Synthesis and characterization of multiresponsive core-shell microgels

We report the synthesis and characterization of temperature and pH responsive hydrogel particles (microgels) with core-shell morphologies. Core particles composed of cross-linked poly(N-isopropylacrylamide) (p-NIPAm) or poly(NIPAm-co-acrylic acid) (p-NIPAm-AAc) were synthesized via precipitation polymerization and then used as nuclei for subsequent polymerization of p-NIPAm-AAc and p-NIPAm, respectively. The presence of a core-shell morphology was confirmed by transmission electron microscopy (TEM). Thermally initiated volume phase transitions were interrogated via temperature-programmed photon correlation spectroscopy (TP-PCS) as a function of solution pH. The p-NIPAm-AAc core hydrogel displays both a strong temperature and pH dependence on swelling. However, both p-NIPAm-AAc (core)/p-NIPAm (shell) and p-NIPAm (core)/p-NIPAm-AAc shell) particles display a more complex pH dependence than the homogeneous particles. Specifically, a multistep volume phase transition appears when the AAc component becomes highly charged at a high pH. It is apparent from the measured deswelling curves that a portion of the particle swelling behavior is dominated by p-NIPAm, regardless of its location in the particle. However, deswelling behavior that is due to a mixture of p-NIPAm-AAc and p-NIPAm is evident, as well as a regime that is largely attributed top-NIPAm-AAc alone. Small differences in the effect of pH on the two core-shell particles indicate that the influence of p-NIPAm is somewhat greater when it is localized in the shell.