Mechanisms of Chain Scission under High Strain Rate Sonication in PS-PIB-PS Triblock Copolymers.

Impedance mismatch block copolymers, combining rubbery polyisobutylene (PIB) with glassy polystyrene (PS) segments, show promise for the design of high-strain-rate material applications. However, the molecular-level processes of energy absorption and dissipation under high strain rates in the solution state are poorly understood. Using a grafting-from approach, we synthesized a library of well-defined PS-PIB-PS triblock copolymers with and without mechanophores at the block interfaces to evaluate the effects of molecular weight, block composition, and dispersity on chain scission induced by high-strain-rate elongational and shear flows under solution-state sonication. Synthesis of block copolymers without the lowest reaction enthalpy bonds identified by DFT analysis allowed interrogation of the site of chain cleavage. GPC and fluorescence spectroscopy analysis revealed that chain scission did not occur preferentially at the block interface, at the lowest enthalpy, or at the chain center in the triblock copolymers but rather was dictated by chain conformation, plasticity, and physical properties of the sonication medium and that stress concentrated at the interface can be tuned by the length and composition of the blocks. Our findings establish an experimental baseline for understanding the effects of factors such as chain architecture, conformation, and sites of stress concentration on the solution-state response of block copolymers under shear and elongational forces.