Isotropic Pressure-Densified Atactic Poly(methyl methacrylate) Glasses: Free-Volume Properties from Equation-of-State Data and Positron Annihilation Lifetime Spectroscopy

Abstract

We modified the sample cell of a commercial pressure−volume−temperature (PVT) apparatus to make glasses by pressure densification (0−200 MPa) in the equilibrium melt. The PVT data of the glasses and the melt were analyzed in terms of the Simha−Somcynsky equation-of-state (EOS) theory and the Tait EOS. Small macroscopic volume changes up to 1.2% were found to yield large microscopic volume changes in isotropic pressure-densified atactic poly(methyl methacrylate) glasses with densities in the range 1.1823−1.1963 g/cm3: the free-volume fraction h from the Simha−Somcynsky theory decreased by up to 19.2%, the orthopositronium (o-Ps) lifetime τ3 from positron annihilation lifetime spectroscopy (PALS) decreased by up to 8.4% whereas the o-Ps intensity I3 remained constant at 29 ± 0.5%, and the volume of the free-volume cavities V(τ3) decreased by up to 16.1%. Moreover, the microscopic volume quantities h and V(τ3) were found to correlate best. The radius of the free-volume cavities R(τ3) at zero h appears to approach the Bohr radius of the o-Ps species. The thermal expansivity α0 remains almost constant, the isothermal compressibility κ0 decreases by up to 10.3%, which corresponds to an 11.5% increase in bulk modulus, and the freezing fraction remains constant up to formation pressures of 200 MPa.