(660d) Stiffness Gradients in Polymer Films and Model Nanocomposites: Characterization By Fluorescence and Nanoindentation

Stiffness
Gradients in Polymer Films and Model Nanocomposites: Characterization by
Fluorescence and Nanoindentation
Shadid
Askar, Min Zhang, L. Catherine Brinson, and John M. Torkelson

While the effects of confinement on polymer stiffness have
been investigated for nearly two decades, disagreement among reports in
literature regarding such effects leaves many fundamental questions unanswered.
For instance, studies using experimental techniques such as nanoindentation or nanobubble inflation have reported modulus enhancements
with confinement. However, other studies involving elastic film wrinkling or nano-beam bending have indicated that modulus decreases
with confinement. Further, there are only a few experimental reports of
stiffness gradients in confined polymer films. The disagreement among reports
of stiffness-confinement effects in conjunction with the lack of reports
investigating stiffness gradients near interfaces leaves a large gap in the
understanding of stiffness-confinement effects. As devices become smaller in
technological applications such as in nanolithography, membranes, coatings, and composites, confinement effects must be understood for
optimal design of such devices.

            There
is a strong need to develop experimental techniques that can characterize
stiffness in polymer films, yet this need is difficult to satisfy due to
experimental constraints. Nanoindentation has shown to be effective in
characterizing stiffness gradients in polymer films, and in this study, it is
used to characterize gradients as a function of distance from the
polymer-substrate interface in polystyrene (PS) and poly(methyl
methacrylate) (PMMA) model nanocomposites. In addition, a novel fluorescence
spectroscopic technique is also used to characterize stiffness gradients near
the substrate interface in the same polymer model nanocomposites to compare the
two techniques. Fluorescence is also utilized to study polymer films containing
a free surface to characterize stiffness gradients near the polymer-air
interface.

While AFM and fluorescence techniques are useful in
characterizing stiffness gradients, each technique is sensitive to stiffness in
different ways. In the case of nanoindentation, an AFM tip probes 5 – 8
nm of surface exposed polymer parallel to the plane of the polymer-substrate
interface. The indenter tip induces a load on the polymer, the force
experienced by the tip is recorded, and modulus values are obtained. As the tip
probes from the substrate interface towards the interior of the polymer,
modulus values are observed to decrease. Using this technique, the indenter
probe is sensitive to stiffness by recording the response of groups of
molecules to the deformation induced by the indenter tip.
The fluorescence technique yields information regarding
stiffness in a different manner. In the particular approach used in this study,
changes in the fluorescence spectral shape of pyrene-dye labeled PS and PMMA
films are analyzed to study stiffness-confinement behavior. After excitation by
UV light, pyrene dye molecules return to the ground state by either
non-radiative (vibrations, rotations, etc.) or radiative (fluorescence) means.
The extent of non-radiative pathways of energy decay determines the extent to
which excited-state pyrene molecules return to the ground state via
fluorescence. The trade-off between the two pathways of energy decay is
manifested in spectral shape changes in the pyrene fluorescence emission
spectrum. More specifically, the ratio between the third and first vibronic band peak intensities (I₃/I₁) in the pyrene fluorescence spectrum is
used to characterize changes in the environment around excited-state pyrene
molecules. The sensitivity of pyrene fluorescence to stiffness originates from
a 'caging' mechanism. In a more 'caged' environment, non-radiative pathways for
energy decay are suppressed thereby enhancing the radiative forms of energy
decay. The pyrene fluorescence spectral shape changes such that the I₃/I₁ decreases in
a more 'caged' environment, or one that is stiffer. The 'caging' mechanism is
also used to rationalize confinement-induced stiffness enhancements in neutron
scattering studies of mean-squared displacement in supported polymer films.

Polymer model nanocomposites were used to characterize
stiffness gradients as a function distance from the polymer-substrate
interface. These multilayer films consisted of polymer layers between two glass
slides, thereby eliminating free surface effects. Stiffness gradients studied
by nanoindentation and fluorescence were characterized and compared at 25 ^oC. Using nanoindentation, it was determined
that the stiffness gradient extended about 60 – 100 nm towards the
interior of the polymer film away from the substrate interface. The advantage
of the fluorescence technique lies in the ability to characterize local
stiffness changes without perturbing the polymer molecules, which could
influence the stiffness. In addition, by using multilayer polymer films with
one dye-labeled layer, stiffness gradients can be characterized as a function
of distance from interfaces. Using the fluorescence technique, it was found
that the stiffness gradients extend about 400 – 600 nm from the substrate
interface. While neutron scattering studies cannot characterize stiffness
gradients, the length scales of stiffness-confinement effects on mean-squared
displacement are similar to those characterized in this study. In addition to
the stiffness gradient characterized near the substrate interface, the
fluorescence/multilayer approach enables characterization of stiffness
gradients near the polymer-air interface—a study that is not possible
using nanoindentation via AFM. It was found that stiffness gradients exist near
the polymer-air interface, and that stiffness decreases within 100 nm of the
free surface interface. It is clear from these results that length scales and
confinement effects associated with stiffness are not coupled with T_g or physical aging.

The difference in the observed stiffness gradient length
scales can be attributed to the manner in which each technique is sensitive to
stiffness. Nanoindentation probes the collective response of many molecules to
deformation, while pyrene fluorescence is sensitive to local stiffness by a 'molecular
caging' mechanism. Since the probing length scales are different, it is not
surprising that the length scales of the stiffness gradients characterized by
these techniques are different. However, this finding is significant since it
may help explain the inconsistencies that are observed in the research literature.
In addition, this investigation shows that fluorescence is a versatile
technique that can be used to characterize polymer properties in ways that
other techniques cannot.