Lignin nanoparticles as sustainable biobased nucleating agents of poly(L-lactic acid): crystallization behavior and effect of particle sizes

Softwood lignin nanoparticles (SLNs) and microparticles (SLMs) were incorporated as bio-additives into poly(L-lactic acid) (PLLA) composite films, and crystallization behaviors were compared with neat PLLA film using polarized optical microscopy (POM), differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS) techniques. The Avrami and Hoffman secondary nucleation theory as well as the Hoffman-Weeks plot were used to determine key information of crystallization behavior. Non-isothermal crystallization results showed an increase in crystallinity (%Xc) of PLLA by 7% and 9% when incorporating a small amount (1.0 wt.%) of SLMs and SLNs, respectively. Spherulite number (N), crystallization rate (G) and final degree of crystallinity ( $$phi^infty $$ ) of neat PLLA film were 4, 0.05 min−1 and 0.48, respectively. These values increased to 116, 0.12 min−1 and 0.52, respectively, when 1.0 wt.% of SLNs was added into PLLA (1%SLNs/PLLA). A Hoffman-Weeks plot was used to examine the equilibrium melting temperature ( $$T_m⁰$$ ), while energy required for nuclei formation (Kg) and free energy on lamellar surface ( $$sigma_e$$ ) values were evaluated. Kg and $$sigma_e$$ values of neat PLLA film were 3.01 × 10−5 K2 and 6.31 × 10−12 J m−2, respectively, and significantly decreased to 2.89 × 10−5 K2 and 6.05 × 10−12 J m−2 when incorporating 1 wt.% of SLMs. Lowest Kg and $$sigma_e$$ values of 2.72 × 10−5 K2 and 5.65 × 10−12 J m−2 were recorded in 1%SLNs/PLLA composite film. Results confirmed lignin nanoparticles (SLNs) as an alternative promising bio-additive for enhancing PLLA crystallizability toward the development of fully biobased and biodegradable plastic.