Quasi-elastic Raman scattering and free volume in isotropic pressure-densified atactic poly(methyl methacrylate) glasses

We have made a series of isotropic pressure-densified atactic poly(methyl m ethacrylate) (PMMA) glasses with densities ranging from 1.1823 g/cm(3) to 1 .1963 g/cm(3), in order to investigate how the glassy dynamics, measured by Raman scattering in the frequency range of 0.1-10 THz, depends on the ther momechanical history of the glasses. In particular, we investigate whether there is a correlation between the strength of the fast relaxation dynamics and the free-volume characteristics, such as the average free-volume size and the free-volume fraction. The fast relaxation strength is deduced from the quasielastic light scattering (QES) intensity; the free-volume size is measured with positron annihilation lifetime spectroscopy (PALS); and the f ree-volume fraction is determined from pressure-volume-temperature (PVT) da ta and the Simha-Somcynsky equation-of-state (EOS) theory. Our temperature- dependent Raman and PALS measurements show that both the QES intensity and the ortho-positronium (o-Ps) lifetime increase smoothly with the temperatur e. On the other hand, the relaxation strength of the densified glasses at r oom temperature is, within experimental error, independent of the density, whereas both the o-Ps lifetime from PALS and the free volume fraction from the PVT data decrease significantly with pressure densification. We therefo re conclude that the fast relaxation intensity cannot be explained on the b asis of free-volume quantities, contrary to a recently suggested correlatio n between the QES intensity and the free volume [V. N. Novikov , J. Chem. P hys. 107, 1057 (1997)]. On the other hand, the observed behavior is consist ent with another recently suggested model [V. N. Novikov, Phys. Rev. B 58, 8367 (1998)] which attributes the fast relaxations to vibration anharmonici ty. Moreover, we show that the measured increase in the refractive index of the pressure-densified glasses fits the Lorentz-Lorenz equation and compar es well to predictions of the relative change in refractive index with form ation pressure from our own as well as published PVT data. (C) 2000 America n Institute of Physics. [S0021-9606(00)51202-1].