Comparison of polylactide/nano-sized calcium carbonate and polylactide/montmorillonite composites: Reinforcing effects and toughening mechanisms

Semicrystalline polylactide (PLA) exhibits high tensile strength and modulus but very low strain-at-break and toughness. In this study, PLA nanocomposites with nano-sized precipitated calcium carbonate (NPCC) and organically modified montmorillonite (MMT) clay were prepared by melt extrusion. Morphologies, tensile mechanical properties, dynamic mechanical and rheological properties, polymer-nanoparticle interactions, and toughening mechanisms of the PLA/NPCC and PLA/MMT nanocomposites were compared. MMT and NPCC showed significantly different effects on the strength, modulus and elongation of the PLA nanocomposites. Different toughening mechanisms were first elucidated for the two types of nanocomposites based on the evidence from both macroscopic and microscopic observations. Under uniaxial tension, large quantities of microvoids were created in both PLA nanocomposites. The microvoids in PLA/NPCC caused massive crazing, while in PLA/MMT they resulted in shear yielding, particularly in the nanocomposite with 2.5 wt% MMT. The MMT stacks and platelets were found to be located between the microvoids in the extended specimens and prevented them from collapsing and coalescing.