Characterization of Phase Behavior in Polymer Blends by Light Scattering

It is well known that most pairs of high molecular weight polymers are immiscible. This is so because the combinatorial entropy of mixing of two polymers is dramatically less than that of two low molecular weight compounds [1–3]. The enthalpy of mixing, on the other hand, is often a positive quantity. Therefore, dissimilar polymers are only miscible if there are favorable specific interactions between them leading to a negative contribution to the Gibbs free energy of mixing. Miscible polymers tend to phase separate at elevated temperatures. This lower critical solution temperature (LCST) behavior is interpreted in terms of the equation of state or the freevolume contribution [1–3]. About 30 pairs of dissimilar polymers have been found to exhibit the LCST behavior.