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Stimuli-responsive materials1,2  are raising major attention due to their wide field of application, such as flexible electronics, drug delivery, detection of pollutants and bioimaging. Moreover, the intrinsic nature of stimuli-responsive materials presents the characteristics to approach and resolve urgent challenges of stability and sensitivity of devices to be used in multifunctional technology, with a possible look toward a better sustainability.

In this context, over the past few decades, polymer-based materials became the most exploited category thanks to their variety and the fine-tuning possibilities of their design, functionalization and mixing, leading to customized properties. To obtain a high-performance material, however, there are critical points to be carefully addressed such as sensitivity, stability, the capability to control the stimulus and the response time. Among the several combinations of external stimuli and material response, mechanoresponsive materials that give an optical answer when subjected to a mechanical action (compressing, stretching, scratching, etc.), i.e. change their transparency, colour, emission intensity or emission spectrum, are particularly convenient and widely investigated. The mechanoresponse can be obtained via different mechanisms that involve a change in morphology, bond breaking (even covalent bond) or energy/electron transfer upon application of a mechanical action. Depending on the type of optical change, these materials can be divided in mechanochromic (MC), mechanoluminochromic (MLC) and mechanoluminescent (ML). Under a mechanical stimulus, MC materials change their absorption spectrum and therefore their colour under white light illumination; MLC systems display variation of their luminescence properties, in particular emission intensity or spectrum; finally, ML systems become chemiluminescent during the mechanical stress, with emission of photons without photoexcitation.3  Endowing polymeric systems with mechanoresponsiveness is a powerful strategy to monitor deformation and stress and to gain information on deformation mechanism, chain orientation, diffusion and relaxation, crack formation, crystallization and residual stress,4–6  and can be useful for a variety of applications, including structural health damage and tamper-proof packaging.

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