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 pr essure 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 beta which, near the double critical point, seem to be larger than twice the sing le-critical-point value 0.325. Contrary to what was expected from prev ious work [F. Monroy et al., Phys. Rev. B 47, 630 (1993)], the use of the thermodynamic field suggested by Malomuzh and Veytsman [N. P. Malo muzh and B. A. Veytsman, Phys. Lett. A 136, 239 (1989)] and simple sca ling did not describe the data with a constant beta = 0.325 for all th e isobars. However, the use of an effective beta(eff) given by a theor y that assumes a fluctuation-isomorphism hypothesis describes very acc urately the crossover from single- to double-critical-point behavior, with a universal beta = 0.325 critical exponent. The theory predicts t hat the region in which simple scaling is valid shrinks, even beyond t he experimentally accessible range of this work, in the proximity of t he double critical point.