A mechanochromic smart material

Abstract

A mechanochromic polyurethane elastomer was synthesized and demonstrated. Azobenzene chromophores were introduced into a copolyamide oligomer to provide conformational latches which stabilized thecis-azobenzenes by the formation of intramolecular hydrogen bonds. The copolyamide oligomer was chemically inserted into a polyurethane copolymer. The chromophores in the resulting copolymer were transformed to thecis-form by ultraviolet (UV) irradiation (λ=365 nm) and then succesively stretched and relaxed to strains of 100%, 200%, and 300%. After 100% strain and relaxation, the absorbance of polyurethane copolymer at λ=380 nm, which indicates transformation to the trans form, was not significantly increased. However, the absorbance increased with 200% and 300% tensile strain followed by relaxation. A mechanochromic relationship between absorbance increase at λ=380 nm in the relaxed sample and the maximum strain to which the sample was subjected was thus demonstrated.