The local dynamics of 1,4 polybutadiene below and above the merging of the α and β relaxations have been investigated by combining neutron spin echo (NSE) and dielectric spectroscopy. The study of the dynamic structure factor measured by NSE over a wide momentum transfer range allows us to characterize the α relaxation as an interchain process while the β relaxation originates from mainly intrachain motions. At temperatures below the merging, the dynamic structure factor can be described by a superposition of elemental processes for the β relaxation as obtained from dielectric spectroscopy. The elemental motions behind this process can be related to rotational jumps of the chain building blocks around their center of mass. Furthermore, we have been able to consistently describe the dynamic structure factor above the merging of the α and β relaxations by assuming that both processes are statistically independent. In the framework of this scenario a procedure for analyzing the dielectric response in the α-β merging region has been developed. Its application to the dielectric data allows us to describe the dielectric response in this region on the basis of the low temperature behavior of the α and β processes and without considering any particular change in the relaxation mechanism of these processes. The temperature dependence found for the relaxation time of the α process follows now the viscosity, a masked feature in the experimental data due to the merging process. In this way, we have been able to consistently describe the relaxation of both, the polarization and the density fluctuations, by using the same scenario, i.e., independent α and β processes, and considering the same functional forms and temperature dependences of the characteristic times of the two processes. © 1996 The American Physical Society.

We present neutron spin echo experiments that address the much debated topic of dynamic phenomena in polymer melts that are induced by interacting with a confining surface. We find an anchored surface layer that internally is highly mobile and not glassy as heavily promoted in the literature. The polymer dynamics in confinement is, rather, determined by two phases, one fully equal to the bulk polymer and another that is partly anchored at the surface. By strong topological interaction, this phase confines further chains with no direct contact to the surface. These form the often invoked interphase, where the full chain relaxation is impeded through the interaction with the anchored chains.

Neutron time-of-flight data of polybutadiene and polyisoprene in a wide temperature range around the glass transition are reported. The anharmonic part of the spectra is evaluated using a recently developed time domain Fourier transform technique. The results corroborate the recent finding of two different time domains in the dynamics, one above a crossover time of about 2 ps and the other below it. Two different interpretations of the short time behavior are considered, namely a fast Debye regime of the α relaxation and a vibrational softening of the boson peak.

The dynamics of a very fragile glass-forming polymer is studied by time of flight neutron scattering in a time scale from ${10}^{\mathrm{-}13}$ to ${10}^{\mathrm{-}11}$ s. We observe a crossover from a Debye-like to a non-Debye behavior at a characteristic time which is effectively independent of temperature and momentum transfer Q. We also confirm the results previously obtained about the non-Debye α relaxation in a different time scale (${10}^{\mathrm{-}11}$ to ${10}^{\mathrm{-}8}$ s). The results here reported open a new outlook for understanding the fast dynamics of glass-forming systems.

The question of whether the primary $\alpha$ relaxation in glass-forming systems is homogeneous or heterogeneous in nature is investigated by incoherent neutron scattering techniques in glass-forming polymers. It is shown that the momentum transfer dependence of the Kohlrausch-Williams-Watts relaxation time allows one to discriminate between these two limiting scenarios. From the results obtained we can conclude that the apparent stretching of the $\alpha$-relaxation function relates dominantly to sublinear diffusion and is not a result of heterogeneities in the material.