Interactions of drugs and spill probes with hydrophobically modified polyelectrolyte hydrogels based on N-isopropylacrylamide

Thermally responsive hydrogels of N-isopropylacrylamide containing hydropho bic comonomers were synthesized by free radical polymerization in both tolu ene and water. Various hydrophobic monomers were used, i.e. methyl methacry late, hexylacrylate, hexafluoroisopropylmethacrylate and hexafluorobutylmet hacrylate. Also polymers containing hydrophilic methacrylic acid were synth esized. The swelling properties of the polymers, as well as polymers contai ning 30 wt% ephedrine or ibuprofen were investigated in pure water at tempe ratures from 20 degrees C to 46 degrees C. The swelling ratios of pure gels depended on the ratio of hydrophilic and hydrophobic structural units in t he polymer, as well as on the network structures of the gels. The hydrophil ic drug ephedrine was bound to polyacid gels causing deswelling of the gels due to electrostatic interactions. Ephedrine did not affect the swelling o f the neural gels, however. Addition of hydrophobic ibuprofen made all the hydrogels collapse at room temperature no matter whether the gels were neut ral or acidic. Interactions between spin probes, 4-benzoxy-2,2,6,6-tetramet hylpiperidine-1-oxyl (BZONO) and 4-amino-2,2,6,6-tetramethylpiperidine-1-ox yl (amino-TEMPO), and the polymer gels were studied using electron paramagn etic resonance spectroscopy. The hydrophobic probe BZONO resided in the gel s below the lower critical solution temperature (LCST) due to hydrophobic i nteractions, but dissolved, at least partially, in the aqueous phase above the critical temperature. The distribution of BZONO between the gel and the aqueous phase depended on the hydrophobicity of the polymer. Also the hydr ophilic probe amino-TEMPO was partially bound into the gels. (C) 1999 Publi shed by Elsevier Science Ltd. All rights reserved.