Viscoelastic study of oil diffusion in molten PP and EPDM copolymer

Abstract

This paper is focused on two important features of the addition of a plasticizer (oil) into a polymer matrix: The influence of its concentration on the viscoelastic properties of the blend, and the diffusion mechanisms that take place during mixing. A rheological approach was chosen for its capacity to support relatively simple modeling. In the first part of the study, we demonstrate that the incorporation of a plasticizer in a PP matrix results in a reduction of entanglements and a global shift of the relaxation time spectrum. A time-concentration superposition is observed. A dilution model, based on the free volume theory, is implemented and predicts successfully the evolution of viscosity of binary PP/oil blends with oil content. This model appears, however, unsuitable for EPDM, whose relaxation mechanisms do not seem to be modified in the same manner. This suggests the presence of intermolecular interactions in EPDM that are not totally released in spite of oil addition. In the second part, the transport of the plasticizer in the polymer matrices is followed during viscoelastic testing. The molecular weight of PP in shown to have no influence on the diffusion kinetics, whereas diffusion appears faster in EPDM. The implementation of a model relying on a Fickian description of diffusion leads to satisfactory agreement with experiments for PP only, and allows for the calculation of the oil/polymer mutual diffusion coefficient. Again, a poor quality of fit is achieved with EPDM, suggesting that elastic diffusion is enhanced with this polymer.