Core-shell microgels synthesized in continuous flow: deep insight into shell growth using temperature-dependent FTIR.

Abstract

While core-shell microgels have been intensively studied in their fully synthesized state, the formation mechanism of the shell has not been completely understood. Such insight is decisive for a customization of microgel properties for applications. In this work, microgels based on a <i>N</i>-isopropylmethacrylamide (NiPMAM) core and a <i>N-n</i>-propylacrylamide (NnPAM) shell are synthesized in a continuous flow reactor. The shell growth is studied depending on the solution's time of residence inside the reactor. PCS experiments reveal a significant decrease of the volume phase transition temperatures of the core and the shell, with increasing residence time. At early stages, a decreased swelling capacity is found before a discrete NnPAM shell is formed. Temperature-dependent FTIR spectroscopy shows that the decreased swelling capacity originates from a pronounced interpenetrated network (IPN) between NnPAM and NiPMAM. AFM images resolve heterogeneously distributed shell material after 3 min, pointing to an aggregation of NnPAM domains before the distinct shell forms. The combination of diffusional properties, AFM images and vibrational information confirms a deeply interpenetrated network already at early stages of the precipitation polymerization, in which the shell material heavily influences the swelling properties.