(43a) Neighboring Domain Perturbation of Glass Transition Temperatures in Multilayer Films and Nanostructured Block Copolymer/Homopolymer Blends
The effects of free surfaces, polymer-substrate (or nanofiller) interfaces, and nanoconfinement itself on the glass transition temperature (Tg) in thin polymer films and polymer nanocomposites have been studied for over a decade. Free surfaces reduce the requirement for cooperative dynamics and tend to decrease Tg; attractive interactions between polymer repeat units and the substrate interface reduce mobility and tend to increase Tg. These interfacial effects are known to induce Tg distributions or gradients across polymer films. However, this Tg gradient across polymer films can be affected by nanoconfinement itself. In PS films, it has been shown that the Tg gradient across films cannot be sustained when the film is too thin to support the full gradient between interfaces.
Here we demonstrate a new effect of nanoconfinement on the Tg of polymers. By employing a multilayer fluorescence technique, we show how the Tg dynamics of polystyrene (PS) layers are perturbed by that of immiscible polymers in a neighboring layer; the Tg of a 14-nm-thick PS film at the free surface of a bilayer film can be tuned over the range of 45 oC to 145 oC by simply varying the polymer species of an underlayer film supporting the ultrathin PS layer. Also, we demonstrate the tunability of Tg of an ultrathin PS layer atop different types of polymers by changing the thickness of the underlayer. Finally, we determine the length scale over which adjoining layers can perturb the PS layer. This shows that the cooperative segmental dynamics of the PS layer are strongly coupled to the neighboring domains over length scales of several tens to hundreds of nanometers. These results suggest a novel route to create new material properties controlled by the type and thickness of polymers in a multilayer film geometry.
Additionally, we have observed similar Tg perturbation behavior for the first time in blends of poly (styrene-block-4-vinylpyridine) and poly(4-vinylpyridine) (P4VP) homopolymer blends, employing a pyrene fluorescence probe which can be strongly segregated into PS domains in the blends. When the PS nanophase is present at less than 10 wt%, the PS domain yields either a Tg value between those of neat PS and neat P4VP or equal to that of neat P4VP. This result suggests that Tg dynamics of the S block (minor domain) is affected by the 4VP block matrix (major domain) dynamics in a manner similar to the results obtained in bilayer films with an ultrathin PS layer sitting atop a P4VP underlayer.