NOVEL SHRINKING-SWELLING HYSTERESIS IN DMSO THF MIXTURES OF COPOLYMERGELS CONTAINING SULFONATE GROUPS/

The swelling volume of copolymer gels made from 2-(acrylamido)-2-methy lpropanesulphonic acid (AMPS; electrolyte monomer), N,N-dimethylacryla mide (DMAA; comonomer) and N,N'-methylenebis(acrylamide) (BIS; crossli nker) was measured in mixtures of two organic solvents, dimethyl sulph oxide (DMSO) and tetrahydrofuran (THF) at 25 degrees C. A volume phase transition was observed in the gels having 43.18 mol% AMPS with diffe rent counterions K+, Na+ and H+, when the THF concentration in the mix ture increased beyond a certain value. We observed a swelling-hysteres is between the shrinking and swelling curves, i.e. more DMSO was requi red 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 decrea sed. An excess of DMSO is required to reduce the ionization energy for making the shrunk gel swell again in the DMSO/THF mixture. These aggr egates and multiplets do not seem to be entirely removed by the swelli ng process and act as additional physical crosslinks, so that the DMSO concentration required to induce the shrunk gel to swell becomes high er in the next swelling process, showing another hysteresis. The relea se of bound water from the solvating layer after shrinkage may be anot her possible reason for this hysteresis. (C) 1998 Society of Chemical Industry.