Quantitative surface bending strain analysis of flexible films by surface labeled grating method

Surface bending strains in various flexible films is quantitatively analyzed by a surface-labeled grating method with a single-nanometer resolution. The real-time strain analysis we achieved has multiple benefits: high resolution, precision, and a wide range of measurable materials. The reliability of the measurements was confirmed using the modified Elastica theory. This method revealed that the cracking of hard coatings on the surface of PET films occurred at surface strains exceeding 1.45%, regardless of the film thickness and curvature. The multilayering of two PET layers with a soft PDMS layer between them reduced the surface bending strain by 50% compared with that of a single-layer film with the same thickness. This triple-layer film successfully suppressed the cracking of the hard coating and the breakdown of the OTFT. The surface-labeled grating method, therefore, is a practical tool that allows the analysis of surface bending strain in the elaborately designed materials.