Mechanochemistry in Block Copolymers: New Scission Site due to Dynamic Phase Separation

H Zhang and AZ Zoubi and MN Silberstein and CE Diesendruck, ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, 62 (2023).

DOI: 10.1002/anie.202314781

Mechanochemistry can lead to the degradation of the properties of covalent macromolecules. In recent years, numerous functional materials have been developed based on block copolymers (BCPs), however, like homopolymers, their chains could undergo mechanochemical damage during processing, which could have crucial impact on their performance. To investigate the mechanochemical response of BCPs, multiple polymers comprising different ratios of butyl acrylate and methyl methacrylate were prepared with similar degree of polymerization and stressed in solution via ultrasonication. Interestingly, all BCPs, regardless of the amount of the methacrylate monomer, presented a mechanochemistry rate constant similar to that of the methacrylate homopolymer, while a random copolymer reacted like the acrylate homopolymer. Size-exclusion chromatography showed that, in addition to the typical main peak shift towards higher retention times, a different daughter fragment was produced indicating a secondary selective scission site, situated around the covalent connection between the two blocks. Molecular dynamics modeling using acrylate and methacrylate oligomers were carried out and indicated that dynamic phase separation occurs even in a good solvent. Such non-random conformations can explain the faster polymer mechanochemistry. Moreover, the dynamic model for end-to-end chain overstretching supports bond scission which is not necessarily chain- centered. Unexpected mechanochemistry kinetics and selective off-center scission behavior is shown in linear block copolymers (BCPs) stressed in solution. Molecular dynamics modeling suggests a dynamic phase separation of the blocks inducing the unusual distribution of applied forces, and promoting force focusing at the interface.image

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