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 random
ly distributed along the network chains. The pH-driven swelling of the
se gels upon ionization in an aqueous medium was studied. The effect o
f 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 hydroph
obicity of the gel. This was explained by the stabilization of the col
lapsed state of the gel by hydrophobic aggregation of n-alkyl side cha
ins. 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 da
ta 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, exce
pt for the most hydrophobic gels.