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很有吸引力但又很困难的研究课题。利用分子模拟的方法对螺旋聚合物的结晶过程进行了深入的研究。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，基本上只由主原子基组成，分子轮廓更光滑；另一种是更一般的螺旋，有很大的侧基，会极大地阻碍分子运动和结晶。(1)我们已经发现，裸螺旋在自由3d空间或晶体基板上迅速结晶。然而，在这种快速结晶的聚合物中，手性识别是模糊的。本文对低聚螺旋聚合物进行了研究，目的是结合MC和MD模拟，建立裸螺旋聚合物中手链结晶的分子机制。我们发现，初始成核过程没有明显的手性选择，但在后期可以观察到手性晶体的逐步有序生长，类似于所谓的奥斯特瓦尔德成熟过程。(2)另一方面，在具有大垂基团的真实等规聚丙烯模型中，结晶速度缓慢，这使得在三维空间中直接进行MD模拟非常耗时。我们在这里采用了MC方法和刚性链模型对寡聚物iPP。我们发现，在中等冷却条件下，最稳定的a型晶体是通过成核和生长形成的，而在非常快的淬火条件下，我们观察到一种无序的结晶相，具有类似于-型的局部链状排列，但整体链状排列为六方对称，这与iPP的所谓近晶相非常符合。

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.