Hydrogel microspheres (microgels) are hydrophilic polymeric particles that can respond rapidly to external stimulus owing to their colloidal size dimensions. Thus, microgels are expected to find numerous applications such as molecular separation, drug-delivery vehicles and sensors. Up to now, the stimulus-responsive behavior of the microgels has been mainly investigated by scattering techniques, calorimetry, sensors, and so forth. However, these techniques provide ensemble-averaged information for the entire system. In order to obtain deeper insight into the stimulus-responsive behavior of individual microgels, a combination of conventional techniques and real-time direct observation is important. In this study, we applied high-speed atomic force microscopy (HS-AFM) to visualize dynamic behavior of individual microgels at nanoscale in real time. Here, we investigated thermoresponsive behavior of poly(N-isopropyl acrylamide) (pNIPAm) based microgels prepared by precipitation polymerization by combining dynamic light scattering (DLS) and HS-AFM observation. Figure (a) shows the temperature dependences of the normalized height of the microgels obtained from the HS-AFM images (nonequilibrium states) and the normalized hydrodynamic diameter (Dh) of the microgels determined by DLS (equilibrium states). The tendencies of the thermoresponsive behavior for the normalized height and Dh of the microgels were similar, indicating the rapid stimulus-responsiveness of microgels. Notably, domain structure (several tens of nanometer in size) was observed in the microgels even below the volume phase transition temperature (VPTT, ~35 °C) of the microgels (Figure (b)) and became clear with deswelling of the microgels. In this presentation, further investigations regarding the domain formation in the microgels synthesized under the different polymerization conditions will be discussed.

Evaluation of thermoresponsive structural changes in hydrogel microspheres by high-speed atomic force microscopy

2019 Fall National Meeting

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