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 compressibilitie s of polystyrene and a mixture of o-terphenyl (67%) and o-phenylphenol (33% ) (abbreviated as OTP-OPP) were measured around their glass transition temp eratures at atmospheric pressure and high pressure (20.8 MPa for polystyren e 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 b etween 80 and 380 K. The heat capacity gap Delta C-p, thermal expansivity g ap Delta alpha, and isothermal compressibility gap Delta kappa at T-g were determined precisely for both samples. The Prigogine-Defay ratio Pi = Delta C(p)Delta kappa/(TV(Delta alpha)(2)) is significantly larger than unity fo r both samples, indicating that more than two internal parameters are neces sary to specify the glassy state thermodynamically. The enthalpy and volume relaxations of the polystyrene and OTP-OPP were also measured simultaneous ly around T-g after the temperature- and pressure-jump processes. The relax ation paths determined in the Delta H-c-Delta V-c plane was inconsistent wi th the fictive temperature concept in the structural relaxation even in the vicinity of the equilibrium state with the volume relaxing faster than the enthalpy.