pH-Responsive Gels of Hydrophobically Modified Poly(acrylic acid)

Abstract

pH-responsive gels of hydrophobically modified (HM) weak polyacid were prepared from acrylic acid and n -alkyl acrylates ( n = 8, 12, 18). The HM gels obtained bear up to 20 mol % of n -alkyl acrylate units randomly distributed along the network chains. The pH-driven swelling of these gels upon ionization in an aqueous medium was studied. The effect of the fraction and of the side chain length of n -alkyl acrylate groups on the equilibrium degree of swelling was examined. It was shown that the swelling transition shifts to alkaline pH with increasing hydrophobicity of the gel. This was explained by the stabilization of the collapsed state of the gel by hydrophobic aggregation of n -alkyl side chains. The formation of such aggregates, which break down in the course of gel ionization, was confirmed by the fluorescent probe method with pyrene as a probe and by NMR spectroscopy. Potentiometric titration data of HM poly(acrylic acid) (PAA) gels and of the corresponding linear copolymers evidence that the introduction of hydrophobic repeat units only slightly affects the apparent dissociation constant of PAA, except for the most hydrophobic gels.