End-Linked Poly(dimethylsiloxane) Elastomers: 2H-Nuclear Magnetic Resonance Investigations of Compression-Induced Segment Anisotropy

2 H-NMR spectroscopy is applied to measure strain-induced orientational anisotropy of polymer segments in three environments in poly(dimethylsiloxane) (PDMS) elastomers: elastic chains, pendant chains, and PDMS(d) chains dissolved in the network. The elastomers were prepared with systematically-varied structures and selective deuterium labeling. Network features were inferred from macroscopic measurements using statistical and thermodynamic models. The segment order parameter, S, was found to be proportional to λ 2 − λ -1, where λ is the compression ratio, for the three environments in uniaxially-compressed PDMS networks: elastic chains, pendant chains, and dissolved probe chains. Furthermore, all three environments show the same dependence of the ratio S /(λ 2 − λ -1 ) on the elastic modulus, in the range 0.089−0.29 MPa. These results agree with the recent theoretical predictions of Brereton and Ries ( Macromolecules 1996, 29, 2644) and thus support the premise that the excluded volume (entropic) interactions between segments subjected to deformation cause anisotropic averaging of the quadrupolar interaction and thus cause a 2 H-NMR line splitting.