(250g) Modeling Polymer Melts Containing Short and Long Fibers: Part I Transient Rheology

In this paper we investigate the transient shear rheology of polypropylene (PP) and polybutylene terephthalate (PBT) containing short glass fibers (30% wt., initial lengths of 1mm, .2mm, and L/D of 80, 16 respectively) and PP containing long glass fibers (40 % wt., initial length of 11mm and L/D of 647). The objectives of this work are to determine the relation between stress growth and relaxation behavior to fiber orientation, interaction and matrix rheology and model the behavior by extending the Doi theory for rigid rod molecules in a Newtonian solvent to glass fibers in a non-Newtonian matrix. Doi's original theory predicts some of the interesting transient phenomena exhibited by the composite materials, but the theory predicts that the relaxation processes are governed by the reorganization of the fibers due to the Brownian motion contribution. Brownian motion can only be a factor if the characteristic dimension is sufficiently small, less than 500nm. In order to correctly predict the transient rheology of these materials, the Phan Thien-Tanner (PTT) model is used to model the rheology of the composite, where the contribution from the fibers is absorbed into the enhanced relaxation spectra. The fiber orientation is then calculated by incorporating the predicted flow field into Doi theory neglecting the effects of Brownian motion. Material parameters for the theory are found by fitting the model to dynamic and steady shear measurements performed on an RMS-800 equipped with cone and plate geometry for small fiber samples, and parallel plate geometry for long fiber samples to allow for gap control. Intermittent stress growth experiments are used to elucidate the contribution of the viscoelastic matrix and glass fiber to the relaxation process. It is confirmed that Brownian motion does not play a role in the relaxation processes but is most likely due to the relaxation of the polymer chains.