PHOTOREGULATED ADSORPTION AND ASSOCIATION OF AMPHIPHILIC COPOLYMERS CONTAINING AZOBENZENE SIDE-CHAINS

The association in aqueous solution and adsorption on polystyrene micr ospheres of N-(2-hydroxypropyl) methacrylamide (HPMA) copolymers conta ining photochromic azobenzene side chains were studied by light scatte ring and spectroscopic methods, respectively. Copolymers with a conten t of azobenzene in side chains, x(a), <7 mol% were water soluble, wher eas copolymers with x(a) > 7 mol% were soluble in water only in the fo rm of aggregates (pseudo-micelles) prepared by a slow increase in copo lymer concentration. The adsorption plateau was greater for copolymers with x(a) < 7 mol% than copolymers with x(a) > 7 mol%. Moreover, the adsorption plateau for molecularly dissolved copolymer (x(a) < 7 mol%) was reached at lower concentrations than for a copolymer present in s olution mainly in the form of aggregates (x(a) > 7 mol%). UV irradiati on of HPMA copolymer influenced the self-association and adsorption pr operties due to cis-trans configuration changes of the azobenzene unit s in the side chains. It was found that aggregates have partially diss ociated in response to UV irradiation. UV irradiation also has increas ed the copolymer adsorption. The effect has been small for molecularly dissolved copolymers (x(a) < 7 mol%) and became significant for solut ions of aggregates (x(a) > 7 mol%). Thus, irradiation induced a 60% in crease in the adsorption for copolymers with x(a) = 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 intramolecu lar association and to conformation changes of adsorbed chains. (C) 19 94 Academic Press, Inc.