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 °C. 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. The redox stability domain of this material spans 5.0 V. “Free” anions and weakly coordinated CF₃SO₃⁻ ions appear to be the main charge carriers at n = 20. 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 prototype 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 state. The corresponding change in colour, evaluated on the basis of the Commission Internationale d'Eclairage (CIE) colour-coordinates system, is reversible: from yellow (0.38, 0.37) for the bleached state to blue (0.29, 0.29) for the coloured state.