Highly contrastive, real-time modulation of light intensity by reversible stress-whitening of spontaneously formed nanocomposites: application to wearable strain sensors

Abstract

Stress-whitening is generally regarded as a sign of irreversible elasticity failure of polymer materials. Here, we report for the first time that in stark contrast to the conventional insight, the full range of stress-whitening occurs prior to a yield point and is even reversible, which can be applied to light intensity modulation. The reversibility of stress-whitening impeccably emerges in highly transparent nanocomposite films spontaneously formed during photocuring of poly(dimethylsiloxane-co-methylvinylsiloxane) solution in tert-butyl acrylate without any addition of nanofillers. This phenomeon is enabled by 'elastic cavitation' at soft elastomer matrix-hard nanoparticle interfaces. The film transmittance is readily controllable from ca. 80 to 0% under small strain (epsilon < 20%) due to a high population of the nanoparticles, high initial modulus difference and low compatibility between the matrix and particles. The ratio of transmittance change to strain is as high as 3.7-8.0 depending on the wavelength in the visible region. A highly sensitive wearable strain sensor is demonstrated as an application of the unprecedented phenomenon. Our optical strain sensor rapidly, repeatedly responds in transmittance and reflectance to strain of around 0-40% in accordance with human body motion. This work may initiate basic and technological research into various applications of the optically or electrically active element-free light intensity modulators which do not place limits on continuous shape-deformations.