A model for chain folding in polymer crystals: rough growth faces are consistent with the observed growth rates
A model for chain folding in polymer crystals: rough growth faces are consistent with the observed growth rates
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DOI：
10.1016/0032-3861(84)90108-3
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发表时间：
1984-10
期刊：
影响因子：
4.6
通讯作者：
D. Sadler;G. Gilmer
中科院分区：
文献类型：
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作者：
D. Sadler;G. Gilmer
假设聚合物晶体的生长表面可以是分子粗糙的，已被测试使用设计来预测片层厚度和晶体生长速率的模型。用于原子生长单元的结晶的蒙特卡罗方法已经被修改以通过包括用于向晶体添加单元的两种类型的限制来考虑沿着链的连通性。首先，不能将新单元添加到已建立的折叠表面。其次，在生长表面处产生的环的效果通过沿晶体茎沿着随机产生的“钉扎”点来模拟，对应于从生长面上的其他地方附着的链生长的茎。不允许沿着结晶茎中断。结果中的趋势不依赖于限制的精确性质，并且与实验结果一致。晶体厚度在小的过冷度下与过冷度大致成反比，然后趋于一个大致恒定的“平台”（没有“灾难性”的增加）。它们的大小比实验值小，因为高折叠能量不包括在内。生长速率服从方程推导二次成核，虽然成核肯定是不存在的。这种解释与晶体整体发展所需的高自由能（低熵）状态有关。低熵依赖于晶体厚度，因为在二次成核的高焓状态。这些结果解决了这样的问题，即层状结构和生长速率曲线的存在都是在分子光滑生长表面的假设下解释的，而聚合物晶体通常是弯曲的。
The hypothesis that growth surfaces of polymer crystals can be molecularly rough has been tested using models designed to predict lamellar thicknesses and crystal growth rates. Monte Carlo methods used for crystallization of atomic growth units have been adapted to take account of the connectedness along the chains by including two types of restrictions for adding units to the crystal. Firstly new units cannot be added to an established fold surface. Secondly the effect of loops created at the growth surface is simulated by ‘pinning’ points generated at random along the crystalline stem, corresponding to stems growing from chains which are attached elsewhere on the growing face. No interruptions are allowed along crystalline stems. The trends in the results do not depend on the precise nature of the restrictions, and agree with the experimental ones. Crystal thick nesses vary approximately inversely with supercooling at small supercooling, and then level off to an approximately constant ‘plateau’ (no ‘catastrophic’ increase). They are smaller in magnitude than experimental ones since high fold energies are not included. The growth rates obey equations derived for secondary nucleation, though nucleation is certainly not present. The explanation is related to the high free energy (low entropy) states that are required for overall advance of the crystal. The low entropy depends on the crystal thickness as does the high enthalpy states in secondary nucleation. These results resolve the problem that both the existence of lamellae and the growth rate curves were explained on the assumption of molecularly smooth growth surfaces, whereas polymer crystals are often curved.