Multi-stimuli responsive free-standing films of DR1- grafted silicones

Abstract Multi-functional materials were synthesized by on chain modification of dimethyl-methylvinylsiloxane copolymers with mercaptopropionic acid and Disperse Red 1 (DR1). After cross-linking, transparent free-standing films were obtained, showing at the same time light-responsive and electromechanical (electrical actuation and piezoelectric) properties. Different characterization tools, such as DSC, dielectric broadband spectroscopy, dynamic contact angle, and indentation, were applied to investigate the impact of polymer functionalities on the intrinsic material properties. The molecular grafting of DR1 enhanced the dielectric permittivity by 63% compared with the starting vinyl-silicones, while high breakdown strength of 84 V/μm was achieved. The films exhibited actuation on three different mechanisms: electrical, piezoelectric and photo-induced. Through the use of specific analyzing techniques (such as electric field actuation, piezoresponse force microscopy -PFM- and photoactuation) direct evidence of these features was brought in each case. The lateral actuation strain reached 8.7% at 40 V/μm, as a consequence of increased dielectric permittivity. The piezoelectric coefficient d33 of about 10 pm/V was determined by PFM. The trans–cis photoisomerization of nitro-azobenzene which occurs during UV irradiation led to surface modifications and preferential actuation of the film (such as, length change or bending), depending on optical and mechanical properties of the materials.