Entanglements in semi-dilute solutions as revealed by elongational flow studies

Abstract

Our methodology of chain stretching using elongational flow enables identification of circumstances in which macromolecules in solution extend cooperatively, in contrast to their extending in isolation. This opens a new window for the detection and study of entanglements. In addition to defining the concentration for the required degree of coil overlap, the present studies also define the time scale on which the geometric entanglement becomes mechanically effective, thus introducing a dynamic element to the identification and classification of entanglements. The entanglements develop through a sequence of patterns, periodic both in time and space with increasing strain rate. The strain patterns thus arising modify the flow field locally, assessed by velocimetry. Further, the macroscopic flow resistance (“elongational viscosity”) is determined and correlated with the various stages of chain stretching and network formation. These findings link molecular behaviour and macrorheology. Examples from engineering applications indicate how entanglements may help to account for the various flow modifying actions of polymeric additives, and conversely, how experience gained in engineering applications can potentially further the study of entanglements.