Coexistence curve exponent of a binary mixture with a high molecular weight polymer

We studied the effects of a high molecular weight polymer (polyacrylic acid, PAA, M(w)=7.5 x 10(5)) on the coexistence curve of a binary liquid mixture (2,6-lutidine + water, LW). A high precision refractometer was built to measure the temperature dependence of the refractive indexes of the two coexisting phases of phase separated LW samples containing less than 1 mg/cc of PAA. From the refractive indexes we mapped out the coexistence curve and obtained the refractive index difference triangle upn between the two coexisting phases. At temperature T close to the critical temperature T(c), the critical exponent beta such that triangle n inversely proportional to (T-T(c))(beta) was found to be 0.41+/-0.02 which is higher than (0.31+/-0.01) that of pure LW.