Isotropic pressure-densified atactic poly(methyl methacrylate) glasses: Free-volume properties from equation-of-state data and positron annihilation lifetime spectroscopy

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/cm(3): the free-volume fraction h from the Simha-Somcynsky theory decreased by up to 19.2%, the orthopositronium (o-Ps) lifetime tau(3) from positron annihilation lifetime spectroscopy (PALS) decreased by up to 8.4% whereas the o-Ps intensity I-3 remained constant at 29 +/- 0.5%, and the volume of the free-volume cavities V(tau(3)) decreased by up to 16.1%. Moreover, the microscopic volume quantities h and V(t3) were found to correlate best. The radius of the free-volume cavities R(tau(3)) at zero h appears to approach the Bohr radius of the o-Ps species. The thermal expansivity alpha(0) remains almost constant, the isothermal compressibility kappa(0) 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.