Simple and Versatile Strategy to Prevent Surface Wrinkling by Visible Light Irradiation

A stiff film bonded to a compliant substrate is susceptible to surface wrinkling when it is subjected to in-plane compression. Prevention of surface wrinkling is essential in many cases to maintain the integrity and functionality of this kind of system. Here we report a simple versatile technique to restrain surface wrinkling of an amorphous poly(p-aminoazobenzene) (PAAB) film by visible light irradiation. The key idea is to use the combined effects of photosoftening of the PAAB film and the stress release induced by the reversible photoisomerization. The main finding given by experiments and dimensional analysis is that the elastic modulus Ef of the film is well modulated by the ratio of light intensity and the release rate, i.e., I/V. Furthermore, the explicit solution describing the correlation of I/V with Ef is derived for the first time. The difference between the calculated critical wrinkling strain εc,t based on Ef and the experimentally measured value εc enables us to quantitatively evaluate the release amount of the compressive stress in the film. These key solutions provide a simple strategy to prevent the undesired surface wrinkling. Additionally, they allow us to propose a wrinkling-based technique to investigate photoinduced changes in the mechanical properties of azo-containing materials.