Study of Multicomponent Diffusion in Entangled cis-Polyisoprene Melts by Normal-Mode Microdielectrometry

Abstract

We combine microdielectrometry (MD) sensors with polymers showing dielectric normal-mode relaxation (type A polymers) to study multicomponent diffusion in melts of entangled, linear, flexible chains. The term normal-mode microdielectrometry (NMMD) reflects this particular combination of ingredients. We consider cis-polyisoprene (cis-PI), a well-characterized type A polymer. Using reptation theory and a linear mixing rule, normal-mode dielectric loss spectra of polydisperse cis-PI samples can be inverted to yield the entire molecular weight distribution (MWD). MD sensors enable probing the dielectric response of the material residing in the immediate vicinity (O(10 μm)) of a solid surface. Therefore, the combination of type A polymers and MD sensors allows us to follow the evolution of the MWD of the material in the immediate proximity of a solid surface. We study the diffusive dissolution of a thin layer of a polydisperse, high-MW cis-PI melt, deposited at the sensor surface, into a polydisperse, semiinfinite medium of smaller chains. The data are first analyzed using a Fickian diffusion model with a molecular weight dependent effective diffusivity. The results are consistent with swelling of the high-MW melt caused by diffusive penetration of smaller chains. We also propose an extension of the Kramer-Sillescu theory of diffusion to polydisperse systems and contrast the results to the kinetic-theory equations of Bird et al. An analytical solution for the evolving near-surface MWD is obtained for a simplistic version of the Bird et al. model. Although showing qualitatively correct trends, the model is unable to quantitatively describe the experimental data.