A Nimmi Das, Begam Nafisa, Ibrahim Mohd, Chandran Sivasurender, Padmanabhan Venkat, Sprung Michael, Basu J K
Department of Physics, Indian Institute of Science, Bangalore, 560012, India.
Nanoscale. 2019 Apr 25;11(17):8546-8553. doi: 10.1039/c8nr10362c.
Viscosity and fragility are key parameters determining the processability and thermo-mechanical stability of glassy polymers and polymer nanocomposites (PNCs). In confined polymers, these parameters are largely dominated by the long relaxation times of the polymers adsorbed at the substrate-polymer interface. On the other hand, for polymer nanocomposites, the interface layer (IL) between the nanoparticles and the surrounding matrix chains often control not only the morphology and dispersion but also various parameters like viscosity and glass transition temperature. Confined PNCs, hence, present a unique opportunity to study the interplay of these two independent interfacial effects. Here, we report the results of X-ray scattering based dynamics measurements of PNC thin films, with a two IL width, unraveling the subtle interplay of these two interfaces on the measured viscosity and fragility. Coupled with coarse-grained molecular dynamics (MD) simulations, our experimental results demonstrate that the viscosity of the PNC films increases with both the IL width and the thickness of the polymer layer adsorbed at the substrate interface. However, while both pristine PS and PNCs with a higher IL width become stronger glasses, as estimated by their fragility, the PNC with a lower IL width shows an increase in fragility with increasing confinement. Our results suggest a novel method to control thermo-mechanical properties and stability of PNC coatings by independently controlling the two interfacial effects in athermal glassy PNCs.
粘度和脆性是决定玻璃态聚合物及聚合物纳米复合材料（PNCs）加工性能和热机械稳定性的关键参数。在受限聚合物中，这些参数很大程度上由吸附在基底 - 聚合物界面的聚合物的长弛豫时间主导。另一方面，对于聚合物纳米复合材料，纳米颗粒与周围基体链之间的界面层（IL）通常不仅控制形态和分散性，还控制诸如粘度和玻璃化转变温度等各种参数。因此，受限的PNCs提供了一个独特的机会来研究这两种独立界面效应的相互作用。在此，我们报告了基于X射线散射的PNC薄膜动力学测量结果，该薄膜具有两种IL宽度，揭示了这两种界面在测量的粘度和脆性上的微妙相互作用。结合粗粒化分子动力学（MD）模拟，我们的实验结果表明，PNC薄膜的粘度随IL宽度和吸附在基底界面的聚合物层厚度的增加而增加。然而，虽然原始PS和具有较高IL宽度的PNCs都变成了更强的玻璃态，根据它们的脆性估计，但具有较低IL宽度的PNC随着受限程度的增加脆性增加。我们的结果提出了一种通过独立控制非热玻璃态PNCs中的两种界面效应来控制PNC涂层热机械性能和稳定性的新方法。
