Abstract

The research on soft materials is interdisciplinary. In the present work, we focus on “smart hydrogels” as the most promising representatives studied by complementary Scanning Force Microscopy (SFM) and high-resolution Scanning Electron Microscopy (SEM). The extremely large range of water uptake of these hydrogels (up to 103 times of their mass) on one hand and their distinct softness (Young´s modulus < some tens of kPa) on the other hand constitute real challenges for the characterization of their local structure, the caging of nano-scaled particles, and some micromechanical properties. SFM images were compared with those obtained by SEM in the dry, swollen and deswollen state of the “smart gel” PNIPAAm [poly-(N-isopropyl acrylamide)]. PNIPAAm reacts to tiny variations of the temperature around 33°C by significantly changing its volume. While both imaging techniques revealed very similar results concerning the surface structure in the dry state, highly resolved structural features remained unapproachable for SFM in the wet state. On the contrary, SEM at high resolution revealed after state-of-the-art cryo-preparation, freeze-drying and subsequent ultrathin Pt/C-coating a sponge-like structure with cavity sizes of around 40 nm in the swollen state and 20 nm in the deswollen state. SFM proved to be an appropriate instrument revealing the local elastic surface properties. For example, at the swollen state at 10 °C, Young’s modulus was found to be more than 100 times lower than for the deswollen state at 35 °C. In addition, SEM also proved to be very suitable for the structural research of microgels and filled hydrogels. The application of both SFM and SEM contributes complimentarily to a characterization of different hydrogel systems at different states.