Novel Photomechanical Thin Films Combining
Photochromic Dithienylethene and Thermoplastic Elastomer: Toward a Better Physical and Photochemical Understanding

This work presents a study on the development of a novel photoactuator based on supramolecular chemistry combining a photochromic dithienylethene A, and a thermoplastic elastomer poly(ethylene-co-butylene) B, both functionalized with ureido‐4[1H]‐pyrimidinone (UPy) units. The objectives were to optimize the thin film fabrication by spin coating and to find the ideal parameters to maximize the photomechanical effect performances, i.e. with respect to the light direction, a reversible bending displacement backward and forward under Visible and UV light respectively.  Then, in order to rationalize the photoresponse of the system, the study focused on the changes produced in the internal structure of the material, as well as in its morphology and photochemistry when it was subjected to alternating UV/visible light irradiation with the aid of several technics (WAXS, SAXS, AFM, SEM, UV-Vis spectrometry, and absorption analysis techniques). The results showed that the system operate mostly according to the DTE photochromic response (open /closed gradient) as long as the irradiation period is short enough to keep the elastomeric mechanical relaxation negligible. Furthermore, it has been found that the amplitude of the photomechanical effect is related with a good viscoelastic properties and  semi-crystallinity o the elastomer. By the way,   the results rationalize the bending toward the light under UV irradiation which is explained in terms of the positive expansion coefficient inside the Timoshenko bimetallic model equation which is a model applied for DTE crystal photoactuators. From the microscopic surface analysis, the photomechanical effect is related to the domains swelling (contraction) size from one photoisomer to the another one. This last result was corroborated by SAXS highlighting the UPy aggregation influence inside domains containing A/B mixture responding reversibly to UV/Visible light.