X-ray diffraction studies of sol-gel derived ORMOSILs based on combinations of tetramethoxysilane and trimethoxysilane

ORMOSILs have been prepared in the series TMOSx.MTMS(100-x) (where TMOS is tetramethoxysilane; MTMS is methyltrimethoxysilane; x is mol% silane with respect to total silane for 0 less than or equal to x less than or equal to 100) by means of acid catalyzed, sol-gel processing. After drying at 60 degrees C, small bulk samples were obtained of excellent optical clarity. Powder X-ray diffraction (XRD) patterns, in the range of 5 to 60 degrees 2 theta, were compared with that of fused silica. All the prepared samples were amorphous. Fused silica exhibits one broad peak, d(2) centered at d-spacing 4.12 Angstrom. For the TMOS100 silica xerogel, the analogous broad peak had shifted slightly, to be centered at 3.88 Angstrom; and remained in about the same position as x was decreased for the series TMOSx.MTMS(100-x). In addition, a second, broad peak, dl, was observed for the ORMOSIL series centered at the d-spacing 8.7 Angstrom for MTMS100 (i.e., x = 0) and increasing smoothly as x was increased, reaching 11.3 Angstrom for x = 70, and >11.3 Angstrom for x > 70. The intensity of d(1) was found to have trebled, relative to the intensity of d(2), on increasing the organic character of the matrix from TMOS70.MTMS30 to MTMS100. The d(2) peak appearing at about 4 Angstrom for both fused silica and the ORMOSILs is assumed to be associated with the spacing between silicon atoms connected by means of an oxygen bridge. The Si-O-Si angle for silica xerogels is known to depend upon the nature of the sol-gel processing and is bigger than that of fused silica. The d(1) peak may be associated with the spacing between silicons attached to methyl groups and indicative of channels of methyl groups in the structure. Alternatively, the d(1) peak may have its origin in a preferred, discrete structural unit in the matrix for instance cubane based on a octameric silicon arrangement.