Sol-gel-derived potassium-based di-ureasils for "smart windows"

Abstract

Sol-gel derived KCF3SO3-doped di-urea cross-linked poly(oxyethylene) (POE)/siloxane (di-ureasil) ormolytes with ∞ > n ≥ 1 (n is the number of oxyethylene units per K+ ion) have been analysed. Samples with n > 40 are thermally stable up to 310 ºC. At n ≥ 100, POE crystallites are present. At n = 5 a crystalline POE/KCF3SO3 complex with stoichiometry 1:1 is formed. In the xerogel with n =1 this complex coexists with free salt. The highest ionic conductivity is reached at n = 20. “Free” anions and weakly coordinated CF3SO3- ions appear to be the main charge carriers. The K+ ions interact with the urea carbonyl oxygen atoms at all salt concentrations. Complexation of the cations by POE occurs at n ≤ 20. In the latter composition range, contact ion pairs and higher ionic aggregates develop. The ormolytes were successfully used in the assembly of electrochromic devices (ECDs) that exhibit good electrochemical stability and demonstrate a memory effect. The ECD incorporating the ormolyte with n = 20 displays an average transmittance in the visible region of ca. 90% in the coloured state and of 85-95% in the bleached atate. The corresponding change in colour, evaluated on the basis of the Commission Internationale d/Eclairage (CIE) colour-coordinate system, is reversible: from yellow (0.38, 0.37) for the bleached state to blue (0.29, 0.29) for the coloured state.