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Characterization of PMMA--3-octanone binary by turbidity and light scattering measurements

We measure the coexistence curve and the critical point properties of a binary mixture composed by polymethylmethacrylate (PMMA) at $M_w = 55900$ g/mol with 3- octanone. This binary mixture which has a demixing transition with an upper critical solution temperature $T_c$ has indeed interesting properties which may be useful for several application : a) its correlation length is larger than that of a liquid-liquid binary mixture, b) it is less viscous than a polymer blend; c) 3- octanone has an evaporation rate much smaller than other solvents. The mixture is first characterized by turbidity, to get the demixing temperature for different volumic fraction of PMMA. The coordinates of the critical point are obtained: $ϕ_c = 12.8\pm0.2$ % and $T_c = 306.5\pm0.1$ K. The correlation length $ξ$ near the critical point is then measured in a solution with a 12.8% volumic fraction of PMMA using static light scattering. Using the fact that PMMA-octanone mixture has scaling exponents compatible with Ising 3D, we determine more precisely the critical temperature $T_c = 306.58\pm0.04$ K and we find that $ξ\simeq ξ_0 [(T-T_c)/{T_c}]^{-0.63}$ with $ξ_0 = 0.97\pm 0.02$ nm. The discrepancy between this value and that extrapolated from other measurements based on turbidity is discussed.