Fully Transparent and Distortion-Free Monotonically Stretchable Substrate by Nanostructure Alignment

Stretchable devices, garnering increasing attention as next-generation form factors, have a crucial problem in that vertical contraction occurs during stretching, causing image distortion of stretchable displays and discomfort in skin-attached devices. Previous structural strategies to mitigate vertical contraction, such as auxetic reentrants and wrinkles, suffer from the drawback that their structure becomes visible during stretching. In this study, this issue is addressed by unidirectionally aligning nanoscopic cylinders within block copolymer elastomer films. Employing a shear-rolling process at high temperatures on thick films of polystyrene-block-polyisobutylene-block-polystyrene thermoplastic elastomers, macroscopic mechanical anisotropy is achieved, resulting in completely transparent and monotonically stretchable substrates devoid of vertical or depth distortion during deformation. Significantly, the unidirectional orientation of high-modulus cylindrical nanostructures induces macroscopic mechanical anisotropy with a modulus ratio exceeding five times. While the Poisson's ratio of conventional elastic materials hovers around ≈0.5, this mechanical anisotropy minimizes vertical contraction, yielding a Poisson's ratio below 0.07. Moreover, owing to the negligible size of the nanocylinders compared to visible-light wavelengths, the substrate can be monotonically uniaxially stretched while maintaining high transmittance without introducing distortions, surface undulations, or haziness, resulting in distortion-free stretchable substrates.