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Summary: Poly(lactide)s [i.e. poly(lactic acid) (PLA)] and lactide copolymers are biodegradable, compostable, producible from renewable resources, and nontoxic to the human body and the environment. They have been used as biomedical materials for tissue regeneration, matrices for drug delivery systems, and alternatives for commercial polymeric materials to reduce the impact on the environment. Since stereocomplexation or stereocomplex formation between enantiomeric PLA, poly(L‐lactide) [i.e. poly(L‐lactic acid) (PLLA)] and poly(D‐lactide) [i.e. poly(D‐lactic acid) (PDLA)] was reported in 1987, numerous studies have been carried out with respect to the formation, structure, properties, degradation, and applications of the PLA stereocomplexes. Stereocomplexation enhances the mechanical properties, the thermal‐resistance, and the hydrolysis‐resistance of PLA‐based materials. These improvements arise from a peculiarly strong interaction between L‐lactyl unit sequences and D‐lactyl unit sequences, and stereocomplexation opens a new way for the preparation of biomaterials such as hydrogels and particles for drug delivery systems. It was revealed that the crucial parameters affecting stereocomplexation are the mixing ratio and the molecular weight of L‐lactyl and D‐lactyl unit sequences. On the other hand, PDLA was found to form a stereocomplex with L‐configured polypeptides in 2001. This kind of stereocomplexation is called “hetero‐stereocomplexation” and differentiated from “homo‐stereocomplexation” between L‐lactyl and D‐lactyl unit sequences. This paper reviews the methods for tracing PLA stereocomplexation, the methods for inducing PLA stereocompelxation, the parameters affecting PLA stereocomplexation, and the structure, properties, degradation, and applications of a variety of stereocomplexed PLA materials. magnified image

Recently, we reported the isothermal crystallization behaviors of poly(L-lactic acid) (PLLA) from the melt and glassy states, respectively [J. ]. Surprisingly, the quite different infrared (IR) spectral evolutions occur in the two crystallization processes at different temperatures in which the same crystal modification is expected to be formed. To clarify this unusual phenomenon, the crystal modifications and thermal behavior of PLLA samples prepared under different crystallization temperatures are investigated in detail by TEM, WAXD, and FTIR techniques. On the basis of the WAXD and IR data, a new crystal modification named the Ŕ form is proposed for the crystal structure of PLLA samples annealed at temperature below 120°C. Such crystal modification with loose 103 helical chain packing is less thermally stable than the standard R form of PLLA. This assignment can explain all the experiment observations well. Other possible mechanisms for the IR spectral difference of bulk PLLA samples annealed at different temperatures are also discussed.