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令人着迷但又非常困难的研究课题。为了更好地理解螺旋聚合物的结晶过程，我们用分子模拟的方法进行了研究。我们采用了将螺旋聚合物分为两种不同类型的策略：一种是简单的裸螺旋，本质上只由主干原子基团组成，具有更平滑的分子轮廓；另一种是更一般的螺旋，具有较大的侧基，这将大大阻碍分子的运动和结晶。(1)我们已经发现，裸螺旋在自由三维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，手性识别的概念是模糊的。我们在这里研究了低聚螺旋聚合物，以便结合MC和MD模拟来建立裸螺旋聚合物中支链结晶的分子机制。我们发现，初始成核过程并没有表现出明显的手性选择，但在后期可以通过所谓的Ostwald成熟化过程的逐步有序残留观察到手性晶体的生长。(2)另一方面，具有大侧基的真实等规聚丙烯模型的结晶速度很慢，这使得直接在三维空间进行分子动力学模拟非常耗时。在此，我们采用MC方法和刚性链模型对低聚iPP进行了研究。我们发现，在适当的冷却条件下，最稳定的a-型晶体是通过形核和长大产生的，而通过快速淬火，我们观察到一个无序的结晶相，它具有类似a-形的局部堆积，但整体堆积是六方对称的，这很好地符合所谓的iPP的近晶相。

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.