The flax and equivalent proportion of poly(l‐lactic acid)/poly(d‐lactic acid) (PLLA/PDLA) were melt compounded and injection molded to prepare flax‐reinforced polylactide stereocomplex (sc‐PLA) bio‐composite, and the effect of alkali treatment on the structure and properties of flax as well as the flax/sc‐PLA composite was investigated. SEM and FTIR results showed hemicellulose in flax was almost completely removed after alkali treatment and the treated flax (ALK‐flax) bundles were more separated with a cleaner surface than untreated flax (UN‐flax). DSC results showed homo‐crystallites (hc, Tm = 160–170°C) and stereocomplex crystallites (sc, Tm ∼210°C) coexisted in sc‐PLA and flax/sc‐PLA composites. Compared with sc‐PLA, the total crystallinity and sc‐crystallinity of flax/sc‐PLA composite increased regardless of whether the flax were treated with alkali, whereas ALK‐flax/sc‐PLA composite showed a little higher crystallinity than UN‐flax/sc‐PLA composite. TGA results confirmed ALK‐flax/sc‐PLA composite had a higher thermal degradation temperature than UN‐flax/sc‐PLA composite. The mechanical tests indicated although the mechanical properties of sc‐PLA increased significantly by reinforcing with flax, the ALK‐flax/sc‐PLA composite showed little lower mechanical properties than UN‐flax/sc‐PLA composite. The alkali treatment of flax had no obvious influence on the Vicat softening temperature (VST) of flax/sc‐PLA composites, a higher heat resistance with VST at ∼155°C could be obtained for flax/sc‐PLA composite. POLYM. ENG. SCI., 55:2553–2558, 2015. © 2015 Society of Plastics Engineers

Poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) with various molecular weights (MW) were melt blended in equivalent proportions, and then followed by injection molding to form polylactide stereocomplex (sc-PLA). The crystalline structures, thermal resistance, and mechanical properties of the formed sc-PLA were then investigated in detail. The results showed stereocomplex crystallites (sc) were formed predominantly in all the sc-PLA regardless of the MW of the PLLA or PDLA pellets, and a small amount of homocrystallites (hc) also coexisted in sc-PLA. The MW of PLLA and PDLA pellets displayed significant effects on the crystallinity, thermal resistance, and mechanical properties of sc-PLA. PLLA and PDLA with low and similar MW facilitated the formation of sc, and also improved the onset degradation temperature and Vicat softening temperature of sc-PLA. For a similar degree of crystallinity, high MW was found to be more favorable in improving the thermal performance of sc-PLA. Although sc-PLA prepared from PLLA and PDLA with low MW showed decreased mechanical properties, the mechanical properties of sc-PLA increased with an increase in the MW of the PLLA or PDLA pellet. When PLLA and PDLA pellets with high and similar MW were used, the mechanical properties of sc-PLA could even exceed those of pure PLLA. K E Y W O R D Sbiodegradable, blending, mechanical properties, thermal properties

Polylactide stereocomplex (sc‐PLA) prepared by blending equivalent proportion of poly(l‐lactic acid)/poly(d‐lactic acid) (PLLA/PDLA) and its composites reinforced with 10, 20, and 30% flax fibers were fabricated by melt compounding and followed by injection molding. The mechanical properties, crystallinity, cross‐section morphology, and heat resistance of sc‐PLA and flax/sc‐PLA composites were compared. The results showed that homocrystallites (hc) and stereocomplex crystallites (sc) were formed simultaneously in sc‐PLA and its composites, with a melting temperature at ∼170 and ∼210°C, respectively. The crystallinity and sc content of composite increased with the increasing content of the flax fibers. The sc content of 30% flax/sc‐PLA composite could reach 98.4%, 32% higher than that of sc‐PLA (66.4%). When compared with nonblended PLLA, heat resistance of sc‐PLA increased slightly, but at the expense of mechanical properties. By the addition of flax fibers, the mechanical properties of flax/sc‐PLA composite improved significantly. The highest tensile strength, Young's modulus, and notched Izod impact strength of flax/sc‐PLA composite were 52.90 MPa, 6.42 GPa, and 5.27 kJ/m2, respectively, improved by 54, 132, and 343% when compared with sc‐PLA. Moreover, the heat resistance of composite was also improved greatly by reinforcing with flax fibers. The Vicat softening temperature of 30% flax/sc‐PLA composite could achieve 162.5°C, nearly 100°C higher than that of PLLA. POLYM. COMPOS., 38:472–478, 2017. © 2015 Society of Plastics Engineers