Homostructured mixed inorganic-organic ion clays: A new approach to epoxy polymer-exfoliated clay nanocomposites with a reduced organic modifier content

A new approach to the preparation of epoxy-clay nanocomposites is reported based on the intercalation and exfoliation of homostructured mixed inorganic/organic cation exchanged forms of a commercially available montmorillonite (PGW) and a synthetic fluorohectorite (FH) clay. In these mixed-ion homostructures both the organic onium ions and the inorganic exchange ions co-occupy the gallery surfaces of the clay, thereby dramatically reducing the amount of organic modifier needed to access the galleries for nanocomposite formation. The homostructures were prepared by ion exchanging the inorganic H⁺ and Li⁺ forms of the smectite clays with diprotonated primary α,ω-diamines of the type H₂NCH(CH₃) CH₂[OCH₂CH(CH₃)]_xNH₂ (denoted Jeffamine D2000 with x = 33.1). Varying the ratio of inorganic cations to onium ions afforded homostructured mixed-ion intercalates with basal spacings ranging from ∼17 Å (25% onium ion exchange) to ∼46 Å (65% onium ion exchange), indicating the Jeffamine D2000 modifier adopted extended chain to folded chain configurations depending on loading. Thermoset glassy epoxy - clay nanocomposites were prepared using EPON 826 resin and Jeffamine D-230 (x = 2.6) as a curing agent. Depending on the fraction of onium ions in the mixed-ion homostructures and on the method of nanocomposite preparation, intercalated and exfoliated clay nanolayers were achieved. The intercalated α,ω-diamine played the dual role of organic modifier of the clay and the curing agent in the thermoset epoxy matrix. Whereas the use of fully exchanged Jeffamine D2000 organoclays compromised the T_g of the matrix, mixed inorganic - organic ion clay homostructures made it possible to limit the plasticizing effect of the long-chain organic modifier and to preserve the glass transition temperature (T_g ∼ 78-85 °C) while improving the storage modulus. Mixed inorganic - organic ion homostructured clays should also provide a useful approach to forming nanocomposites with other engineering polymers, while reducing the need for an organic clay surface modifier.