Date of Award

1-1-2011

Document Type

Campus Access Dissertation

Department

Mechanical Engineering

First Advisor

Sarah Baxter

Abstract

Nanocomposite materials hold the power to revitalize and revolutionize the field of composite materials. Nanocomposites can exhibit strikingly different material properties than their macroscale counterparts; often at significantly lower volume fractions. A key mechanism contributing to these novel effects is the scale of the included phase itself. From a composites perspective the small interfacial zone, which surrounds the included phase in all composites, becomes a significant third phase with respect to nanoscale particles. Also, the large number density of particles, present at small volume fractions results in a close packed, in absolute terms, composite microstructure, with the potential to produce interface &ndash particle composites, or fully percolated, connected particle microstructures.

There exists a strong body of work in the literature modeling the effects of interfacial regions on the effective properties of polymer nanocomposites. In particular, the usefulness and validity of using continuum scale micromechanics models to describe the properties of nanocomposites has been demonstrated. Less has been done to examine the effect of scale with respect to the large number density, and close packing of the particles. The scale of particle separation may serve to confine or eliminate matrix material between particles, producing a pseudo-percolated, or connected microstructure, which has the potential to enhance mechanical stiffness as well as electrical conductivity.

In this work the usefulness of the unit cell model known as the Generalized Method of Cells (GMC) to model these scale effects is demonstrated. The model is then used to predict the effective composite viscoelastic response under static creep, for varying interfacial elastic stiffnesses. The model suggests that an elastically stiff interface greatly increases the stiffness of the polymer in response to an &lsquo instantaneous &rsquo step load, reduces the rapid creep response, and results in a rapid leveling off of the time-dependent strain curves. The response of the composite to increasing stiffness of the interface region eventually reaches a plateau or threshold value, where further increases in the stiffness of the interface produces negligible increases in stiffness, or further reduction in creep response.

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