Tunable swelling kinetics in core-shell hydrogel nanoparticles

Thermoresponsive, core-shell poly-N-isopropylacrylamide (p-NIPAm) nanoparti cles (microgels) have been synthesized by seed and feed precipitation polym erization, and the influence of chemical differentiation between the core a nd shell polymers on the phase transition kinetics and thermodynamics has b een examined. The results suggest that the core-shell architecture is a pow erful one for the design of colloidal "smart gels" with tunable properties. To examine these materials, differential scanning calorimetry (DSC), H-1 N MR, and temperature-programmed photon correlation spectroscopy (TP-PCS) hav e been employed. These measurements show that the addition of small concent rations of a hydrophobic monomer (butyl methacrylate, BMA) into the particl e shell produces large decreases in the rate of thermo-induced particle col lapse. Conversely, these low levels of hydrophobic modification do not pert urb the thermodynamics of the particle phase transition. When these results are examined in light of previous studies of macroscopic hydrogels, they s uggest that the formation of a thin, stable skin layer at the particle exte rior during the early stages of particle collapse is the rate limiting fact or in particle deswelhng. Finally, the hydrophobicity (BMA content) of the shell determines the magnitude of the hydrogel collapse rate, while the thi ckness of the BMA containing region does not impact the observed kinetics. Together, these results suggest that control over the kinetics of microgel deswelling events can be accomplished simply by modification of the particl e periphery, and therefore do not require homogeneous modification of the e ntire polymer structure.