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

The solution properties and the phase behaviour of random copolymers of N-i sopropylacrylamide (NIPAM) with acrylic acid (AA) have been investigated. T he mole fraction of NIPAM in these copolymers (x) varies from 0 to 0.29. At pH = 3.00, i.e. when the AA groups an in the undissociated form, the intri nsic viscosity decreases substantially as x increases. This chain shrinkage is attributed to the formation of intrachain hydrogen bonds between the tw o complementary groups, AA and NIPAM. The weakening of the hydrophilic char acter and the appearance of hydrophobic properties with increasing x is fur ther supported by fluorescence probing studies and potentiometric measureme nts of the aqueous copolymer solutions. Moreover, the phase behaviour of th ese copolymers in salt solution changes dramatically with x. The copolymer with x = 0.10 presents, like the homopolymer poly(acrylic acid), an Upper C ritical Solution Temperature (UCST) behaviour. On the contrary, the copolym er with x = 0.29 presents, similar to the homopolymer poly(N-isopropylacryl amide), a Lower Critical Solution Temperature (LCST) behaviour. Finally, th e copolymer with the intermediate NIPAM content, x = 0.17, presents both an UCST and an LCST behaviour. (C) 2000 Elsevier Science Ltd. All rights rese rved.