Synthesis And Characterization Of Lamella Urea-formaldehyde Resin-Silica Hybrid Material

The development of the organic-inorganic material with structure variety of organic species and stability of inorganic species is currently an area of extensive research, particularly with regard to potential applications in areas such as adsorption, chromatography, catalysis, optical and magnetic materials. Organic-silica hybrid material is a typical organic-inorganic material. Its synthesis approach refers to "grafting" and "co-condensation" route. The "grafting" method has the advantage that, under the synthetic conditions used, the mesostructure of the starting silica phase is usually retained, however, this can lead to complete closure of the pores (pore blocking). Pore blocking is not a problem in the co-condensation method. Furthermore, the organic units are generally more homogeneously distributed than in materials synthesized with the grafting process. But the co-condensation method also its disadvantage: in general, the degree of mesoscopic order of the products decreases with increasing concentration of (R'O)₃SiR in the reaction mixture, which ultimately leads to totally disordered products. Herein, a novel active templating method without surfactants is presented for synthesis of silica materials, in which the mesopore structure, as well as morphology of hybrid silica was induced by formaldehyde - urea polymerization.Polymer-silica hybrid precipitate produced from the urea (U) and formaldehyde (F) polymerization reaction in silica sol-gel at 55℃was investigated. Its primary particles (ca. 1-3μm) with network structures become more compact as the reaction time scale prolonged or the acidity increased. A few large spheres (ca. 5μm) appear in the primary particles and tend to crack if the U/F molar ratio slightly deviates from the value of 1.00. The FT-IR characteristic absorption verifies the formation of linear structure of -HNCONHCH₂-, which is remarkable in the sample prepared with the U/F ratio of 1.00. Diffraction peaks with 2θvalues of ca. 21.9°, 24.8°, and 31.5°are observed in each XRD pattern of the samples that is correlated with the linear structure -[NHCONHCH₂]_n-. Removal of the polymeric species in TGA occurs in the temperature range of 200-400℃, and the linear structure still remained until 300℃. After calcination in air at 600℃, the samples maintain morphologies of the lamellas or particles of the original hybrids. All nitrogen isotherms of the calcined samples are of type IV with a clear hysteresis loop that seems to lie between type H4 and H2/H1. It is believed that the structural characteristics of the silica or hybrids come from peculiar templating of the UF polymer semicrystallization via hydrogen bonding interaction (C=O---HNRR'). A typical formation mechanism is proposed: the urea-formaldehyde semicrystalline polymer formed in the reaction system serves as structure-directing agent to orient the different hybrid materials, which well replicate the morphology and structure of the semicrystalline polymer. Therefore, we can control the silica morphology and structure by regulating the morphology and structure of the semicrystalline polymer.