ON THE DETECTION OF FLOW-INDUCED FRACTIONATION IN MELTS OF HOMOPOLYMERS BY NORMAL-MODE MICRODIELECTROMETRY

A new technique is introduced that enables us to probe main-chain relaxation of polymers in the immediate vicinity of a solid surface. The method combines microdielectrometry sensors with polymers for which normal-mode relaxation can be measured dielectrically (i.e., Stockmayer's type-A polymers). The term "normal-mode microdielectrometry", or NMMD, was coined to reflect this particular combination of ingredients. The penetration depth of the electric field on a microsensor scales with the spacing between the electrode lines and can be reduced considerably by miniaturizing the device. This enables one to follow the evolution of the relaxation times of type-A polymers in an extremely localized region near the sensor surface. In this preliminary work we apply this tool to the study of flow-induced fractionation in melts of cis-polyisoprene undergoing plane Poiseuille-type flow. Dielectric relaxation measurements during flow start-up showed the onset of a remarkable transient, consistent with the buildup of a layer of low molar mass chains near the solid surface. The real and the imaginary components of the dielectric permittivity showed both pronounced overshoots before approaching an apparent steady state. An advective mechanism could explain only the earliest stages of the transient. Upon cessation of flow, the shape and magnitude of the dielectric spectra slowly relaxed back to the equilibrium (preflow) values. The time scale of the relaxation was consistent with a reequilibration by chain diffusion of the melt composition within the dielectric control volume. A simple dielectric model of a type-A polymer mixture could qualitatively describe the general features of the data. Other possible applications of NMMD to study chain relaxation and diffusion in melts and concentrated solutions, at polymer-solid interfaces, and in polymeric liquid crystals are proposed.