Adiabatic compressibility of polyelectrolytes: Effect of solvents on copolymers of vinyl pyrrolidone with acrylic acid and N-dimethylaminoethyl methacrylate

Abstract

The results of adiabatic compressibility measurements for two copolymers, acrylic acid-vinyl pyrrolidone (AA—VP) and N-dimethylaminoethyl methacrylate-vinyl pyrrolidone (DAM—VP), in three different solvents, namely, water, methanol, and dioxane, have been described. The molecular weight of copolymers was determined by the light scattering method and the IR and NMR spectra of the polymers and copolymers were examined to establish that the alternating acrylic acid–vinyl pyrrolidone and N-dimethylaminoethyl methacrylate–vinyl pyrrolidone structure exists in the copolymers. The AA—VP copolymer behaves as a slightly weaker acid than the homopolymer of acrylic acid, while DAM—VP copolymer is very feebly basic and has the same strength as that of the homopolymer of N-dimethylaminoethyl methacrylate. The reduced viscosity for the two copolymers in aqueous solution is very low (∼0.08 dL/g for AA—VP copolymer). In methanol solution AA—VP and DAM—VP copolymers show a decrease of oK°2 and oV°2 by 61.6 × 10−4 cc/bar/mol and 8.0 cc/mol, and 191.0 × 10−4 cc/bar/mol and 20.0 cc/mol, respectively, over that of the values of aqueous solution. The void space around the solute is smaller in methanol than in water, and accordingly this decrease has been attributed to geometric effect. Only one copolymer, DAM—VP is soluble in dioxane, and the values are seen to have increased in this solution by 71.0 × 10−4 cc/bar/mol and 18.7 cc/mol, respectively, compared to the values obtained from aqueous solution. The experimentally determined oK°2 and oV°2 for AA—VP and DAM—VP copolymer are 0.6 × 10−4 cc/bar/mol, and 102.4 cc/mol and −61.0 × 10−4 cc/bar/mol, 94.4 cc/mol, respectively, in aqueous solution, and −12.0 × 10−4 cc/bar/mol, 211.0 cc/mol and −203.0 × 10−4 cc/bar/mol, 191.0 cc/mol, respectively, in methanol solution. In dioxane solution the values for DAM—VP copolymer are 59.0 × 10−4 cc/bar/mol and 229.7 cc/mol, respectively. These experimentally determined values for AA—VP copolymer show an increase by 0.04 × 10−4 cc/bar/mol, 4.4 cc/mol and 28.3 × 10−4 cc/bar/mol, 8.0 cc/mol in aqueous and methanol solution, respectively, compared to calculated values determined on the basis of no interaction between acid and the pyrrolidone group. In contrast, the DAM—VP copolymer shows a decrease of 27.6 × 10−4 cc/bar/mol and 10.3 cc/mol, 149.3 × 10−4 cc/bar/mol and 20.2 cc/mol, and 23.0 × 10−4 cc/bar/mol and 4.1 cc/mol in aqueous, methanol, and dioxane solutions, respectively. In aqueous solution these differences between calculated and observed values have been attributed to a change of water structure around the copolymer chain. A similar effect is responsible for the difference of the values in the methanol solution also. In the dioxane solution the difference is rather small, and the solvent structure has probably not altered much due to the presence of the DAM unit in the chain.