Dielectric relaxation in PMMA revisited

Broad-band dielectric spectroscopy (10(-2)-10(9) Hz) has been performed on a mainly syndiotactic sample of poly(methyl methacrylate) (PMMA) of molecular weight (Mw) 50 000. The syndiotactic (85%) PMMA was chosen to study the merging of the alpha and beta relaxations in a system where the dielectric loss is dominated by a beta (or Johari-Goldstein) relaxation. Contrary to other polymers the measured beta relaxation has an asymmetrical shape that cannot be described by a Cole-Cole function, not even at low temperatures where the alpha and beta relaxations are well separated in frequency. Therefore the more general Havriliak-Negami equation (HN) was used to fit the beta-loss. At temperatures close to T-g (= 404 K) the influence of the alpha relaxation complicates the analysis. To investigate this merging of the alpha and beta processes the data were analyzed in different ways. Both directly in frequency domain and by means of a regularization method also in the time domain. We have also analyzed the data adopting the William's ansatz, i.e, instead of a simple addition of the alpha and beta relaxations the processes are convoluted in the frequency domain. (C) 1998 Elsevier Science B.V. All rights reserved.