Impact of stereochemistry on rheology and nanostructure of PLA–PEO–PLA triblocks: stiff gels at intermediate l/d-lactide ratios

Abstract

We report rheology and structural studies of poly(lactide)–poly(ethylene oxide)–poly(lactide) (PLA–PEO–PLA) triblock copolymer gels with various ratios of L-lactide and D-lactide in the PLA blocks. These materials form associative micellar gels in water, and previous work has shown that stereoregular triblocks with a L/D ratio of 100/0 form much stiffer gels than triblocks with a 50/50 L/D ratio. Our systems display an unexpected maximum in the storage modulus, G′, of the hydrogels at intermediate L/D ratio. The impact of stereochemistry on the rheology is very striking; gels with an L/D ratio of 85/15 have storage moduli that are ∼1–2 orders of magnitude higher than hydrogels with L/D ratios of 100/0. No stereocomplexation is observed in the gels, although PLLA crystals are found for gels with L/D ratios of 95/5 and 90/10, and SANS results show a decrease in the intermicellar spacing for intermediate L/D ratios. We expect the dominant contribution to the elasticity of the gels to be intermicellar bridging chains and attribute the rheology to a competition between an increase in the time for PLA endblocks to pull out of micelles as the L/D ratio is increased and PLLA crystallization occurs, and a decrease in the number of bridging chains for micelles with crystalline PLA domains, as formation of bridges may be hindered by crowded crystalline PLA domains. These results provide a new strategy for controlling the rheology of PLA-based hydrogels for potential applications in biomaterials, as well as fundamental insights into how intermicellar interactions can be tuned via stereochemistry.