Experimental study of the approach to a double critical point in the poly(styrene) plus acetone system: Test of fluctuation-isomorphism theory

The phase-separation temperature has been measured as a function of pressure and composition for the poly(styrene) plus acetone system. From the experimental data, the coexistence curves at different pressures have been interpolated. The analysis of the curves in terms of simple scaling equations leads to values of the critical exponent {β} which, near the double critical point, seem to be larger than twice the single-critical-point value 0.325. Contrary to what was expected from previous work [F. Monroy et al., Phys. Rev. B 47, 630 (1993)], the use of the thermodynamic field suggested by Malomuzh and Veytsman [N. P. Malomuzh and B. A. Veytsman, Phys. Lett. A 136, 239 (1989)] and simple scaling did not describe the data with a constant {β}=0.325 for all the isobars. However, the use of an effective βeff given by a theory that assumes a fluctuation-isomorphism hypothesis describes very accurately the crossover from single- to double-critical-point behavior, with a universal {β}=0.325 critical exponent. The theory predicts that the region in which simple scaling is valid shrinks, even beyond the experimentally accessible range of this work, in the proximity of the double critical point.