The molecular weight dependence of nuclear spin correlations in entangled polymeric liquids

We present experimental proton NMR data for poly(dimethylsiloxane) (PDMS) and poly(ethylene oxide) (PEO) 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 in entangled melts. Theoretical expressions for the beta-echo can be computed with a closed-form expression that depends on the correlation function describing macromolecular motion. We use a simple correlation function for reptation that depends on only two parameters, (M) over bar(2), the pre-averaged dipolar interaction strength and tau(d), the tube disengagement time. From fits of the molecular weight dependence of the proton NMR measurements of beta in PDMS melts at 300 K, we conclude that the classical reptation model (tau(d) similar to M(z) where z = 3) applied to the decay of residual intramolecular dipolar interactions is applicable in high molar mass melts (M greater than or equal to 500 000) but intermolecular interactions, described by a lower molar mass scaling process (M(z), z = 1-2), dominate the data for smaller (entangled) chains. The temperature dependence of tau(d) suggests an Arrhenius behavior with an activation energy of 13 kJ mol(-1), the same as that observed for bulk viscosity. PEO appears to exhibit dynamics dominated by processes with a smaller scaling exponent.