From poly(N-isopropylacrylamide)-block-poly(ethylene oxide)-block-poly(N-isopropylacrylamide) triblock copolymer to poly(N-isopropylacrylamide)-block-poly(ethylene oxide) hydrogels: Synthesis and rapid deswelling and reswelling behavior of hydrogels

Poly(N-isopropylacrylamide)-block-poly(ethylene oxide)-block-poly(N-isopropylacrylamide) (PNIPAAm-b-PEO-b-PNIPAAm) triblock copolymer was synthesized via the reversible addition-fragmentation chain transfer/macromolecular design via the interchange of xanthate (RAFT/MADIX) process with xanthate-terminated poly(ethylene oxide) (PEO) as the macromolecular chain transfer agent. The successful synthesis of the ABA triblock copolymer inspired the preparation of poly(N-isopropylacrylamide)-block-poly(ethylene oxide) (PNIPAAm-b-PEO) copolymer networks with N,N'-methylenebisacrylamide as the crosslinking agent with the similar approach. With the RAFT/MADIX process, PEO chains were successfully blocked into poly(N-isopropylacrylamide) (PNIPAAm) networks. The unique architecture of PNIPAAm-b-PEO networks allows investigating the effect of the blocked PEO chains on the deswelling and reswelling behavior of PNIPAAm hydrogels. It was found that with the inclusion of PEO chains into the PNIPAAm networks as midblocks, the swelling ratios of the hydrogels were significantly enhanced. Furthermore, the PNIPAAm-b-PEO hydrogels displayed faster response to the external temperature changes than the control PNIPAAm hydrogel. The accelerated deswelling and reswelling behaviors have been interpreted based on the formation of PEO microdomains in the PNIPAAm networks, which could act as the hydrophilic tunnels to facilitate the diffusion of water molecules in the PNIPAAm networks. (C) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012