Microthermal characterization of segmented polyurethane elastomers and a polystyrene-poly(methyl methacrylate) polymer blend using variable-temperature pulsed force mode atomic force microscopy

Pulsed force mode atomic force microscopy (AFM) has been used to study the morphology of three segmented polyurethane elastomers. High-contrast images are presented, showing their phase-separated structure on a scale of several tens of nanometers to a few hundred nanometers. The adhesion-dependent pull-off force signal is found to be far more sensitive to local variations in mechanical properties than the stiffness-dependent indentation force signal. An insight into the size, shape, and distribution of phases can be obtained from images constructed from spatial variations in surface adhesion properties. Comparison of images obtained both above and below the T-g of the soft-segment material allows the spatial distribution of different phases to be characterized and the effects of surface topography to be decoupled from real changes in localized mechanical properties. Typical domain sizes are found to be at least an order of magnitude larger than those obtained by small-angle X-ray scattering and AFM in previous studies. A novel method of constructing and interpreting AFM images of multiphase materials is used. The equipment and methodology are first demonstrated on a model polystyrene-poly(methyl methacrylate) blend.