Investigation of Nanodispersion in Polystyrene-Montmorillonite Nanocomposites by Solid-State NMR.
Investigation of Nanodispersion in
Polystyrene-Montmorillonite Nanocomposites by
Bourbigot, S.; Vanderhart, D. L.; Gilman, J. W.; Awad,
W. H.; Davis, R. D.; Morgan, A. B.; Wilkie, C. A.
Journal of Polymer Science: Part B: Polymer Physics,
Vol. 41, No. 24, 3188-3213, December 15, 2003.
Sponsor:Federal Aviation Administration, Washington, DC
Air Force Office of Scientific Research
Nanocomposites result from combinations of materials
with vastly different properties in the nanometer scale.
These materials exhibit many unique properties such as
improved thermal stability, reduced flammability, and
improved mechanical properties. Many of the properties
associated with polymer-clay nanocomposites are a
function of the extent of exfoliation of the individual
clay sheets or the quality of the nanodispersion. This
work demonstrates that solid-state NMR can be used to
characterize, quantitatively, the nanodispersion of
variously modified montmorillonite (MMT) clays in
polystyrene (PS) matrices. The direct influence of the
paramagnetic Fe3+, embedded in the aluminosilicate
layers of MMT, on polymer protons within about 1 nm from
the clay surfaces creates relaxation sources, which, via
spin diffusion, significantly shorten the overall proton
longitudinal relaxation time (T). Deoxygenated samples
were used to avoid the particularly strong contribution
to the T of PS from paramagnetic molecular oxygen. We
used T as an indicator of the nanodispersion of the clay
in PS. This approach correlated reasonably well with
X-ray diffraction and transmission electron microscopy
(TEM) data. A model for interpreting the
saturation-recovery data is proposed such that two
parameters relating to the dispersion can be extracted.
The first parameter, f, is the fraction of the
potentially available clay surface that has been
transformed into polymer-clay interfaces. The second
parameter, , is a relative measure of the homogeneity of
the dispersion of these actual polymer-clay interfaces.
Finally, a quick assay of T is reported for samples
equilibrated with atmospheric oxygen. Included are these
samples as well as 28 PS/MMT nanocomposite samples
prepared by extrusion. These measurements are related to
the development of high-throughput characterization
techniques. This approach gives qualitative indications
about dispersion; however, the more time-consuming
analysis, of a few deoxygenated samples from this latter
set, offers significantly greater insight into the clay
dispersion. A second, probably superior, rapid-analysis
method, applicable to oxygen-containing samples, is also
demonstrated that should yield a reasonable estimate of
the f parameter. Thus, for PS/MMT nanocomposites, one
has the choice of a less complete NMR assay of
dispersion that is significantly faster than TEM
analysis, versus a slower and more complete NMR analysis
with sample times comparable to TEM, information
rivaling that of TEM, and a substantial advantage that
this is a bulk characterization method.