Environmental concerns and advances in polymerization technologies have lead to a growing presence of commercial sustainable polymers, the most ubiquitous of which is poly(lactic acid) (PLA). Unfortunately, PLA is inherently brittle, impeding its applications in many fields. Recently, it has been shown that incorporating rubbery blocks in an alternating multiblock architecture greatly increases the toughness of PLA, providing a robust and sustainable route to prepare plastics with enhanced mechanical properties.
In order to process these materials, it is imperative to understand and control their nonlinear shear and extensional rheological behavior. Here, we synthesize both linear poly(D,L-lactide-b-ε-decalactone) triblock polymers and star poly(D,L-lactide-b-ε-decalactone) diblock polymers, and we attempt to introduce long chain branching via coupling to form star-like multiblocks. Variable temperature small angle x-ray scattering reveals microphase separation with no long-range order and a broad and accessible microphase separation transition regime. Linear shear rheology reveals shear thinning behavior reminiscent of commercial PLA, while strain hardening in extensional rheology gives strong evidence for long chain branching for the star-like multiblock. Tensile testing confirms that the toughness values of the multiblocks are orders of magnitude greater than those for PLA homopolymer. This unique approach of coupling star block polymers simultaneously improves the processability and mechanical properties of PLA.