Novel smart core-shell microgels : synthesis, characterization and applications

Abstract

A novel method has been developed to synthesize well-defined temperature- and pH-sensitive core-shell microgels via a graft copolymerization of N-isopropylacrylamide (NIPAM) or N-vinylcaprolactam (NVCL) from amino-containing water-soluble polymers such as branched poly(ethyleneimine) (b-PEI), chitosan (CTS) or gelatin. The microgel particles are consisted of poly(N-isopropylacrylamide) (PNIPAM) or Poly(N-vinylcaprolactam) (PNVCL) cores crosslinked with 1% of N,N'-methylene-bisacrylamide (MBA) and water-soluble polymers shells. The hydrodynamic diameters of these smart microgels ranged from 100 to 400 nm with narrow size distributions. The unique core-shell nanostructures of microgels exhibited individual responsiveness with tuneable properties to pH and temperature. Optimal reaction conditions were systematically investigated including effects of reaction temperature, stirring rate, addition method of the initiator, electrolyte concentration, pH of reaction media, initiator concentration, crosslinker concentration, solid content and pH-sensitive polymer to temperature-sensitive monomer charge weight ratio. The compositions of the core-shell microgels were characterized with FTIR spectroscopy and high resolution proton NMR. The particle size, size distribution, surface charge and morphology of the microgel particles were determined with dynamic laser light scattering, zeta-potential measurement and scanning electron microscopy (SEM). Transmission electron microscopic (TEM) images of the particles clearly showed well-defined core-shell morphologies where PNIPAM or PNVCL cores were coated with hairy PEI, CTS or gelatin shells. The temperature-sensitive properties of these microgels were studied through determinations of the lower critical solution temperature (LCST) or the volume phase transition temperature (VPTT) using an UV-visible spectrometry and dynamic laser light scattering methods. The pH-sensitive properties were characterized by measuring variation of particle size as a function of pH. Electrolyte concentration was also found to affect the particle size, volume phase transition temperature and particle stability. Applications of PNIPAM/PEI and PNIPAM/chitosan microgels in affinity protein separation and deliveries of ionic and non-ionic drugs were explored.