| Abstract: | The dynamics of three glass-forming polymers, PVC, PB and PI, has been investigated by time of flight (TOF) neutron scattering in a time scale from 10?13s to 1011s looking for the crossover from microscopic dynamics to segmental dynamics (α relaxation). A new analysis procedure has been applied to TOF data in order to separate harmonic vibrational and relaxational contributions. Due to the involved assumptions, this procedure can be considered only as a first approximation adequate for the case of “fragile” systems (in the Angell's meaning) like the polymers here investigated. The behaviour obtained was the same for the three polymers studied. The intermediate scattering function corresponding to the relaxational dynamics, Io(Q,t), shows two different dynamical regimes separated by a crossover time tc (≈ 2 ps), which hardly depends on Q (momentum transfer) and temperature. At t < t<c, Io(Q,t) displays a Debye-like behaviour (exponential decay). The activation energy found for the relaxation time corresponding to this regime was in the range of 2–5 Kcal/mol, i.e., in the range of the activation energy for local conformational transitions in isolated macromolecular chains. At t > tc and, at least at high temperature, Io(Q,t) shows a Kohlrausch-Williams-Watts (KWW) behaviour similar to the obtained one by means of backscattering neutron techniques in the mesoscopic time scale (10?11s to 10?7s) and dielectric measurements in the macroscopic time scale (10?7s to 10°s). This KWW regime can be associated to the segmental dynamics involved in the α relaxation. A phenomenological interpretation is outlined. In this framework, the Debye-like regime is interpreted to be the segmental dynamics free from intermolecular hindrances. Therefore, tc should be the time at which intermolecular interactions start to play a significant role concerning to the segmental dynamics. This interpretation recalls some of the basic ideas of the so called “Coupling-Model” proposed a long time ago by Ngai. |
