Model calculations for wetting transitions in polymer mixtures

Partially compatible binary mixtures of linear flexible polymers are considered in the presence of a wall which preferentially adsorbs one component. Using a Flory-Huggins type mean field approach, it is shown that in typical cases at two-phase coexistence the wall is always « wet », i.e. coated with a macroscopically thick layer of the preferred phase, and the transition to the non wet state occurs at volume fractions of the order of 1/~N (where N is the chain length) at the coexistence curve. Both first and second order wetting transitions are found, and the variation of the surface layer thickness, surface excess energy and related quantities through the transition is studied. We discuss both the validity of the long wavelength approximation involved in our treatment, and pos- sible fluctuation effects for « critical wetting », comparing our results to Monte Carlo simulations of wetting in Ising models. The relation of our results to previous work and possible experimental consequences are also briefly mentioned.

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