Effect of stereochemistry on nanoscale assembly of ABA triblock copolymers with crystallizable blocks. (3rd May 2021)

• Record Type: Journal Article
• Authors: Yin, Xuechen; Hewitt, David R.O.; Zheng, Bingqian; Quah, Suan P.; Stanley, Christopher B.; Grubbs, Robert B.; Bhatia, Surita R.
• Is Part Of: Polymer. Volume 223(2021)
• Publication Date: 2021-05-03
• Subjects: Poly(lactic acid) -- Hydrogel -- Block copolymer; Polymers -- Periodicals; Polymerization -- Periodicals; Polymères -- Périodiques; Polymérisation -- Périodiques; 547.7
• DOI: 10.1016/j.polymer.2021.123683
• Languages: English
• ISSNs: 0032-3861

Abstract

Abstract: We report on the nanoscale assembly of poly(lactic acid)- b- poly(ethylene oxide)- b- poly(lactic acid) (PLA-PEO-PLA) triblock copolymers in water, focusing on the effect of stereochemistry, where the PLA blocks are statistical copolymers of l -lactide and d -lactide with l/d ratios of 100/0, 95/5, 90/10, 85/15, 75/25, and 50/50. Small-angle neutron scattering (SANS) shows a nearly constant d -spacing as concentration varies in triblock systems with l/d ratios of 90/10 and 95/5, which we attribute to inhomogeneity in the structure of these gels, supported by previous USANS and confocal microscopy studies. The SANS data fit well to a core-shell ellipsoid form factor model with a hard-sphere structure factor. Polymeric micelles with l/d ratios from 75/25 to 85/15 displayed very high aggregation numbers, consistent with a strong interaction between PLA chains and the enhanced storage modulus observed in rheological studies of these systems. While the 90/10 and 95/5 samples showed lower aggregation numbers, their SANS profiles shows close spacing between micelles, which may promote a high fraction of intermicellar bridging chains, also consistent with a higher storage modulus. Overall, these results provide insight into the micellar assembly behavior of block copolymers with a crystallizable block, and indicate that tuning stereochemistry of PLA-based block copolymers is an effective means of modifying micellar properties for specific applications. Graphical abstract: Image 1 Highlights: Assembly of block copolymer gels with varying block stereochemistry was explored. SANS shows a nearly constant d -spacing with concentration for intermediate l/d ratios. Profiles are consistent with inhomogeneity in the gel structure. Data also suggest a high fraction of bridging chains at intermediate l/d ratios. Results give insight into the unique rheology observed in these systems.