Coexistence curves for the ternary system polystyrene (1)+polystyrene (2)+methylcyclohexane have been determined from measurements of refractive index. The polystyrene fraction of longer chain length was the same in all the measurements (M2=929000), but the shorter chain length fractions were varied (M1=13700, 23700, 29400, and 34000). The coexistence curves (in terms of total mass fraction of polymer) can be fitted to the same extended (classical) theory used in previous studies of quasi-binary hydrocarbon mixtures and can be extrapolated to a tricritical point at M2/M1=23 (M1=40000) and 36℃.

The turbidities of four bimodal mixtures mixtures of polystyrene in methylcyclohexane have been mea-sured in the three-phase region near the tricritical point, The results fromt eight different wave-lengths have been analyzed to obtain the correlation lengths. and the susceptibilities χ.χ and are strongly correlated. so it is their ratio x/that is determined most precisely. As the tricritical point is approached, alt three quantities diverge, as expected from the classical mean-field theory. Al-though the uncerainties are larger than wished for. it appears that the Griffiths first sums ger in-creasingly larger as the tricritical point is approached, and probably diverge in the same way as has been observed for other tricritical systems.

The coexistence curves of (NN dimethylaeet,tmide+hexane) have been determined within 10 K from the critical temperature by measurements of refractive index The crilical exponent βdeduced from (T, n), (T. x), and (T, φ) curves are consistent wlih theoretical predictions The results also have been analysed to obtain critical amplitudes and Wegner-eorreclion terms whenβ is fixed at the theoretical value and to compare the goodnesses of fit of the diameters of coexistence curves with τ1-2 andτ2β[τ=(T2-T)/T2]. It has been shown that the first Wegner-correction term is just significant and volume fraction possibly is a better order pararneter than is mole fraction.

The power-law dependences of critical amplitudes either on the molar volume of cycloalkane or on the interaction parameter per unit volume for coexistence curve, correlation length and susceptibility have been derived from a Landau-Ginsburg-Wilson model and the Scatchard-Hildebrand theory for solutions of cycloalkanes in a polar solvent. Coexistence curves of (T, n), (T, x) and (T, φ), where n, x, andφare refractive index, mole fraction and volume fraction respectively, for the binary mixtures of methylcyclopentane and cycloheptane in N-methyl-2-pyrrolidone (NMP) have been determined in the critical regions by measurements of n. The critical amplitude B and the critical exponent b have been deduced. The experimental results reported in this work and previous work are in good agreement with theoretical predictions.