SWELLING CHARACTERISTICS OF A BLEND HYDROGEL MADE OF POLY(ALLYLBIGUANIDO-CO-ALLYLAMINE) AND POLY(VINYL ALCOHOL)

A hydrogel was prepared by mixing poly(allylbiguanido-co-allylamine) h ydrochloride (PAB) with poly(vinyl alcohol) (PVA) and by repeatedly fr eezing and thawing the blend. The swelling behaviour of the gel was st udied as a function of the pH and ionic strength of the medium. The pe rformance of the gel was compared to that of a blend hydrogel made fro m poly(allylamine) hydrochloride (PAA) and PVA. In a salt-free aqueous medium, the PAB/PVA hydrogel displayed a remarkable pH response: a dr astic reduction in size was observed above pH 9 and below pH 3, but th e size was little affected in the pH range 3-9. In a medium of constan t ionic strength (mu = 0.1), the hydrogel's pH response was much diffe rent: it was significantly reduced in size above pH 10, but the size w as affected only moderately below pH 10. When the ionic strength of th e medium was varied at a fixed pH, the size change of the hydrogel was gradual. For instance, in sodium chloride solutions (at a pH of 6.0), the hydrogel shrank by about 30% as the electrolyte concentration was raised from 10(-6) to 2 moll(-1). All these phenomena could be unders tood by observing that: (i) PAB is subject to multiple acid-base equil ibria (with widely spaced pK(a) values); and (ii) the electrostatic in teractions among the charges on the polymer backbone are shielded by t he added electrolyte as the ionic strength of the medium is raised.