(618j) Synthesis and Characterization of Phase Transition of Aqueous Poly-(N-isopropylacrylamide-ran-acrylic acid) Solutions As a Function of pH

The synthesis and characterization of a random copolymer of N-isopropylacrylamide and acrylic acid (pNIPAAm-ran-AA) has been presented. The polymer is synthesized by RAFT (radical addition fragmentation chain transfer) polymerization. Liquid state 1H-NMR is used to determine extent of monomer conversion. Molecular weight and weight distribution has been estimated by MALDI-MS. The polymer molecular weights agree with those predicted and have narrow weight distributions with PDIs less than 1.1. Copolymer composition has been determined via estimation of acrylic acid content by potentiometric titration. The estimated copolymer composition matches fairly well with the feed composition. RAFT polymerization thus provides good control over polymer molecular weight and weight distribution. Also, the monomer incorporation is governed by the relative abundance of monomers in the feed.
NIPAAm undergoes an inverse phase solubility transition with a rise in temperature at 32 degrees C. This transition for NIPAAm by itself, is pH insensitive but polymerization with an ionizable comonomer causes the transition to become pH-dependent. The effect of acrylic acid, on polymer critical solution temperature has been studied by transmittance measurements on samples varying in copolymer composition and degree of ionization (a). The lower critical solution temperature (LCST) of pNIPAAm-ran-AA increases with increasing acrylic acid content and ionization. Also, the rate of change of LCST with acrylic acid ionization is higher for samples containing higher amounts of acrylic acid. This suggests that the phase transition of pNIPAAm-ran-AA can be set to a desired temperature by changing solution pH and the degree of pH dependence can be modified by changing the acrylic acid incorporation in the copolymer.