Investigation of the Dielectric -Process in Polyisobutylene by Incoherent Quasielastic Neutron Scattering A. Arbe and J. Colmenero* Departamento de Fı ´sica de Materiales, Universidad del Paı ´s Vasco, Apartado 1072, 20080 San Sebastia ´ n, Spain B. Frick Institut Laue-Langevin, 156X, 38042 Grenoble CEDEX, France M. Monkenbusch and D. Richter Institut fu ¨ r Festko ¨ rperforschung, Forschungszentrum Ju ¨ lich, 52425 Ju ¨ lich, Germany Received December 8, 1997; Revised Manuscript Received May 5, 1998 ABSTRACT: 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.; Monkenbusch, M.; Farago, B.; Faust, R. Macromolecules 1998, 31, 1133]. Using high resolution neutron backscattering, we investigated the quasielastic neutron spectra from protonated PIB which are related to this process as a function of momentum transfer and temperature. In addition we studied the elastically scattered intensity over a wide temperature range. After multiple scattering corrections all results can be described consistently in terms of a local jump process with the distribution parameters from the dielectric -relaxation. The spatial extent of the associated protonic motion was determined to be d ) 2.7 Å. A comparison with the existing body of data for PIB leads to the conclusion that the dielectric -process and the earlier found δ-process must be identical, thereby revising the assignment of the δ-process as due to methyl group rotation. Finally, we remark on aspects of the relation between quasielastic coherent and incoherent scattering and address the seemingly contradictory result of different length scales revealed for the same process with the two techniques. I. Introduction The viscoelastic properties of polyisobutylene (PIB) have been investigated continuously over the last 50 years, rendering it one of the best studied polymers. 1-14 PIB exhibits an R-process which has been thoroughly explored by Ferry et al., who established a shift factor which approaching the glass transition exhibits a relatively weak temperature dependence and thereby qualifies PIB as the least fragile polymer (τ R ) τ o exp [DT o /(T - T o )] with the fragility index D ) 49 and the Vogel-Fulcher temperature T o ) 89.2 K). 1 Later experiments applying spectroscopic techniques such as NMR and ESR did not reproduce the rheologically based temperature dependence but rather indicated a more fragile behavior. 6,11,12 Very recently, however, neutron spin-echo (NSE) experiments were undertaken at the momentum transfer of the first structure factor peak, thereby studying the polymer motion at interchain distances. 15 There, the dynamic response is selective for the relative motion of different chains and reveals the R-relaxation. These NSE experiments demon- strated that at length scales corresponding to the distance between adjacent chains the neutron spectra superimpose with the shift factors from viscous flow, thereby supporting strongly the original findings of Ferry et al. Obviously, the spectroscopic results, which are also reflecting microscopic motions, are not process selective. Probably as a consequence of the very weak electric dipole moment of the PIB chain, the experimental picture for the secondary relaxations is much less clear. Some time ago To ¨rma ¨la ¨ 6 compiled experimental and theoretical results for PIB: Theoretically, two relaxation processes γ and γ′ are predicted. 14 Experimentally by NMR 11,13 and ESR 6 a so-called δ-process was observed which was interpreted as rotations of CH 3 groups. Very recently, by dielectric spectroscopy, 15 we were able to find and characterize a secondary relaxation in PIB which has all the signatures of a Johari-Goldstein -process. 16 The distribution of jump times τ  can be well described by a log-normal distribution with a temperature-dependent width where T denotes the temperature in Kelvin, τ o  ) 1.5 × 10 -13 s, the average activation energy amounts to E o ) 260 meV and k B is the Boltzmann constant. Both the temperature dependence as well as the time scale of this process disagree significantly with the earlier theoretical speculations. Some traces of it could also be identified in the dynamic structure factor of PIB at large momentum transfers. Apparently the process had to be associated with a short jump distance. The aim of the present study was to find and to characterize this dielectric -process by incoherent quasielastic neutron scattering (IQENS). IQENS ac- cesses the self-correlation function of the moving protons on the appropriate atomic length and time scales, thereby facilitating molecular space-time observa- tions. 17 The experiments were carried out with the goal to explore the length scale of the -process, to identify g  (ln τ  ) ) k B T  πσ exp { - [ k B T ln(τ  /τ o  ) - E o σ ] 2 } (1) σ ) 130 - 0.29T [meV] (2) 4926 Macromolecules 1998, 31, 4926-4934 S0024-9297(97)01781-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/10/1998