Mamoru Okada

Phase diagram of star-shaped polystyrene/cyclohexane system : location of critical point and profile of coexistence curve

Abstract Coexistence curves of cyclohexane solutions of star-shaped polystyrenes (PSs) having 6.3 and 11.1 arms (arm molecular weight 1.88 × 105) were measured to investigate the effects of molecular shape on the phase diagram near the critical point. The reduced coexistence curve ф ф c versus τ τ c , where volume fraction ф and dimensionless temperature τ calculated from τ = (T − Θ) Θ with a usual temperature T and the theta temperature Θ were reduced by the values at the critical point, were compared with those of linear PSs. The reduced coexistence curves of star-shaped PSs do not superpose on the reduced universal curve of linear PS when the same Θ is assumed for linear and star-shaped PSs. The critical temperature Tc and concentration ф c were compared with those of linear, randomly branched, and comb-shaped PSs. The location of the critical point of the star-shaped PS is very different from that of linear PS of the same molecular weight Mw, indicating failure of the mean-field theory of polymer solutions. The same simple two-parameter scaling, which predicts the relations ф c ∼ ф∗ (= overlap concentration) and τ c ∼ (M w ф∗) −1 , does not hold for the star-shaped and linear PSs, when the same Θ is used for linear and star-shaped PSs. The scaling relations do not hold either even when we use different Θs for star-shaped PSs which were evaluated so that the reduced coexistence curves were superposed on the universal coexistence curve of linear PS. The ф c of star-shaped PS lies at a lower concentration than expected from its ф∗ and Tc lies at a lower temperature than expected from ( M w ф∗) −1 .

Molecular weight and heating rate dependence of the cloud points of polystyrene/poly(2-chlorostyrene) blends

Abstract Cloud points of polystyrene/poly(2-chlorostyrene) blends of narrow molecular weight distribution were measured with changing heating rate and molecular weight of polystyrene. Cloud points were also measured for blends containing various amounts of the plasticizer di-n-butyl phthalate. A large heating rate dependence of cloud points is observed at very small heating rates when no or a small amount of the plasticizer is contained. Cloud points extrapolated to zero plasticizer content are very different from directly observed cloud points of the binary blends, especially for large molecular weight samples. This is explained by the very slow phase separation kinetics of this system. The Flory-Huggins interaction parameter χ was calculated from the extrapolated cloud points using the spinodal condition. The χ parameter is positive and has a very weak temperature dependence.

Phase separation induced by a two-step temperature jump in polystyrene/poly(2-chlorostyrene) blends: dependence on duration and temperature of the first step

Abstract Phase separation induced by a two-step temperature jump was studied by time-resolved light scattering and scanning electron microscopy for polystyrene/poly(2-chlorostyrene) blends. A co-continuous domain structure was formed by the first-step jump, with small droplets being formed in these co-continuous domains by the second-step jump made in the intermediate stage, as well as in the late stage, of phase separation at the first-step temperature. When the duration of the first step was long enough, the growth of co-continuous domains formed after the second jump was well described by the same power function as that found for the corresponding single-step jump if the phase-separation time was shifted; this feature was confirmed for different first-step temperatures.

Critical solution point and chain dimension of branched-polystyrene solutions

SummaryCritical solution point and chain dimension were measured for branched polystyrene(BPS) in solution as a function of molecular weight(M) and compared with those for linear polystyrene(LPS). The critical concentration φc of BPS was quite different from that of LPS at a fixed M, but the same at a fixed overlap-concentration φ*, i.e., plots of φc vs. φ* fall on a single straight line for both BPS and LPS (gfc ∝ φ*). Reduced critical temperature τc defined by gtc=(θ−Tc)/θ [Tc: critical temperature, θ: the θ-temperature] was related to φc as τc ∝ φc∼2 for BPS, whereas τc ∝ φc for LPS.

Phase equilibrium of a star-shaped polymer solution

Abstract A simple theoretical model was proposed to explain the solubility difference between star-shaped and linear polymers in a solvent. The basic assumption of the model was that probability of intermolecular contacts was low near the centre of a star-shaped polymer because of the higher local segment density. The mixing free energy was obtained by modifying the enthalpy term of the Flory–Huggins free energy. Coexistence curves of star-shaped and linear polymer solutions were calculated, and the results were compared with available experimental data. The present model could predict that star-shaped polymer had a better solubility, but quantitatively it slightly overestimated the difference between the critical temperatures of star-shaped and linear polymer solutions.

Arm-number dependence of dimensions of star-shaped polystyrene in a good solvent

Abstract Radii of gyration Rg and hydrodynamic radii Rh of star-shaped polystyrenes with different arm numbers and their arm polymers were measured in toluene solution by static and dynamic light scattering. Rh depends on the number of arms f stronger than Rg. The ratios g a g = R g ( star ) R g ( arm ) and g a h = R h ( star ) R h ( arm ) were compared with other experimental and computer simulation results to examine the universality of these ratios. Universality of the relation of gagversus number of arms f is confirmed. No clear indication of failure of gah in universality is observed, and it was suggested that gah is not sensitive to systems. Experimental results were also compared with available theories. The renormalized two-parameter theory by Douglas and Freed for static quantities gag and g A (= A 2 ( star ) A 2 ( linear ) , where A2 is the second virial coefficient) reproduces the experimental data very well at lower fs and agreement in gag is excellent over a wide range of f.

Critical exponents and phase diagram of polymer blend solutions : polystyrene/poly(methyl methacrylate)/d6-benzene system

Abstract Critical behaviour of blend solutions of polystyrene (Mw = 3.55 × 105)/poly(methyl methacrylate) (Mw = 3.27 × 105) in d6-benzene was investigated by static light scattering. The phase diagram (spinodal, binodal and cloud points) was determined at a fixed total polymer concentration. The system had the lower critical solution temperature. The inverse isothermal osmotic compressibility ( ∂π ∂c 1 T ) and correlation length ζ of concentration fluctuations were determined as functions of the reduced temperature ϵ= | T s T−1 | near the stability limit, where T is the absolute temperature and Ts is the spinodal temperature. Both ( ∂π ∂c 1 T ) and ξ were described by single exponents over the investigated temperature range. The critical exponents γ and ν for ( ∂π ∂c 1 T ) and ζ were 1.23 and 0.63, respectively. They were very close to the three-dimensional Ising exponents and obviously different from the mean-field or Fishers renormalized Ising exponents, as theoretically predicted by Broseta et al.