A metastable nematic precursor accelerates polyethylene oligomer crystallization as determined by atomistic simulations and self-consistent field theory.
A metastable nematic precursor accelerates polyethylene oligomer crystallization as determined by atomistic simulations and self-consistent field theory.
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根据原子模拟和自洽场论确定，亚稳态向列前驱体加速聚乙烯低聚物结晶。
DOI：
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发表时间：
2019
影响因子：
4.4
通讯作者：
R. Larson
中科院分区：
文献类型：
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作者：
Wenlin Zhang;R. Larson
采用PYS、TraPPE、OPLS-L和FW力场模型，在450 ~ 600 K温度范围内对正戊烷（C50）的熔体密度ρ、持续长度Np、结晶耦合常数α和结晶动力学进行了原子模拟.耦合常数α来源于棒状Kuhn段的堆积熵，并随ρ和Np的增大而增大。结合自洽场理论，用Np和α预测了聚乙烯（PE）低聚物的各向同性到均质（IN）转变温度随链长的变化。发现该相是亚稳态的，因为IN转变温度低于使用不同力场的模拟中C50的晶体熔化温度。最后，等温模拟PE C50低聚物和C1000聚合物的结晶表明，晶体成核可以大大加速淬火低于IN转变温度，其中链在各向同性状态下首先迅速形成有序域，其中结晶顺序然后增长。我们还发现，PYS，TraPPE和FW模型高估了C50的熔化温度约50 K，而最灵活的OPLS-L模型给出的熔化温度在实验值的10 K左右。虽然给出了更准确的熔融温度，但OPLS-L模型的缓慢结晶动力学可能限制其在PE结晶直接模拟中的应用。
Using PYS, TraPPE, OPLS-L, and Flexible-Williams (FW) force field models, atomistic simulations at temperatures ranging from 450 K to 600 K are performed to predict the melt density ρ, the persistence length Np, the nematic coupling constant α, and crystallization dynamics for pentacontane (C50). The coupling constant α arises from packing entropy of rodlike Kuhn segments and increases with increasing ρ and Np. Together with a self-consistent field theory, Np and α are then used to predict the isotropic-to-nematic (IN) transition temperature for polyethylene (PE) oligomers as a function of chain length. The nematic phase is found to be metastable since the IN transition temperature lies below the crystal melting temperatures for C50 in simulations using different force fields. Finally, isothermal simulations of crystallization for PE C50 oligomers and C1000 polymers show that crystal nucleation may be much accelerated by quenching below the IN transition temperature, where chains in the isotropic state first rapidly form nematic ordered domains, within which crystalline order then grows. We also find that the PYS, TraPPE, and FW models overpredict the melting temperature for C50 by around 50 K, while the most flexible OPLS-L model gives a melting temperature within around 10 K of the experimental value. Although giving a more accurate melting temperature, the slow crystallization kinetics of the OPLS-L model may limit its application in direct simulations of PE crystallization.