PNIPAAM [poly-(N-isopropylacrylamide)] is one of the most interesting and promising "smart" gels. It undergoes a reversible phase transition in response to external temperature changes. The PNIPAAM matrix, swollen in aqueous solution, collapses as the temperature is increased above the lower critical solution temperature (LCST), which is about 33°C. Due to this thermoresponsive ability, these gels are promising candidates for thermal switches, micro/nanoactuators or controlled-release systems.
In order to provide information on the local structural and mechanical properties of PNIPAAM, we employ scanning force microscopy (SFM) in air or in water at various temperatures below and above the LTSC. SFM images of the gel surface are compared with those obtained in dry, swollen, and collapsed states using field emission scanning electron microscopy (FESEM).
SFM and FESEM images of the dry hydrogel surface reveal similar structural features (cooperation with R. Reichelt, IMPB, University of Münster, Germany). The surface is rather smooth except for small spherically shaped protrusions with a diameter and a height ranging from 10 to 50 nm and from 5 to 15 nm, respectively. FESEM of a cryogenically dried PNIPAAM sample swollen in water at 20°C reveals a coral-like structure with cavities of ~40 nm.
Force vs. cantilever displacement curves are measured with both spherical (µm-sized) and commercial probes. Indentation of the PNIPAAM surface as a function of the probe load is evaluated using the Hertz model to determine the local elastic moduli at different temperatures. For the swollen state at 10°C Young's modulus is found to be 1.71 kPa, which is more than 100 times lower than for the collapsed state at 35°C. More generally, this modulus is significantly lower than the moduli measured for biological cells.
Spongy structure of a swollen PNIPAAm gel surface as obtained by FESEM of a freeze-dried sample. The image size is 2 µm x 2 µm.
Schematic view of a glass sphere (left) and a sharp tip (right) on a cantilever before indenting the PNIPAAm gel surface. The images are photo assembled on the basis of FESEM micrographs after rescaling (IMPB).