Polymers: When S(Q,t) is not what you think! The role of dynamic RPA

Abstract

Investigating a (polymeric) soft-matter system with several components often is aimed at the observation of the motions/mobilities of one of the components. The normal approach to this request in the context of quasielastic neutron scattering (e.g. NSE) is to put contrast on the targeted component, typically by leaving it hydrogenated and have the rest (surrounding, matrix) deuterated. In simple systems with only one molecular species as a homo-polymer melt with a few of the molecules contrasted (e.g. 10% h in 90% d) -provided the h-and d-molecular varieties behave sufficiently equalthis strategy yields a valid single chain (molecule) structure factor S(Q,t)/S(Q), even for a 50/50% mixture. However, if the component of interest and the matrix are of different kind, unexpected distortions of the observed S(Q,t) may occur. Interpretation of such results in terms of the single chain structure factor would then lead to erroneous conclusions. In this contribution the conditions, under which these distortions will occur, are discussed and how dynamic RPA may help to cope with them is explained. A practical method to apply this correction to polymer and similar problems is presented and an experimental verification is discussed [1, 12].