Synergistic reinforcing and toughening of poly(l-lactide) composites with surface-modified MgO and chitin whiskers

In order to improve the mechanical properties of poly(l-lactide) (PLLA), two types of whiskers, rod-like MgO whiskers (MgO-Ws) and needle-like chitin whiskers (CHN-Ws), were prepared and then surface modified with l-lactide to obtain grafted MgO-Ws and CHN-Ws (g-MgO-Ws and g-CHN-Ws), respectively. Subsequently, the initial and grafted MgO-Ws and CHN-Ws were introduced into a PLLA matrix together or singly to prepare PLLA/MgO-Ws/CHN-Ws and PLLA/g-MgO-Ws/g-CHN-Ws composites via injection molding. As compared to pure PLLA, the increase in the degree of crystallinity and hydrophilicity and decrease in the thermal stability of the resulting composites were confirmed by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and water contact angle (WCA) testing. After surface modification, the dispersion of the whiskers and the interfacial adhesion between the whiskers and the PLLA matrix were significantly improved, which was clearly observed by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). As a result, higher values for tensile strength (84.2 ± 0.9 MPa), flexural strength (102.7 ± 0.6 MPa), notch impact strength (36.7 ± 0.8 J/m), and fracture energy (576 ± 31 J/m2) were obtained for the PLLA/g-MgO-Ws/g-CHN-Ws composites as compared to those for pure PLLA and PLLA/MgO-Ws/CHN-Ws composites. Moreover, the crystallization ability and mechanical properties of PLLA were more improved by loading both types of whiskers as compared to loading only one.