Synthesis And Characterization Of Smart Core/Shell Nanohydrogels

Smart hydrogels comprising environmentally responsive polymers continue to attract attention due to their potential applications in numerous fields, including drug delivery, chemical separations, sensors and catalysis. Perhaps the most widely study class of responsive nanohydrogel are pH-, temperature-, or both responsive hydrogels. Poly(N-isopropylacrylamide) (PNIPAM) is the most representative temperature-responsive poymer. Poly(methacrylic acid) (PMAA) is a pH-respensive water soluble polymer. We have successfully synthesized the surfactant-free poly(methacrylic acid) (PMAA) nanogels using a natural polymer hydroxypropylcellulose (HPC) as a template in aqueous solution. Finally, pH- and temperature-sensitive PMAA core/PNIPAM shell nanogels using PMAA nanogels as a template were synthesized in aqueous solution. The main achievements are as follows:(1)Different MAA concentrations affected the phase transition temperature of HPC solution and PNIPAM nanohydrogel. UV-Vis spectrophotometer measured absorbance of HPC solution at different MAA concentrations. After adding a small amount of MAA, the phase transition temperature of HPC decreased quickly. It is noted that an increase in MAA concentration resulted in the decreasing phase transition temperature of the HPC solution.(2) A series of pH-sensitive PMAA nanohydrogels were prepared by surfactant-free emulsion polymerization(SFEP) with different temperatures, HPC concentrations, MAA concentrations, BIS concentrations. The size of the resulting PMAA nanohydrogel ranged from 100 nm and 240nm. An increase in BIS and HPC concentration resulted in a decrease in the particle size. As monomer MAA concentration was increased, the size of the PMAA nanogels became larger. Specifically, As the PMAA nanogels were prepared at 24℃,0.20 wt% HPC,0.30 wt% MAA,0.1 wt% BIS, the size of the nanogels is 149.8 nm and its polydispersion index (PDI) is 0.119. Using the PMAA nanohydrogel as a template, monomer NIPAM in the PMAA nanogel dispersion was polymerized to form PMAA/PNIPAM core/shell nanohydrogel. The morphology of the nanodydrogels was characterized by atomic force microscope (AFM) and transmission electron microscope (TEM). The structure of PMAA nanohydrogels was studied by fourier trandform infrared spectrometer (FTIR). Dynamic light scattering (DLS) and UV-vis spectrophotometer indicate that PMAA nanohydrogels had good pH sensitivity.(3) Smart PMAA core/PNIPAM shell nanohydrogels with pH- and temperature- sensitivity were synthesized by using PMAA nanohydrogels as templet and N-isopropylacrylamide as monomer. The size of the resulting PMAA/PNIPAM nanohydrogels ranged from 200 nm to 658.3 nm were prepared with different NIPAM and BIS concentrations. The results showed that the size of PMAA/PNIPAM nanohydrogels increased with an increasing NIPAM monomer concentration but decreased as increasing crosslinker concentration. The size of PMAA/PNIPAM nanohydrogels synthesized with 0.267wt% BIS,1 wt% NIPAM,0.267 wt% APS initiator and 0.10 wt% accelerator TMEDA is 338.8 nm. The narrow dispersivity of PMAA/PNIPAM nanohydrogels is 0.164. The size variation of PMAA/PNIPAM nanohydrogels was measured by dynamic light scattering (DLS) and inner structure was characterized by fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR). The morphology of PMAA/PNIPAM nanohydrogels was characterized by atomic force microscope (AFM) and the formation of core/shell was characterized by transmission electron microscopy (TEM). Ultraviolet/visible spectrophotometer (UV) and DLS show that PMAA/PNIPAM nanohydrogels have been synthesized in which PMAA nanohydrogels are core and PNIPAM nanohydrogels are shell. PMAA/PNIPAM nanohydrogels demonstrated excellent pH responsiveness and temperature sensitivity.