PRESSURE-DEPENDENCE OF THE ELASTIC-CONSTANTS OF POLY(METHYL METHACRYLATE)

We report measurements of the hydrostatic pressure dependence of the e lastic constants of poly(methyl methacrylate) using high resolution Br illouin spectroscopy. We have measured the density rho(p) up to 500 MP a, and the longitudinal and transverse sound velocities up to 300 MPa. We have calculated the bulk modulus, Youngs modulus, shear modulus an d Poisson's ratio. Initially there is a strong increase in the intensi ty of the shear phonons which levels off at p > 100 MPa. At this press ure a dispersion-like step in the shear modulus as well as a decrease in the bulk modulus is found, which is tentatively discussed as a 'wea k glass transition' within the glass.