Structural Relaxations of Glassy Polystyrene and o-Terphenyl Studied by Simultaneous Measurement of Enthalpy and Volume under High Pressure

The heat capacities, thermal expansivities, and isothermal compressibilities of polystyrene and a mixture of o -terphenyl (67%) and o -phenylphenol (33%) (abbreviated as OTP−OPP) were measured around their glass transition temperatures at atmospheric pressure and high pressure (20.8 MPa for polystyrene and 28.8 MPa for OTP−OPP). The measurements were performed simultaneously with a novel calorimeter recently developed by our group. This calorimeter works under constant pressure up to 100 MPa and in the temperature range between 80 and 380 K. The heat capacity gap Δ C p, thermal expansivity gap Δ α, and isothermal compressibility gap Δ κ at T g were determined precisely for both samples. The Prigogine−Defay ratio Π ≡ Δ C p Δ κ /( TV (Δ α ) 2 ) is significantly larger than unity for both samples, indicating that more than two internal parameters are necessary to specify the glassy state thermodynamically. The enthalpy and volume relaxations of the polystyrene and OTP−OPP were also measured simultaneously around T g after the temperature- and pressure-jump processes. The relaxation paths determined in the Δ H c −Δ V c plane was inconsistent with the fictive temperature concept in the structural relaxation even in the vicinity of the equilibrium state with the volume relaxing faster than the enthalpy.