Molecular dynamics modeling of polymer crystallization from the melt
Molecular dynamics modeling of polymer crystallization from the melt
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DOI：
10.1016/j.polymer.2003.04.006
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发表时间：
2004-02
期刊：
影响因子：
4.6
通讯作者：
Takashi Yamamoto
中科院分区：
文献类型：
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作者：
Takashi Yamamoto
用分子动力学模拟方法研究了聚合物结晶的分子途径和结晶-熔体界面的结构。我们采用一个简化的分子模型的聚亚甲基链，链是由CH 2类珠连接的谐波弹簧，最低能量构象是一个线性拉伸序列的珠轻微的弯曲刚度被施加。考虑两个分子系统，一个由640条C100链组成，另一个由64条C1000链组成，两个分子系统都放置在两个平行的基底之间，这两个基底代表相互生长的薄片的生长表面。初始熔体保持在一个足够高的温度高于熔点迅速冷却到各种结晶温度，和随后的结晶的分子过程进行了研究。在这两个系统中，我们清楚地观察到从基板的堆叠链折叠laminates的增长。生长的薄片呈一定的锥形，沿链轴沿着和垂直于链轴的方向都有明显的增厚生长，总的结晶速率对结晶温度非常敏感，在320-330 K附近有一个明显的最大值。我们发现，laminates不增长保持步伐，但在独立的增长率，特别是在较高的温度。我们还研究了横向生长表面的结构，发现生长表面是局部平坦的，晶体生长的Kossel机制似乎是可操作的。此外，发现折叠表面覆盖有相对短的链折叠;至少约60-70%的折叠连接最近或下一个最近的相邻晶茎。在C100和C1000的过冷熔体中没有发现明显的键取向顺序。
Molecular pathways to polymer crystallization and the structures of crystal-melt interfaces are investigated by molecular dynamics simulation. We adopt a simplified molecular model for polymethylene-like chains; the chain is made of CH2-like beads connected by harmonic springs, and the lowest energy conformation is a linear stretched sequence of the beads with slight bending stiffness being imposed. Two molecular systems are considered, one is made of 640 chains of C100 and the other is made of 64 chains of C1000, both being placed between two parallel substrates that represent the growth surfaces of the lamellae growing toward each other. The initial melt kept at a sufficiently high temperature above the melting point is rapidly cooled down to various crystallization temperatures, and the molecular processes of crystallization that follow are investigated. In both systems, we clearly observe the growth of stacked chain-folded lamellae from the substrates. The growing lamellae have a definite tapered shape, and they show marked thickening growth along the chain axis as well as usual growth perpendicular to it. The overall crystallization rate is found to be very sensitive to the crystallization temperature, showing an apparent maximum around 320–330 K for C100. We find that the lamellae do not grow keeping pace with each other but grow in independent rates especially at higher temperatures. We also examine the structures of the lateral growth surfaces and find that the growth surfaces are locally flat and the Kossel mechanism of crystal growth seems to be operative. In addition, the fold surfaces are found to be covered with relatively short chain-folds; at least about 60–70% of the folds are connecting the nearest or the next nearest neighbor crystalline stems. No appreciable bond orientational order is found in the undercooled melt of C100 and C1000.