A*Star researchers toughen up polylactic acid

Researchers at the Agency for Science, Technology and Research (A*Star, Singapore) have toughened up polylactic acid (PLA) while maintaining its elasticity by adding core-shell nanoparticles as a filler.

Chaobin He and fellow A*Star researchers
Chaobin He (center) and fellow A*Star researchers. Image courtesy A*Star Institute of Materials Research and Engineering.

A biodegradable and highly biocompatible polymer with good thermal processability, PLA has found widespread use in biomedical applications and as a packaging material. Because the polymer is brittle and has poor mechanical stability, it is often modified by adding reinforcing materials and by incorporating different polymerization methods, reports A*Star Research. These modifications also may reduce the material's strength and elastic modulus, thus limiting its applications.

Chaobin He, Beng Hoon Tan and colleagues at the A*Star Institute of Materials Research and Engineering were able to increase the toughness of PLA while maintaining its strength and modulus by the addition of core-shell nanoparticles as a filler. The nanoparticles have silica cores with rubber chains covalently linked to the silica, and poly(D-lactic acid) (PDLA) grafted onto the outer shell. These sequential steps were realized by using a technique called “ring-opening polymerization.”

On addition of the nanoparticles to a poly(L-lactic acid) (PLLA) matrix using a solution blending process, a complex structure forms between the pendant PDLA chains and the PLLA  matrix. Interestingly, thermal analysis of the polymer nanocomposite indicates that the material reassembles perfectly after recrystallization from the melt. This melt-memory effect is enhanced by the incorporation of rubber chains in the nanoparticles. 

"The presence of silica-rubber-PDLA nanoparticles in the PLA matrix and its complex formation with PLLA provide stress relief and bridging effects during deformation, thus improving toughness without sacrificing strength and modulus,” He told A*Star Research. The increased facile stress relief is probably due to the ability of rubber to act as a stress concentrator during plastic deformation. From microscopic analysis of the polymer nanocomposite, the deformation mechanisms of the material were identified as “crazing,” which involves the formation of microvoids in the material and fibrillation at sites of local plastic deformation. 

“The large scale production of PLA from renewable resources makes our environmentally friendly material a promising candidate to replace petroleum-based thermoplastics,” said He. “While our approach has significantly overcome the shortcomings of pure PLA, such as brittleness and poor mechanical stability, further optimization of the materials and process as well as enhancement in the nanoparticles-polymer matrix compatibility need to be carried out.”

The research will pave the way for the further development of sustainable polymers in consumer electronics, automotive and packaging applications, according to He.

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