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Ionic liquids feature a combinationof properties that make them very interesting solvents for polymers,but many questions remain regarding the thermodynamics of polymer/ionicliquid solutions. In this work, the lower-critical-solution-temperature(LCST) phase behavior of poly(n-butyl methacrylate)(PnBMA) in mixtures of the ionic liquids 1-butyl-3-methylimidazolium:bis(trifluoromethylsulfonyl)imide([BMIm][TFSI]) and 1-ethyl-3-methylimidazolium:TFSI ([EMIm][TFSI])is characterized by transmittance, light scattering, and small-angleneutron scattering measurements. Several relevant thermodynamic parametersare easily tuned by varying the ionic liquid composition. In particular,the cloud point, spinodal, and Θ temperatures are all foundto increase linearly with [BMIm] content. The interaction parameters, χ,are determined as a function of temperature and concentration usingthree different methods, and the results from each method are compared.The entropic and enthalpic components of the interaction parameter,χ, are also found to vary linearly with [BMIm] content in theionic liquid, increasing and decreasing, respectively. The increasein the enthalpic driving force for mixing with increasing [BMIm] contentdominates over the decrease in the entropic penalty for mixing, leadingto improved solubility. This result is discussed in terms of molecularinteractions and oriented solvation. This work characterizes the solutionthermodynamics of one representative system in a very interestingclass of polymer solutions, and provides insight into the molecularmechanisms underlying its LCST phase behavior. [ABSTRACT FROM AUTHOR]