Intermediate length scale dynamics in glass forming polymers: coherent and incoherent quasielastic neutron scattering results on polyisobutylene

In the complex dynamics of glass forming systems, and in particular of polymers, there exists a region in length scales which remains basically unexplored: that of the so-called intermediate length scales. These correspond to several times the intermolecular distances. For polymer systems, the dynamics under question lays between the large scale Rouse regime and the <f>α</f> or structural relaxation, which is universal for glass forming systems. In this work we have made an experimental effort by neutron spin echo to extend as much as possible the study of the single chain dynamic structure factor towards short length scales and the study of the self and collective atomic motions towards large length scales for one and the same polymer, polyisobutylene. Despite the difficulties to interpret results corresponding to the different correlation functions investigated, a common process appears to be reflected in all of them at intermediate length scales. This process seems to be the main responsible for the deviations from entropy driven (Rouse) dynamics and for the relaxation of the longitudinal stress. It would be originated by local conformational transitions taking place by correlated jumps across the rotational barriers in the main chain. [Copyright &y& Elsevier]