This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: How can we predict the way a real material, such as a polymer, responds mechanically over timescales ranging from the ultrafast motion of atoms to the slow deformations measured in a laboratory? This is a deceptively difficult problem.
A glassy polymer such as poly(methyl methacrylate), or PMMA, does not have a single mechanical stiffness. Its response depends strongly on how fast we deform it. At extremely high frequencies, atoms and molecular bonds respond almost instantaneously.
At lower frequencies, molecules have progressively more time to rearrange, and the material becomes softer. Eventually, slow molecular relaxation processes begin to dominate. The difficulty is that no single experimental or computational technique can normally follow all of these regimes.
Molecular dynamics simulations can resolve only the fastest atomic motions, but the shortness of the time step involved in atomic motions makes it extremely difficult to reach the timescales of ordinary mechanical experiments. Dynamic mechanical analysis, at the opposite end, probes much slower deformation. Other techniques—including Brillouin light scattering, ultrasonic measurements and high-strain-rate tests—fill in portions of the enormous gap between them.
As a result, we normally obtain separate snapshots of the same material taken at very different "speeds." In our recent work, published in The Journal of Chemical Physics, my colleagues and I asked whether all of these apparently disconnected regimes could instead be predicted starting from the atomic structure of the polymer itself. We studied PMMA using an atomistic model containing 9,920 atoms. Rather than trying to simulate every possible deformation directly over increasingly long periods of time—which would rapidly become computationally impossible—we extracted information about the material's atomic vibrations and about how those vibrations couple to an imposed deformation.
The theoretical framework we use is based on non-affine lattice dynamics (NALD), a framework that I contributed to develop along with my longtime collaborator Dr. Army Research Laboratory. The word "non-affine" is important.
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