Validation of theoretical analysis of surface bending strain in polymer films by surface-labeled grating method

Quantitative analysis of tension and compression imposed on surfaces of bending polymer films plays a key role in the design of flexible electronic devices. For over a decade, the analysis has relied on the classical beam theory that mainly deals with metals, glass, and cement; however, the applicable limit of the theory to largely bending polymer films has never been validated. We present that the classical beam theory accurately analyzes surface bending strains in single-layer and double-layer polymer films through measuring the strains by a surface-labeled grating method. The experimental analysis reveals that the bending strains on the outer and inner surfaces of the single-layer film are symmetrical, whereas those of the double-layer film are asymmetrical. These results are well explained by the classical beam theory considering stress–strain curves of polymer films. This approach will further advance the strain design of polymer films, which aids in the development of mechanically durable devices.