Self-Assembly Behavior of Thermo- and Ph-Responsive Diblock Copolymer of Poly(N-isopropylacrylamide)- Block-Poly(acrylic acid) Synthesized Via Reversible Addition-Fragmentation Chain Transfer Polymerization

Abstract

Diblock copolymer of poly(N-isopropylacrylamide)-block-poly(acrylic acid) (PNIAAm-b-PAA) with designed molecular weight and narrow molecular weight distributions was synthesized by sequential reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide (NIPAAm) and tert-butyl arcylate (tBA) using trithiocarbonate as a chain transfer agent (CTA), followed by selective hydrolysis of the PtBA block. The polymerization kinetic exhibited pseudo first-order behavior and the molecular weight increased linearly with the increase of the monomer conversion, which confirmed a controlled polymerization. Well defined homopolymer PNIPAAm and copolymer poly(N-isopropylacrylamide)-block-poly(tert-butyl arcylate) (PNIAAm-b-PtBA) were synthesized and characterized with 1H -NMR and GPC analysis. Temperature- and pH-sensitive block copolymer of PNIAAm-b-PAA was obtained after hydrolysis of PNIAAm-b-PtBA. The block copolymers formed large micelles in aqueous solutions in dependence on temperature or pH. The self-assembly behavior and morphology of the double responsive block copolymers in aqueous solution were investigated by dynamic light scattering (DLS) and atomic force microscopy (AFM). Although the self-assembly behavior was strongly influenced by hydrogen bonding interaction between the PNIPAAm and PAA blocks in aqueous solutions, the reversible micelles were observed above the phase transition temperature or below the critical aggregation pH value.