We measure the coexistence curve and the critical point properties of a binary mixture composed by polymethylmethacrylate (PMMA) at Mw = 55900 g/mol with 3- octanone. This binary mixture which has a demixing transition with an upper critical solution temperature Tc has indeed interesting properties which may be useful for several application: a) its correlation length is larger than that of a liquidliquid 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: φc = 12.8 ± 0.2 % and Tc = 306.5 ± 0.1 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 Tc = 306.58±0.04 K and we find that ξ ≃ ξ0[(T −Tc)/Tc] −0.63 with ξ0 = 0.97±0.02 nm. The discrepancy between this value and that extrapolated from other measurements based on turbidity is discussed.
