Morozov Konstantin I, Köhler Werner
Department of Chemical Engineering, Technion─Israel Institute of Technology, Haifa 32000, Israel.
Physikalisches Institut, Universität Bayreuth, D-95440 Bayreuth, Germany.
Langmuir. 2025 Jun 10;41(22):14220-14228. doi: 10.1021/acs.langmuir.5c01186. Epub 2025 May 30.
Thermodiffusion of entangled molecules in an inhomogeneous temperature field is determined by their length. With increasing length, the thermophoretic velocity increases in magnitude and sometimes changes its direction, depending on the nature of the polymer and the solvent. Thus, the theoretical description of thermodiffusion, already a multifactorial phenomenon, is complicated by the appearance of another important property. Here, we generalize to chained molecules an approach recently proposed for molecular systems and calculate the thermodiffusion coefficients of polymer molecules, starting from oligomers up to the high polymer limit. The calculations were performed for two types of chained molecules─polystyrene (PS) and alkanes─dissolved in one of three nonpolar solvents─toluene, ethylbenzene, or cyclohexane. For both types of chain molecules, the thermodiffusion coefficient saturates with length, but with different scaling exponents. The predicted values of of PS in toluene and ethylbenzene are in excellent agreement with experimental data over the entire range of chain lengths from monomer to high polymer. In particular, the plateau value of the product η (where η is a solvent viscosity) proves to be close to the experimentally observed universal value ≈ 6 · 10 N/K. The theoretical dependencies for of alkanes in the same solvents also agree well with the data, although they slightly overestimate it. More significant deviations of the predictions from the measurements occur when cyclohexane is used as a solvent. This behavior is similar to that found earlier for molecular mixtures. In all solvents studied, alkane molecules manifest the negative values of the thermodiffusion coefficients and migrate to the hotter layers.
在非均匀温度场中，纠缠分子的热扩散取决于它们的长度。随着长度增加，热泳速度在大小上会增加，有时还会改变方向，这取决于聚合物和溶剂的性质。因此，热扩散这一原本就具有多因素性质的现象，由于另一个重要性质的出现而变得更加复杂。在此，我们将最近针对分子系统提出的一种方法推广到链状分子，并计算聚合物分子的热扩散系数，范围从低聚物到高聚物极限。计算针对两种类型的链状分子——聚苯乙烯（PS）和烷烃——溶解在三种非极性溶剂之一——甲苯、乙苯或环己烷——中进行。对于这两种类型的链状分子，热扩散系数随长度饱和，但具有不同的标度指数。聚苯乙烯在甲苯和乙苯中的预测值在从单体到高聚物的整个链长范围内与实验数据高度吻合。特别是，乘积η（其中η是溶剂粘度）的平稳值被证明接近实验观察到的通用值≈6·10 N/K。烷烃在相同溶剂中的热扩散系数的理论依赖性也与数据吻合良好，尽管略微高估了数据。当使用环己烷作为溶剂时，预测值与测量值出现更显著的偏差。这种行为与早期在分子混合物中发现的行为相似。在所研究的所有溶剂中，烷烃分子表现出热扩散系数的负值，并迁移到较热的层。
