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Title: Does melt viscosity cross over to pure reptation scaling
Plain: The viscosity of melts of long linear chains grows as chain length to the power 3.4, while the basic sliding-in-a-tube picture (reptation) predicts power 3. Corrections explain the gap, but it is unconfirmed whether very long chains finally reach power 3.
Precise: For monodisperse linear polymer melts, eta_0 ~ $M^3$.4 is observed while pure reptation gives M^3; contour-length fluctuations and constraint release account for the difference at moderate $M / M_e$. Determine whether $\eta_0$ crosses over to $M^3$ at very high $M / M_e$, and at what $M / M_e$, in experiment and in simulation.
Settled by: Measurements of $\eta_{0}$ for narrowly distributed melts extending to $M / M_{e}$ of order 1e3 or more, or equivalent simulations, showing the local exponent approaching 3 or not.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: contested
Status note: Exponents near 3 were reported for very high M polybutadiene (Colby, Fetters and Graessley 1987) and ultrahigh-molecular-weight polyethylene (Vega et al. 2004), but polydispersity and equilibration make these claims disputed; tube theory explains the 3.4 regime quantitatively.
Posed since: (empty)