Mya Warren

Simulations of aging and plastic deformation in polymer glasses

We study the effect of physical aging on the mechanical properties of a model polymer glass using molecular dynamics simulations. The creep compliance is determined simultaneously with the structural relaxation under a constant uniaxial load below yield at constant temperature. The model successfully captures universal features found experimentally in polymer glasses, including signatures of mechanical rejuvenation. We analyze microscopic relaxation time scales and show that they exhibit the same aging characteristics as the macroscopic creep compliance. In addition, our model indicates that the entire distribution of relaxation times scales identically with age. Despite large changes in mobility, we observe comparatively little structural change except for a weak logarithmic increase in the degree of short-range order that may be correlated with an observed decrease in aging with increasing load.

Microscopic View of Accelerated Dynamics in Deformed Polymer Glasses

A molecular level analysis of segmental trajectories obtained from molecular dynamics simulations is used to obtain the full relaxation time spectrum in aging polymer glasses subject to three different deformation protocols. As in experiments, dynamics can be accelerated by several orders of magnitude, and a narrowing of the distribution of relaxation times during creep is directly observed. Additionally, the acceleration factor describing the transformation of the relaxation time distributions is computed and found to obey a universal dependence on the strain, independent of age and deformation protocol.

Mechanical rejuvenation and overaging in the soft glassy rheology model

Mechanical rejuvenation and overaging of glasses is investigated through stochastic simulations of the soft glassy rheology (SGR) model. Strain- and stress-controlled deformation cycles for a wide range of loading conditions are analyzed and compared to molecular dynamics simulations of a model polymer glass. Results indicate that deformation causes predominantly rejuvenation, whereas overaging occurs only at very low temperatures, small strains, and for high initial energy states. Although the creep compliance in the SGR model exhibits full aging independent of applied load, large stresses in the nonlinear creep regime cause configurational changes leading to rejuvenation of the relaxation time spectrum probed after a stress cycle. During recovery, however, the rejuvenated state rapidly returns to the original aging trajectory due to collective relaxations of the internal strain.

Atomistic mechanism of physical ageing in glassy materials

Using molecular simulations, we identify microscopic relaxation events of individual particles in ageing structural glasses, and determine the full distribution of relaxation times. We find that the memory of the waiting time tw elapsed since the quench extends only up to the first relaxation event, while the distribution of all subsequent relaxation times (persistence times) follows a power law completely independent of history. Our results are in remarkable agreement with the well-known phenomenological trap model of ageing. A continuous-time random walk (CTRW) parametrized with the atomistic distributions captures the entire bulk diffusion behavior and explains the apparent scaling of the relaxation dynamics with tw during ageing, as well as observed deviations from perfect scaling.

Time Dependent Parallel Resistance in an Organic Schottky Contact

The DC characteristics of a Schottky contact between regioregular poly (3-hexylthiophene) and aluminum are studied in forward and reverse bias regimes. Current-voltage curves of the junction in reverse bias show a resistive path in parallel with the expected Schottky contact. This is the sign of a nonuniform junction between the metal and semiconductor that exhibits ohmic behavior in some regions. Reduction of this parallel resistance and degradation of the Schottky junction are observed over a period of two weeks. Accumulation of undesired ions in the polymer or diffusion of aluminum atoms into the semiconductor are two possible mechanisms which may explain the time dependent behavior of these Schottky junctions.