Nieto Simavilla D, Ramakrishnan V, Smoukov S K, Venerus D C
Basque Center for Applied Mathematics, 48009 Bilbao, Bizkaia (Basque-Country), Spain.
SABIC, Plasticslaan 1, 4612PX Bergen op Zoom, The Netherlands.
Soft Matter. 2022 Aug 24;18(33):6200-6208. doi: 10.1039/d2sm00759b.
Investigations on the diffusion of small molecules or particles in polymeric materials are important to numerous technologies and can also be used to gain insight on polymer chain dynamics. Systems where the probe size is comparable to (or smaller than) a characteristic length of the polymer chain, the tube diameter for example, are of particular interest because the diffusion coefficient of the probe can be orders of magnitude larger than the value predicted by the Stokes-Einstein relation. In the present study, we employ the optical technique known as forced Rayleigh scattering to study the diffusion of a molecular probe (dye) in several entangled polymer melts over a wide range of length and time scales. The probe size is much smaller than the tube diameter for the systems studied. We find the diffusion coefficient is larger by four to five orders of magnitude than the Stokes-Einstein prediction. More interestingly, we observe anomalous, non-Fickian, diffusion where the value of the measured diffusion coefficient can abruptly change by as much as 50%. We suggest that this unexpected behavior occurs when the time scale for diffusion is larger than the relaxation time associated with the constraint release mechanism for polymer chain dynamics.
小分子或颗粒在聚合物材料中的扩散研究对众多技术而言至关重要，还可用于深入了解聚合物链动力学。当探针尺寸与聚合物链的特征长度相当（或更小）时，例如管直径，这类体系尤为引人关注，因为探针的扩散系数可能比斯托克斯 - 爱因斯坦关系预测的值大几个数量级。在本研究中，我们采用称为受激瑞利散射的光学技术，在广泛的长度和时间尺度范围内，研究分子探针（染料）在几种缠结聚合物熔体中的扩散。对于所研究的体系，探针尺寸远小于管直径。我们发现扩散系数比斯托克斯 - 爱因斯坦预测值大四到五个数量级。更有趣的是，我们观察到反常的、非菲克扩散，即测量的扩散系数值可能会突然变化多达50%。我们认为，当扩散的时间尺度大于与聚合物链动力学的约束释放机制相关的弛豫时间时，就会出现这种意外行为。
