Mesostructured silica for the reinforcement and toughening of rubbery and glassy epoxy polymers

Mesoporous forms of silica with wormhole framework structures prepared from tetraethylorthosilicate (denoted MSU-J-TEOS) or from sodium silicate (denoted MSU-J-SS) and an amine surfactant as the structure-directing porogen are highly effective reinforcing and toughening agents for rubbery and glassy epoxy polymers. The improvements in tensile strength and modulus provided by MSU-J silicas with a large average framework pore size (e.g., 5.9 and 21.3 nm) are superior to those provided by the corresponding silicas made from the same TEOS or SS precursors but with smaller framework pore sizes (e.g., 4.2 and 5.2 nm). The improved performance of the larger pore structures is realized even though the surface areas (∼670 m2/g) are substantially lower than the surface areas of the smaller pore analogs (812–1025 m2/g), most likely, because of more efficient polymer impregnation of the particle mesopores. In comparison to the MSU-J-TEOS silica assembled from TEOS, MSU-J-SS silica made from SS exhibits a more uniform pore distribution and smaller particles. These latter textural features lead to improved tensile strength and modulus without compromising the strain-at-break for both rubbery and glassy epoxy polymers. The exceptional strength and toughness provided by MSU-J-SS silica in comparison to MSU-J-TEOS silica are correlated with the high degree of dispersion of the mesophase particles in the epoxy matrix for the more effective distribution of stress and the deflection of microcracks.