Random Copolymers: Their Solution Thermodynamics as Compared with That of the Corresponding Homopolymers

Vapor pressure data (at 50 °C) of solutions of poly(methyl methacrylate) [PMMA], polystyrene [PS], and poly(styrene-ran-methyl methacrylate) [P(S-ran-MMA)], with different weight fractions f of styrene units, in either CHCl3, acetone [AC], methyl acetate [MeAc], or toluene [TL] were evaluated with respect to the dependence of the Flory−Huggins interaction parameter χ on polymer concentration and on f. For all solutions under investigation, χ varies considerably with the composition of the mixture, and only for four of them [CHCl3/PS, AC/PMMA, MeAc/PS, and TL/P(S-ran-MMA) f = 0.5] is this dependence linear; another four systems exhibit a minimum [CHCl3/PMMA, CHCl3/P(S-ran-MMA) f = 0.5, TL/PMMA, and TL/PS], and only one [MeAc/PMMA] shows a maximum. With the exception of CHCl3/P(S-ran-MMA) and f = 0.5, the χ values of the copolymers do not fall reasonably between the data obtained for the corresponding homopolymers. In most cases, the incorporation of a small fraction of the monomer that interacts less favor...