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Enthalpy relaxation of polystyrene (PS) and four modified polystyrene copolymers containing co-monomers capable of forming hydrogen bonds of different strengths is described. Values of enthalpy lost (?H(Ta,ta)) were calculated from experimental data plotted against log (ta) and modelled using the Cowie-Ferguson (CF) semi-empirical model. This gives a set of values for three adjustable parameters, ?H8(Ta), log (tc) and ß . Each of the parameters defines the relaxation process, which was found to be sensitive to changes in hydrogen bond strength. The introduction of hydrogen bonding causes a slower relaxation compared with PS, with a greater overall enthalpy lost measured for the all the copolymers except the styrene-co-4-hexafluoro-2-hydrox isopropyl styrene (SHFHS). Interestingly, the free volume of this copolymer measured using Positron Annihilation Lifetime Spectroscopy (PALS) was greater than that of PS. Furthermore, the SHFHS copolymer had the lowest change in heat capacity (?Cp) of any of the systems on passing through the glass transition,Tg. All experiments indicate that the enthalpy lost by the fully relaxed glass (?H8(Ta)) is less than the theoretical amount possible on reaching the state defined by the equilibrium liquid enthalpy line (?Hmax(Ta)). The results are discussed with reference to the strength of interaction and free volume. [ABSTRACT FROM AUTHOR]

The thermodynamic phase behaviour of blends composed of two random copolymers of polystyrene-co-methyl methacrylate (SMMA) and polystyrene-co-acrylonitrile (SAN) was studied using a modified Flory''s equation of state (EOS) theory. Light scattering measurements indicated that SMMA and SAN are miscible within certain copolymer composition ranges and exhibit lower critical solution temperature behaviour. The temperature, at which phase separation is observed, changes with the copolymer composition. The segmental interaction energy parameters XS–MMA, XS–AN and XMMA–AN were calculated using a binary interaction model by fitting the lowest temperature in the calculated spinodal boundary from EOS theory to the experimental data for several copolymer compositions. It was found that EOS theory, originally used for binary polymer solutions, could be modified and successfully extended to simulate the phase boundaries of random copolymer blends, when using Flory''s mixing rule to calculate the equation of state parameters for the copolymers from those of the constituent monomer segments. [Copyright &y& Elsevier]