Jamie Michael Kropka

Local polymer dynamics in polymer-C60 mixtures.

The complexity of intermolecular interactions in polymer-nanoparticle systems leads to spatial variations in structure and dynamics at both the meso- and nanoscale. Much of this behavior is manifested in properties such as the glass transition and the viscosity. Incoherent neutron scattering measurements of C60-polymer mixtures reveal that local polymer chain backbone motions in the glassy state are suppressed relative to those of the pure polymer. Moreover, the scattering spectrum of the melt suggests that the influence of C60 on polymer dynamics is limited to the vicinity of the particles at nanosecond time scales. A model is presented to reconcile these observations with the bulk dynamical properties exhibited by the mixtures.

A Process and Environment Aware Sierra/SolidMechanics Cohesive Zone Modeling Capability for Polymer/Solid Interfaces

The performance and reliability of many mechanical and electrical components depend on the integrity of po lymer - to - solid interfaces . Such interfaces are found in adhesively bonded joints, encapsulated or underfilled electronic modules, protective coatings, and laminates. The work described herein was aimed at improving Sandias finite element - based capability to predict interfacial crack growth by 1) using a high fidelity nonlinear viscoelastic material model for the adhesive in fracture simulations, and 2) developing and implementing a novel cohesive zone fracture model that generates a mode - mixity dependent toughness as a natural consequence of its formulation (i.e., generates the observed increase in interfacial toughness wi th increasing crack - tip interfacial shear). Furthermore, molecular dynamics simulations were used to study fundamental material/interfa cial physics so as to develop a fuller understanding of the connection between molecular structure and failure . Also reported are test results that quantify how joint strength and interfacial toughness vary with temperature.