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.