Di-ureasil ormolytes doped with Mg2+ ions: Part 2. Cationic and anionic environments
Journal article, 2005
Fourier Transform mid-infrared and Raman (FT-IR and FT-Raman, respectively) spectroscopies were used to investigate the cation/polymer, cation/cross-link, cation/anion and hydrogen bonding interactions in sol-gel derived ormolytes doped with magnesium triflate (Mg(CF3SO3)2). The framework of these xerogels (di-ureasils) contains poly(oxyethylene) (POE) segments with about 40.5 repeat units bonded at both ends to a siliceous backbone through urea cross-links. Materials with compositions ∞ > n ≥ 1 (where n is the ratio of oxyethylene moieties per Mg2+ ion) were studied. FT-IR data revealed that, while the urea carbonyl oxygen atoms bond to the cations in all the compounds analyzed, the POE chains complex the Mg2+ ions only at high salt content. Although the beginning of the POE/Mg2+ interaction was detected spectroscopically at n = 10, thermal data obtained in Part 1 of this series of papers suggest that this threshold composition is probably located at lower salt content. Bonding of the Mg2+ ions to the ether oxygen atoms of the POE chains is accompanied by the destruction of the urea/POE hydrogen-bonded aggregates formed in the d-U(2000) medium. Proofs of the existence of a crystalline POE/Mg(CF 3SO3)2 complex in samples with n ≤ 10 were found. "Free" and weakly coordinated CF3SO3- ions, present in all the samples examined, appear to be the main charge carriers of the conductivity maximum of this family of ormolytes (i.e., d-U(2000)20Mg(CF3SO3)2). At n ≤ 40, along with those anionic species, another anionic configuration, attributed to contact ion pairs, emerges. Higher ionic aggregates are formed in di-ureasils with n ≤ 5.