Volume recovery and physical aging of pressure-densified glasses

When a glass is formed by cooling under high pressure, the resulting pressure densified glass exhibits a higher density and different thermal and mechanical properties than the corresponding glass prepared by conventional cooling at low pressure. Understanding, manipulating, and exploiting the full potential of pressure densified glasses offers the possibility of better properties. The effects of pressure densification on 1,3,5-tri(1-naphthyl)benzene (TNB) are assessed from volumetric and calorimetric measurements. The pressure densified glass (PDG) exhibits anomalous physical aging. Rather than evolving monotonically towards the equilibrium density, there is an overshoot to a lower density state. Only when the density of the PDG becomes equivalent to the corresponding CG does the density begin a slow approach towards equilibrium. Using molecular dynamics simulations of a simplified model of TNB, we show that the effects of vitrification pressure and subsequent volume recovery and aging of the glass are comparable to experimental results for real TNB. A two-parameter description of the underlying non-equilibrium structure, entailing a fictive temperature and fictive pressure, appears adequate to interpret the “anomalous” aging behavior of pressure-densified TNB.