Lamellar Salt-Doped Hybrids with Two Reversible Order/Disorder Phase Transitions

Abstract

A lamellar bilayer hierarchically structured amide cross-linked alkyl/siloxane hybrid matrix (mono-amidosil, m-A(14)) was doped with a wide concentration range of potassium triflate (KCF3SO3), magnesium triflate (Mg(CF3SO3)2), and europium triflate (Eu(CF3SO3)(3)). In the Kt, Met, and Eu3+-based samples with n > 5, 20, and 60 (where n is the molar ratio of amide C = O groups per cation), respectively, the original lamellar structure of m-A(14) coexists with a new lamellar phase with lower interlamellar distance. The texture of the mono-amidosils doped with K+, Mg2+, and Eu3+ ions mimics cabbage leaves, foliated schist, and sea sponges, respectively. In the three series of materials, the cations bond to the oxygen atoms of the amide carbonyl groups. The amide amide hydrogen-bonded array of m-A(14) is less perturbed by the inclusion of KCF3SO3 and Mg(CF3SO3)(2) than by the incorporation of Eu(CF3SO3)(3). The degree of ionic association is low for n > 20. The cations coordinate to the oxygen atoms of the triflate ions, forming contact ion pairs at higher salt content. In the Mg(CF3SO3)2- and Eu(CF3SO3)(3)-containing materials with n = 5 and 10, respectively, crystalline salt is formed. The structural changes undergone by the alkyl chains of selected mono-amidosils in a heating/cooling cycle are reversible, are time-independent, and exhibit two distinct hysteresis domains, one associated with the order/disorder phase transition of the original lamellar bilayer structure of m-A(14) and the second one associated with the order/disorder phase transition of the new lamellar bilayer structure formed in the presence of the salts.