(704f) Nanovoid Formation and Mechanics: A Simulation Study of Poly(dicyclopentadiene) and Epoxy Cross-Linked Networks

Nanovoid formation and
mechanics: A simulation study

of poly(dicyclopentadiene) and epoxy cross-linked networks

Robert M. Elder, Daniel B.
Knorr, Jr., Joseph L. Lenhart,

Jan W. Andzelm, and Timothy W. Sirk*

US Army
Research Laboratory, Aberdeen Proving Ground, MD 21005 

ABSTRACT

Cross-linked polymer
networks are widely used in protective and structural applications under extremes
of temperature, pressure, and strain rate. Typical materials for these applications,
like epoxy resins, have high stiffness but inferior toughness. Recent
experiments showed that cross-linked poly(dicyclopentadiene) (pDCPD), a
hydrocarbon, can overcome this trade-off between toughness and stiffness,
resulting in excellent protective performance. Based on the physicochemical
properties of pDCPD and epoxy, it was hypothesized that the superb toughness of
pDCPD is related to the formation and growth of nanovoids at extreme strain
rates. Voids can dissipate energy that might otherwise lead to failure. Here,
we use atomistic molecular dynamics to study pDCPD and epoxy networks
undergoing high strain rate deformation. We quantify the networks’ mechanical
properties (tensile modulus and yield strength), and the prevalence and characteristics
of nanoscale voids. We identify molecular-level properties – such as the polar
chemistry of epoxy – that influence the behavior of the networks. The strong
non-covalent interactions (e.g., hydrogen bonds) that accompany polar moieties increase
the modulus and yield strength of epoxy networks. Conversely, the weak non-covalent
interactions accompanying the aliphatic (non-polar) chemistry of pDCPD can
account for its lower modulus and strength. We examine the local chemical
environment of nanovoids, finding that nanovoids are more prevalent in regions
of non-polar chemistry. Consequently, pDCPD generally has more, larger
nanovoids that grow more readily during deformation. Thus nanovoids may be
related to the higher fracture toughness and more ductile behavior observed in
pDCPD.

*Corresponding author: timothy.w.sirk.civ@mail.mil