Discovery of a unique dual response in acrylonitrile copolymers

Poly(acrylonitrile) is not reversibly sensitive to either changing temperature or changing pH. However, the addition of acrylic acid (AA) as a comonomer induces the pH response to this polymer. In this study, for the first time, the copolymers of acrylonitrile and acrylic acid were shown to possess a unique dual response to pH and temperature. For this, a series of acrylonitrile-based copolymers were synthesized with varying AA content from 10 to 50 mol% using free radical copolymerization. These copolymers were solution-spun from a DMF–water system and heat-set to give stable fine fibers of 120 ± 3 µ diameter without any chemical crosslinking. The fibers showed temperature sensitivity in an aqueous alkaline bath. At pH 10, the volumetric swelling of fibers containing 30 mol% of AA initially increased from 3810% to 6440% with increase in temperature of the bath from 30 to 49 °C, and suddenly decreased to 4200%, near to the original swelling level, when the temperature reached 50 °C. The copolymer was found to have a clear and sharp transition temperature between 49 and 50 °C. The behavior shown was unlike other temperature-responsive polymers, where it is a step transition from the swollen to deswollen state. The transition temperature could be tuned in the range of 40 to 62 °C by varying the composition of acrylic acid from 10 to 50 mol% in the copolymers. The transition temperature was also found to increase with increasing pH of the surrounding bath. All transitions were reversible and stable to repeated cycling.