Modelling and scaling of dichroism during relaxation in emulsions and polymer blends

Abstract

Relaxation after cessation of flow is studied in mixtures of immiscible polymer fluids by means of linear conservative dichroism. Depending on the previous shear history, relaxation is known to proceed by either droplet retraction or break-up. Here, scaling relations for the evolution of the dichroism for both mechanisms of relaxation are presented. In addition a direct analysis of the rheo-optical relaxation curves is given. Starting from a suitable model for droplet retraction and applying Rayleigh–Gans–Debye scattering theory, this particular type of dichroism relaxation can be described quite accurately. A similar approach has been applied to relaxation by droplet break-up as induced by interfacial instabilities (Rayleigh instabilities). Tomotika theory is used to model the evolution of the droplet shape. The theoretical result deviates systematically from the experiments which is attributed to end-pinching. Lacking a suitable model for the latter, it is taken into account by an empirical correction similar to that used to describe the normal stress relaxation. With the rheo-optical technique the evolution of the relaxation time as a function of the initial aspect ratio of the droplets can be studied. In addition the effect of concentration on the relaxation time is investigated.