快，但发现折叠表面主要是相邻重入。对分子途径的详细检查给出了一个有洞察力的图像，可以解释明显矛盾的结果。此外，具有特定空间异质性的折叠结构导致结晶区域内的异质迁移。另一方面，研究发现，在玻璃态薄膜缓慢加热通过 Tg 时的冷结晶会产生由小晶体组成的粒状纹理。发现晶核优先在 Tg 较低的自由表面附近形成，并且主要是边缘取向，表现出将其链轴平行于自由表面的明确趋势。
