Compatibilization of PLA/PEBA Blends via Reactive Extrusion: A Comparison of Different Coupling Agents

Poly(lactic acid) (PLA)/poly(ether-b-amide) (PEBA) blends were prepared by melt extrusion using a co-rotating twin-screw extruder. PEBA elastomer, with content varying between 0-24 wt%, was used to toughen PLA. PLA/PEBA blends are found to be immiscible on all the composition range studied. Rigidity and strength of PLA/PEBA blends are strongly decreased by the incorporation of PEBA. The fragile-ductile transition was observed in the 8-12 wt% PEBA range. A variety of reactive coupling agents have been used in low (2 wt%) and high proportions (4-12 wt%) to compatibilize PLA/PEBA blends. Their efficiency was evaluated by comparing their thermal, mechanical and morphological properties. Poly(ethylene-co-methyl acrylate-co-glycidyl methacrylate) (PEAGM), poly(maleic anhydride-alt-1-octadecene) (PMAOD), 1,1'-carbonyl biscaprolactam (CBC) and 4,4-methylene diphenyl diisocyanate (MDI) were incorporated during melting process. Compatibilized PLA/PEBA blends were also compared with PLA/PEBA blends with various amounts (0-8 wt%) of polyethylene glycol (PEG). In the blends, PEG acted exclusively as a plasticizer in the PLA rigid phase; it drastically increased fluidity, ductility and impact strength. PMAOD and PEAGM bridged the two immiscible polymers at the interface. When used in small amounts, PMAOD and PEAGM reactive copolymers compatibilized, to some extent, PLA/PEBA blends as testified by a moderate increase in the impact strength (similar to 25 %); both reduce the tensile strength and the Young's modulus. CBC also had a moderate compatibilizing effect and provided an antiplasticizing-like effect by acting essentially on the PLA rigid phase. An excellent toughening effect was achieved with the use of MDI without penalizing Young's modulus, tensile strength or elongation at break. The strength needed to break the notched Izod and the Garner impact was about 7 and 63 times higher than the corresponding blend without MDI, respectively. The super toughening effect was attributed to the crosslinking of the rigid segments of the soft PEBA phase, together with the various efficient coupling reactions making the optimal interfacial adhesion possible.