Abstract
The production and utilization of polymers have been widely implemented into diverse applications that benefit modern human society, but one of the most valuable properties of polymers, durability, has posed a long-standing environmental challenge from its inception since plastic waste can lead to significant contamination and remains in landfills and oceans for at least hundreds of years. Poly(lactic acid) (PLA) derived from renewable resources provides a sustainable alternative to traditional polymers due to its advantages of comparable mechanical properties with common plastics and biodegradability. However, the poor thermal and hydrolytic stability of PLA-based materials limit their potential for durable applications. Stereocomplex crystallization of enantiomeric poly (l-lactide) (PLLA) and poly (d-lactide) (PDLA) provides a robust approach to significantly enhance material properties such as stability and biocompatibility through strong intermolecular interactions between L-lactyl and D-lactyl units, which has been the key strategy to further PLA applications. This review focuses on discussing recent progress in the development of processing strategies for enhancing the formation of stereocomplexes within PLA materials, including thermal processing, additive manufacturing, and solution casting. The mechanism for enhancing SC formation and resulting material property improvement enabled by each method are also discussed. Finally, we also provide the perspectives on current challenges and opportunities for improving the understanding of processing-structure-property relationship in PLA materials that could be beneficial to their wide practical applications for a sustainable society.
Highlights
Traditional petroleum-derived plastics have been “ideal” materials and widely used in a variety of industrial and consumer products over past decades due to their high strength, flexibility, and durability and low manufacturing cost (Andrady and Neal, 2009)
Additive manufacturing stratgey can be combined with low-temperature sintering method to further enhance the formation of SC crystallites at a relatively low melt processing temperature. He et al introduced a trace amount of untwisted carbon nanotubes (CNTs) onto polylactic acid (PLA)-SC particle surfaces, and these unentangled CNTs act as nucleating agents for promoting SC formation of PLLA/PDLA homopolymer blend on the surfaces, as well as favor SC crystals to strongly weld the interfaces during low pressure sintering process (Akagi et al, 2012)
This review highlights recent advances in material preparation and processing technologies for enhancing SC crystallization, which is a key parameter for material property improvement
Summary
Traditional petroleum-derived plastics have been “ideal” materials and widely used in a variety of industrial and consumer products over past decades due to their high strength, flexibility, and durability and low manufacturing cost (Andrady and Neal, 2009). Young’s modulus, and elongation-at-break of PLASC film all exhibit almost two times higher than those of homopolymer film since 3D micro crystal networks from SCs serve as intermolecular crosslinks for mechanical property enhancement (Shirahama et al, 2005) These results all indicate the significance of forming SC within PLA matrix for material property improvement and in general, a higher degree of crystallinity can provide better material physicochemical properties. While a wide variety of approaches have been demonstrated to enhance the SC formation within PLLA/PDLA blend (Bai H. et al, 2017), advanced processing techniques that can offer distinct advantages of scalability and low cost for raw materials are more relevant for industrial applications compared with molecularlevel engineering of PLA that involves sophisticated chemistry for controlling MW and chain architecture. We briefly discuss the challenges that need to be addressed in this field in order to allow further advancement of PLA-based materials that warrants a green and sustainable future
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