Effect of side-chains on solution properties of N-(2-hydroxypropyl)methacrylamide copolymers in aqueous solvents

Abstract

The solubility and association of water-soluble copolymers of N-(2-hydroxypropyl)methacrylamide (HPMA) containing both p-nitroaniline-terminated oligopeptide side-chains and N-2-hydroxyethylpiperazine- N′-ethanesulfonic acid (HEPES) side-chains in aqueous solvents were investigated using light scattering methods. The weight-average molar masses, second virial coefficients, diffusion coefficients and hydrodynamic sizes of the copolymer molecules and their aggregates were studied as a function of the copolymer concentration, temperature and solvent pH. The copolymers with a low content (<2 mol%) of hydrophobic side-chains terminated in p-nitroaniline were soluble molecularly in aqueous solutions. The water solubility was substantially enhanced by the presence of HEPES side-chains. The copolymers with a high content (∼7 mol%) of p-nitroaniline-containing side-chains associated in water, generally forming two types of micelles of different sizes. Micelle formation depended on hydrophobic and amphiphilic side-chain content, polymer concentration, temperature and solvent pH. Thus, macromolecules containing a higher amount of p-nitroaniline and a small amount of HEPES side-chains tended to form small, compact micelles with the hydrophobic p-nitroaniline molecules oriented into the core, while macromolecules with a high content of HEPES side-chains tended to form large micelles. The formation of large micelles was strongly dependent on solvent pH, due to the electrolyte behaviour of the HEPES moieties. The solution properties of the copolymers generally reflected the competition between these two association processes.