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

Abstract

Abstract In an attempt to clarify the criteria satisfied at the glass transition ( T g ) the effect of pressure on T g 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 (∂ T g /∂ P ). The In τ D (where τ D is dielectric relaxation time) is linear in the pressure. This dependence of In τ D on pressure is different from the dependence of In τ D on ( T − T g ), which can be described by the WLF equation. By assuming f = ( f 0 + α f Δ T )/( a + bP ) and using the free volume model, we find In a p = bP/(f 0 + α f ΔT ). If a = 1, f 0 = 0.025; then the calculated value of b is 3.1 × 10 −4 atm., and 1/ b = 3.2 × 10 3 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 T g was used to follow ∂ T g /∂, and ∂ T g /∂ 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 ∂ T g /∂P = T g V Δα/Δ C p , to the glass transition is justified.