Enhanced electrochromic switching in multilayer thin films of polyaniline-tethered silsesquioxane nanocage

Polyaniline (PANI)-tethered cubic polyhedral oligomeric silsesquioxane (POSS) in emeraldine base (EB) form is synthesized via oxidative copolymerization of octa(aminophenyl) silsesquioxane and aniline in the presence of HCl followed by the treatment with triethylamine. The chemical structures of POSS-PANI-EB are elucidated by Fourier transform infrared spectroscopy and elemental analysis. POSS-PANI/poly(2-acrylamido-methane-2-propanesulfonic acid) (PAMPS) multilayer thin films are successfully fabricated via layer-by-layer (LBL) assembly. Cyclic voltammetry studies show that the redox reactions in the thin film containing 50 POSS-PANI/PAMPS bilayers ((POSS-PANI/PAMPS)50) are close to non-diffusion-controlled processes in the studied scan-rate range of 10-100 mV/s, and the reactions are also more reversible than that in the linear counterpart (PANI/PAMPS)50. Under a dynamic switching condition of 40 s per cycle, the electrochromic contrast of (POSS-PANI/PAMPS)50 is increased by more than 30% over that of (PANI/PAMPS)50 and the switching time of (POSS-PANI/PAMPS)50 is also significantly shorter than that of (PANI/PAMPS)50. In addition to the better control of film thickness and morphology, the electrochromic performance of the POSS-PANI/PAMPS multilayer films is also superior to the previously studied spin-coated thin films of SS-PANI/polymeric acid complexes. The improvements can be attributed to the unique morphology brought by the synergistic combination of the starlike structure of POSS-PANI and the LBL assembly method, which allows for more favorable interactions between PANI chains and polymeric dopants as well as faster ion transport during the redox switching.