(722af) Maximizing or Eliminating the Effects of Nanoscale Confinement on the Glass Transition In Freely Standing, Supported, and Multilayer Polymer Films

Using fluorescence spectroscopy and ellipsometry, we are able to characterize the effects of nanoscale confinement on the glass transition temperature (Tg) in supported single-layer and multilayer films and in freely standing polymer films. We have recently demonstrated several ways in which we can maximize or minimize the perturbations from bulk Tg response caused by confinement and in which we can address fundamental issues regarding the fundamental reasons behind the effects of nanoconfinement on polymer properties.

First, we have shown that we can eliminate the effect of nanoconfinement on polymer Tg via the addition of several weight percent of low molecular weight anti-plasticizer molecules to the polymer. In the case of poly(vinyl acetate) (PVAc), water, which is naturally sorbed at levels of several weight percent in PVAc, is an anti-plasticizer. Bone-dry PVAc films exhibit a significant Tg reduction with decreasing film thickness. However, PVAc films containing several weight percent water exhibit a film-thickness-independent Tg down to thicknesses of ~10 nm. Also, we determined via ellipsometry that water molecules increase the density of PVAc films, and this effect indicates that water acts as an anti-plasticizer in PVAc. These results are consistent with a recent theoretical analysis by Riggelman et al. (PRL 97, 045502 (2006)) and related experimental studies done previously in our group (Ellison et al., PRL 02, 095702 (2004) and Mundra et al., Eur Phys J Spec Topics 141, 143 (2007)) indicating that the presence of anti-plasticizers leads to a suppression of confinement effects on the polymer dynamics. Our results also indicate that researchers must be aware of these effects when developing polymeric systems used under conditions of nanoconfinement (e.g., as ultrathin films or in nanocomposites) that may sorb significant amount of water from the atmosphere.

Second, we have developed a new fluorescence method to measure the effect of confinement on the properties of freely standing polymer films, i.e., films that are not supported by a substrate. Related studies have been reported by only a small number of other research groups around the world because of experimental difficulties associated with freely standing films, especially when the films are less than 100 nm thick. In our studies, we measure Tg via the temperature dependence of the shape of the fluorescence spectrum of a dye covalently attached to the polymer at trace levels. We have found that a 40-nm-thick polystyrene (PS) freely standing film exhibits a Tg value that 36 K below that of the bulk Tg. We are exploiting our novel characterization method to obtain a better fundamental understanding of the large effect of confinement on Tg in freely standing films and in particular to test the theory proposed in 2000 by de Gennes (Eur Phys J E, 2, 201 (2000)).

Finally, we have discovered that the Tg of a 14-nm-thick PS film at the free surface of a bilayer film can be tuned over the range of 318 K to 418 K by simply varying the polymer species of an underlayer film supporting the ultrathin PS layer. This shows that the cooperative segmental dynamics of two immiscible polymers are strongly coupled over length scales of several tens to hundreds of nanometers and that sufficiently thick underlayers of selected polymers can essentially cause the Tg dynamics of the ultrathin PS layer to be ?slaved? to the dynamics of the underlayer. This discovery suggests that nanostructured multilayer films may yield properties that cannot be achieved by conventional polymer blends, providing a new route to the design of novel materials.