Synthesis and Protein Adsorption Resistance of PEG-Modified Poly(N-isopropylacrylamide) Core/Shell Microgels

Thermoresponsive poly(N-isopropylacrylamide) (pNIPAm) nanoparticles grafted with poly(ethylene glycol) (PEG) chains were prepared via free-radical precipitation polymerization, using PEG monomethyl ether monomethacrylate (PEG-MA) as a comonomer. The breadth of the particle size distributions was found to increase with the amounts of the PEG-MA used, along with a broadening of the volume phase transition of the particles and a transition shift to higher temperatures. However, these effects were minimized by spatially localizing the PEG to the particle periphery in core−shell pNIPAm particles prepared using a two-stage precipitation polymerization method. Reduced protein adsorption on the particle surface was observed as a result of incorporation of PEG chains into the particles, especially when the PEG chains were located in the particle shell. The protein adsorption measurements also suggest that the PEG side chains may stretch outward from the particle surface as the particles collapse at temperature above the transition temperature. The increased mobility of the PEG chains, presumably due to this surface extension, was confirmed by variable temperature 1H NMR studies for both core and core−shell particles. Such effects are also observed for particles where the PEG chains are localized in the particle core, which is then surrounded by a pNIPAm shell. These results suggest that the PEG grafts can penetrate the pNIPAm shell when it is in its phase-separated state.