Molecular weight dependence of nuclear spin correlations in PDMS networks

We present experimental proton NMR data for poly(dimethylsiloxane) (PDMS) m odel networks in the form of a beta-echo (sine correlation) function which is particularly sensitive to modulations of nuclear dipolar interactions ca used by slow molecular reorientations. The beta-echo can be computed with a correlation function which allows for preaveraging of local dipolar intera ctions-methyl group rotation, intra-Kuhn-segment isomerization, extra-segme nt reorientations due to (restricted) Rouse modes, and slower modulations d ue 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 mas s, M-e. Fits of the beta-echo give quite reliable estimates of the preavera ged dipolar interaction strength <(M-2)over bar>, and these indicate a clea r dependence on M-X, in which the role of physical entanglements is dramati cally evident. Network dynamics cannot be reliably ascertained due to the e ffect of higher-order spin interactions.