Topology and the Dynamics and Transport in Polymer Glasses

Bradley Frieberg (Materials Science & Engineering Department, NIST)

Polymeric glasses are used for a variety applications ranging from structural components of airplanes and automobiles to active membranes for ion/gas/water transport. While they are widely used by many different industries, there are still a number of unanswered questions about glasses and glass formation that stem from the fact that they are inherently not in equilibrium. In this work we use different polymer topologies to tackle two long standing problems with glassy physics by measuring their dynamic properties using neutron scattering. Because of their non-equilibrium nature, the physical properties of polymer glasses are time dependent which leads to a structural relaxation over the lifetime of the material. The physical mechanism in which structural relaxation occurs is still a topic of debate. Using star-shaped polymers the structural relaxation rate can be tailored without changing the chemical structure, and we observe a strong correlation between the relaxation rate and dynamics measured by neutron scattering. This correlation can shed light on the mechanism behind the structural relaxation process. Polymeric glasses are also often used as active materials in applications such as desalination membranes or fuel cells where ion or small molecule transport is critical. Understanding the connection between polymer structure and dynamics with the transport of small molecules is crucial for furthering the understanding and design of future materials. We leverage a series of epoxy networks as a means to study the connection between structure, dynamics and water transport, through control of the cross link density and porosity of the material. By comparing these two polymer systems we can begin to make some connections between the polymer structure, transport properties and polymer chain dynamics in the glassy state.

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