Theory, thermal-induced phase separation

Since the deGennes-Pincus analysis was developed for blends in which the two components are the same molecular weight, we made a scaling modification of their theory to account for the fact that our experimental blends have different PS and PVME molecular weights. Thermally induced phase separation was followed in the same manner as previously [9] for 10% and 50% PS blends of both molecular weights. It is significant to note that excimer fluorescence is a more sensitive measure of phase separation at short times because films annealed for about one minute showed no visual signs of opalescence while R increased between 10 and 50% for the different blends. 

The model athermal blend is defined [59,62] as the hypothetical limit of vanishing interchain attractive potentials relative to the thermal energy, i.e., Pvmm-W = 0- For this situation the atomic site-site Percus-Yevick closure approximation of Eq. (2.7) is employed where the subscripts now refer to the spedes type. The constant volume athermal blend is of theoretical interest since it isolates the purely entropic packing effects. However, as emphasized by several workers [2,62,63,67], the athermal reference blend is not an adequate model of any real phase separating system. Its primary importance is as a reference system for the theories of thermally-induced phase separation discussed in Sect 8. 

GRA Graham, P.D., Barton, B.F., and McHugh, A.J., Kinetics of thermally induced phase separation in ternary polymer solutions, n. Comparison of theory and experiment, J. Polym. Sci. PartB Polym. Phys., 37,1461,1999. 

In the first category, polystyrene (PS) and poly(vinyl methyl ether) (PVME) have been reported to be compatible when films are cast from suitable solvents.5 But phase separation can be induced in a compatible film simply by heating the mixture to about 150°C. Subsequent slow cooling of the phase-separated mixture resTilts in a compatible film. The thermally induced phase separation behavior of PS-PVME mixture indicates the existence of LOST. The presence of UCST was not reported, althoTigh, according to Patterson s theory, if LOST exists, UCST should also exist. 

From the observation that free volumes—due to the difference between specific volumes at a given temperature T and at 0°K—can be very different, depending upon the nature of the components, theories based on the concept of free volume and including the equations of state were thus proposed. Polymer connectivity implies the existence of free volume and a lower coefficient of thermal expansion than that of a solvent, and under these conditions an increase of temperature necessarily induces differentiation in the densities of the components, which cannot adjust one with another to make a homogeneous solution. As a result of an increase in temperature, the solution demixes and this phase separation can occur at an even lower temperature if the polymer molar mass is higher. 

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