Substrate interactions, polymer blend phase separation

The surface chemistry of the substrate, even if the substrate is homogeneous, will affect the film phase segregation (see Fig. 6.2). In a two component blend film of two polymers—let us call them polymer A and polymer B— if the surface tension of polymer A and the substrate is less than the surface tension of polymer B and the substrate, polymer A will tend to sink towards the substrate and polymer B will tend to rise from the substrate, producing a vertically phase segregated film. Substrate interactions tend to dominate the atmospheric interactions, though a similar process can lead to vertical phase separation at the atmospheric interface. 

In practice, it is not easy to control the phase separation process in polymer blend films. The phase separation process in the polymer blends is very complicated, and the final morphology in the blend films is highly sensitive to many factors, such as solvent evaporation rate, solubility parameter, film-substrate interaction, surface tension of each component, and film thickness. Thus, the vertical phase separation can take place only under very extreme conditions [4B, 41], and, alternatively, lateral phase separation is more typical than vertical phase separation because the forces that contribute to the formation of lateral structures minimize the interface area. 

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