The Study On The Poly(N-Isopropylacrylamide) Hydrogel Microspheres With Narrow Phase Transition Temperature Range

Temperature-responsive poly(N-isopropylacrylamide) (PNIPAM) hydrogel microspheres have intriguing potential applications in on-off intelligent systems, such as sensors, controlled release system for drug, chemical valve. However, compared with discontinuous volume phase transition behavior of PNIPAM bulk hydrogels, PNIPAM hydrogel microspheres undergo continuous volume phase transition, that is, they swell or shrink in certain temperature range. The wide phase transition temperature range means their blunt temperature sensitivity, which is detrimental to their applications. Based on the sufficient investigation of the internal structure of PNIPAM hydrogel microspheres reported by other authors, the inhomogenous crosslinking structure from the interior to outside was thought to be the reason why they undergo continuous volume phase transition in this thesis, and the PNIPAM hydrogel microspheres of narrow phase transition temperature range were synthesized by semi-batch or continuous surfactant-free emulsion polymerization process or self-crosslinking process.PNIPAM hydrogel micorspheres were previously prepared by batch (surfactant-free or traditional) emulsion polymerization based on N-isopropylacrylamide (NIPAM) as monomer and N, N'-methylenebisacrylamide (MBA) as crosslinker. Continuous or semi-batch surfactant-free emulsion polymerization (SFEP) process was used to synthesize the PNIPAM hydrogel microspheres in this thesis, and their phase transition temperature ranges were markedly narrower than ones synthesized by batch SFEP process, among which continuous SFEP process had the strongest effect in narrowing the phase transition temperature range. In the case of the same crosslinker amount used, the PNIPAM hydrogel microspheres synthesized by continuous SFEP process had the greatest particle size and swelling ratio, while the batch PNIPAM hydrogel microspheres had the smallest ones. In comparison with the PNIPAM hydrogel microspheres synthesized by batch SFEP, the PNIPAM hydrogel microspheres by continuous or semi-batch SFEP had more homogeneous internal crosslinking structure, which was confirmed by the measurement of their swelling ratios against reaction time.The PNIPAM hydrogel microspheres can also be prepared by SFEP without crosslinker. The comparative analysis of solid ¹³C NMR spectra of linear PNIPAM and the self-crosslinked PNIPAM microspheres indicated that there are self-crosslinking points formed by chain transfer reaction in the microspheres. The phase transition temperature range of the self-crosslinked PNIPAM hydrogel microspheres was very narrow, and they had nearly noncontinuous phase transition behavior. The particle size and swelling ratio of the PNIPAM microspheres were more influenced by the reaction temperature, reaction time, and the amount of initiator used, while their phase transition temperature range was less effected by these factors.The PNIPAM microspheres prepared by batch, semi-batch and continuous surfactant-free emulsion polymerization process all had the ability to lower the surface tension of water medium below or above their phase transition temperature, and the rate of lowering surface tension was the greatest for continuous microspheres, and the lowest for the batch ones. The rheological measurement results showed that the storage modulus of the PNIPAM microsphere dispersion was very sensitive to the change of the environmental temperature.Guting(Material Science)Supervised by Professor Zha Liusheng...