Use of environmental scanning electron microscopy to image poly(N-isopropylacrylamide) microgel particles

Poly(N-isopropylacrylamide) microgel samples previously characterized with several techniques [M.J. García-Salinas, M.S. Romero-Cano, F.J. de las Nieves, J. Colloid Interface Sci. 241 (2001) 280-285] are directly observed in this work using an Environmental Scanning Electron Microscope (ESEM). This is a novel approach to microgel studies, because the particles are close to their "natural" or hydrated state. In the ESEM sample chamber, the relative humidity can be chosen by controlling independently water vapour pressure and sample temperature. Microgel particle size is affected by changes in these variables. The changes in diameter are followed for a set of individual, specific particles, detecting for each of them a size increase with relative humidity. This shows the possibility of following a dynamic process (i.e. hydration) in situ, for a specific particle. Several sets of images in different conditions are obtained for two microgels with different amount of cross-linker and co-monomer. The swelling behaviours are compared. A linear relation is found between particle diameter measured from ESEM images and relative humidity, the slope being positive and higher for particles with a lower level of cross-linker. It is shown that extrapolating data from ESEM measurements is a good method to estimate collapsed diameters. The collapsed particle diameters measured from Transmission Electron Microscopy (TEM) and ESEM are in good agreement. Thus, ESEM technique provides size data similar or complementary to previous measurements obtained by other techniques, i.e. photo-correlation spectroscopy (PCS) and TEM. In this way, the whole range of microgel sizes, from the biggest (swollen) diameter to the collapsed one, that is, the complete shrinking behaviour is studied combining different devices.