John L. Barber

A new paradigm for the molecular basis of rubber elasticity

The molecular basis for rubber elasticity is arguably the oldest and one of the most important questions in the field of polymer physics. The theoretical investigation of rubber elasticity began in earnest almost a century ago with the development of analytic thermodynamic models, based on simple, highly-symmetric configurations of so-called Gaussian chains, i.e. polymer chains that obey Markov statistics. Numerous theories have been proposed over the past 90 years based on the ansatz that the elastic force for individual network chains arises from the entropy change associated with the distribution of end-to-end distances of a free polymer chain. There are serious conceptual objections to this assumption and others, such as the assumption that all network nodes undergo a simple volume-preserving linear motion and that all of the network chains have the same length. Recently, a new paradigm for elasticity in rubber networks has been proposed that is based on mechanisms that originate at the molecular level. Using conventional statistical mechanics analyses, Quantum Chemistry, and Molecular Dynamics simulations, the fundamental entropic and enthalpic chain extension forces for polyisoprene (natural rubber) have been determined, along with estimates for the basic force constants. Concurrently, the complex morphology of natural rubber networks (the joint probability density distributions that relate the chain end-to-end distance to its contour length) has also been captured in a numerical model (EPnet). When molecular chain forces are merged with the network structure in this model, it is possible to study the mechanical response to tensile and compressive strains of a representative volume element of a polymer network. As strain is imposed on a network, pathways of connected taut chains, that completely span the network along strain axis, emerge. Although these chains represent only a few percent of the total, they account for nearly all of the elastic stress at high strain. Here we provide a brief review of previous elasticity theories and their deficiencies, and present a new paradigm with an emphasis on experimental comparisons.

Shock-front broadening in polycrystalline materials

We analyze a model for the evolution of shock fronts in polycrystalline materials. This model is based on the idea of Meyers and Carvalho [Mater. Sci. Eng. 24, 131 (1976)] that the shock velocity anisotropy within the polycrystal is the most important factor in shock front broadening. Our analysis predicts that the shock front width increases as the

Studying Shocked Material Dynamics with Ultrafast X-rays

1∕2

Fresnel-regime coherent diffractive imaging with with a 13 nm high harmonic source

power of the front penetration distance into the crystal. Our theoretical prediction is in plausible agreement with previous experimental results for the elastic precursor rise time, and it should therefore provide a useful shock width estimate. Furthermore, our theoretical framework is also applicable to other problems involving front propagation in heterogeneous media.

The theoretical strength of rubber: numerical simulations of polyisoprene networks at high tensile strains evidence the role of average chain tortuosity

The response of micron-scale inhomogeneities dictates the overall dynamic, structural and chemical response of many materials. Of particular interest is the response of micron scale voids. It is believed that such micron scale voids are responsible for the nucleation of damage leading to structural failure in metals and to initiation of detonation in explosive material under high strain-rates. A critical step towards developing safer, stronger, and longer lasting materials in a range of applications from energy to defense requires understanding the dynamic response of these inhomogeneties on the micron-scale.

The entropy of the rotational conformations of (poly)isoprene molecules and its relationship to rubber elasticity and temperature increase for moderate tensile or compressive strains

Coherent x-ray diffractive imaging (CXDI) is a powerful technique for wavelength-limited, ultrafast images. We present a novel analysis and demonstration of Fresnel-regime (near field) CXDI with a tabletop 13 nm high harmonic generation source.