Poly(N‐isopropylacrylamide) grafted to a strongly charged backbone: Thermoresponsive behavior in aqueous solution

The thermoresponsive properties in aqueous solution of the graft copolymer poly(acrylic acid‐ co ‐2‐acrylamido‐2‐methyl propane sulfonic acid)‐ g ‐poly( N ‐isopropylacrylamide) [P(AA‐ co ‐AMPSA)‐ g ‐PNIPAM] were studied and compared to the corresponding behavior of the poly(acrylic acid)‐ g ‐poly( N ‐isopropylacrylamide) (PAA‐ g ‐PNIPAM) graft product. Both products contain about 40% (w/w) of PNIPAM, whereas the backbone, P(AA‐ co ‐AMPSA), of the first copolymer contains about 40% of AMPSA mole units. The strongly charged P(AA‐ co ‐AMPSA)‐ g ‐PNIPAM graft copolymer was water soluble over the whole pH range, whereas the PAA‐ g ‐PNIPAM copolymer precipitated out from water at pH < 4. As a result, the first product exhibited a temperature‐sensitive behavior in a wide pH range, extended in the acidic region, whereas in semidilute aqueous solutions, an important thermothickening behavior was observed, even at low pH (pH = 3.0).