Novel compatibilized nylon-based ternary blends with polypropylene and poly(lactic acid): morphology evolution and rheological behaviour

In this paper, the interaction between nylon 6 (PA6), polypropylene (PP) and poly(lactic acid) (PLA) is reported. To improve the compatibility between these immiscible polymers, a reactive compatibilization approach was used through extrusion with maleic anhydride grafted polypropylene (PP-g-MA). To further improve the compatibility of the phases, PLA was selected as a semi-polar polymer and a low molecular weight was used to assure a good droplet dispersion. All the blends were twin-screw extruded in the melt at different compositions. The morphologies of binary and ternary blends were investigated using microscopic techniques by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM). The PP-g-MA grafting process was observed to have a dramatic effect on the compatibility of both the binary and ternary blends and while a drastic reduction of the PP dispersed phases particle size was observed, the affinity between PLA and PP was also highlighted. The surface tension of the homopolymers and the interfacial tension of the pairs of polymers were measured to characterize the interaction at their interfaces. The interaction of PA6/PLA appeared preferable to PLA/PP, explaining the thinner dispersion obtained for PLA phase. The morphologies observed were compared to the predictions of spreading coefficient and minimum free energy models. While both models predicted the encapsulation of PP by the PLA phase, disagreeing with the morphological results, the rheological measurement gave an explanation for this phase separation. Using rheology measurements, the interaction between the phases was further investigated and the viscosity ratios were measured for the different pairs of polymers, stressing the high interaction between PA6 and PLA with and without compatibilizer. The droplet size of the dispersed phases appeared to substantially influence the chain relaxations in the melt.