END-LINKED POLY(DIMETHYLSILOXANE) ELASTOMERS - H-2-NUCLEAR MAGNETIC-RESONANCE INVESTIGATIONS OF COMPRESSION-INDUCED SEGMENT ANISOTROPY

H-2-NMR spectroscopy is applied to measure strain-induced orientationa l anisotropy of polymer segments in three environments in poly(dimethy lsiloxane) (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. Ne twork features were inferred from macroscopic measurements using stati stical and thermodynamic models. The segment order parameter, S, was f ound to be proportional to lambda(2) - lambda(-1), where lambda is the compression ratio, for the three environments in uniaxially-compresse d PDMS networks: elastic chains, pendant chains, and dissolved probe c hains. Furthermore, all three environments show the same dependence of the ratio S/(lambda(2) - lambda(-1)) on the elastic modulus, in the r ange 0.089-0.29 MPa. These results agree with the recent theoretical p redictions of Brereton and Res (Macromolecules 1996, 29, 2644) and thu s 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 H-2-NMR line splittin g.