Photoregulated Adsorption and Association of Amphiphilic Copolymers Containing Azobenzene Side Chains

Abstract

The association in aqueous solution and adsorption on polystyrene microspheres of N-(2-hydroxypropyl)methacrylamide (HPMA) copolymers containing photochromic azobenzene side chains were studied by light scattering and spectroscopic methods, respectively. Copolymers with a content of azobenzene in side chains, xa, <7 mol% were water soluble, whereas copolymers with xa > 7 mol% were soluble in water only in the form of aggregates (pseudo-micelles) prepared by a slow increase in copolymer concentration. The adsorption plateau was greater for copolymers with xa < 7 mol% than copolymers with xa > 7 mol%. Moreover, the adsorption plateau for molecularly dissolved copolymer (xa < 7 mol%) was reached at lower concentrations than for a copolymer present in solution mainly in the form of aggregates (xa > 7 mol%). UV irradiation of HPMA copolymer influenced the self-association and adsorption properties due to cis-trans configuration changes of the azobenzene units in the side chains. It was found that aggregates have partially dissociated in response to UV irradiation. UV irradiation also has increased the copolymer adsorption. The effect has been small for molecularly dissolved copolymers (xa < 7 mol%) and became significant for solutions of aggregates (xa > 7 mol%). Thus, irradiation induced a 60% increase in the adsorption for copolymers with xa = 11.5 and 12.5 mol% of azobenzene moieties. The changes of the copolymer adsorption upon irradiation have been related to changes in the inter- and intramolecular association and to conformation changes of adsorbed chains.