SWELLING BEHAVIOR OF PH-SENSITIVE COPOLYMERS BASED ON STYRENE AND 4-VINYLPYRIDINE OR 2-VINYLPYRIDINE

Two series of lightly cross-linked copolymer beads based on styrene (St) and 4-(or 2-)vinylpyridine (4VP or 2VP) were synthesized, and their equilibrium swelling behavior in aqueous solution was studied as a function of pH and ionic strength. A wide range of pH-sensitive swelling behavior could be obtained using a range of compositional changes of copolymers. Over a fairly narrow pH range, the polymer swells as the medium becomes more acidic, consistent with protonation of the VP units. With an increase in the ratio of St to 4VP, the pH at which the swelling transition occurs shifts to lower values and the extent of swelling decreases. An increase in the amount of cross-linking agent, divinylbenzene (DVB), generally does not change the pH of the swelling transition but, as expected, leads to a decrease in the extent of swelling at pH's below the transition. Comparing 4VP and 2VP, some subtle changes have been found in the pH of the swelling transition and also the pH interval for the transition. A nonmonotonic ionic strength dependence of swelling behavior has been observed, and the swelling maxima have been found to shift to higher ionic strength values as the pH increases.