PHASE-TRANSITION IN SWOLLEN GELS .21. EFFECT OF ACRYLAMIDE QUATERNARY-SALTS WITH VARIOUS ALKYL LENGTHS ON THE COLLAPSE, MECHANICAL, AND SAXS BEHAVIOR OF POLY(ACRYLAMIDE) NETWORKS

A series of ionic networks were prepared by copolymerization of acrylamide, methylenebis-(acrylamide), and N-[2-(alkyldimethylammonio)ethyl]acrylamide with C1, C4, C6, C8, C12, and C16 straight-chain alkyls (mole fraction of the last comonomer x(I) = 0-0.15). Small-angle X-ray scattering, swelling, and mechanical behavior of the networks were investigated in water-ethanol mixtures. For the gels with C1-C8 alkyls collapse was found; both the volume jump and the critical ethanol concentration at which the transition takes place, e(c), increase with increasing content of the ionic component, x(I). Increasing the alkyl length stabilizes the expanded state of the gel and increases the e(c) values, probably due to preferential sorption of ethanol by hydrophobic regions. Different swelling behavior was found for gels with C12 and C16 alkyls, where mostly a decrease in swelling in water at low ethanol concentrations was observed with increasing x(I). This is caused by a distinct amphiphilic character of salts with the two longest alkyls; in networks with the C16 alkyl formation of micelles was proved by SAXS. Mechanical behavior of the networks is predominantly determined by the degree of swelling; a jumpwise change in the gel volume is accompanied by a similar change in the equilibrium modulus.