Doubly Temperature Sensitive Core−Shell Microgels

We report on synthesis and characterization of doubly temperature sensitive core−shell microgels. These core−shell microgels are composed of a core of chemically cross-linked poly(N-isopropylacrylamide) (PNIPAM) and a shell of cross-linked poly(N-isopropylmethacrylamide) (PNIPMAM). PNIPAM exhibits in water a lower critical solution temperature (LCST) of ca. 34 °C, and the LCST of PNIPMAM is ca. 45 °C. Solution properties were investigated by means of dynamic light scattering (DLS), optical transmission, differential scanning calorimetry (DSC), and small-angle neutron scattering (SANS). Core−shell microgels of this composition display a temperature-dependent two-step shrinking behavior. The influences of the content of the cross-linking agent N,N‘-methylenbis(acrylamide) (BIS) and of the thickness of the PNIPMAM shell on the thermosensitive response of the PNIPAM/PNIPMAM core−shell microgels were investigated. Core−shell microgels with a thick shell do not show a size transition at the PNIPAM LCST anymore. The volume transition is adjustable by varying the cross-link density of the shell. The swelling behavior of the core−shell microgels is compared to that of pure PNIPAM and PNIPMAM. Additionally, an inverse system consisting of a PNIPMAM core and a PNIPAM shell was prepared and investigated by DLS. Here the collapsed shell at intermediate temperatures strongly restricts the core swelling so that the overall size of the core−shell microgel is smaller as compared to the pure core.