By Josh Perry, Editor
[email protected]

Researchers from the Agency for Science, Technology, and Research (A*STAR) in Singapore have demonstrated that adding core-shell nanoparticles to polylactic acid (PLA) toughens the material without negatively impacting its elasticity.

Polymeric nanocomposites comprising poly(lactic acid) with nanoparticles of silica-rubber-poly(D-lactic acid). (A*STAR)

In an announcement from A*STAR that was published on Phys.org, the researchers explained that this is an enhancement on the standard processing of PLA, which includes mixing other polymer strands to avoid the material’s inherent brittleness but that also limit the elasticity of PLA.

The nanoparticles that have been added to PLA have silica cores, rubber chains linked covalently to the silica, and poly(D-lactic acid) (PDLA) attached to the outer shell. The process for creating these nanoparticles is called ring opening polymerization.

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

Researchers saw that the nanoparticles provided stress relief (probably from the rubber) and created bridging effects during deformation, which enhanced the material’s toughness while preserving its strength and modulus.

“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 article added.

The research was recently published in Composites Science and Technology. The abstract stated:

“In the effort to overcome the shortcomings such as brittleness and poor mechanical stability, and increase the competitive edge of renewable poly(lactic acid) PLA over conventional petroleum-based thermoplastics, silica rubber core-shell nanoparticles for effective PLA toughening were successfully synthesized by sequential ring opening polymerization (ROP).

“The core-shell structure was designed with silica as inner core, P(CL-mLA) as ‘rubber’ middle layer and terminal PDLA chains (SiO₂-r-PDLA), to facilitate the stereocomplex formation with PLLA matrix for enhanced interface control. The PLLA/SiO₂-r-PDLA nanocomposites were fabricated through solution blending-injection molding process. 

“Nuclear magnetic resonance (¹H NMR and ¹³C NMR) results confirmed the presence of grafted ’rubber’ and PDLA chains from the surface of silica particle. In addition, PLLA/SiO₂-r-PDLA nanocomposites showed tremendous improvement in thermal and mechanical properties using differential scanning calorimetry (DSC) and tensile testing, respectively. Besides the formation of stereocomplex in the nanocomposites, a detailed study on the melt stability of these stereocomplex nanocomposites revealed a ‘memorized’ stereocomplex behavior, i.e., having the ability to perfectly reassemble after re-crystallization from melt (melt memory effect), when rubber segment is present.

“Finally, structure-deformationmechanisms were studied using scanning electron microscopy (SEM) and small angle x-ray scattering (SAXS). From SAXS, crazing was clearly observed whereas SEM revealed fibrillated structures.

“Thus, crazing and fibrillation are the key deformation mechanisms in PLLA/SiO₂-r-PDLA system. The exciting new fillers could open up new horizons for PLA advanced composites applications.”