Sol−Gel Derived Urea Cross-Linked Organically Modified Silicates. 1. Room Temperature Mid-Infrared Spectra

Abstract

Organic−inorganic hybrids prepared by the sol−gel process are investigated by infrared spectroscopy at room temperature. The matrix of the so-called ureasils is a silica network to which oligopolyoxyethylene chains are grafted by means of urea cross-links. The ureasils prepared U(2000), U(900), and U(600) are obtained by reacting three diamines (containing about 40.5, 15.5, and 8.5 oxyethylene units, respectively) with 3-isocyanatepropyltriethoxysilane. The mid-infrared spectra of the diamines are examined. The FTIR spectrum of U(2000) shows that the polyether chains of the parent diamine become less ordered upon incorporation into the inorganic backbone. The assignment of the absorption bands originating from the urea moieties in the ureasils is proposed. Spectroscopic data reveal that the number of oxyethylene units present affects dramatically the amide I and amide II bands and indicate that the N−H groups of the urea linkage are involved in hydrogen bonds of different strength. The existence of non-hydrogen-bonded urea groups and hydrogen-bonded urea−urea and urea−polyether associations is suggested. The formation of urea−urea structures is apparently favored in U(600), whereas the number of “free” carbonyl groups is greatest in U(2000).