Mechanochromic Polymer Film with High Sensitivity toward Tensile Strain by the Post-Curing Ring-Closure Induced Pre-Stretching.

Mechanochromic materials have received broad research interests recently, owing to its ability to monitor the in situ stress/strain in polymer materials in a straightforward way. However, one major setback that hinders the practical application of these materials is their low sensitivity toward tensile strain. Here a new strategy for pre-stretching of the mechanochromic agent in a polymer film on the molecular scale, which can effectively enhance the mechanochromic sensitivity of a polymer film toward tensile strain, is shown. In situ fluorescent measurement during tensile test shows an early activation of the mechanochromic agent at tensile strain as low as 50%. The pre-stretching effect is realized by first inducing ring-opening of the mechanochromic agent by molecular functionalization, and then compelling the ring-closure process in the cured film by elevated temperature. This post-curing ring-closure process will result in pre-stretched mechanochromic agent in a crosslinked network. The mechanism for mechanochromic activation of polymer films with different composition is elaborated by visco-elastic measurements, and the effect of pre-stretching is further confirmed by films with other compositions. Combined with the simplicity of the method developed, this work could offer an alternative strategy to enhance the sensitivity of different mechanochromic agents toward tensile strain.