Influence of Cross-Link Density on Electrochromic Properties of Conjugated Polymer

Cross-linked conjugated polymers show many unique performance advantages in ionic transport, stretchability, and cycle stability. However, the relationship between cross-link density and performance is always ignored in designing cross-linked conjugated polymers. Here, a series of poly(3,4-ethylenedioxythiophene) (PEDOT) derivatives with different cross-link densities were prepared by two steps: (1) free radical polymerization of flexible side chain (methacrylate); (2) electrochemical copolymerization of conjugated units (thiophene derivatives). These cross-linked conjugated copolymers except for poly(2,1) with high cross-link density exhibit a similar optical absorption region with a definite peak, indicating that low cross-link density has little influence on the absorption region. Electrochromic results indicate that poly(1,10) with low cross-link density possesses a higher optical contrast (70%) and faster response time (0.5 s) than linear poly(ProDOT-Me) and other cross-linked conjugated copolymers with relatively higher cross-link density. When cross-link density is beyond a critical value, it could be also found that the electrochromic properties of conjugated polymers decrease gradually as the cross-link density increases. And the flexible electrochromic device (10 cm x 6 cm) based on poly(1,10) was constructed, whose color could reversibly turn between purple and transmissive blue. These results demonstrate that controlling cross-link density is essential in designing cross-linked conjugated polymers as electrochromic materials.