Mechanochemical reactions within graft copolymeric assemblies: ultrasound-induced multimechanophore activation, arm chain scission, and aggregation morphology/size changes

Abstract

The development of nanosized aggregates that respond to mechanical stimuli is of great importance for applications related to ultrasound imaging, sensors, etc. In this work, we fabricate model assemblies via the self-assembly of amphiphilic graft copolymers with multiple maleimide–anthracene cycloadduct-based (DA) mechanophores situated at the backbone–arm junctions. The presence of dimethylformamide (DMF) in the DMF/water assembly solutions compared with the pure water system is found to play an important role in promoting the ultrasonic activation of the DA mechanophore. The DMF content in the solutions is also found to have a significant influence on the ultrasound-induced mechanochemical reaction characteristics, including the mechanoactivation level of the DA mechanophore and the mechanical reaction rate constants, of the assemblies. Due to the irreversible arm chain scission inconsistent with DA adduct cleavage, the disruption and reassembly of the aggregates in the DMF/water solution were found to occur upon sonication. Meanwhile, the diameters of the assemblies were found to gradually increase as the sonication time was extended; when subjected to long sonication times, the aggregates presented irregular shapes and a wide range of sizes. The mechanofluorescence and changes in the morphology and size of the assemblies due to ultrasound stimulation were found to be effective and closely controllable. These results establish the characteristics of the mechanical response of the multimechanophore-functionalized graft copolymeric assembly on the nanoscale. This work thus expands the field of polymer mechanochemistry toward self-assembled aggregates of polymers with complicated topological architectures.