Zhou Yuxing, Schweizer Kenneth S
Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois 61801, USA.
J Chem Phys. 2020 Sep 21;153(11):114901. doi: 10.1063/5.0021954.
We present an integrated theoretical study of the structure, thermodynamic properties, dynamic localization, and glassy shear modulus of melt polymer nanocomposites (PNCs) that spans the three microstructural regimes of entropic depletion induced nanoparticle (NP) clustering, discrete adsorbed layer driven NP dispersion, and polymer-mediated bridging network. The evolution of equilibrium and dynamic properties with NP loading, total packing fraction, and strength of interfacial attraction is systematically studied based on a minimalist model. Structural predictions of polymer reference interaction site model integral equation theory are employed to establish the rich behavior of the interfacial cohesive force density, surface excess, and a measure of free volume as a function of PNC variables. The glassy dynamic shear modulus is predicted to be softened, reinforced, or hardly changed relative to the pure polymer melt depending on system parameters, as a result of the competing and qualitatively different influences of interfacial cohesion (physical bonding), free volume, and entropic depletion on dynamic localization and shear elasticity. The localization of polymer segments is the dominant factor in determining bulk PNC softening and reinforcement effects for moderate to strong interfacial attractions, respectively. While in the athermal entropy-dominated regime, the primary origin of mechanical reinforcement is the stress stored in the aggregated NP subsystem. The PNC shear modulus is often qualitatively correlated with the segment localization length but with notable exceptions. The present work provides the foundation for developing a theory of segmental relaxation, T changes, and collective NP dynamics in PNCs based on a self-consistent treatment of the cooperative activated motions of segments and NPs.
我们对熔融聚合物纳米复合材料（PNC）的结构、热力学性质、动态局域化和玻璃态剪切模量进行了综合理论研究，该研究涵盖了由熵耗引起的纳米颗粒（NP）聚集、离散吸附层驱动的NP分散以及聚合物介导的桥接网络这三种微观结构状态。基于一个极简模型，系统地研究了平衡和动态性质随NP负载量、总堆积分数以及界面吸引力强度的演变。采用聚合物参考相互作用位点模型积分方程理论的结构预测来确定界面内聚能密度、表面超额以及作为PNC变量函数的自由体积量度的丰富行为。由于界面内聚（物理键合）、自由体积以及熵耗对动态局域化和剪切弹性的竞争且性质不同的影响，预测玻璃态动态剪切模量相对于纯聚合物熔体而言，根据系统参数会变软、增强或几乎不变。对于中等至强的界面吸引力，聚合物链段的局域化分别是决定本体PNC软化和增强效应的主导因素。而在无热熵主导的状态下，机械增强的主要来源是聚集的NP子系统中储存的应力。PNC剪切模量通常与链段局域化长度定性相关，但也有显著例外。本工作为基于对链段和NP协同活化运动的自洽处理，开发PNC中链段弛豫、T变化和集体NP动力学的理论奠定了基础。
