Kozuch Daniel J, Zhang Wenlin, Milner Scott T
Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802, USA.
Polymers (Basel). 2016 Jun 22;8(6):241. doi: 10.3390/polym8060241.
The Flory⁻Huggins χ parameter describes the excess free energy of mixing and governs phase behavior for polymer blends and block copolymers. For chemically-distinct nonpolar polymers, the value of χ is dominated by the mismatch in cohesive energy densities of the monomers. For blends of chemically-similar polymers, the entropic portion of χ, arising from non-ideal local packing, becomes more significant. Using polymer field theory, Fredrickson et al. predicted that a difference in backbone stiffness can result in a positive χ for chains consisting of chemically-identical monomers. To quantitatively investigate this phenomenon, we perform molecular dynamic (MD) simulations for bead-spring chains, which differ only in stiffness. From the simulations, we apply a novel thermodynamic integration to extract χ as low as 10 - 4 per monomer for blends with stiffness mismatch. To compare with experiments, we introduce a standardized effective monomer to map real polymers onto our bead-spring chains. The predicted χ agrees well with experimental values for a wide variety of pairs of chemically-similar polymers.
弗洛里－哈金斯χ参数描述了混合超额自由能，并决定了聚合物共混物和嵌段共聚物的相行为。对于化学性质不同的非极性聚合物，χ的值主要由单体内聚能密度的不匹配决定。对于化学性质相似的聚合物共混物，由非理想局部堆积引起的χ的熵部分变得更加显著。弗雷德里克森等人利用聚合物场论预测，主链刚性的差异会导致由化学性质相同的单体组成的链产生正的χ。为了定量研究这一现象，我们对仅在刚性上有所不同的珠簧链进行了分子动力学（MD）模拟。通过模拟，我们应用一种新颖的热力学积分方法，对于具有刚性不匹配的共混物，提取出低至每单体10⁻⁴的χ。为了与实验进行比较，我们引入了一个标准化的有效单体，将实际聚合物映射到我们的珠簧链上。对于各种化学性质相似的聚合物对，预测的χ与实验值吻合得很好。
