Weiguo Shen

Molar mass dependence of critical amplitudes for chain-molecule solutions

Abstract The power-law dependences of critical amplitudes on molar mass for coexistence curve, correlation length and susceptibility have been derived from a Landau–Ginsburg–Wilson type model for chain-molecule solutions of both small molecules and polymers. A series of turbidity measurements and determinations of coexistence curves for solutions of n-alkane in polar liquids and polymethylmethacrylate in 3-octanone have been conducted. Our experimental results and those of polystyrene solutions indicate that this model is satisfied within experimental errors.

Turbidity measurements and amplitude scaling of critical solutions of polystyrene in methylcyclohexane

We report turbidity measurements for a series of solutions of polystyrene in methylcyclohexane with varying polymer molecular weights. The obtained correlation length ξ and the osmotic compressibility χ shows power law dependence on both the reduced temperature t (=|T−Tc|/Tc) and the molecular weights Mw of the polymers. We find that the relations ξ0(1−φc)0.85∝Mw0.18, and χ0(1−φc)−1.20∝Mw−0.09 proposed by An, Jiang, Chen, and Shen [Chem. Phys. Lett. 282, 403 (1998)] can be used to describe the present experimental results. Moreover, these relations appear to be better suited than a simple scaling for extracting the Mw-exponents. The exponents so obtained are found to be in excellent agreement with theoretical predictions.

Critical phenomena of chain-molecule solutions

A model of Landau-Ginsburg-Wilson type is used to describe the dependence of the critical amplitude (associated with coexistence curve, or correlation length, or Susceptibility) on molar mass for chain-molecule solutions of both small molecules and polymers. Experimental methods for measurements of the critical volume fraction, coexistence curve, and turbidity are discussed. Some experimental results are summarized and compared with the theory, which shows that the theory is satisfied within experimental uncertainties.

The coexistence curves of {xN-methyl-2-pyrrolidone + (1 x)cyclohexane} and {xN-methyl-2-pyrrolidone + (1 x)cyclooctane} in the critical region

Abstract Coexistence curves of (T,n), (T,x) and (T,φ), where n, x, and φ are refractive index, mole fraction and volume fraction, respectively, for the binary mixtures {xN-methyl-2-pyrrolidone (NMP)xa0+xa0(1−x)cyclohexane} and {xNMPxa0+xa0(1−x)cyclooctane} have been determined in the critical regions by measurements of n. The critical amplitude B and the critical exponent β have been deduced and the values of β are consistent with the theoretical ones. The experimental results have been analysed to examine the Wegner correction terms and the behaviour of the rectilinear diameter of the coexistence curves. The coexistence curves have been successfully described by a combination of the Wegner equation and the expression for the diameter. The power law dependence of critical behaviour either on the molar volume of cycloalkane or on the interaction parameter per unit volume has also been discussed.