Interfacial Relaxation Leading to Coexisting Phases

Thin polymer films composed of two layers with different composition have been used for almost two decades to determine the diffusion coefficient [12,78-82] on the basis of observed broadening of their initial profiles ( (z). When the two layers are built of two fully miscible phases (T TC regime for blends with upper critical solution temperature UCST), a free interdiffusion takes place with the interface growing with time t as w1/2°=t1/2. This process proceeds without limits and results in a single homogeneous phase. 

A different behavior is observed [76] for bilayers composed of partially miscible polymers below their critical temperature Tc. In this case two pure blend components interdiffuse until the equilibrium of two coexisting phases is established. The above equilibrium state is characterized by the coexistence compositions ( q and (] 2 and the interfacial width w. The relaxation of the initial interface between pure constituents involves two processes (see Fig. 3)  

Formation of the equilibrium interface characteristic for coexisting phases 

Mass transport across the interface until coexisting concentrations are reached 

The first phenomenon has been analyzed theoretically by various workers [83-86]. Most of them start with the Cahn-Hilliard equation relating the flux J of the polymer species A  

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