Pressure‐jump volume‐relaxation studies of polystyrene in the glass transition region

Abstract

An apparatus has been constructed that permits the measurement of time‐dependent changes in pressure near the point of vitrification. The same instrument is used for measuring steady‐state PVT properties, which are necessary for a proper analysis of the dynamic measurements. The former experiments are referred to as pressure‐jump volume‐relaxation (PJVR) measurements and serve as a direct probe of the structural relaxation process that occurs in all glasses. Experiments have been performed on polystyrene from 110 to 150°C and up to 2 kbar using pressure steps of 500 bars. The qualitative observations are analogous to those obtained at atmospheric pressure by rapid changes in temperature, namely (1) nonlinearity, (2) asymmetry, and (3) memory effects associated with complicated temperature or pressure histories. Each of these effects is accounted for semiquantitatively by a phenomenological order‐parameter model that has been extended to include the effect of pressure. Deviations between theory and experiment increase as temperature and pressure increase, this being manifest mostly in a predicted recovery curve (expansion isobar) that recovers the equilibrium volume more quickly than the experimental data; the contraction isobars are in most cases predicted within experimental error. The adjustable parameters of the model are found to vary somewhat with pressure and temperature, apparently due to variations in δ and Δκ. The activation volume suggests that 10–20 monomer segments are involved in the recovery process, assuming that the activation volume itself represents only a fraction of the dynamic unit (as observed in molecular glasses).

Pressure dependence of density and structural relaxation of glass near the glass transition region

“…Several phenomenological models 14,15,16,17 describe the time dependent frozen-in state and departure from equilibrium () 19 of glass forming materials depending on temperature history 4,5,18 and also pressure history 14,15,16,17 Here, we present new experimental data to study the density and structural relaxation of glass under elevated pressure of up to 1.5 GPa and elevated temperature, which were obtained using a piston-cylinder apparatus. We describe the new experimental procedure developed for this as well as a two-internal-parameter model that can overall fit the data.…”

“…At the beginning of the isothermal volume recovery, the time constant increases approximately linear with fictive pressure (Fig.6) 17 (14) where is the characteristic time when the uncompressed fast-cooled sample first reaches 525 °C. Based on the Maxwell relationship 29 on shear modulus relaxation time (15) where is the shear viscosity and is the unrelaxed shear modulus (34 GPa of SCHOTT N-BK7  glass), the viscosity of glass increases with pressure as does the characteristic time .…”

Isochoric and isobaric glass formation: Similarities and differences

“…[31][32][33] In addition, pressure jumps, [34][35][36][37] dynamic pressure changes, 38 and metometry, and (3) time-dependent relaxation (recovery) experiments. PVT experiments that lie in chanical perturbations [39][40][41] have been used to explore polymer relaxation behavior and the results the first category are performed by heating a material above T g and cooling at a constant rate unreveal similar time-dependent behavior to the Kovacs results.…”

Structural relaxation in the glass: Evidence for a path dependence of the relaxation time

“…Although these models are able to capture many of the experimental observations associated with the glass transition and structural recovery, model parameters vary with thermal history [24,26,[28][29][30][31][32][33][34][35][36][37]. Hence the models may be little better than empirical fitting functions.…”

“…s is known as the characteristic relaxation time (although it is actually a retardation time), and b is the non-exponentiality or stretching parameter. Various phenomenological equations have been used to describe the dependence of the characteristic relaxation time s on temperature and structure and sometimes pressure, including the TNM equation [22], equations derived by Hodge [40] and Scherer [41], both based on the approach of Adam and Gibbs [42], the KAHR and similar equations [23,43] based on free volume, and several others [33,44]. The most widely used forms are the TNM [22] and KAHR [23] equations for isobaric structural recovery; the KAHR equation is written in terms of d, whereas the TNM equation is written in terms of T f :…”