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Dissolution and wetting of hydrosoluble polymers

François Lequeux, Laurence Talini, Emilie Verneuil et Pauline Valois
 
Dissolution of powders is an experiment of daily life which, as everybody knows, can be difficult and result in the formation of lumps. Although frequent and of dramatic importance in several industrial processes, lump formation is a poorly understood problem. When a powder is put in contact with a liquid, capillary imbibition drives the liquid between and within grains. Imbibition may be heterogeneous leading to remaining dry parts in grains, which results in lumps. At a smaller scale, imbibition is ruled by the advance of the contact line in a pore. Understanding of how the soluble solid matter is wetted by the liquid is therefore necessary to fully comprehend dissolution of powders.
 
A first study deals with wetting by water of hydrosoluble polymers : we have performed simple experiments that consists in depositing a droplet of solvent on a polymer layer and in measuring both the contact angle and spreading velocity of the droplet. We focus on the case of amorphous polymers for which glass transition temperature is larger than room temperature. We have shown the solvent content is a key parameter for the wetting dynamics. According to that content, rich wetting behaviours emerge, owing to the possible large variations of the polymer properties. At low solvent fraction, the polymer is glassy and behaves as a rigid solid in which solvent uptake is small and solvent diffusion is slow, whereas at large solvent content it turns into a viscous liquid with a large solvent sorption and diffusion coefficient. Between those two situations, the polymer behaves as a soft viscoelastic medium that can easily be deformed. Such dramatic changes have important consequences on the wetting behaviour.
 

Variations of the contact angle with contact line velocity during the spontaneous spreading of a water droplet on a thin hydrosoluble polymer. At first the contact angle is very large : although constituted by a hydrophilic polymer, the substrate behaves as a hydrophobic one. As spreading proceeds, the substrate is hydrated by water transferred from the droplet, and becomes more and more hydrophilic, resulting in smaller contact angle values.
 
A second study deals with the dissolution of polymer powders. When solvent is put in contact with the polymer, a gel is formed (figure below). We have shown disappearance of the gel phase is driven by water diffusion rather than polymer specific relaxation processes such as reptation. Full dissolution occurs when the water content is large enough for polymer concentration to become smaller than the overlap concentration of the polymer.
 

Pictures taken at different times after a piece of a glassy polymer has been immersed in water. A gel quickly forms as water diffuses in the polymer. The limiting step for dissolution is the disappearance of the gel that is a much slower process than diffusion by water in the glassy polymer.
 
Publications

  • Wetting of polymers by their solvent, F. Lequeux, L. Talini, E. Verneuil, G. Delannoy, P. Valois, to appear in Eur. Phys. J. E, topical issue on “Wetting and Drying : Physics and Pattern Formation” (2015).
  • Glass Transition Accelerates the Spreading of Polar Solvents on a Soluble Polymer, J. Dupas, E. Verneuil, M. Van Landeghem,B. Bresson,L. Forny, M. Ramaioli, F. Lequeux and L. Talini, Phys. Rev. Lett. 112, 188302 (2014).
  • Diffusion and evaporation control the spreading of volatile droplets onto soluble films, J. Dupas, E. Verneuil, M. Ramaioli, L. Forny, L. Talini, and F. Lequeux, Interfacial Phenomena and Heat Transfer 1, 231 (2013).
  • Dynamic wetting on thin film of soluble polymer : Effects of non-linearities in the sorption isotherm, J. Dupas, E. Verneuil, L. Talini, F. Lequeux, M. Ramaioli and L. Forny, Langmuir 29, 12572 (2013).