Low-Temperature Sintering of Stereocomplex-Type Polylactide Nascent Powder: Effect of Crystallinity

Stereocomplex (SC) crystallization between high-molecular-weight poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) provides a promising route to substantially improve the properties of polylactide (PLA), but conventional melt processing of the SC-type PLA (SC-PLA) is nearly impossible primarily due to the poor crystallization memory effect as well as serious thermal degradation after complete melting of SC crystallites with high melting temperatures of above 220 °C. Recently, we reported an innovative low-temperature (180–210 °C) sintering technology for fabricating SC-PLA products from its nascent powder. Unfortunately, its practical application has been significantly hindered by an extremely high pressure of 1 GPa, which must be utilized to ensure good surface wetting of the densified powder particles. With this challenge in mind, herein, the role of powder crystallinity in the low-temperature sintering has been investigated. Interestingly, we first demonstrate that depressing powder crystallinity is favorable for the particle wetting under a much lower pressure during the densification stage of the nascent powders because the deformation of the powders becomes easier with the decrease in the fraction of rigid crystal network. Moreover, during the subsequent interface welding stage, more PLLA/PDLA chains could be involved in the interdiffusion and SC crystallization across particle interfaces, thus forming large amounts of new SC crystallites capable of tightly welding the interfaces. As a consequence, SC-PLA sheets with excellent heat resistance and mechanical properties have been successfully fabricated by sintering under a pressure of as low as 300 MPa. Overall, these fascinating findings not only provide new fundamental understandings on the role of initial crystallinity in the low-temperature sintering of SC-PLA powder but also indicate an avenue toward industrial-scale fabrication of SC-PLA products from low-crystallinity nascent powder using conventional polymer sintering equipment.