Research

There has been an increased interest in polymeric materials over the last three decades, and polymer blends in particular. Vital to the development of these materials is a basic understanding of how local motions affect material properties. This research aims to improve the molecular-scale understanding of how both chain architecture and local environment affect the short-time dynamics of these materials. The polyolefins: head-to-head poly propylene, polypropylene, poly(ethylene-butene) and poly(ethylene-propylene) represent an excellent family of polymer for this particular study since they exhibit similar chain architectures with very systematic changes in pendant group density and placement. Additionally, this family of polymers forms a wide variety of miscible blends.

In order to provide the simultaneous spatial and temporal resolution of dynamic events that is necessary to fully probe molecular motions, we combine quasielastic neutron scattering (Q.E.N.S.) with molecular dynamics (M.D.) simulation. Neutron scattering measures the exchange of energy between the sample and the scattered neutrons, while M.D. provides the time evolution of the atomic positions. From both of these observables, we can extract the dynamic structure factor, S(Q,t) (where Q is the scattering vector and inversely proportional to distance), allowing the verification of simulation data and a deeper interpretation of the dynamics unobtainable from experiment alone.

Currently, dynamic measurements have been made on two polymer species (poly (ethylene-propylene and atactic polypropylene) at the NIST Center for Neutron Research using the high flux backscattering spectrometer (H.F.B.S.) and the disk-chopper time of flight spectrometer (D.C.S.). The first spectrometer probes dynamics occurring in the 250-2500 p.s. time range while the latter probes dynamics in the 1 - 40 p.s. time range. In conjunction with these experiments, united atom (hydrogen atoms are considered as a portion of a single unit centered on the carbon to which they are attached) M.D. simulations have been performed to assess the slower process in the H.F.B.S. window. E.A. simulations will be used to assess the faster processes in the D.C.S. window.

Publications