The effect of pressure on glass temperature and dielectric relaxation time of polyvinyl acetate

Abstract

In an attempt to clarify the criteria satisfied at the glass transition (Tg) the effect of pressure on Tg of polyvinyl acetate has been measured by dielectric and volumetric techniques. Dielectric constant and loss has been measured as a function of temperature (25–120°C.), pressure (0–3300 atm.) and frequency (0.06–10 kcycles/sec.). At fixed frequency the temperature at which ε″max occurs increases with pressure by 0.022°C. atm. and this value is identified with (∂Tg/∂P). The In τD (where τD is dielectric relaxation time) is linear in the pressure. This dependence of In τD on pressure is different from the dependence of In τD on (T − Tg), which can be described by the WLF equation. By assuming f = (f0 + αfΔT)/(a + bP) and using the free volume model, we find In ap = bP/(f0 + αfΔT). If a = 1, f0 = 0.025; then the calculated value of b is 3.1 × 10−4 atm., and 1/b = 3.2 × 103 atm. is the same order of magnitude as an internal pressure. Volume measurements were made by the piston displacement method and by use of an Instron tester for recording force and length. The change in compressibility at Tg was used to follow ∂Tg/∂, and ∂Tg/∂P = 0.021°C./atm. in good agreement with the dielectric measurements. It was found that the volume of the sample at the same final pressure is smaller when compressed at high temperature than at low temperatures. In other words, vitrification at high temperatures and pressures produces a more dense sample than can be achieved by compression at low temperatures and seems to be a property of many glass forming systems. These results and other examples were used to show that the application of thermodynamic equations, namely ∂Tg/∂P = TgVΔα/ΔCp, to the glass transition is justified.

Temperature and Volume Effects on Local Segmental Relaxation in Poly(vinyl acetate)

“…This falls within the range of literature values for the pressure coefficient of the transition temperature of PVAc, obtained by various methods. 3,5,59 In terms of T R and the Avramov parameters, the fragility is given by m ) R(log(τ(T R ) -log τ 0 ), yielding m ) 78 ( 4 herein. This value, calculated from all 10 isotherms in Figure 3, is about 10% smaller than the fragility obtained by restricting the analysis to the atmospheric temperature data in Figure 1.…”

“…Previous workers have measured τ(P) for PVAc over a more limited frequency range at pressures below 300 MPa. 3,48,49 Over this smaller range, the relaxation times can be taken to be proportional to exp P, yielding an activation volume defined as ∆V ) 2.303RT(∂ log τ R /∂P) T . O'Reilly 3 reported activation volumes ranging from 130 to 200 mL/mol for temperatures from 390 to 340 K. These values are ca.…”

“…The two characteristic temperatures have also been shown to have the same pressure dependence. [3][4][5][6] The fragility method of classifying glass-formers derives from a configurational entropy approach to supercooled dynamics. 7 Other schemes have been utilized, in particular plotting the relaxation times as a function of T -T g , as suggested by free volume interpretations of the glass transition.…”

Pressure coefficient of the glass transition temperature in the thermodynamic scaling regime

“…1 is generally not fulfilled [3,19,20,21,22,23,24,25], whereas the eq. 2 seems to hold reasonably well for many systems, although not for all.…”

Viscoelastic Relaxation Modulus Characterization Using Prony Series

“…In order to perform the pressure superposition throughout time, the material must be piezorheologically simple, in other words, all relaxation times must be affected by pressure in the same way, allowing the calculation of a shift factor (Ferry, 1980). O'Reilly (1962) studied the effect of pressure in polyvinyl acetate behavior in the glass transition temperature region, Tg, by using dielectric and volumetric measurement techniques. Subsequently, this author developed a shift factor that considers the effect of pressure -α -which contains an exponential relation between a characteristic material Constant and the pressure applied to the material.…”