Lee Sanghun, Frank Curtis W, Yoon Do Y
Department of Chemistry, Gachon University, Seongnam 13120, Gyunggido, Korea.
Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA.
Polymers (Basel). 2020 May 6;12(5):1059. doi: 10.3390/polym12051059.
Molecular dynamics simulations of free-standing thin films of neat melts of polyethylene (PE) chains up to CH and their binary mixtures with n-CH are performed employing a united atom model. We estimate the surface tension values of PE melts from the atomic virial tensor over a range of temperatures, which are in good agreement with experimental results. Compared with short n-alkane systems, there is an enhanced surface segregation of methyl chain ends in longer PE chains. Moreover, the methyl groups become more segregated in the surface region with decreasing temperature, leading to the conclusion that the surface-segregation of methyl chain ends mainly arises from the enthalpic origin attributed to the lower cohesive energy density of terminal methyl groups. In the mixtures of two different chain lengths, the shorter chains are more likely to be found in the surface region, and this molecular segregation in moderately asymmetric mixtures in the chain length (CH + CH) is dominated by the enthalpic effect of methyl chain ends. Such molecular segregation is further enhanced and dominated by the entropic effect of conformational constraints in the surface for the highly asymmetric mixtures containing long polymer chains (CH + CH). The estimated surface tension values of the mixtures are consistent with the observed molecular segregation characteristics. Despite this molecular segregation, the normalized density of methyl chain ends of the longer chain is more strongly enhanced, as compared with the all-segment density of the longer chain itself, in the surface region of melt mixtures. In addition, the molecular segregation results in higher order parameter of the shorter-chain segments at the surface and deeper persistence of surface-induced segmental order into the film for the longer chains, as compared with those in neat melt films.
采用联合原子模型对高达 CH 的聚乙烯（PE）链纯熔体及其与正己烷（n-CH）的二元混合物的独立薄膜进行了分子动力学模拟。我们通过原子维里张量在一系列温度范围内估算了 PE 熔体的表面张力值，这些值与实验结果吻合良好。与短链正构烷烃体系相比，较长 PE 链中甲基链端的表面偏析增强。此外，随着温度降低，甲基在表面区域的偏析程度更高，这表明甲基链端的表面偏析主要源于焓效应，这是由于末端甲基的内聚能密度较低。在两种不同链长的混合物中，较短的链更易出现在表面区域，并且在链长适度不对称的混合物（CH + C18H38）中这种分子偏析受甲基链端的焓效应主导。对于含有长聚合物链的高度不对称混合物（CH + C60H122），这种分子偏析在表面因构象限制的熵效应而进一步增强并占主导。混合物的估算表面张力值与观察到的分子偏析特征一致。尽管存在这种分子偏析，但在熔体混合物的表面区域，与较长链自身的全链段密度相比，较长链甲基链端的归一化密度增强得更为显著。此外，与纯熔体薄膜相比，分子偏析导致较短链段在表面具有更高的序参数，并且对于较长链，表面诱导的链段有序在薄膜中持续得更深。
