Characterization of PDMS model junctions and networks by solution and solid‐state silicon‐29 NMR spectroscopy

The application of 29 Si solution and solid‐state cross‐polarization/magic‐angle spinning (CP/MAS) nuclear magnetic resonance (NMR) techniques to the study of structural features in polydimethylsiloxane (PDMS) model endlinked elastomeric networks is explored. The relationship between the topological (network) functionality of a structural moiety, which determines network mechanical properties, and its chemical (spectral) functionality, which is reported by the NMR, is discussed. The second‐order spectral shifts corresponding to topographical functionality variation within a chemical functionality class are usually sufficiently well resolved in these networks to allow positive identification of a variety of structural features. The basic PDMS repeat unit, OSi(CH 3 ) 2 , is found to possess an axially symmetric chemical shift tensor with σ ∥ = −56.8 ppm downfield from TMS, and σ ⊥ = −4.4 ppm. This axial symmetry does not result from rapid reorientation about the chain axis. The NMR spectrum reveals defects in model endlinked networks. In the case of vinyl‐endlinked systems, the defects are ascribed to the formation of elastically ineffective loops. Hydroxyl‐endlinked systems contain either loops or else trifunctional junctions (hydrolyzed before chain coupling could take place) and dangling chain ends. The CP/MAS technique provides an order‐of‐magnitude reduction over standard solution techniques in the time to acquire a spectrum from a network not containing paramagnetic doping. 13 C spectra of PDMS systems are not as informative as 29 Si spectra.