For the increasing demands of multifunctional materials in applications such as drug delivery system, a pH- and temperature-responsive polyelectrolyte copolymer gel system was studied using rheometry. Rheological properties, determined by plate-plate rheometry in oscillatory shear, of hydrogels formed by free radical initiated copolymerization of N-isopropylacrylamide (NIPA) and 2-acrylamido-2-methylpropanesulphonic acid (AMPS) in the presence of methylene bisacrylamide (MBAA) as crosslinker are compared with the properties of semi-interpenetrating network (SIPN) polyelectrolyte gels made by incorporation of poly(ethylene glycol) with molar mass 6000 g mol(-1) (PEG6000). Based on our systematic studies for this PEG/SIPN system, the effects of initiator and crosslinker concentration, relative proportions of comonomer units in the main chains, PEG6000 content and temperature on viscoelastic properties, unusual high storage moduli at small strain for the SIPN were discussed. The SIPN gel with characteristics of PEG molecules as well as pH and temperature responsiveness from AMPS and NIPA units has potential application in drug delivery system design. Ice-like rheological behavior of the PEG/AMPS-NIPA SIPN gels at low temperature was first time reported and water remains homogeneous without phase separation in PEG/AMPS-NIPA SIPN hydrogels at low temperature may be considered as an ideal candidate for water storage material. (c) 2008 Wiley Periodicals, Inc.