Effect of shear rate on orientation of cellulosic nanofibers and nanocrystals in poly(butylene adipate‐co‐terephthalate) based composites

Poly(butylene adipate‐co‐terephthalate) with or without thermoplastic starch is often used as a biodegradable matrix in composites reinforced with 5 and 10 wt% of either microfibrillated cellulose or cellulose nanocrystals. If dispersion of the fillers is well studied, their orientation in melt blended composites requires further understanding. In this study, the effect of a controlled shear rate on the orientation of the filler was investigated to understand how shear rate affects orientation and how orientation affects mechanical properties of the composites. To this end, composites prepared by melt mixing and then compressed were taken as a reference state of low orientation. On the contrary, to orient the fillers, extrusion through a slit die with a determined shear rate was carried out. Results of tensile tests, microscopic observations, atomic force microscopy and dynamic mechanical analysis in the melt showed that orientation of fillers in the flow axis was possible for shear rates higher than 13 s−1. The orientated samples presented enhanced mechanical properties in the elastic domain as opposed to unoriented samples. In general, orientation of fillers leads to uniaxial stiffness at lower filler content with much better ductility. However, this was observed only on samples for which the percolation of the filler was not obtained in the unoriented state, otherwise, orientation proved to be detrimental to the elastic mechanical properties because of the rupture in the formed network.Highlights Orientation can be controlled during processing. Sufficient shear rate is required. Orientation improves mechanical properties. Orientation might have a negative effect on rigidity in case of existing network.