Cai Tao, Ma Yu, Yin Pengcheng, Hu Wenbing
Department of Polymer Science and Engineering, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, 210093 Nanjing, China.
J Phys Chem B. 2008 Jun 26;112(25):7370-6. doi: 10.1021/jp800500g. Epub 2008 May 29.
Polymer materials often contain a polydispersity of molecular lengths. We studied the linear growth rates of polymer lamellar crystals in the binary mixtures of different chain lengths by means of dynamic Monte Carlo simulations. Both chain lengths were chosen large enough to perform chain folding upon crystal growth but not very large to avoid the effect of chain entanglement in the bulk phase. We found that the crystal growth rates exhibit a linear dependence upon the compositions of mixtures. This linear relation implies that the overall crystal growth rates are integrated by the separate contributions of variable-length single polymers, supporting the model of intramolecular crystal nucleation. In each event of crystal growth of single polymers, long chains yield more crystallinity than short chains. This high efficiency explains higher crystal growth rates of long chains than that of short chains, and the explanation is quite different from the traditional view on the basis of their different melting points. In addition, with a partial release of sliding diffusion for crystal thickening, a new dependence of crystal growth rates occurs near the dilute end of long-chain compositions at high temperatures, which can be attributed to the preference of integer-number chain folding at the crystal growth front. The preferred fold lengths may vary with chain lengths and thus influence the crystal growth rates.
聚合物材料通常包含分子长度的多分散性。我们通过动态蒙特卡罗模拟研究了不同链长的二元混合物中聚合物片晶的线性生长速率。两种链长都选择得足够大，以便在晶体生长时进行链折叠，但又不是非常大，以避免本体相中链缠结的影响。我们发现晶体生长速率对混合物的组成呈线性依赖关系。这种线性关系意味着总体晶体生长速率是由可变长度的单链聚合物的单独贡献积分而成的，这支持了分子内晶体成核模型。在单链聚合物的每次晶体生长过程中，长链比短链产生更多的结晶度。这种高效率解释了长链比短链更高的晶体生长速率，并且这种解释与基于它们不同熔点的传统观点有很大不同。此外，随着晶体增厚过程中滑动扩散的部分释放，在高温下长链组成的稀端附近出现了晶体生长速率的新依赖关系，这可归因于晶体生长前沿整数链折叠的偏好。优选的折叠长度可能随链长而变化，从而影响晶体生长速率。
