Ultrafast Dynamics of Shock Waves in Polymers and Proteins: The Energy Landscape

Abstract

A 4.5 GPa shock pulse producing a cycle of compression heating ͑,25 ps͒ and expansion cooling ͑ϳ1.5 ns͒ is used to study fast mechanical dynamics of solid organic polymers and proteins. Coherent Raman spectroscopy of a dye in the sample shows that compression occurs by an instantaneous part followed by a second, ϳ300 ps, structural relaxation process. After expansion, a mechanically distorted structure is produced which does not relax on the ,15 ns time scale. The results are interpreted with an energy landscape model. PACS numbers: 61.41. + e, 62.50. + p, 78.47. + p, 87.15.He In this paper we use the nanoshock spectroscopy technique [1] to investigate the microscopic response of organic polymers and proteins, subjected to a cycle of shock compression heating followed by expansion cooling, which produces ultrafast large amplitude structural dynamics. The results are interpreted with an energy landscape model.The nanoshock [1] is a shock pulse with a duration of about 1 ns. The peak pressure is 4.5͑60.5͒ GPa. The rise time is ,25 ps. The pressure relaxes in a few ns. In a typical organic polymer [e.g., polymethylmethacrylate (PMMA)], shock volume compression is ϳ20%, the shock velocity is ϳ4 km͞s, and the temperature rise is ϳ125 K [2,3]. A microscopic understanding of fast large amplitude dynamics of materials helps extend current models beyond linear response, and might be relevant to practical problems such as impact resistance of polymers or biological effects of shock waves generated in pulsed laser surgery or lithotripsy.Structural relaxation of amorphous materials is usually treated in the context of potential energy landscape theories [4]. Relaxation response functions have been measured for weak external mechanical, acoustic, electrical, thermal, and optical perturbations which result from transitions between local energy minima [4,5]. Shock waves allow us to study fast structural evolution processes in the higher energy regions of the landscape difficult or impossible to access by conventional techniques. Figure 1 diagrams the energy landscape model for shock waves in amorphous materials. First consider the effects of slow, reversible isothermal compression or expansion [ Figs. 1(a) and 1(b)]. This problem has been treated recently in the context of molecular dynamics simulations of glasses [6,7]. With reversible compression, the ambient landscape's total potential energy is increased by an amount equal to the work done on the system. The additional potential energy is distributed among atom pairs in a complicated way which changes the entire topography [6,7], created a new "compressed landscape," whose local minima represent quite different structures from the corresponding ambient landscape. The global minima of compressed landscapes are displaced from the ambient landscapes, because compression favors configurational coordinates which increase the density. Slow reversible compression or expansion is a gradual evolution from the lower energy region of one landscape to another.Instantaneous shock com...

Dynamics of polymer response to nanosecond shock compression

“…The combination of PDV and time-resolved dye emission spectroscopy provides both well-characterized shocks and detailed measurements of the polymer dynamical response. 9,10 A simplified description of shock compression of polymers, such as PMMA, [1][2][3][4]19,20 relies on a viscoelastic paradigm. [3][4][5] In this view, expressed here as a basis for further discussion, the initial response to a planar (uniaxial) shock is an instantaneous elastic compression, creating a (transverse) shear stress.…”

On the Shock Response of Polymers to Extreme Loading

“…However, the work nevertheless shows an indication of behaviour across the class of materials. Laser compression with ultrafast spectroscopy as a diagnostic has also probed structural stretching and relaxation processes with polymers subject to femptosecond loads [39][40][41][42].…”

The role of momentum transfer during incoherent neutron scattering is explained by the energy landscape model

“…Additional support for the elastic spring model comes from shockwave studies (31)(32)(33)(34)(35). In an experiment by Dlott and coworkers, a 50-µm-thick aluminum plate hits a solid polymer target with a velocity of 1 km/s (35).…”