IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Applied Surface Analysis Wednesday Sessions
       Session AS-WeM

Paper AS-WeM3
Micromechanical Properties of 'Smart' Gels: A Study of PNIPAAm by Scanning Force and Scanning Electron Microscopy

Wednesday, October 31, 2001, 9:00 am, Room 134

Session: Biomaterials and Polymers
Presenter: T.R. Matzelle, Universite Libre de Bruxelles, Belgium
Authors: T.R. Matzelle, Universite Libre de Bruxelles, Belgium
R. Reichelt, University of Muenster, Germany
N. Kruse, Universite Libre de Bruxelles, Belgium
Correspondent: Click to Email
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 employed scanning force microscopy (SFM) in air or in water at various temperatures below and above the LTSC. 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 hydrogel surface revealed similar structural features. 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 revealed a coral-like structure with cavities of @tdA@40 nm. Force vs. cantilever displacement curves were measured with both, spherical (µmm-sized) and commercial probes. Indentation of the hydrogel surface as a function of the probe load was evaluated using the Hertz model to determine the local elastic moduli at different temperatures. For the swollen state at 10°C Young's modulus was found to be 1.11 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.