NMR CHARACTERIZATION OF ELASTOMERS REINFORCED WITH INSITU PRECIPITATED SILICA

Poly(dimethylsiloxane) (PDMS) model networks were reinforced by in situ precipitated silica following the usual sol - gel methods. Relatively large sample size and short reaction times were chosen to increase the probability of observing inhomogeneities. These elastomers were characterized by H-1 and Si-29 magic-angle spinning (MAS) NMR spectroscopy and H-1 NMR two-dimensional Fourier transform (2DFT) spin-echo imaging techniques. Bulk spin - lattice (T1) and spin - spin (T2) relaxation times were measured for all samples. The specific gravity of small pieces from regions of interest in selected specimens was measured. Only a small fraction (approximately 5%) of the total protons present in the sample is visible in the NMR imaging experiments. The proton density and T2 maps show clear and significant variations of NMR signal intensity throughout the sample due to the nonuniform hydrolysis of the tetraethylorthosilicate (TEOS) in the rubber. Under these precipitation conditions, silica precipitates mainly in the sample periphery as indicated by the proton density images, specific gravity, and Si-29 MAS NMR results. These studies indicate the importance of mass transport phenomena and the use of nondestructive test methods such as NMR imaging in the production of bulk specimens of in situ precipitated materials.