John M. Torkelson, Northwestern University
October 10, 2013
11:00 AM - 12:00 PM
Effects of Nanoscale Confinement in Polymer Films: Glass Transition Temperature and Diffusion
Abstract:
In 1995, Philip Anderson wrote, “The deepest and most interesting unsolved problem in solid state theory is … the nature of glass and the glass transition” (Science 1995, 267, 1615). One year earlier, Keddie, Jones and Cory (Europhys. Lett. 1994, 27, 59) discovered that a 15-nm-thick polystyrene film supported on silica exhibits a glass transition temperature, Tg, that is reduced by more than 20 K relative to bulk Tg. Since then, dozens of studies have characterized how Tg values of polymers are modified by nanoscale confinement. However, most studies characterized only average Tgs across films and focused on a single polymer — polystyrene. In 2000, Pierre de Gennes (Eur. Phys. J. E 2000, 2, 201) challenged the glass transition and confinement research communities by stating, “Future experiments should aim not at the determination of a single Tg, but at a distribution of Tgs.” In response to this challenge, we developed a simple fluorescence/multilayer method to determine distributions of Tg in polymer films and nanocomposites. We have found that perturbations to Tg at a free surface or polymersubstrate interface (with attractive interactions) can propagate several tens to hundreds of nanometers into a film and that the strength of the gradient in Tg is strongest at the free surface or interface. Our studies of a dozen different polymer species also indicate which fundamental polymer properties play key roles in defining the Tg-confinement effect, with fragility being an important example. Finally, we have discovered that translational diffusion coefficients of small dye molecules dispersed in polymer can be reduced by as much as a factor of 1000 with confinement. Our studies have led to understanding why the effects of surfaces and interfaces in perturbing Tg depend strongly on polymer species. We have found that the Tg reduction associated with free surface effects is enhanced with an increasing requirement for cooperativity of segmental mobility associated with Tg; that is, the free-surface effect is enhanced with increasing polymer fragility. In contrast, H-bond formation between chain segments and hydroxyl groups on nanofiller or substrate surfaces results in increasing values of Tg with nanoconfinement. Novel experiments involving templated nanorods and nanotubes will also be discussed. With this geometry, it is possible to explore Tg-confinement effects by conventional differential scanning calorimetry, which is the technique used to characterize thermal transitions in bulk polymers in both industry and academia. We will show that confinement in this novel geometry leads to Tg confinement effects similar to those observed in polymer films via non-calorimetric techniques.
Date posted
Jun 17, 2019
Date updated
Jun 17, 2019