Super-tough poly(lactic acid)/polycarbonate (PLA/PC)
(50/50) blends
with an excellent balance of stiffness, toughness, and thermal stability
were systematically designed and characterized. Poly(methyl methacrylate)
(PMMA) was utilized as a novel, highly effective nonreactive interphase
to promote PLA–PC phase compatibility. Partial miscibility
of PMMA with both PLA and PC produced strong molecular entanglements
across the PLA–PC phase boundary followed by an excellent phase
adhesion. This was predicted from interfacial energy measurements
and supported by dynamic mechanical thermal analysis, morphological
observations, and mechanical tests. Ternary PLA/PC/PMMA blends exhibited
an exceptional set of stiffness, tensile and flexural strength, tensile
and flexural ductility, and thermal stability together with improved
impact strength compared with neat PLA and uncompatibilized PLA/PC
blends. Addition of nonreactive polybutadiene-g-styrene-co-acrylonitrile (PB-g-SAN) impact modifier
to the compatibilized blend resulted in further dramatic improvements
in the dispersion state of PC and PMMA phase domains followed by the
development of an interconnected structure of PC, PMMA, and PB-g-SAN domains in the PLA matrix. Such a network-like morphology,
with rubbery particles percolated at the interface between the dispersed
structures and surrounding PLA matrix, produced a tremendous increase
in impact resistance (≈700 J/m) and tensile ductility (≈200%
strain) while maintaining excellent stiffness (≥2.1 GPa). The
combined effects of interfacial localization of impact modifier particles,
network-like morphology (extended over the entire volume of the blend),
and strong phase interactions between the components (due to mutual
miscibility) are described to be responsible for super-tough behavior.
The role of PMMA as an efficient interphase adhesion promoter in the
toughened quaternary blends is also clarified. Impact fractography
revealed multiple void formations, plastic growth of microvoids, and
the formation of void-fibrillar structures around as well as inside
the dispersed structures as the main micromechanical deformation processes
responsible for massive shear yielding and plastic deformation of
blends. Blends designed in this work offer remarkable improvements
in tensile and flexural ductility, impact resistance, and heat deflection
temperature compared with neat PLA resin. The overall characteristics
of these blend systems are comparable and/or superior to those of
several commercial thermoplastic resins.
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