Synthesis Of Poly(N-isopropylacrylamide) Microgels By Dispersion Polymerization

Temperature-sensitive microgels can undergo volume phase transition in response to environmental temperature change stimuli, which is often accompanied by the change of their many physical and chemical properties such as, refractive index, voidage, surface charge density, colloid stability, rheology, etc. Thus, it has been widely utilized in many fields, for example, colloidal crystals, sensors, biological substance sepetation, drug controlled delivery, etc. Till now, the most intensely studied temperature sensitive microgel is poly(N-isopropylacrylamide) (PNIPAM) microgel, which undergo a lower critical solution temperature of approximately 32℃in aqueous solution close to human body temperature. In this paper, the synthesis of poly(tert-butyl methacrylate) (PtBMA) macromonomer was investigated in order to synthesize PNIPAM microgel by dispersion polymerization using macromonomer. Then, temperature-sensitive PNIPAM microgels were prepared by dispersion polymerization in the medium of water or water/ alcohol mixture. Finally, doubly temperature-sensitive core-shell microgels were obtained using one-step or two-step soapless emulsion polymerization. The detailed results are as follows:(1) PtBMA macromonomer with one vinyl end group per polymer chain was synthesized by chain transfer free radical polymerization in the presence of 2-mercaptethanol as chain transfer agent and end capping reaction with p-chloromethyl styrene. The molecular weight of PtBMA oligomer or PtBMA macromonomer was well-controlled by the feed amount of 2-mercaptethanol.The PtBMA macromonomer,of which the introducing probability of the vinyl end group was 85 % , could be prepared by controlling the amount of p-chloromethyl styrene and TBPB(tetrabutytphonium bromide) as phase transfer reaction catalyst.(2) Temperature-sensitive PNIPAM microgels were synthesized by dispersion polymerization in the presense of poly(N-vinylpyrrolidone) (PVP) as stabilizer and water or alcohol/water mixture as dispersion medium. The results indicated that the particle size and swelling ratio of the PNIPAM microgels can be controlled by changing the amount of the stabilizer and the polarity of the medium, but their phase transition behavior was not influenced by the two factors. The particle size, swelling ratio and phase transition behavior of the PNIPAM microgels were markedly influenced by the amount of crosslinker used. It was speculated that the PNIPAM microgel nucleus may be formed by graft copolymer coalescence mechanism, and their growth were conducted by the aggregate between the primary PNIPAM particles . (3)Doubly temperature-sensitive PNIPAM/poly(N-isopropylme thacrylamide (PNIPMAM) (or PNIPMAM/PNIPAM) core-shell microgels were obtained using one-step or two-step soapless emulsion polymerization. The results obtained by dynamic light scattering and spectrometer indicated the microgels have double temperature-sensitivity, regardless of the polymerization process used. SEM observation result showed that the microgels exhibited spherical shape, and their size distribution was relatively homogeneous.