Morphology of layered silicate- (NanoClay-) polymer nanocomposites by electron tomography and small-angle x-ray scattering

A basis for quantitative analysis of layered silicate(-) (nanoclay(-)) polymer nanocomposite morphology using two characterization methods, electron tomography and small-angle X-ray scattering (SAXS), is provided. For tilt greater than 15 degrees, the contrast of a single montmorillonite layer experimentally decreases below the detectable limit of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Calculations based on Z-contrast imaging of a I nm thick aluminosilicate layer predict this tilt angle (15 degrees) should produce 17% contrast, consistent with a reasonable limit of HAADF-STEM detection for this system. This result implies that segmentation or thresholding of 2-dimensional Z-contrast projection images of randomly oriented, highly anisotropic nanoparticles, such as layered silicates in polymer nanocomposites, will be extremely inaccurate. For example, nearly 75% of the volume of montmorillonite layers in an epoxy matrix will not be identified in the segmentation, owing to their orientation alone. Using electron tomography, this number is reduced to below 15% and tomographic reconstruction reveals three-dimensional information. The corresponding 3D fast Fourier transformation (FFT) indicates that the image volume (10(-1) mu m(3)) does not contain sufficient distribution of local environments (interlayer correlation length similar to 16.1 nm) to directly correspond to the global average as revealed by SAXS (scattering volume, 10(7) mu m(3); interlayer correlation length similar to 12.3 nm). Nevertheless, in contrast to SAXS, the tomographic reconstruction provides precise details of the distribution of morphological features, in addition to statistical averages over the sample volume.