Solution properties and phase behaviour of copolymers of acrylic acid with N-isopropylacrylamide: the importance of the intrachain hydrogen bonding

Abstract

The solution properties and the phase behaviour of random copolymers of N-isopropylacrylamide (NIPAM) with acrylic acid (AA) have been investigated. The mole fraction of NIPAM in these copolymers (x) varies from 0 to 0.29. At pH=3.00, i.e. when the AA groups are in the undissociated form, the intrinsic viscosity decreases substantially as x increases. This chain shrinkage is attributed to the formation of intrachain hydrogen bonds between the two complementary groups, AA and NIPAM. The weakening of the hydrophilic character and the appearance of hydrophobic properties with increasing x is further supported by fluorescence probing studies and potentiometric measurements of the aqueous copolymer solutions. Moreover, the phase behaviour of these copolymers in salt solution changes dramatically with x. The copolymer with x=0.10 presents, like the homopolymer poly(acrylic acid), an Upper Critical Solution Temperature (UCST) behaviour. On the contrary, the copolymer with x=0.29 presents, similar to the homopolymer poly(N-isopropylacrylamide), a Lower Critical Solution Temperature (LCST) behaviour. Finally, the copolymer with the intermediate NIPAM content, x=0.17, presents both an UCST and an LCST behaviour.