Micromechanical Properties of “Smart” Gels: Studies by Scanning Force and Scanning Electron Microscopy of PNIPAAm

Abstract

Mechanical properties of PNIPAAm [poly-(N-isopropylacrylamide)] gel surfaces were studied using scanning force microscopy (SFM) in air or in water at various temperatures below and above the reversible phase transition at ∼33 °C. SFM images of the gel surface were compared with those obtained in dry, swollen, and collapsed states using field emission scanning electron microscopy (FESEM). Images of SFM and FESEM of the dry gel surface revealed similar structural features. Force vs displacement curves were measured with both spherical (micrometer sized) and commercial probes. Indentation of the gel surface as a function of the probe load was evaluated to determine the local elastic modulus. For the swollen state at 10 °C, Young's modulus was found to be more than 100 times lower than for the collapsed state at 35 °C.