Continuity of solvent quality in polymer solutions. Poor-solvent to Theta-solvent continuity in some polystyrene solutions

Critical temperatures for polystyrene/methyl acetate (PS/MA) and polystyrene/ethyl formate (PS/EF) solutions were measured at positive (PS/MA, PS/EF) and negative (PS/MA) pressure. The results confirm that solvent quality is sensitive to pressure; some solvents, designated Theta, at P-nominal similar to 0, undergo a Theta-to-poor transition at negative pressure, and others, nominally designated ''poor,'' show a poor-to-Theta transition at positive pressure. Thus, any dichotomous division into sets of ''poor'' and ''Theta'' solvents is inaccurate, unless it accounts for the effects of pressure and other variables on solvent quality. (C) 1997 John Wiley & Sons, Inc.