SWELLING OF POLY(N-ISOPROPYLACRYLAMIDE) GELS IN WATER-APROTIC SOLVENTMIXTURES

The swelling volume of poly(N-isopropylacrylamide) (PIPAAm) gel in apr otic solvents (acetonitrile (AcN)-, tetrahydrofuran (THF)-, 1,4-dioxan e (DO)- and dimethylsulfoxide (DMSO)) -water mixtures was measured at 25-degrees-C. The gel swollen in water shrank first and then reswelled with addition of the aprotic solvents. At an intermediate mole fracti on (XDMSO) range of DMSO-water mixtures, the gel demonstrated a ''reen trant'' swelling phenomenon: the hydrated gel shrank first on addition of a small amount of solvent, showed a typical wide reentrant transit ion, and gradually-reswelled in the range near pure solvent. On the ot her hand, the gels in AcN-, THF-, and DO-water mixtures demonstrated a ''reentrant-convex'' swelling phenomenon: the gels reswelled after a reentrant phase transition in low Xorg (XAcN, XTHF and XDO), showed a maximum swelling in the intermediate Xorg region, and shrank again gra dually in the high Xorg region. Such a swelling behavior of the gel wa s interpreted by correlating with solution properties of the aqueous a protic solvent mixtures. The strength of hydrogen bonding around amide groups of the homopolymer was examined in pure solvents (water, THF, and DMSO) and in all proportion of aqueous THF to observe the relation with swelling behavior of gel by spectrum analysis of the amide I and II bands of Fourier Transform Infrared Spectroscopy (FT-IR). The swel ling properties of gels in solvents and the aqueous mixtures were well correlated with the peak shifts of amide groups of the homopolymer.