Unusual Thermal Diffusion in Polymer Solutions

Department of Physics, University of Akron, Akron, Ohio, United States
Physical Review Letters (Impact Factor: 7.51). 01/2004; 91(24):245501. DOI: 10.1103/PhysRevLett.91.245501
Source: PubMed


Thermal diffusion forced Rayleigh scattering results on thermal diffusion of poly(ethylene oxide) (PEO) in ethanol/water mixtures are presented. In water-rich solvent mixtures, PEO is found to migrate towards regions of lower temperature. This is typical for polymer solutions and corresponds to a positive Soret coefficient of PEO. In solvent mixtures with low water content, however, the polymer is found to migrate towards higher temperatures, corresponding to a negative Soret coefficient of PEO in ethanol-rich solutions. To our knowledge, this is the first observed sign change of the Soret coefficient of a polymer in solution. We also present a simple lattice model for the polymer solvent system and calculate Soret coefficients with statistical mechanics methods. The calculated values agree qualitatively with the experimental results.

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Available from: Simone Wiegand, Nov 19, 2015
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    • "Another optical method employs a transient holographic technique. It is based on thermal diffusion forced Rayleigh scattering and is actively used nowadays [12] [13]. A grating created by the interference of two laser beams of equal intensity is written into a sample. "
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    ABSTRACT: The paper presents the development of an experimental technique based on optical digital interferometry for measuring thermodiffusion (Soret) coefficients. The experiments are performed in a cubic cell containing an aqueous solution and being heated from above. This is the first report on the method that allows to record the temperature and concentration fields in the entire cell rather than just between two points. Such information is not only available at the steady state, but within the whole diffusion process. The time-dependent evolution of the concentration field allows the measurement of molecular diffusion coefficient in addition to the Soret coefficients. The measurements of transport coefficients have been performed in water/ethanol mixture and water/isopropanol mixtures.
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    • "In 1977, Giglio and Vendramini found a negative Soret coefficient for poly(vinyl alcohol) in water [7]. Recently, we investigated poly(ethylene oxide) (PEO) in ethanol/water mixtures [8] [9]. It turned out that the observed sign change of the Soret coefficient of PEO in the solvent mixture at a water weight fraction of x water = 0.92 corresponded with the breakdown of the hydrogen bond network formed in pure water by addition of ethanol. "
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    ABSTRACT: We present measurements of aqueous low molecular weight mixtures and aqueous macromolecular solutions. The Soret coefficient S-T for ethanol, acetone, and DMSO in water was measured by an optical grating technique, and all systems showed a sign change around a molar fraction of water X-water 0.85 +/- 0.05. ST for poly(ethylene oxide), poly(N-isopropylacrylamide), and boehmite rods in ethanol/water mixtures showed also a sign change close to the same concentration. The occurring sign change concentration in the systems will be related to structural reorganizations in the solvent mixture and the Hildebrandt solubility parameter. We will also compare the experimental data within lattice calculations and simulations, which indicate that strong cross-interactions are important for a sign change of the Soret coefficient.
    Full-text · Article · Jan 2007 · Journal of Non-Equilibrium Thermodynamics
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    • "Thermal diffusion of water-soluble polymers often exhibit peculiar transport phenomena depending on the temperature and the solvent compositions. For instance, the ternary system of poly(ethylene oxide) (PEO) in the mixed solvent, water/ethanol, shows a sign change of the Soret coefficient S T [1] [2] [3]. The S T is defined as , (1) where DT and D are the thermal diffusion and the translational diffusion coefficients, respectively, and w and T are the weight fraction of solution and temperature, respectively. "
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