Fast Dynamics in Glass-Forming Polymers Revisited
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coupling theory (MCT) [1]. This theory predicts an intermediate regime for the time dependence of the density-density correlation connecting the first microscopic decay with the mesoscopic dynamics known as the ax-relaxation. It is worthy of remark that in the framework of the MCT this intermediate regime is not an actual process, i. e., a process with a well defined characteristic time scale (relaxation time). The intermediate regime of the MCT was unfortunately called f3process producing a big confusion due to the fact that the same term was associated for a long time to another universal process in glass-forming systems, the 3 (Johari-Goldstein) relaxation [2]. This 3 (Johari-Goldstein) relaxation is, in contrast to the MCT-j3-process, an actual process (i. e., with a well defined time scale) which can be observed separated from the aX-process at temperatures around and below the calorimetric glass transition temperature, Tg, by means of
relaxation techniques [3]. Very recently this process has also been observed by neutron spin echo (NSE) techniques in a glass-forming polymer (1,4-polybutadiene) [4]. From an experimental point of view, the first observations associated to the MCT-fast dynamics correspond to neutron scattering experiments also carried out in the last years of the 80's. There were two kinds of different experiments. One set corresponds to NSE techniques (see as representative examples [5,6]). These techniques directly measure the time dependence of the normalized density-density correlation function S(Q,t) at different values of the momentum O-12 0-9 transfer Q. The time scale covered by NSE ranges roughly between 5 x 10 1 and 10 s, and the first experiments were made at the Q-value corresponding to the first maximum of the static structure factor S(Q). The values obtained for S(Q,t) at the shortest time available (= 5 ps) were always less than one (= 0.85). This was considered as an indirect proof of the existence of the MCT-13-regime, without taking into account the possible decay of S(Q,t) due to vibrations or, even in the framework of the MCT, to the microscopic regime whatever it may be. The other set of experiments corresponds to TOF-measurements techniques (see as a representative example [7]). Several investigations by TOF found a quasielastic broadening in different glass-forming systems setting up around and above Tg. However, in the case of TOF measurements the presence of vibrational contributions including the additional excitations, which are known as the Boson peak, was clear. The first papers tried to separate the effect of vibrations subtracting from the high temperature spectra the low (T
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