Molecular weight dependence of nuclear spin correlations in PDMS networks

We present experimental proton NMR data for poly(dimethylsiloxane) (PDMS) model networks in the form of a beta-echo (sine correlation) function which is particularly sensitive to modulations of nuclear dipolar interactions caused by slow molecular reorientations. The beta-echo can be computed with a correlation function which allows for preaveraging of local dipolar interactions-methyl group rotation, intra-Kuhn-segment isomerization, extra-segment reorientations due to (restricted) Rouse modes, and slower modulations due to network fluctuations-to yield a residual proton second moment, <(M-2)over bar>. The data were obtained from networks with inter-cross-link molar mass, M-X, covering a wide range both below and above the entanglement mass, M-e. Fits of the beta-echo give quite reliable estimates of the preaveraged dipolar interaction strength <(M-2)over bar>, and these indicate a clear dependence on M-X, in which the role of physical entanglements is dramatically evident. Network dynamics cannot be reliably ascertained due to the effect of higher-order spin interactions.