Volume phase transition of polyelectrolyte gels with different charge distributions

Four polyelectrolyte gels consisting of acrylic acid (AA) and N-isopropylacrylamide (NIPA) were prepared using the following methods: (i) the usual redox polymerization of an aqueous solution containing NIPA, AA, and N,N'-methylenebis(acrylamide) (cross-linker) which was initiated by a pair of ammonium persulfate and N,N,N',N'-tetramethylethylenediamine; (ii) the same method as employed in (i) except for the use of poly(acrylic acid) (PAA) instead of the AA monomer; (iii) the gelation of an aqueous solution containing the polymer of NIPA (PNIPA) and PAA with gamma-ray irradiation from Co-60; (iv) the same method as used in (iii) except for the use of an AA/NIPA copolymer instead of the PAA/PNIPA mixture. The gels prepared by (i) and (iv) contain the AA residues randomly distributed within the network, while the AA residues in the gels prepared by (ii) and (iii) are localized along the PAA chain within the network. This difference in the AA distribution between the former and latter gels was found to result in two clearly different sets of swelling curves when the degree of swelling was examined at pHs 3 and 10 as a function of temperature. At pH 10, at which a complete dissociation of the COOH groups takes place, the gels prepared by (ii) and (iii) underwent a volume phase transition at around 33 degrees C, whereas the gels obtained via (i) and (iv) were in a swollen state over the temperature range measured (25-50 degrees C). At pH 3, at which most of the COO- ions are protonated, all the gels collapsed at temperatures >33 degrees C. However, the locally distributed AA residues reduced the gel volume in both the swollen (<33 degrees C) and the collapsed states (>33 degrees C), which suggests that hydrogen bonding, in addition to hydrophobic interaction, plays an important role in the gel collapse. The possibility that hydrogen bonding occurs under acidic conditions was supported by studying the effects of pH and urea on the complexation between PNIPA and PAA using the dynamic Light scattering technique. In conclusion, the present study suggests a strong influence of the "distribution" of ionic groups in the network on the volume phase transition of ionic gels, the nature of which can no longer be explained in terms of the concept of osmotic pressure arising from mobile counterions within the gel phase.