Effect of chain rigidity on block copolymer micelle formation and dissolution as observed by 1H-NMR spectroscopy

Abstract

Amphiphilic copolymers poly(methyl methacrylate-b-acrylic acid), poly(methyl methacrylate-b-methacrylic acid), poly(methyl acrylate-b-acrylic acid) and poly(methyl acrylate-b-methacrylic acid) were prepared by reversible addition fragmentation chain-transfer (RAFT) polymerization. The hydrophilic polyacid blocks were either synthesized directly or formed by the hydrolysis of poly(tert-butyl acrylate) or poly(tert-butyl methacrylate) blocks. The hydrophobic blocks consisted of either the more rigid, high glass transition temperature (Tg) poly(methyl methacrylate) or more flexible, low Tg poly(methyl acrylate) material. The hydrophilic blocks were either poly(methacrylic acid) (rigid, high Tg) or poly(acrylic acid) (flexible, low Tg). The micellization behavior of the polymers was studied by proton nuclear magnetic resonance (1H-NMR) spectroscopy in mixtures of 1,4-dioxane-d8 and D2O. All four polymers were soluble in neat dioxane. In solutions of higher water content, the polymers with the more rigid hydrophobic blocks formed into micelles as was evidenced by broadening of the resonances resulting from the protons in those blocks. At moderate water concentration (25–50%), dissolution of the micelles was observed upon heating the solution. No micellization was observed in polymers containing the less rigid poly(methyl acrylate) hydrophobic block regardless of the identity of the hydrophilic block. As further evidence of micellization formation and dissolution, the spin-lattice (T1) and spin-spin (T2) relaxation times of protons in the hydrophobic and hydrophilic blocks were measured. Significant differences in the relaxation times as functions of temperature and solvent concentration were observed between the hydrophilic and hydrophobic blocks of the micelle-forming polymers.