Progress of Theoretical Physics Supplement No. 126, 1997 Incoherent Inelastic Neutron Scattering on Polybutadiene under Pressure B. FRICK,*) C. ALBA-SIMIONEsco,* J. HENDRICKs** and L. WILLNER** Inst. Laue-Langevin, Grenoble, France *CPMA Paris, France **IFF /Forschungszentrum Julich, Germany We report on first results from incoherent inelastic neutron scattering on PB-h6 (T 9 = 178 K at Patm) between T = 2 K and 340 K and for pressures between Patm and 4 kbar, employing the new backscattering spectrometer IN16 and the TOF spectrometer IN6 at the ILL. The strictly elastic scattering ( t 10- 8 s ), observed by temperature scans on IN16, shows indirectly the onset of the fast relaxation process and the onset of the a-relaxation at the isobars of Patm, P = 1, 2, 3 and 4 kbar. The observed temperature dependence may be interpreted by a change towards lower fragility when applying pressure. Clearly, the apparent activation energy changes in the temperature range of the a-relaxation. Quasielastic spectra were measured for several isotherms as well as for an isochore. The results allowed the deter- mination of an isochrone on the ns time scale, which has a slope of dT; 00 /dP::::: 18 K/kbar, compared to a T 9 variation of dT 9 /dP::::: 13 K/kbar . Isochoric spectra certainly do show different relaxation behaviour. Using IN6 we observe for the first time a shift of the Boson peak with pressure by about 0.13 meV /kbar between ambient pressure and 4 kbar. This is visible at low temperatures where relaxations are not yet active. classification: polymer glasses, glass transition, scattering, viscoelastic-systems §1. Introduction 213 Over the recent years the glass transition in polybutadiene was studied in quite some detail by neutron spectroscopy. l), 2 ) For all these experiments temperature was used to drive the sample through the glass transition. Temperature changes affect the thermal energy and the volume. Here we present first data for protonated polybutadiene where we use pressure as additional control parameter in order to locate specific points in the P- T diagram via pressure and temperature changes. The general scenario for the glass transition found by inelastic neutron scat- tering is the appearance of a fast process on the ps time scale close to T 9 and a slow process above T 9 . The origin of the fast process is much under discussion, whereas the slower process is related to the macroscopic viscosity and thus to the a-relaxation. Explanations for the fast process reach from the 'mode-coupling /3- relaxation' 3 ) to 'vibrational softening'. 4 ) Furthermore the reason for the existence of specific excitations in glasses called the Boson peak, is not well understood. Application of pressure as a new external variable might help in future to discern between different models. Here we can only present preliminary results from such a study. The hope is to separate by application of pressure the slow and fast relax- •) Correspondence to: Dr. Bernhard Frick, Institut Laue Langevin, Avenue des Martyrs, BP 156, F-38042 Grenoble, France. Downloaded from https://academic.oup.com/ptps/article-abstract/doi/10.1143/ptp.126.213/1944443 by guest on 18 June 2020