Hall Lisa M, Schweizer Kenneth S
Department of Chemical & Biomolecular Engineering, University of Illinois, 1304 West Green Street, Urbana, Illinois 61801, USA.
J Chem Phys. 2008 Jun 21;128(23):234901. doi: 10.1063/1.2938379.
Liquid state theory is employed to study phase transitions and structure of dense mixtures of hard nanoparticles and flexible chains (polymer nanocomposites). Calculations are performed for the first time over the entire compositional range from the polymer melt to the hard sphere fluid. The focus is on polymers that adsorb on nanoparticles. Many body correlation effects are fully accounted for in the determination of the spinodal phase separation instabilities. The nanoparticle volume fraction at demixing is determined as a function of interfacial cohesion strength (or inverse temperature) for several interaction ranges and nanoparticle sizes. Both upper and lower critical temperature demixing transitions are predicted, separated by a miscibility window. The phase diagrams are highly asymmetric, with the entropic depletion-like lower critical temperature occurring at a nanoparticle volume fraction of approximately 10%, and a bridging-induced upper critical temperature at approximately 95% filler loading. The phase boundaries are sensitive to both the spatial range of interfacial cohesion and nanoparticle size. Nonmonotonic variations of the bridging (polymer-particle complex formation) demixing boundary on attraction range are predicted. Moreover, phase separation due to many body bridging effects occurs for systems that are fully stable at a second order virial level. Real and Fourier space pair correlations are examined over the entire volume fraction regime with an emphasis on identifying strong correlation effects. Special attention is paid to the structure near phase separation and the minimum in the potential of mean force as the demixing boundaries are approached. The possibility that nonequilibrium kinetic gelation or nanoparticle cluster formation preempts equilibrium phase separation is discussed.
采用液态理论研究硬纳米颗粒与柔性链（聚合物纳米复合材料）的致密混合物的相变和结构。首次在从聚合物熔体到硬球流体的整个组成范围内进行计算。重点是吸附在纳米颗粒上的聚合物。在确定旋节线相分离不稳定性时充分考虑了多体相关效应。对于几种相互作用范围和纳米颗粒尺寸，确定了 demixing 时的纳米颗粒体积分数作为界面内聚强度（或逆温度）的函数。预测了上临界温度和下临界温度 demixing 转变，它们由一个混溶窗分隔。相图高度不对称，熵耗尽型下临界温度出现在纳米颗粒体积分数约为 10%处，而桥联诱导的上临界温度出现在填料负载约为 95%处。相界对界面内聚的空间范围和纳米颗粒尺寸都很敏感。预测了桥联（聚合物 - 颗粒复合物形成）demixing 边界在吸引范围内的非单调变化。此外，对于在二级维里水平完全稳定的系统，会发生由于多体桥联效应导致的相分离。在整个体积分数范围内研究了实空间和傅里叶空间对关联，重点是识别强关联效应。特别关注相分离附近的结构以及接近 demixing 边界时平均力势中的最小值。讨论了非平衡动力学凝胶化或纳米颗粒团簇形成抢先于平衡相分离的可能性。
