Dynamics at the Glass Transition in Polymers: Results from Neutron Spectroscopy
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relaxation process termed /fat is found. Both, the Boson peak as well as the 8fat process exhibit only very little temperature dependence. Though the relaxation processes, in particular the a,,o, and Aslow relaxations, have been investigated by spectroscopic methods like dielectric and mechanical measurements since at least 50 years, still not very much is known about the molecular mechanism behind these relaxations. This deficiency relates to the fact that these spectroscopic methods although providing a very large dynamic range and high sensitivity in the time or frequency domain they do not reveal spatial information. Such space-time sensitivity can be provided by inelastic and quasielastic neutron scattering because cold and thermal neutrons possess wavelengths corresponding to the interatomic distances and at the same time thermal energies. Other than X-rays providing structural information neutrons reveal knowledge where an atom is situated and in which direction it moves at what speed. In this way space-time observation of individual molecules is facilitated. In particular, the neutron spin echo (NSE) method reveals information about the time-dependent dynamic structure factor S(Q, t) which reflects the time-dependent density-density correlations S(Q,t)0 =sp(Q, tjp(Qo)( S(Q)
Mat. Res. Soc. Symp. Proc. Vol. 455 ©1997 Materials Research Society
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Boson peak
P fast 10
5
-\
PSIOW
,
Fig. 1: Relaxation map for (1,4) polybutadiene covering the prevailing dynamical features around the glass transition. The frequency range for the Boson peak and the associated fast relaxation like dynamics (ffast) are indicated schematically [15]. The full and open circle along the a relaxation trace represent dielectric [7] and mechanical [8] results respectively. The full squares display characteristic rates obtained from neutron spin echo
0 Tg 3
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5
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spectroscopy [5,6]. The time scale of the a process has been shifted to match that of the microscopic data. The dashed line represents the temperature dependence of the Aim process observed by dielectric spectroscopy [9].
1000 K/ T
Sp(Q,t) is the Fourier component of a density fluctuation to the wave vector Q at a time t. S(Q) denotes the static structure factor and Q=4m'2 sin(O/2) is the scattering vector, where 2 is the neutron wave length and o9 the scattering angle.
In this work we present neutron spin echo results on the Q-dependent dynamic structure factor of polybutadiene [9,10] and some new results on PVC [11]. First, these data are analyzed in the 8Jrelaxation regime revealing spatial information about this relaxation process. Thereafter, we consider the merging regime of the a and 81relaxation and show the statistical independence of both relaxation processes. We revisit dielectric results on polybutadiene and interpret them consistently on the basis of the insight gained from the NSE investigations. Finally, we present some new results on PVC which are interpreted in the same spirit resolving an old puzzle on the seemingly very strongly stretched rela
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