Impact of stereochemistry on rheology and nanostructure of PLA–PEO–PLA triblocks: stiff gels at intermediate l/d-lactide ratios. Issue 35 (23rd August 2018)

• Record Type: Journal Article
• Authors: Yin, Xuechen; Hewitt, David R. O.; Quah, Suan P.; Zheng, Bingqian; Mattei, Gerard S.; Khalifah, Peter G.; Grubbs, Robert B.; Bhatia, Surita R.
• Is Part Of: Soft matter. Volume 14:Issue 35(2018)
• Page Start: 7255
• Page End: 7263
• Publication Date: 2018-08-23
• Subjects: Soft condensed matter -- Periodicals; 530.413
• DOI: 10.1039/c8sm01559g
• Languages: English
• ISSNs: 1744-683X

Abstract

Abstract : Hydrogels of PLA–PEO–PLA display a striking enhancement of the stiffness at intermediatel/d ratio, demonstrating the impact of block stereochemistry on the rheology. Abstract : We report rheology and structural studies of poly(lactide)–poly(ethylene oxide)–poly(lactide) (PLA–PEO–PLA) triblock copolymer gels with various ratios ofl -lactide andd -lactide in the PLA blocks. These materials form associative micellar gels in water, and previous work has shown that stereoregular triblocks with al/d ratio of 100/0 form much stiffer gels than triblocks with a 50/50l/d ratio. Our systems display an unexpected maximum in the storage modulus, G ′, of the hydrogels at intermediatel/d ratio. The impact of stereochemistry on the rheology is very striking; gels with anl/d ratio of 85/15 have storage moduli that are ∼1–2 orders of magnitude higher than hydrogels withl/d ratios of 100/0. No stereocomplexation is observed in the gels, although PLLA crystals are found for gels withl/d ratios of 95/5 and 90/10, and SANS results show a decrease in the intermicellar spacing for intermediatel/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 thel/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.