Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
J Chem Phys. 2023 Jul 7;159(1). doi: 10.1063/5.0148247.
The diffusion of small molecular penetrants through polymeric materials represents an important fundamental problem, relevant to the design of materials for applications such as coatings and membranes. Polymer networks hold promise in these applications because dramatic differences in molecular diffusion can result from subtle changes in the network structure. In this paper, we use molecular simulation to understand the role that cross-linked network polymers have in governing the molecular motion of penetrants. By considering the local, activated alpha relaxation time of the penetrant and its long-time diffusive dynamics, we can determine the relative importance of activated glassy dynamics on penetrants at the segmental scale vs entropic mesh confinement on penetrant diffusion. We vary several parameters, such as the cross-linking density, temperature, and penetrant size, to show that cross-links primarily affect molecular diffusion through the modification of the matrix glass transition, with local penetrant hopping at least partially coupled to the segmental relaxation of the polymer network. This coupling is very sensitive to the local activated segmental dynamics of the surrounding matrix, and we also show that penetrant transport is affected by dynamic heterogeneity at low temperatures. To contrast, only at high temperatures and for large penetrants or when the dynamic heterogeneity effect is weak, does the effect of mesh confinement become significant, even though penetrant diffusion more broadly empirically follows similar trends as established models of mesh confinement-based transport.
小分子渗透物通过聚合物材料的扩散是一个重要的基本问题，与涂层和膜等应用材料的设计相关。聚合物网络在这些应用中具有前景，因为网络结构的微小变化可以导致分子扩散的显著差异。在本文中，我们使用分子模拟来理解交联网络聚合物在控制渗透物分子运动中的作用。通过考虑渗透物的局部、活化的 α 松弛时间及其长时间扩散动力学，我们可以确定在介观尺度上，活化玻璃态动力学对渗透物的相对重要性与渗透物扩散的熵网格限制。我们改变了几个参数，如交联密度、温度和渗透物尺寸，以表明交联主要通过改变基质玻璃化转变来影响分子扩散，而局部渗透物跳跃至少部分与聚合物网络的链段松弛相耦合。这种耦合对周围基质的局部活化链段动力学非常敏感，我们还表明，在低温下，渗透物输运受到动态异质性的影响。相比之下，只有在高温下、对于大的渗透物或当动态异质性效应较弱时，网格限制的效应才变得显著，尽管渗透物扩散更广泛地遵循与网格限制基于传输的经验模型类似的趋势。
