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

Abstract

A series of ionic networks were prepared by copolymerization of acrylamide, methylenebis-(acrylamide), and N-[2-(alkyldimethylammonio)ethyl]acrylamide with Cl, 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.