Swain Aparna, Begam Nafisa, Chandran Sivasurender, Bobji M S, Basu J K
Department of Physics, Indian Institute of Science, Bangalore 560012, India.
Institut Für Angewandte Physik, Universtitat Tübingen, 70276, Tübingen, Germany.
Soft Matter. 2020 Apr 29;16(16):4065-4073. doi: 10.1039/d0sm00019a.
Thin polymer and polymer nanocomposite (PNC) films are being extensively used as advanced functional coating materials in various technological applications. Since it is widely known that various properties of these thin films, especially their thermo-mechanical behavior, can be considerably different from the bulk depending on the thickness as well as interaction with surrounding media, it is imperative to study these properties directly on the films. However, quite often, it becomes difficult to perform these measurements reliably due to a dearth of techniques, especially to measure mechnical or transport properties like the viscosity of thin polymer or PNC films. Here, we demonstrate a new method to study the viscosity of PNC thin films using atomic force microscopy based force-distance spectroscopy. Using this method we investigated viscosity and the glass transition, Tg, of PNC thin films consisting of polymer grafted nanoparticles (PGNPs) embedded in un-entangled homopolymer melt films. The PGNP-polymer interfacial entropic interaction parameter, f, operationally controlled through the ratio of grafted and matrix molecular weight, was systematically tuned while maintaining good dispersion even at very high PGNP loadings, φ. We observed both a significant reduction (low f) and giant enhancement (high f) in the viscosity of the PNC thin films with the effect becoming more prominent with increasing φ. Significantly, none of the established theoretical models for viscosity changes observed earlier in suspensions or polymer nanocomposites can explain the observed viscosity variation. Our results thus not only demonstrate the tunability of the interfacial entropic effect to facilitate a dramatic change in the viscosity of PNC coatings, which could be of great utility in various applications of these materials, but also suggest a new regime of viscosity variation in athermal PNC films indicating the possible need for a new theoretical model.
薄聚合物和聚合物纳米复合材料（PNC）薄膜作为先进的功能涂层材料，正广泛应用于各种技术领域。众所周知，这些薄膜的各种性能，尤其是它们的热机械行为，可能会因薄膜厚度以及与周围介质的相互作用而与本体材料有很大不同，因此直接研究这些薄膜的性能至关重要。然而，由于缺乏相关技术，尤其是用于测量薄聚合物或PNC薄膜的机械性能或传输性能（如粘度）的技术，往往很难可靠地进行这些测量。在此，我们展示了一种基于原子力显微镜力-距离光谱法研究PNC薄膜粘度的新方法。利用这种方法，我们研究了由嵌入非缠结均聚物熔体薄膜中的聚合物接枝纳米颗粒（PGNP）组成的PNC薄膜的粘度和玻璃化转变温度Tg。通过接枝和基体分子量的比例可操作地控制PGNP-聚合物界面熵相互作用参数f，即使在非常高的PGNP负载量φ下，也能在保持良好分散性的同时系统地调整该参数。我们观察到PNC薄膜的粘度显著降低（低f）和大幅增强（高f），且随着φ的增加，这种效应变得更加明显。值得注意的是，之前在悬浮液或聚合物纳米复合材料中观察到的粘度变化的既定理论模型都无法解释所观察到的粘度变化。因此，我们的结果不仅证明了界面熵效应的可调性，有助于显著改变PNC涂层的粘度，这在这些材料的各种应用中可能非常有用，而且还表明了无热PNC薄膜中粘度变化的新机制，这表明可能需要一个新的理论模型。
