Coexistence curve near the tricritical point in ternary polymer solutions

Two-phase coexistence curves were measured for ternary solutions of bimodal polystyrene in methylcyclohexane with the molecular weight ratio near the tricritical value 23. Coexistence curves were determined by the refractive index method on a diagram of temperature versus volume fraction of total polystyrene. The diameter was strongly curved near the top. The double logarithmic plots of volume fraction difference between two coexisting phases versus reduced temperature yielded the critical exponent beta=0.250+/-0.005 for the tricritical solution and, 0.412+/-0.005 and 0.383+/-0.016 for solutions not far from the tricritical one. The former value could be compared with the classical tricritical exponent beta=1/4 and the latter values near 0.40 could be explained by a crossover between the ordinary critical exponent 0.32 or the tricritical exponents 1/4 and the classical exponent 1/2.