Over the past decade, there has been considerable interest in the effect that nanoscale confinement has on the properties of glass forming materials. The field has been driven because of both scientific and technological reasons. Examining the properties of glass formers confined to the nanoscale may provide vital information about the underlying physics of the material. With the emergence of nanotechnology, some glass formers, in particular, polymeric glass formers, will be used at increasingly smaller length scales. An understanding of how polymeric properties are impacted by nanoconfinement is essential to their full utilization in nanotechnology applications. The focus of my research has been investigating how confinement and interfaces impact the alpha relaxation dynamics, the glass transition temperature (Tg), and physical aging of polymeric glasses using fluorescence and dielectric spectroscopy.

The study of the effects of confinement on Tg of polymers began more than a dozen years ago. Since that time, it has been observed that Tg can undergo major changes relative to bulk Tg with confinement. In freely suspended films and films supported on a substrate that lack attractive substrate interactions, Tg decreases with nanoscale confinement. However, for films supported on a substrate that possess attractive interactions, Tg increases with nanoscale confinement. For suspended films and films supported on a substrate that lack attractive substrate interactions, the effect is associated with how the free surface locally reduces Tg, as evidenced by a fluorescence/multilayer method. Using the previously developed fluorescence/multilayer method, we have extensively investigated why Tg increases in a system (poly(methyl methacrylate)/silica) that possesses attractive interactions. The effect is associated with how the attractive substrate interactions increase Tg at the substrate interface. The attractive substrate interactions not only perturb Tg at the substrate but also do so in way that overcomes the opposing effects of the free surface. In addition, we have investigated related systems such as poly(ethyl methacrylate), poly(propyl methacrylate), and poly(isobutyl methacrylate) supported on silica.

Although the glass transition is associated with the cooperative dynamics (alpha relaxation dynamics) of many repeat units, how it is impacted by nanoconfinement has not been intensively investigated. Using dielectric spectroscopy few studies have reported a reduction in the peak temperature of the alpha process (TĄ) and a broadening of the alpha relaxation dynamics in polystyrene with confinement at temperatures above Tg. Such studies are extremely cumbersome since polystyrene possesses a weak dipole. To address this issue we have attached molecular dipoles to polystyrene to enhance its dielectric activity. In agreement with previous studies we observe a reduction TĄ and a broadening of the alpha relaxation dynamics with nanoscale confinement. Since the glass transition is related to the alpha relaxation dynamics, it must be that these dynamics at the interfaces are different from the bulk. However, because of experimental limitations no direct measurement of the complete alpha relaxation time distribution at the interfaces has been conducted. We have developed a novel multilayer/dielectric spectroscopy technique to allow for such measurements. For the first time, a reduction in the peak temperature of the alpha process and a broadening of the alpha-relaxation process are observed at the polystyrene interface. The results suggest that the impact of nanoconfinement on the alpha process originates from interfacial effects just as the impact of nanoconfinement on Tg originates from interfacial effects.

When cooled below Tg polymers are in a non-equilibrium state of higher specific volume and specific energy. As a result, if annealed below Tg, polymers will undergo a spontaneous structural relaxation process toward their equilibrium state. This relaxation process, often referred to as physical aging, can result in a time dependence of end-use properties of critical technological importance such as increased modulus, increased brittleness and reduced permeability. To date, conventional techniques for monitoring aging of bulk polymers are incapable of doing so for nanoconfined polymers. We have developed novel fluorescence methods to monitor aging in thin polymer films. More importantly, our technique allows for the monitoring of aging at specific locations in films near interfaces. For systems that lack attractive interactions with the substrate, aging is unaffected by confinement. However, for systems that possess attractive interactions with the substrate, aging is strongly suppressed. A novel use of the fluorescence/multilayer approach allows for monitoring aging for the first time at the free surface and substrate polymer interface. At both interfaces aging is reduced with a nearly complete suppression of aging observed at the substrate interface when aged deep in the glassy state. Our work indicates that nanoconfinement and interfacial effects strongly alter aging and that the development of polymer glasses that do not physically age may be possible. Using dielectric spectroscopy the molecular origins for the suppression of physical aging in nanoconfined systems have been investigated and will be presented as well.