Tracking the Connection between Disorder and Energy Landscape in Glasses Using Geologically Hyperaged Amber

Abstract

Fossil amber offers the unique opportunity of investigating an amorphous material which has been exploring its energy landscape for more than 110 Myears of natural aging. By applying different x-ray scattering methods to amber before and after annealing the sample to erase its thermal history, we identify a link between the potential energy landscape and the structural and vibrational properties of glasses. We find that hyperaging induces a depletion of the vibrational density of states in the THz region, also ruling the sound dispersion and attenuation properties of the corresponding acoustic waves. Critically, this is accompanied by a densification with structural implications different in nature from that caused by hydrostatic compression. Our results, rationalized within the framework of fluctuating elasticity theory, reveal how upon approaching the bottom of the potential energy landscape (9% decrease in the fictive temperature T f ) the elastic matrix becomes increasingly less disordered (6%) and longer-range correlated (22%).Glasses are metastable systems whose microscopic properties vary depending on their thermal history and aging. The way their structure and dynamics relate to thermodynamic metastability is one of the current challenges for the glass transition understanding. From the experimental point of view, one of the major hurdles is the limited stability range normally accessible on the laboratory timescale. The glassy state is conventionally obtained by cooling the liquid below the melting point, avoiding its crystallization. The slower is the cooling rate, the lower is the energy of the basins (inherent structures) of the potential energy landscape [1, 2] in which the system gets trapped. The stability range covered by tuning the cooling rate is however limited: upwards by thermal conductivity and heating/cooling technology (hyperquenched glasses), downwards by crystal nucleation. Nevertheless, being metastable materials, glasses spontaneously evolve towards states of lower configurational energy and enthalpic content on a timescale dictated by the relaxation time. Accordingly, the stabilization process, or aging, becomes increasingly slow upon lowering the temperature below the glass transition temperature T g .Amber, a fossilized resin resulting from the chemical vitrification [3] of conifers and angiosperms exudates, represents a rare example of glass which has been aging for a time as long as tens of million years (hyperaging) at temperatures well below T g , experiencing extreme thermodynamic stabilization [4][5][6]. Such a long-aging history enables a unique opportunity to investigate how microscopic properties depend on energy landscape topology.Glasses are characterized, at the microscopic scale, by the lack of long-range structural order and, at the macroscopic scale, by thermodynamical and transport properties strikingly different from those of the corresponding crystalline phase. Among them, there are the thermal conductivity plateau and a specific heat C p (T ) excess over the Deby...

Reaching the Ideal Glass in Polymer Spheres: Thermodynamics and Vibrational Density of States

“…This result is in line with the finding that the magnitude of the boson peak scales with the degree of disorder in the glass, i.e., the configurational entropy, which is directly connected to T f [33]. Hence, the boson peak progressively decreases from hyperquenched glasses [80] to hyperaged [21,37], vapor deposited [38,39] and pressure densified [81] glasses, passing from weakly aged systems [33,82,83] and eventually disappears in the ideal glass of the present study. The connection between VDOS and T f shown in the inset of Fig.…”

“…The boson peak has been interpreted either as arising from (quasi)localized modes [35] or being associated with random fluctuations of force constant originating from the intrinsic disordered nature of glasses [36]. Several studies on low T f glasses, including amber aged during geological time scales [21,37] and vapor deposited glasses [38,39], show significant depletion of the boson peak, which is explained by the decrease in the disorder of the glass. Hence, a question of utmost importance is whether the absence of disorder originating from the vanishing configurational entropy-the entropy depleted of its vibrational component-of the ideal glass with T f ¼ T K would result in the complete fading of the boson peak, in analogy to crystals with zero configurational entropy.…”

Microscopic analysis of sound attenuation in low-temperature amorphous solids reveals quantitative importance of non-affine effects

“…The Rayleigh scattering scaling was obtained [4] using an "isotopic scattering" [5] model in which every atom of the glass is an independent source of scattering. Several recent experimental and simulational results were analyzed within the framework of the fluctuating elasticity theory [10][11][12], which posits that an amorphous solid can be represented as a continuous medium with spatially varying elastic constants. The inhomogeneity of the elastic constants causes sound scattering and attenuation.…”

“…For all three parent temperatures there is excellent agreement between results of Eqs. (12,13) and independently estimated [7] transverse and longitudinal sound speeds, and transverse and longitudinal sound damping coefficients. At small wavevectors we recover Rayleigh scaling, Γ ∝ k 4 , but the theory also accurately predicts Upper panels: the transverse (a) and longitudinal (b) speed of sound obtained from the theory, Eq.…”

Low Frequency Vibrations and Diffusion in Disordered Polymers Bearing an Intrinsic Microporosity as Revealed by Neutron Scattering

“…As evidenced by results found for low molecular weight glass formers and polymers also including confinement investigations, the Boson peak of amorphous materials is related to collective effects for instance sound waves [43][44][45][46][47]. Moreover, direct experimental relationships between the Boson peak and the thermodynamic state were discussed for geological aged amber [48,49], vapor deposited ultra-stable glasses [50,51] and polymer spheres [52]. Recently a correlation was found of the maximum frequency of the Boson peak and the microporosity described by the BET surface area for microporous polynorbornenes and PIM-1 [53].…”