386793 Nanoscale Confinement Effects on Polymer Properties: Recent Advances in Characterization and Understanding and Important Questions to be Addressed
Polymeric materials are employed in confined states in many important technological applications. These include nanoscale and nanostructured films used in lithographic applications for manufacturing microelectronic chips as well as well-dispersed and/or exfoliated nanocomposites where all polymer chains are less than ~100 nm from a nanofiller interface. The study of how nanoscale confinement affects the properties of polymeric materials is now 20 years old, dating back to a 1994 publication by Keddie, Jones and Cory in which ellipsometry was used to demonstrate a dramatic reduction in the glass transition temperature (Tg) of polystyrene films supported on silica when thickness decreased below ~40 nm. Since that time, major nanoscale confinement effects have been demonstrated in a variety of polymer properties, including Tg, physical aging and modulus. Furthermore, a variety of studies have contributed to demonstrating the tunability of such effects based on changes in polymer repeat unit structure, the presence (and strength) or absence of attractive polymer/substrate or nanofiller interactions, and the addition of small molecule diluent. Nevertheless, major questions have remained unanswered in the field, including why different polymer molecules that lack attractive interactions with a substrate (or have no substrate, as with a free-standing film) exhibit starkly different magnitudes of the Tg-confinement effects. As well, major stumbling blocks to certain types of characterization, such as the use of conventional differential scanning calorimetry (DSC) for quantifying the effects of confinement on Tg and physical aging, have hampered progress in the field.
Here, I shall present recent progress from my research group that has positively addressed such questions and overcome such stumbling blocks. In particular, I shall describe work that demonstrates the important role of the fragility of a polymer in influencing both the magnitude of the Tg-confinement effect in supported polymers and the film thickness at which such effects first become noticeable. The observed effect is that the more fragile the polymer, the greater the Tg-confinement effect. I shall explain the underlying cause behind this key role of fragility on confinement effects and also demonstrate that some polymer system systems with very low fragilities exhibit no confinement effect for films as thin as 20 nm. I shall also describe how we have developed a new method to characterize the effect of confinement on polymer fragility via ellipsometry. With this new approach, we have demonstrated a dramatic decrease in fragility with nanoscale film thickness and that limiting values are approximately achieved in sufficiently thin films.
I shall also show how differential scanning calorimetry can be easily employed to characterize Tg-confinement effects in polymeric nanotubes made using anodized alumina templates. The Tg-confinement effects measured by DSC for polystyrene nanotubes made by melt infiltration show excellent correspondence with Tg-confinement effects determined from fluorescence measurements on the same nanotubes as well as with Tg-confinement effects of supported polystyrene films. With this advance, it will be relatively easy in the future for any research team having access to DSC to study and characterize confinement effects. Additionally, we shall demonstrate how changing the substrate can result in Tg-confinement effects for polystyrene films changing sign. For silica substrates which have no attractive interactions with polystyrene, Tg decreases with decreasing thickness below ~40 nm. We have found that carbon-based substrates can result in increases in Tg of polystyrene films of the same thickness that show decreases in Tg when placed on silica. This is because of pi-pi bonding interactions between polymer and the carbon-based substrate, a result that has important implications in polymer/carbon-black based composites found in the tire industry as well as novel graphene-based nanocomposites.
Although significant progress has been made very recently in this field, major new questions have emerged. Possibly most important among these is why thermodynamic and pseudo-thermodynamic methods of Tg measurement, including ellipsometry, x-ray and neutron reflectivity, fluorescence, capacitive dilatometry, and DSC, report strong effects of confinement on polymer properties such as Tg while measures of dynamics related to the alpha-relaxation process show suppressed effects. In bulk materials, there is a strong correlation between the measured Tg and dynamics that is apparently less robust or missing at the nanoscale. I shall highlight this issue and other questions that will be important for researchers to explore in the coming decade.
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