Anwar Muhammad, Graham Richard S
School of Mathematical Sciences, University of Nottingham, Nottingham, NG9 4DP, UK.
Soft Matter. 2021 Mar 18;17(10):2872-2882. doi: 10.1039/d0sm01361g.
Modelling of flow-induced nucleation in polymers suggest that long chains are enriched in nuclei, relative to their melt concentration. This enrichment has important consequences for the nucleation rate and mechanism, but cannot be directly observed with current experimental techniques. Instead, we ran united atom molecular dynamics simulations of bimodal polyethylene blends, comprising linear chains at a 50 : 50 mix of long (1000 carbon) and short (500-125 carbon) chains, under shear flow. We developed a method to extract the nucleus composition during a transient start-up flow. Our simulations show significant and systematic enrichment of long-chains for all nucleus sizes up to and beyond the critical nucleus. This enrichment is quantitatively predicted by the recent polySTRAND model [Read et al. Phys. Rev. Lett. 2020, 124, 147802]. The same model parameters also correctly capture the nucleus induction time in our simulations. All parameters of the model were fitted to a small subset of our data in which long chain enhancement was absent. We conclude that long-chain enrichment is central to the mechanism of flow-induced nucleation and that this enrichment must be captured to correctly predict the nucleation rate.
聚合物中流动诱导成核的模型表明，相对于其熔体浓度，长链在晶核中更为富集。这种富集对成核速率和机制具有重要影响，但目前的实验技术无法直接观察到。相反，我们对双峰聚乙烯共混物进行了联合原子分子动力学模拟，该共混物由长链（1000个碳原子）和短链（500 - 125个碳原子）以50 : 50的比例混合而成的线性链组成，并处于剪切流之下。我们开发了一种方法来提取瞬态启动流过程中的晶核组成。我们的模拟结果表明，对于所有直至并超过临界晶核大小的晶核，长链都有显著且系统的富集。这种富集由最近的polySTRAND模型[Read等人，《物理评论快报》，2020年，第124卷，第147802页]进行了定量预测。相同的模型参数在我们的模拟中也正确地捕捉到了晶核诱导时间。该模型的所有参数都拟合到了我们数据中的一个小子集，在这个子集中不存在长链增强现象。我们得出结论，长链富集是流动诱导成核机制的核心，并且必须捕捉这种富集才能正确预测成核速率。
