Effect of N-isopropylacrylamide thermoresponsive blocks on the rheological properties of water-soluble thermoassociative copolymers synthesized via RAFT polymerization

Abstract

The effect of poly N-isopropylacrylamide (PNIPAM) blocks on the thermoresponsive behavior of multiblock copolymers with polyacrylamide (PAM) backbones synthesized via RAFT polymerization in aqueous solution is examined. These multiblock copolymers (or multistickers) were prepared by the sequential polymerization of acrylamide and N-isopropylacrylamide. Initially, a hydrophilic macro-RAFT PAM is synthesized using acrylamide (AM), [S, S′-bis(α,α′-dimethyl-α″-acetic acid)-trithiocarbonate (DMAT)] as a chain transfer agent, and 4,4′-azobis(4-cyanopentanoic acid) (ACPA) as an initiator. Chain extensions were carried out by the sequential polymerization of N-isopropylacrylamide (NIPAM) or acrylamide (PAM). Multiblock copolymers (tri-, penta-, hepta- and nonablock) were acquired by the insertion of 1, 2, 3 or 4 NIPAM blocks, respectively. The resultant copolymers were characterized by nuclear magnetic resonance (NMR) spectroscopy, size-exclusion chromatography (SEC), dynamic light scattering (DLS), ultraviolet-visible (UV-vis) spectroscopy and rheometry. Rheological analyses were performed at different temperatures from 25 to 70 °C, and the data showed an increase in the apparent viscosity of the copolymers from the macro-chain transfer agent to the nonablock copolymer. The lower critical solution temperature (LCST) was measured for each polymer, and the resulting values were found fluctuate as a function of the number of thermoresponsive blocks incorporated into the thermoresponsive copolymers.