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

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.