RAYLEIGH-SCATTERING OF MOSSBAUER GAMMA-RAYS IN THE AMORPHOUS POLYMER POLYMETHYL METHACRYLATE

The Mössbauer effect was used for measuring the elastic and inelastic Rayleigh scattering of the57Fe 14.4 keV γ-rays in the amorphous polymer polymethyl methacrylate. The observed dependence of the elastic- and inelastic-scattering intensities on sin θ/λ indicates that the inelastic scattering by thermal vibrations is a considerable fraction of the total intensity just outside the first diffraction maximum. The temperature dependence of both elastic and inelastic intensities has been measured up to the melting point. A strong decrease of I el and a consequent increase of I in were observed above the glass transition temperature T g. This behaviour may be attributed to the excitation at T >T g of librational motions of part of the polymeric chain. A small broadening in the energy distribution of the diffracted γ-rays was detected very near the molting point. Prom these data a diffusion relaxation time τ= 2.7 · 10-7s was deduced. © 1979 Società Italiana di Fisica.