Charge density dependence of solution and gel behaviors of maleic acid-containing polyelectrolytes

Two kinds of polyelectrolytes containing maleic acid component (MA copolymers), i.e., poly(styrene-alt-maleic acid) (PSMA) and poly(vinyl methyl ether-alt-maleic acid) (PVMEMA), were investigated on their polymer chain dimensional changes in solution, and also in gel phase for the latter, as a function of the polymer charge density or pH of the (immersing) solution. Being different from common poly(carboxylic acid)s such as poly(acrylic acid), both of the MA copolymers showed maximum in the reduced viscosity or the gel size with increasing the charge density or the solution pH. The maximum in the reduced viscosity was much more significant for PSMA than PVMEMA. To see if intramolecular hydrogen bond between a pair of dissociated and undissociated carboxyl groups in an MA residue contributes to the emergence of the peak or not, similar measurements were performed also in the presence of concentrated urea (5 M). Almost the same values were obtained up to the peak position for the reduced viscosity and the gel swelling degree with increasing pH of the (immersing) solutions, which strongly suggested that the supposed effect is negligible, if any. All the results including those for PVMEMA system were interpreted in terms of the ionomer-like conformational change (ion cluster formation) of polyelectrolytes that has been often observed when the polymer charges are in rather less polar circumstances.