Growing environmental concerns are calling for nondegradable petroleum-derived plastics to be replaced with biodegradable, more sustainable options. Poly(L-lactide) (PLLA), a product of polymerization of L,L-lactide and a different form of poly(lactide) (PLA) by virtue of its chirality, is one of the many viable alternatives with high tensile strength and high elastic modulus, but its intrinsically brittle characteristic must be overcome for applications requiring toughness, impact resistance, and optical clarity. A research group with the Center for Sustainable Polymers at the University of Minnesota have now successfully toughened PLLA by uniformly dispersing short cylindrical micelles of a low-molecular-weight poly(ethylene oxide)-b-poly(butylene oxide) (PEO-PBO) diblock in commercial high-molecular-weight glassy PLLA. Introduction of these micelles into the PLLA results in a greater than tenfold increase in tensile toughness and notched Izod impact strength.
“Despite standard polymer physics, the attempt was to try and toughen PLA by creating the same type of nanoscale micelles that the group has been investigating for years in epoxy resins. This finding has the potential to be a facile and economical way to toughen PLA and perhaps produce low-density porous films,” says chief scientist Frank Bates.
As reported in a recent issue of ACS Macro Letters (doi:10.1021/acsmacrolett.6b00063), Tuoqi Li, a graduate student in the group, synthesized a series of PEO-PBO diblock copolymers (a polymer with distinct blocks of monomers PEO and PBO grafted together to form a single copolymer chain), identified as EB1–3, as well as a PLLA-PBO diblock, identified as LB, by anionic polymerization with fixed composition but with varying total molecular weight. Blends of PLLA and both the diblock PLLA copolymers were then prepared by solvent and melt blending routes, followed by rapid cooling and aging for two days prior to characterization.
The resulting blend, identified as EB1/PLLA, showed a dispersion of the EB1 copolymer into small micelle-like particles as shown in the figure. In general, increasing the overall molecular weight of the EB additives in PLLA led to coarser dispersions and larger domain sizes than those expected from the assembly of individual micelles. The blend prepared using EB1 exhibited the greatest improvement relative to neat PLLA with a 1300% increase in tensile toughness and a 2500% increase in strain at break. By adding 1.25 wt% and 5 wt% EB1 to the PLLA, the notched Izod impact strength (measured by impacting the specimen by an arm from a height, and measuring the amount of energy absorbed by the sample) could be improved by 600% and 1500%, respectively.
“This is an exciting advance by Bates and co-workers that shows how diblock polyether additives can have a profound influence on toughness and impact strength without degrading other properties,” explains Geoffrey Coates, an expert in defined structure polymers from Cornell University. He further says, “These additives have great potential to create poly(lactides) that have performance characteristics that match their already impressive environmental attributes.”
The favorable enthalpic dispersion of EB1 into bulk PLLA as a micelle microstructure was attributed to a negative Flory–Huggins interaction parameter, which means that the mixing is of an exothermic nature. Study of the tensile specimens before and after deformation suggested that the improved performance derives from the formation of micron and submicron holes, which are caused by cavitation of the rubbery core micelles. Along with this kind of cavitation, micromechanical mechanisms of crazing and shear yielding are believed to produce a synergistic toughening effect in the PLLA-EB1 blends.
Transmission electron microscope image of 5 wt% EB/PLLA blend showing, in the inset, the micelle structure at a higher magnification. Credit: ACS Macro Letters.
This work represents a significant step toward developing a low-cost approach for toughening sustainable glassy PLLA materials based on a facile processing route. The controlled cavitation and void formation observed offers a new method for producing low-density porous materials with a host of potential applications. Future research would entail a more comprehensive study of the toughening mechanisms for the series of EB diblocks.
