Novel shrinking-swelling hysteresis in DMSO/THF mixtures of copolymer gels containing sulphonate groups

Abstract

The swelling volume of copolymer gels made from 2-(acrylamido)-2-methylpropanesulphonic acid (AMPS; electrolyte monomer), N,N-dimethylacrylamide (DMAA; comonomer) and N,N′-methylenebis(acrylamide) (BIS; crosslinker) was measured in mixtures of two organic solvents, dimethyl sulphoxide (DMSO) and tetrahydrofuran (THF) at 25°C. A volume phase transition was observed in the gels having 43·18mol% AMPS with different counterions K+, Na+ and H+, when the THF concentration in the mixture increased beyond a certain value. We observed a swelling-hysteresis between the shrinking and swelling curves, i.e. more DMSO was required to make a shrunk gel start to swell again than to cause the volume shrinking. This hysteresis stems from the formation of ion pairs and/or multiplets in the ionizable gels when the medium polarity is decreased. An excess of DMSO is required to reduce the ionization energy for making the shrunk gel swell again in the DMSO/THF mixture. These aggregates and multiplets do not seem to be entirely removed by the swelling process and act as additional physical crosslinks, so that the DMSO concentration required to induce the shrunk gel to swell becomes higher in the next swelling process, showing another hysteresis. The release of bound water from the solvating layer after shrinkage may be another possible reason for this hysteresis. © 1998 Society of Chemical Industry