Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India.
J Phys Chem B. 2013 Jun 27;117(25):7771-82. doi: 10.1021/jp401704e. Epub 2013 Jun 18.
Studies on diffusion dynamics of single molecules (SMs) have been useful in revealing inhomogeneity of polymer thin films near and above the glass-transition temperature (T(g)). However, despite several applications of polymer thin films where exposure to solvent (or vapor) is common, the effect of absorbed solvent molecules on local morphology and rigidity of polymer matrices is yet to be explored in detail. High-T(g) hydrophilic polymers such as poly(vinylpyrrolidone) (PVP) are used as pharmaceutical coatings for drug release in aqueous medium, as they readily absorb moisture, which results in effective lowering of the T(g) and thereby leads to plasticization. The effect of moisture absorption on swelling and softening of PVP thin films was investigated by visualizing the diffusion dynamics of rhodamine 6G (Rh6G) tracer molecules at various ambient relative humidities (RH). Wide-field epifluorescence microscopy, in conjunction with high-resolution SM tracking, was used to monitor the spatiotemporal evolution of individual tracers under varied moisture contents of the matrix. In the absence of atmospheric moisture, Rh6G molecules in dry PVP films are translationally inactive, suggestive of rigid local environments. Under low moisture contents (RH 30-50%), translational mobility remains arrested but rotational motion is augmented, indicating slight swelling of the polymer network which marks the onset of plasticization. The translational mobility of Rh6G was found to be triggered only at a threshold ambient RH, beyond which a large proportion of tracers exhibit extensive diffusion dynamics. Interestingly, SM tracking data at higher moisture contents of the film (RH ≥ 60%) reveal that the distributions of dynamic parameters (such as diffusivity) are remarkably broad, spanning several orders of magnitude. Furthermore, Rh6G molecules display a wide variety of translational motion even at a fixed ambient RH, clearly pointing out the extremely inhomogeneous environment of plasticized PVP network. Intriguingly, it is observed that a majority of tracers undergo anomalous subdiffusion even under high moisture contents of the matrix. Analyses of SM trajectories using velocity autocorrelation function reveal that subdiffusive behaviors of Rh6G are likely to originate from fractional Brownian motion, a signature of tracer dynamics in viscoelastic medium.
对单分子（SMs）扩散动力学的研究对于揭示聚合物薄膜在玻璃化转变温度（Tg）附近和以上的非均质性非常有用。然而，尽管聚合物薄膜在许多应用中都需要暴露在溶剂（或蒸汽）中，但吸收的溶剂分子对聚合物基质的局部形态和刚性的影响仍有待详细研究。高 Tg 亲水性聚合物，如聚乙烯吡咯烷酮（PVP），被用作药物在水介质中的释放涂层，因为它们很容易吸收水分，这有效地降低了 Tg，从而导致塑化。通过在不同环境相对湿度（RH）下可视化罗丹明 6G（Rh6G）示踪分子的扩散动力学，研究了水分吸收对 PVP 薄膜溶胀和软化的影响。宽场荧光显微镜结合高分辨率 SM 跟踪，用于监测基质不同水分含量下单个示踪剂的时空演化。在没有大气水分的情况下，干燥 PVP 薄膜中的 Rh6G 分子没有平动，这表明局部环境僵硬。在低水分含量（RH 30-50%）下，平动迁移率仍然被阻止，但旋转运动增强，表明聚合物网络稍有溶胀，标志着塑化的开始。只有在环境 RH 超过阈值时，Rh6G 的平动迁移率才会被触发，超过此阈值，很大一部分示踪剂会表现出广泛的扩散动力学。有趣的是，在膜的较高水分含量（RH≥60%）下的 SM 跟踪数据显示，动态参数（如扩散率）的分布非常广泛，跨越几个数量级。此外，即使在固定的环境 RH 下，Rh6G 分子也表现出多种平动运动，这清楚地指出了塑化 PVP 网络的极端非均相环境。有趣的是，观察到大多数示踪剂即使在基质的高水分含量下也会经历异常的亚扩散。使用速度自相关函数对 SM 轨迹进行分析表明，Rh6G 的亚扩散行为可能源自分数布朗运动，这是粘弹性介质中示踪剂动力学的特征。
