Thermodiffusion ternärer Polymerlösungen
Zusammenfassung der Projektergebnisse
We have investigated thermodiffusion in ternary systems composed of a polymer and a mixed binary solvent with the focus on polystyrene dissolved in mixtures of toluene and cyclohexane. The initial plan to develop a holographic TDFRS setup with simultaneous twocolor detection turned out to be more complex than initially expected, but high quality measurements could eventually be performed by means of an adapted two-color optical beam deflection setup. The program suffered a setback of almost one year by initially overlooked microconvection in the beam deflection cell. In a first step, the concept of the universality of the thermophoretic mobility of highly asymmetric systems, whose applicability to ternary mixtures should be investigated, could be extended to the situation of vanishing mass of the solvent molecules by extrapolating results for the homologous series of the n-alkanes. Initially, this universal thermophoretic mobility was only observed in the opposite limit of a high mass of the solute. The work towards the theoretical description of polymer thermophoresis in binary and ternary systems started with the insight that any model must be based on units of the size of the Kuhn segment, which makes the problem similar to the problem of thermophoresis of uncharged colloids. Since it is known from small molecules that the Soret effect is a multi-cause phenomenon, where both the so-called (pseudo)isotopic contribution and the chemical contribution are of comparable magnitude, it appeared necessary to take both contributions also for larger entities into account. This led to two publications. The first one treats the equivalent of the isotope effect for colloids on the basis of hydrodynamic fluctuations. The second one considers a thermophoretic drift that is caused by dispersion interactions, which formally corresponds to the chemical contribution known from small molecules. In ternaries with a low to moderate polymer concentration, the diffusion eigenvalues are sufficiently separated and an effective solvent model could be developed. By assigning the slow mode to the motion of the polymer and the fast mode to the solvent dynamics, the eigenvectors of the diffusion matrix can be fixed and the inversion problem of the contrast factor matrix vanishes. Based on this assumption, all diffusion, thermodiffusion and Soret coefficients could be determined with an accuracy that is otherwise only achievable for binary systems. By comparing the coefficients for the slow and the fast mode with data for corresponding binaries, where available, the consistency of the model could be proven. Transformation invariant Soret coefficients could be obtained by a transformation suggested by Ortiz de Zárate in 2019. The large number of 616 individual experiments have been performed in order to systematically map the composition and temperature dependence of all diffusive and thermodiffusive transport coefficients for two different polystyrene molar masses. These measurements could be completed by the end of 2022 and their evaluation is still underway.
Projektbezogene Publikationen (Auswahl)
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An Unreasonable Universality of the Thermophoretic Velocity. The Journal of Physical Chemistry Letters, 11(11), 4498-4502.
Pur, B.; Schock, F.; Köhler, W. & Morozov, K. I.
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Brownian Particle Thermal Drift Caused by Hydrodynamic Fluctuations. The Journal of Physical Chemistry B, 125(27), 7462-7469.
Morozov, Konstantin I. & Köhler, Werner
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Can the Thermophoretic Mobility of Uncharged Colloids Be Predicted?. Langmuir, 38(8), 2478-2485.
Morozov, Konstantin I. & Köhler, Werner
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Thermodiffusion of polymer solutions and colloidal dispersions in mixed solvents. The Journal of Chemical Physics, 157(19).
Sommermann, D.; Schraml, M. & Köhler, W.
