Computer simulation of molecular recognition in self organization of helical polymers

螺旋聚合物结晶的分子机理是一个有趣而又困难的研究课题。我们已经作出努力，更好地了解在螺旋聚合物的结晶通过使用分子模拟方法。我们采用了一种策略，将螺旋聚合物分为两种不同的类型;一种是简单的裸螺旋，其基本上仅由骨架原子团组成，并且具有更光滑的分子轮廓，另一种是具有大的侧基的更一般的螺旋，这将大大阻碍分子运动和结晶。(1)我们已经发现裸露的螺旋在自由3D空间或晶体衬底上快速结晶。然而，在这种快速结晶的聚合物中，手性的概念是模糊的。我们在这里研究了低聚螺旋聚合物，以建立在裸螺旋聚合物结合MC和MD模拟手性结晶的分子机制。发现成核初期没有明显的手性选择，但在成核后期，可以观察到手性晶体的生长是通过逐步有序进行的，类似于Ostwald熟化过程。(2)另一方面，在现实的全同立构聚丙烯模型具有大的侧基的结晶被证明是缓慢的，这使得在3D空间中的直接MD模拟非常耗时。我们在这里采用了MC方法和刚性链模型的低聚聚丙烯。我们发现，最稳定的α-型晶体通过成核和适度冷却生长而出现，而通过非常快速的淬火，我们观察到具有α-型局部链堆积但具有六方对称的整体链堆积的无序结晶相，这与iPP的所谓近晶相很好地一致。

The molecular mechanism of crystallization in helical polymers is a fascinating but very difficult subject of research. We have made efforts toward better understanding of the crystallization in helical polymers by use of molecular simulation methods. We have adopted a strategy of categorizing the helical polymers into two distinct types ; one is a simple bare helix which is essentially made of backbone atomic groups only and has smoother molecular contours, and the other is a more general helix having large side groups that would considerably hamper molecular motion and crystallization.(1) We already found that the bare helix rapidly crystallize in free 3D-space or on the crystalline substrate. However, the chirality recognotion was obscure in this rapidly crystallizing polymer. We have here investigated oligomeric helical polymers in order to establish the molecular mechanism of chirail crystallization in bare helical polymers combining both the MC and MD simulations. We have found that the initilal nucleation process doest not show marked chiraility selection, but at the late stage can be observe the growth of chirail crystals through stepwise ordering remniscent of the so called Ostwald Ripening Process.(2) On the other hand, crystallization in realistic isotactic polypropylene model having large pendant groups was shown to be sluggish which renders the direct MD simulation in 3D-space extremely time consuming. We have here adopted a MC approach and the rigid chain model for the oligomeric iPP. We have found that the most stable a-form crystasl emerge through nucleation and growth by moderate cooling, while by very quick quench we have observed a disordered crystalline phase having a -form-like local chain packing but with overall chain packing of hexagonal symmetry, which conforms well to the so called smectic phase of iPP.