Mapping Polymer Heterogeneity Using Atomic Force Microscopy Phase Imaging and Nanoscale Indentation

Abstract

Polymer coatings often contain degradation-susceptible regions, and corrosion of the metallic substrate can occur directly underneath these regions. In this paper, the microstructure of model coating materials is investigated using atomic force microscopy (AFM). Specifically, AFM is used to study heterogeneity in thin film blends of polystyrene (PS) and polybutadiene (PB) as a function of annealing time at 80 °C. PS/PB blend films with thicknesses of approximately 250 nm are prepared by spin casting from solutions onto silicon substrates. Both topographic and phase imaging in tapping mode AFM are performed on these films under ambient conditions and at different force levels using a silicon tip. For certain force levels, phase imaging provides good contrast between the phase-separated PS and PB regions, primarily because of the large compliance difference between the two materials. This contrast decreases with increasing annealing time because thermal oxidation causes cross-linking in PB, and thus, the compliance of the PB region increases toward that of PS. Nanoscale indentation measurements are then made on the observed phase-separated regions to identify these regions as PS-and PB-rich and to better understand the influence of relative surface stiffness on the phase images. Cast and free-standing films of pure PS and pure PB are also studied as a function of annealing time using AFM, contact angle measurements, Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). Results from studies of the individual PS and PB films are related to the AFM results for the blend films. The use of phase imaging for cure monitoring of polymers and for studies of chemically heterogeneous polymer systems is also discussed.

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…Similarly, stiffness measurements have been exploited to characterize heterogeneity in thin film blends of polystyrene and polybutadiene [288,289] and in polymer brush layers from poly(styrene-co-pentafluorostyrene) and polymethylacrylate [290]. In this last study the temperature dependence of the elastic response of polymer brushes also is addressed, showing that changes in the elastic behavior of the polymer sample, probed through force-displacement curves, can be exploited to determine the glass transition temperature of the polymer.…”

Phase behavior of UCST blends: Effects of pristine nanoclay as an effective or ineffective compatibilizer

“…The phase contrast images of the scanning force microscopy have been deduced to be sensitive to the sample surface properties such as modulus, viscoelastic characteristics and chemical composition [28]. Thus, different components of PE/EVA/MMT films having highly different stiffness and viscoelasticity were contrasted with each other sharply, especially in the phase-mode image of PE70EVA30MMT3 sample.…”

Solubilization of Genistein by Caseinate Micellar System

“…In the presence of genistein, the sub-micelle diameters are twice those of caseinate alone 24 nm versus 12 nm; phase images to hard or soft domains is not always straightforward 30 . However, in this study, dark and bright spots clearly indicate softer and more rigid materials, respectively.…”