Abstract
In the removal of a good solvent from immiscible polymer blends, the solution is quenched into a particular location in the two-phase region. The resulting phase separation alters the local solvent concentration profile in the solution. However, few physical models are currently available for describing the dynamic interplay between the solvent diffusion and the polymer phase separation. Here we present experimental evidence that the diffusive resistance at the interface of separating polymers, rather than that in bulk phase, can be a source of the characteristic retardation of solvent evaporation. Two immiscible polymers of polycarbonate and polystyrene were dissolved in tetrahydrofuran at different weight fractions. The polymer solution was deposited on a substrate as a thin liquid film and then dried in the condenser drier to promote the evaporation-induced phase separation. The results indicated that the polymer phase separation significantly retards the solvent evaporation rate. At high solvent contents, the evaporation rates in ternary solutions agreed well with those in binary solutions. However, the ternary solutions exhibited lower evaporation rates than the non-separating binary systems after the beginning of phase separation. Microstructure visualization showed that an increase in polystyrene weight fraction resulted in a transition of the phase morphology from a random distribution of "disk-like" polystyrene-rich droplets to that of polycarbonate-rich droplets. The maximum decrease in evaporation rate was found to be proportional to the peripheral interfacial length between the separated domains. These facts imply that the interfacial contribution of each polymer domain is essential for describing the retarded solvent diffusion in phase-separating polymer coatings.
