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 β-echo (sine correlation) function which is particularly sensitive to modulations of nuclear dipolar interactions caused by slow molecular reorientations. The β-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, . 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 β-echo give quite reliable estimates of the preaveraged dipolar interaction strength, 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.