Nmr Imaging of Silica-Silicone Composites
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NMR IMAGING OF SILICA-SILICONE COMPOSITES LEONCIO GARRIDO-, JEROME L. ACKERMAN' AND JAMES E. MARK" "NMR Center, Massachusetts General Hospital, 149 13th St., Charlestown, MA 02129. "Departmentof Chemistry and the Polymer Research Center, University of Cincinnati, Cincinnati, OH 45221.
ABSTRACT Polydimethylsiloxane (PDMS) model networks reinforced by in situ precipitated Si0 2, and polymer-modified silica glasses were obtained following the usual sol-gel meth-
ods. The conditions were chosen to increase the probability of observing inhomogeneities: (i) bulky samples, and (ii) limited reaction times. These composites were characterized by measuring bulk spin-lattice (T 1 ) and spin-spin (T 2) relaxation times and using 1H NMR two-dimensional Fourier transform (2DFT) spin echo imaging techniques. The Ti and T 2 maps show clear and significant variations of NMR signal intensity throughout the sample due to nonuniform hydrolysis of the tetraethylorthosilicate (TEOS) in the specimens.
INTRODUCTION The search for materials with optimal properties for specific applications, including new pathways for their synthesis and processing, is a continuing process. Over the past 20 years, sol-gel processes have been extensively studied as alternatives to the existing preparation methods for composites [1,2]. The ability to manipulate the composite microstructure by the sol-gel reaction results in materials with improved and sometimes unexpected physical and chemical properties. The understanding of a material's behavior requires knowledge of how the properties of interest depend upon its chemical composition and molecular structure. Nuclear magnetic resonance (NMR) spectroscopy is very sensitive to both the chemical composition and the structure of a substance. These material properties are reflected in the chemical shift spectrum as well as in the NMR relaxation parameters. NMR imaging techniques [3-11], by producing visual pictures of the spatial variation of selected NMR properties, offer the possibility of selectively mapping the distribution of particular chemical species in a region of interest. Moreover, NMR imaging can also provide spatial information about changes in NMR properties that can be correlated with alterations in molecular structure and dynamics. The aim of this work is to develop NMR imaging techniques for the characterization of sol-gel prepared organic-inorganic composites by mapping the organic phase distribution and the degree of alkoxide hydrolysis. We have obtained 1H NMR images of intentionally heterogeneous polydimethylsiloxane (PDMS) model networks reinforced by in situ precipitated silica (Si0 2) and polymer-modified silica glasses. In these images the variations in NMR signal intensity between different regions in the sample (image contrast) are a function of proton density, spin-lattice (TI) and spin-spin (T 2) relaxation times. Such maps of NMR parameters provide a measure of the molecular mobility, which can in turn be related to the spatial variation of the relationship between the organic and
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