Molecular Origins of Macroscopic Mechanical Properties of Elastomeric Organosiloxane Foams

Molecular compositions, structures, interaction, polymer chain dynamics, and Micron-scale cell structures of elastomeric organosiloxane foams have been analyzed and correlated with their macroscopic mechanical properties. Open-cell organosiloxane foams were synthesized within a narrow range of relative densities (+/- 5% relative uncertainty) and with similar micron-scale pore structures, as determined from quantitative analyses of micro-X-ray computed tomography. (MXCT) images. Network cross-linking densities, polymer molecular weights; organic side-chain moieties, and inorganic filler contents were varied systematically, resulting in materials with significantly different mechanical properties. Solid-state single-pulse H-1 and Si-29 magic-angle-spinning (MAS), two-dimensional (2D) Si-29{H-1} hetereonuclear correlation (HETCOR), and transverse H-1 relaxation (T-2) nuclear magnetic resonance (NMR) spectroscopy measurements establish significant differences in molecular and polymer network characteristics that are correlated with the bulk mechanical properties of the organosiloxane foams. These Characteristics include differing extents of polymer cross-linking, concentrations of phenyl side-chain groups, mass fractions of low- to high-molecular-weight cross-linking chains, and polymer chain dynamics. The mechanical properties of the organosiloxane foams are accounted for by the differences in the molecular compositions, structures, and polymer chain dynamics of the foam frameworks, independent of cell microstructures.