Department of Chemical Engineering, Columbia University, New York, New York 10027, USA.
Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29201, USA.
Phys Rev Lett. 2019 Oct 11;123(15):158003. doi: 10.1103/PhysRevLett.123.158003.
The tracer diffusion coefficient of six different permanent gases in polymer-grafted nanoparticle (GNP) membranes, i.e., neat GNP constructs with no solvent, show a maximum as a function of the grafted chain length at fixed grafting density. This trend is reproduced for two different NP sizes and three different polymer chemistries. We postulate that nonmonotonic changes in local, segmental friction as a function of graft chain length (at fixed grafting density) must underpin these effects, and use quasielastic neutron scattering to probe the self-motions of polymer chains at the relevant segmental scale (i.e., sampling local friction or viscosity). These data, when interpreted with a jump diffusion model, show that, in addition to the speeding-up in local chain dynamics, the elementary distance over which segments hop is strongly dependent on graft chain length. We therefore conclude that transport modifications in these GNP layers, which are underpinned by a structural transition from a concentrated brush to semidilute polymer brush, are a consequence of both spatial and temporal changes, both of which are likely driven by the lower polymer densities of the GNPs relative to the neat polymer.
六种不同永久气体在接枝纳米颗粒（GNP）膜中的示踪扩散系数，即在无溶剂的纯 GNP 结构中，在固定接枝密度下随接枝链长度呈最大值。这种趋势在两种不同的 NP 尺寸和三种不同的聚合物化学中得到了重现。我们假设，局部、链段摩擦随接枝链长度（在固定接枝密度下）的非单调变化必须是这些效应的基础，并使用准弹性中子散射来探测相关链段尺度上聚合物链的自运动（即，采样局部摩擦或粘度）。这些数据，用跳跃扩散模型解释时，表明除了局部链动力学的加速外，片段跳跃的基本距离也强烈依赖于接枝链长度。因此，我们得出结论，这些 GNP 层中的传输修饰是由从浓刷到半浓聚合物刷的结构转变所支撑的，这是空间和时间变化的结果，这两者都可能是由于与纯聚合物相比 GNP 的聚合物密度较低所驱动的。
