The inherent shortcomings of polylactide (PLA) including brittleness, low glass transition temperature, and melt strength during processing were addressed through a facile melt blending of PLA with polybutadiene-g-poly­(styrene-co-acrylonitrile) (PB-g-SAN) core–shell impact modifier and poly­(methyl methacrylate) (PMMA). Highly tough PLA-based ternary blends with drastically enhanced glass transition temperature (≈ 21 °C) and melt strength were successfully prepared. The effect of PMMA content (ranging from 0 to 30 wt %) on the phase miscibility, morphology, mechanical properties, thermal behavior, rheological properties, and toughening mechanisms of PLA/PB-g-SAN/PMMA blends with 30% PB-g-SAN was systematically investigated. It was found that PMMA can effectively tune the interfacial interactions, phase morphology and performance of incompatible PLA/PB-g-SAN blend owing to its partial miscibility with PLA matrix and miscibility with SAN shell of PB-g-SAN, as evidenced by DMTA analysis. Increase in PMMA content promoted the phase adhesion and dispersion state of PB-g-SAN terpolymer in the blends and highly toughened blends were achieved which showed incomplete break of impact specimen. The significant effect of phase morphology on imparting tremendous improvement in impact toughness was clarified. The maximum impact strength (about 500 J/m), elongation-at-break and glass transition were obtained for ternary blend with 25% PMMA. The PLA crystallinity was gradually suppressed in ternary blends upon progressive increase in PMMA content. Rheological studies showed solid-like behavior with enhanced viscosities for ternary blends. Micromechanical deformations and toughening mechanisms were studied by post-mortem fractography. Massive matrix shear yielding was found as the main source of energy dissipation triggered by suitable interfacial adhesion and microvoid formation.

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The relaxation behavior of PMMA-PS core-shell particles in PP matrix was studied. The effects of composition and viscosity ratio of core and shell forming components on relaxation processes of PP/(PMMA+PS) 80/20 ternary blends were evaluated at three different temperatures (200, 225 and 250°C) and discussed. For the ternary blends containing low-viscosity PMMA (PP/L-PMMA/PS), the shape relaxation time, λ, sharply reduced at first with PS content and then increased after passing through a minimum with further increase in the PS content. The corresponding relaxation strength, λ.H(λ), increased suddenly at first and then monotonically decreased upon further incorporation of PS. The changes of λ and λ.H(λ) with PS content were attributed to a change in the interfacial energy (shell formation) and average viscosity of the dispersed composite droplets upon progressive addition of PS into PP/L-PMMA blend so that the minimum λ corresponding to the maximum λ.H(λ) was observed at shell completion composition with finest core-shell- particles (12 wt% of PS phase). The core-shell particles of all the ternary blends containing L-PMMA exhibited a shape relaxation at 200°C, while at the same temperature for the blends containing high-viscosity PMMA (H-PMMA) no shape relaxation peak was detected when the PS shell content was lower than 62 wt%. This was attributed to restriction effect of non-deformable H-PMMA core droplets, which led to no droplet relaxation. At higher PS contents, appearance of the shape relaxation peak was related to the relaxation of PS shell, suggesting that there is a critical shell thickness above which the relaxation behavior of core-shell droplets is dominated by the low-viscous PS shell. Increase in temperature from 200 to 250°C reduced the PS shell content corresponding to onset of deformation and accelerated the relaxation of composite droplets.