Neutron Scattering and Polymer Dynamics

Abstract

Nowadays polymers are ubiquitous in our daily life because they are durable, cheap to produce, easy to process and exhibit very versatile and favorable mechanical properties. For example, depending on temperature or time the same polymer may be viscous, rubber elastic, very tough with high impact strength or even brittle. Polymers are composed of macromolecules which are built up by a large number monomer units linked together by covalent bonds. Due to this connectivity, the relevant processes driving the dynamics in polymers depend on the length scale of observation. While at typical inter- and intra-macromolecular length scales (≈ 10 Å and below) polymer dynamics display the universal features of glass-forming systems, at larger distances the macromolecular character of the polymer chains prevails –there, entropy and topological constraints (‘entanglements’) play the major role. Carbon and Hydrogen are in most cases the main constituents of polymers. Therefore, neutron scattering combined with isotopic substitution (H/D labeling) is an extremely well-suited technique to study the dynamical processes of polymeric materials at a molecular level. Motions of particular molecular groups or given polymer chains, e. g. in a polymer blend, can be selectively investigated by neutron scattering. However, the restricted dynamic window offered by this technique prevents a complete characterization of the complex polymer dynamics, including several multi-scale processes the characteristic times of which span over many orders of magnitude. The combination with other experimental techniques is, in most cases, essential to fully characterize the processes; in addition, the use of complementary MD-simulations has proved to be crucial for the interpretation of the neutron-scattering results.

Acknowledgments

We are indebted to our research group in San Sebastián, in particular to A. Alegría, A. J. Moreno and S. Arrese-Igor. We also thank the group of D. Richter in Jülich and our colleagues at the different neutron facilities, in particular B. Frick and B. Farago at the ILL, for many years of fruitful collaboration.