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


pdf icon Mapping Polymer Heterogeneity Using Atomic Force Microscopy Phase Imaging and Nanoscale Indentation. (594 K)
Raghavan, D.; Gu, X.; Nguyen, T.; VanLandingham, M. R.; Karim, A.

Macromolecules, Vol. 33, No. 7, 2573-2583, 2000.

Keywords:

microscopy; mapping; polystyrene; polybutadiene; FT-IR; blends; coatings; heterogeneity; nanoindentation; phase imaging

Abstract:

Polymer coatings contain degradation-susceptible regions, and corrosion of 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 phase separating polystyrene (PS) and polybutadiene (PB) as a function of annealing time at 80 deg C. The PS/PB blend films are prepared by spin casting from solutions onto silicon substrates with thicknesses of approximately 250 nm. 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 PS and PB regions, and this contrast decreases with increases in the degree of oxidation of the PB region. A microphase of PB "droplets" within the PS domains is also observed at certain force levels. To understand the influence of relative surface stiffness on the phase images, nanoscale indentation measurements are made on the observed domains. Heated and unheated cast and free-standing PS films and PB films are also studied 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 films 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.