Skountzos Emmanuel N, Karadima Katerina S, Mavrantzas Vlasis G
Department of Chemical Engineering, University of Patras and FORTH-ICE/HT, GR 26504 Patras, Greece.
Particle Technology Laboratory, Department of Mechanical and Process Engineering, ETH Zürich, CH 8092 Zürich, Switzerland.
Polymers (Basel). 2021 Aug 16;13(16):2749. doi: 10.3390/polym13162749.
Detailed molecular dynamics (MD) simulations are employed to study how the presence of adsorbed domains and nanoparticle bridging chains affect the structural, conformational, thermodynamic, and dynamic properties of attractive polymer nanocomposite melts in the semi-dilute regime. As a model system we have chosen an unentangled poly(ethylene glycol) (PEG) matrix containing amorphous spherical silica nanoparticles with different diameters and at different concentrations. Emphasis is placed on properties such as the polymer mass density profile around nanoparticles, the compressibility of the system, the mean squared end-to-end distance of PEG chains, their orientational and diffusive dynamics, the single chain form factor, and the scattering functions. Our analysis reveals a significant impact of the adsorbed, interfacial polymer on the microscopic dynamic and conformational properties of the nanocomposite, especially under conditions favoring higher surface-to-volume ratios (e.g., for small nanoparticle sizes at fixed nanoparticle loading, or for higher silica concentrations). Simultaneously, adsorbed polymer chains adopt graft-like conformations, a feature that allows them to considerably extend away from the nanoparticle surface to form bridges with other nanoparticles. These bridges drive the formation of a nanoparticle network whose strength (number of tie chains per nanoparticle) increases substantially with increasing concentration of the polymer matrix in nanoparticles, or with decreasing nanoparticle size at fixed nanoparticle concentration. The presence of hydroxyl groups at the ends of PEG chains plays a key role in the formation of the network. If hydroxyl groups are substituted by methoxy ones, the simulations reveal that the number of bridging chains per nanoparticle decreases dramatically, thus the network formed is less dense and less strong mechanically, and has a smaller impact on the properties of the nanocomposite. Our simulations predict further that the isothermal compressibility and thermal expansion coefficient of PEG-silica nanocomposites are significantly lower than those of pure PEG, with their values decreasing practically linear with increasing concentration of the nanocomposite in nanoparticles.
采用详细的分子动力学（MD）模拟来研究吸附域和纳米颗粒桥连链的存在如何影响半稀溶液中具有吸引力的聚合物纳米复合熔体的结构、构象、热力学和动力学性质。作为一个模型系统，我们选择了一种未缠结的聚乙二醇（PEG）基质，其中含有不同直径和不同浓度的无定形球形二氧化硅纳米颗粒。重点关注诸如纳米颗粒周围聚合物质量密度分布、系统的压缩性、PEG链的均方末端距、它们的取向和扩散动力学、单链形状因子以及散射函数等性质。我们的分析表明，吸附的界面聚合物对纳米复合材料的微观动力学和构象性质有显著影响，特别是在有利于更高表面积与体积比的条件下（例如，对于固定纳米颗粒负载量下的小纳米颗粒尺寸，或对于更高的二氧化硅浓度）。同时，吸附的聚合物链呈现出接枝状构象，这一特征使它们能够从纳米颗粒表面显著延伸以与其他纳米颗粒形成桥连。这些桥连驱动了纳米颗粒网络的形成，其强度（每个纳米颗粒的连接链数量）随着纳米颗粒中聚合物基质浓度的增加或在固定纳米颗粒浓度下随着纳米颗粒尺寸的减小而显著增加。PEG链末端羟基的存在在网络形成中起关键作用。如果羟基被甲氧基取代，模拟结果表明每个纳米颗粒的桥连链数量会急剧减少，因此形成的网络密度较小且机械强度较低，对纳米复合材料性质的影响也较小。我们的模拟进一步预测，PEG-二氧化硅纳米复合材料的等温压缩性和热膨胀系数显著低于纯PEG，其值实际上随着纳米复合材料在纳米颗粒中浓度的增加而呈线性下降。
