Mechanically robust and flame-retardant polylactide composites based on molecularly-engineered polyphosphoramides

Abstract Intrinsic flammability significantly impedes practical applications of polylactide (PLA), despite many merits. Phosphoramides have shown exceptional flame retardancy efficiency in PLA, but to date it remains a grand challenge to create strong, tough and flame-retardant PLA based on phosphoramides due to lack of fundamental understanding of structure−property correlation. Herein, we design a series of polyphosphoramides (PPDA-x) with tunable structures and compositions. With 1.0 wt% of PPDA-8, tensile strength and toughness of PLA are increased by 11% and 44%, respectively, because of balanced hydrogen-bonding and interfacial tension. Meanwhile, the final PLA achieves a desirable UL-94 V-0 rating and a high limited oxygen index (LOI) of ∼26.8% because phosphorus contents and interfacial tension govern flame retardancy. This work offers a general methodology for creating robust and flame-retardant polymers by molecularly tailoring flame retardants and shedding light on their structure−property relationship.