Mapping Polymer Heterogeneity Using Atomic Force Microscopy Phase Imaging and Nanoscale Indentation.
Mapping Polymer Heterogeneity Using Atomic Force
Microscopy Phase Imaging and Nanoscale Indentation.
Raghavan, D.; Gu, X.; Nguyen, T.; VanLandingham, M. R.;
Macromolecules, Vol. 33, No. 7, 2573-2583, 2000.
microscopy; mapping; polystyrene; polybutadiene; FT-IR;
blends; coatings; heterogeneity; nanoindentation; phase
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.