Biodegradable and renewable poly(lactide)-lignin composites: Synthesis, interface and toughening mechanism

Poly(lactide) (PLA)-lignin composites were fabricated by blending lignin-g-rubber-g-poly(d-lactide) copolymer particles and commercial poly(l-lactide) (PLLA) in chloroform. To synthesize the copolymer, a poly(ε-caprolactone-co-lactide) (PCLLA) rubbery layer was formed via the lignin-initiated ring opening copolymerization of an ε-caprolactone/l-lactide mixture, followed by the formation of poly(d-lactide) (PDLA) outer segments via the polymerization of d-lactide. The PDLA segments may contribute to strong interfacial interactions between lignin-rubber-PDLA and PLLA matrix by stereocomplexation, which was observed using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR) and wide angle X-ray scattering (WAXS). The quasi-random structure of PCLLA and the formation of the outer PDLA segments were characterized by nuclear magnetic resonance (NMR). A T_g of ∼-36 °C for PCLLA was detected by DSC, which confirms the rubbery character of the synthesized copolymer. The resulting renewable and biodegradable composites exhibited a six-fold increase of elongation at break and a simultaneous improvement in their tensile strength and Young's modulus, though to a lesser extent. Light scattering, WAXS, small angle X-ray scattering (SAXS) and scanning electron microscope (SEM) studies suggested that good lignin dispersion, rubber-initiated crazing and strong filler/matrix interactions due to stereocomplexation are the effective mechanisms behind the excellent mechanical performance of these composites.