INVESTIGATION OF THE DIELECTRIC BETA-PROCESS IN POLYISOBUTYLENE BY INCOHERENT QUASI-ELASTIC NEUTRON-SCATTERING

Recently by dielectric spectroscopy a new secondary relaxation process in polyisobutylene (PIB) was detected showing all the signatures of a Johari-Goldstein relaxation [Richter, D; Arbe, A.; Colmenero, J.; Mon kenbusch, M.; Farago, B.; Faust, R. Macromolecules 1998, 31, 1133]. Us ing high resolution neutron backscattering, we investigated the quasie lastic neutron spectra from protonated PIE which are related to this p rocess as a function of momentum transfer and temperature. In addition we studied the elastically scattered intensity over a wide temperatur e range. After multiple scattering corrections all results can be desc ribed consistently in terms of a local jump process with the distribut ion parameters from the dielectric beta-relaxation. The spatial extent of the associated protonic motion was determined to be d = 2.7 Angstr om. A comparison with the existing body of data for PIE leads to the c onclusion that the dielectric beta-process and the earlier found delta -process must be identical, thereby revising the assignment of the del ta-process as due to methyl group rotation. Finally, we remark on aspe cts of the relation between quasielastic coherent and incoherent scatt ering and address the seemingly contradictory result of different leng th scales revealed for the same process with the two techniques.