Liquid-liquid (LL) demixing ofweakly interacting polymer/solvent solutions such as polystyrene(PS)/ acetone(AC), PS/ cyclohexane(CH), PS/methylcyclohexane(MCH), etc. is characterized in the temperature/ segment -fraction (T,y) plane by the presence of upper and lower demixing branches. 1-4 Some solvents dissolve some polymers at all accessible temperatures ( i.e. between the melting point ofthe solvent and its liquid/vapor critical point), no matter the length ofthe chain. These are the so called "good solvents", and the solutions, while viscous and perhaps hard to handle, are homogeneous across the entire concentration range, (0< 1), e.g. polystyrene(PS) in benzene. Other solvents (e.g. CH, MCH) dissolve very long chains (inthe limit, infinitely long chains) for (0< ), but only within a limited range of temperature |[email protected]<_T<_T ©i). Here T0u and T© ,L are the upper and lower Flory ©-

temperatures, respectively. Finally there exists a class of poor solvents which are unable to dissolve long polymer chains (and in some cases even short ones) at any appreciable concentration, Agood example is PS/acetone. Acetone does dissolve short chain PS, but the limit (192 monomer units at the critical concentration) is small enough to destroy the utility of this solvent in all but special cases.5

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