Physics Colloquium - Monday, January 22nd, 2007, 4:00 P.M.

E300 Math/Science Center; Refreshments at 3:30 P.M. in Room E200

Connie Roth
Northwestern University

Exploring Polymer Dynamics in Ultrathin Films Subjected to High Shear Rates and Dominating Interfacial Interactions due to Nanoscale Confinement

Polymers are ubiquitous in everyday life from plastic bags and unseen antireflection coatings to biological polymers such as proteins and DNA. Current research on designing polymers for technological applications is focused on reducing structures to the nanoscale (e.g., nanocomposites, nanostructured blends, multilayer films, and nanolithography). Research on single-layer homopolymer films over the past 15 years has demonstrated that properties such as chain conformations and relaxation times are greatly affected by the presence of a nearby interface. For instance, changes in the glass transition temperature Tg of many tens of degrees Kelvin have been observed for film thicknesses of a few tens of nanometers. I will demonstrate using two examples how polymer ultrathin films can be used as model systems to gain new insight into the properties of polymers subjected to high shear rates and dominating interfacial interactions due to nanoscale confinement. Hole growth measurements in freely-standing polymer films act as a micro-rheological probe of nonlinear viscoelastic dynamics at high shear rates allowing time-dependent strain measurements to be collected at temperatures and over time scales not accessible by traditional rheological techniques. Multilayer films composed of nanoscopic alternating layers of different polymers provide a unique geometry for determining how the cooperative segmental dynamics of an individual layer/domain are influenced by neighboring domains. The extent to which interfacial interactions at the free surface and substrate interface are mediated by the narrow immiscible polymer-polymer interface is investigated using fluorescence to selectively measure the Tg of individual layers within the ultrathin multilayer films.