Study Of PH/Temperature Dually Responsive Of Polymer Microspheres With Interpenetrating Polymer Network Structure

As pH and temperature are two environmental conditions, which are very common and changed very easily in biological and chemical systems, the intelligent polymer microspheres which can respond to both pH and temperature stimuli have potential applications in many fileds, such as biological sensor, drug release, biomass separation, enzyme carrier, etc. In this thesis, the pH/temperature dually responsive polymer microspheres with interpenetrating polymer network (IPN) structure were synthesized, which overcame the drawbacks of the pH/temperature dually responsive polymer microspheres prepared by random copolymerization method, core/shell structure method or graft copolymerization method. Then, the microenvironment change of macromolecular chains of pH/temperature dually responsive IPN polymer microspheres during their phase transition were investigated by temperature-variable Fourier transform infrared (FTIR) spectrscopy. Finally, the dynamic rheological properties of the pH/temperature dually responsive IPN polymer microspheres latexes were studied. The main results obtained are as follows:(1) The pH/temperature dually responsive polymer microspheres with interpenetrating polymer network (IPN) structure based on poly(N-isopropylacrylamide) (PNIPAM) and poly(acrylic acid) (PAA) were synthesized by seed-emulsion polymerization using redox initiator at room temperature. The polymer microspheres were determined to be composed of PNIPAM and PAA by Fourier transform infrared spectrometer(FTIR). The content of PAA in the polymer microspheres increased with the used amount of acrylic acid (AA) monomer in feed recipe. The IPN structure in the microspheres was confirmed by transmission electron microscopy (TEM). The results obtained by dynamic laser light scattering (DLLS) under different pH and temperature conditions showed that the polymer microspheres had good pH and temperature stimuli responsiveness with little interference. The pH stimuli responsiveness of the microspheres was enhanced by the increase in PAA content within them.(2) The microenvironment change of macromolecular chains of pH/temperature dually responsive PNIPAM/PAA IPN polymer microspheres in D2O media during their phase transition were investigated by temperature variable Fourier transform infrared (FTIR) spectroscopy under different temperatures and pH values. The results indicated that, with the increase of pH value of D2O media, the COOH groups of PAA chains in the microspheres dissociated into COO-, leading to the phase transition induced by pH variation. However, the microenvironment of PNIPAM chains within the microspheres was not evidently changed with the pH variation, showing that the PNIPAM network holds relative independence. The redshift of the amideⅡbands and the blueshift of the amideⅠbands of PNIPAM component after temperature induced phase transition of the microspheres exhibited that the hydrogen bond between amide groups and water was broken, and the number of free amide groups was increased. In addition, the redshift of the C-H stretching bands for the isopropyl groups of PNIPAM component may be due to the breakdown of water cage structure around the groups.(3) The effect of temperature and pH on the dynamic rheology of PNIPAM/PAAIPN polymer microspheres aqueous dispersions were investigated by rotation viscometer. When the pH value of the dispersion was 7.0, the viscoelastic properties of polymer microspheres dispersion exhibited viscosity of liquid predominately. As the temperature increased and exceeded 32℃, their viscoelastic properties exhibited elastic property of soild, and the polymer microsphere dispersion became thicker and even underwent sol-gel transition. When the pH value of the aqueous dispersion was 3.0, the loss moduli G" of the dispersions was always larger than the storage moduli G'over the entire temperature range studied. The loss tangents (tan8) measured under different temperatures indicated the gelation temperature for the polymer microsphere dispersion was 33℃, which was in accordance with the VPTT of PNIPAM polymer microspheres.