Swelling equilibrium of poly(acrylic acid) (PAA) gels in aqueous poly(ethylene glycol) (PEG) solutions has been studied. First order volume transition was observed as the concentration of PEG was varied. The pH of the solution and the molecular weight of PEG are the key factors in determining the phase behavior of the PAA gel in this system. The transition concentration of PEG varies many orders of magnitude for different molecular weights. PEG with molecular weight 600 does not induce a transition. Reentrant phase transition was observed for the molecular weight of 18thinsp;500. Namely, at low PEG concentrations, the PAA gel is swollen; as the concentration is increased the gel collapses, but swells again as PEG is further added to the solution. PEG and PAA are known to form a polycomplex through hydrogen bonds. As the hydrogen bonds are formed, the hydrophilic side of polymers are hidden and the polymers become effectively hydrophobic, inducing the collapse. A simple theoretical model is presented to qualitatively explain the phenomena.
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