Polymeric materials that change color in response to various external stimuli such as heat, light, electric field and magnetic field have been widely known and applied to various types of display devices and sensors. However, there exist a few polymeric materials that change color in response to mechanical stimuli. The National Institute of Advanced Institute from Japan has recently developed polymer films that change colour instantaneously and reversibly in response to changes in the tension. The films were prepared on elastic substrates by spin-coating from solutions of substituted polyacetylenes. The colour of these polymer films changed instantaneously and reversibly when the polymer films were stretched and contracted using a stretching machine. The change in colour was repeatable. If such polymers can be put to practical use, mechanical stimuli can be visualized easily and inexpensively. For example, such polymers can be applied to a tension sensor, which will indicate danger spots evidently by detecting stresses acting on structures of buildings and then indicating them as a change in color.
It has been difficult to put polyacetylene, which is renowned as a conducting polymer, to practical use because it is not stable in air. However, substituted polyacetylenes into which substituents are introduced, are suitable for practical applications because they are stable in air and allow the production of films from their solutions. The National Industrial Science and Technology from Japan has been studying the syntheses of new substituted polyacetylenes and control of the optical properties of their films. Polymerization of acetylene substituted with a substituted phenyl group with Norborane used as a catalyst produces a polymer in which the main chain is in the cis conformation and has a helical structure. A film of this polymer was prepared by spin-coating from a chloroform solution of the polymer on a colourless elastic sheet. The colour of the obtained film of the substituted polyacetylene was yellow at the time of formation. By stretching this film with the sheet by use of a stretch machine, the substituted polyacetylene molecules were oriented along the direction of stretching. Further stretching led to a colour change in the film from yellow to red. Measurement of the ultraviolet-visible absorption spectrum indicated an increase in absorption from approximately 500 to 600 nm. Removal of the tension and contracting the film led to the return of the colour of the film from red to yellow, and the absorption spectrum matched with the spectrum before stretching. Thus, the change in colour due to stretching and contracting was reversible. The colour of the film changed instantaneously in response to quick manual stretching and contraction. The change in colour was repeatable, that is, the change in colour between yellow and red could be repeated by stretching and contraction repeatedly.