Dissolution of symmetric diblock copolymers with neutral solvents, a selective solvent, a nonsolvent, and mixtures of a solvent and nonsolvent monitored by FT-IR imaging

FT-IR imaging was used to study the dissolution of symmetric diblock copolymers of poly(styrene-b-methyl methacrylate) in neutral solvents (toluene and benzene), a selective solvent (2-ethoxyethanol), a nonsolvent (cyclohexane), and mixtures of the selective solvent and nonsolvent. 2-Ethoxy-ethanol is a good solvent for poly(methyl methacrylate) (PMMA) but a nonsolvent for polystyrene (PS), and cyclohexane is a nonsolvent for polystyrene below 35 degreesC and a nonsolvent for PMMA. Copolymer samples of three different molecular weights were tested with the neutral solvents, and one molecular weight was tested with the selective solvent, nonsolvent, and solvent mixtures. FT-IR images and concentration profiles were obtained, and dissolution rates were calculated from the concentration profiles. For the neutral solvent experiments, the block copolymer dissolution mechanism was governed by either solvent diffusion, which resulted in stress cracking for low molecular weights, or disentanglement of the polymer chains, which resulted in normal dissolution for higher molecular weights. The selective solvent dissolved the copolymer, and swollen, gel, and liquid layers were observed. The nonsolvent penetrated the bulk copolymer, but no dissolution occurred during the time of the experiments. Both the selective solvent and nonsolvent in the solvent mixtures penetrated the copolymer and produced similar results compared to the selective solvent or nonsolvent, depending on the weight fraction of each solvent in the mixture. The dissolution rate of the block copolymer was higher with mixtures of the selective solvent with the nonsolvent than for the neat selective solvent, but it decreased with increased weight percent of the nonsolvent.