Achieving all-polylactide fibers with significantly enhanced heat resistance and tensile strength via in situ formation of nanofibrilized stereocomplex polylactide

Polylactide (PLA), a promising biodegradable and renewable polymer, finds many applications, particularly, used as biodegradable fiber in textile industry. The key is to improve its heat resistance. In this work, small amount of poly (D-lactide) (PDLA) was added firstly into poly (L-lactide) (PLLA) matrix to form stereocomplex polylactide (SC-PLA), which has much better heat resistance than that of PLLA matrix. Then two melting temperatures were adopted for fiber spinning, the one (210 °C) is far below the melting temperature of SC-PLA (i.e., 220–230 °C), and the other (225 °C) is close to the melting temperature of SC-PLA. The boiling water shrinkage and tensile strength of PLLA/PDLA fibers were investigated as function of PDLA content. It was found, the boiling water shrinkage could be largely reduced to as low as 5.6% and a satisfactory tensile strength (628 MPa) is achieved for fibers spun at 225 °C, compared with fibers spun at 210 °C (7.5% and 500 MPa, respectively), as the PDLA content is 3 wt%. SEM observation suggests a formation of nanofibrillar like SC-PLA structure as spun at 225 °C, while only spherical SC-PLA particles as spun at 210 °C. DSC result indicates that adding PDLA could largely enhance the crystallization rate of PLLA, but almost a constant crystallinity and SC-PLA content were observed for both spun fibers, disregarding the spinning temperatures. Herman's orientation parameters of spun fiber were obtained from 2D-WAXD, which suggests that fiber spun at 225 °C has higher orientation than that of fiber spun at 210 °C. Thus it can be concluded that the formation of nanofibrillar like SC-PLA and enhanced orientation degree contribute mainly to the improved heat resistance and tensile strength. This work presents a feasible and effective way to fabricate all-PLA-based fibers with the outstanding heat resistance by adding a small amount of PDLA and using a suitable spinning temperature.