Local Strain Relaxation Dictates Polymer Melt Viscosity

A remarkable manifestation of slow dynamics in dense polymeric systems is that their viscosity scales with their molecular weight as a power-law, with the “viscosity exponent” mη, experimentally measured to be 3.4 ± 0.2. Following De Gennes’ work in 1971, current understanding of this slow dynamics is based upon an effective single-polymer theory that pictures every polymer in such systems moving within a rigid static network formed by the surrounding ones, yielding mη = 3. We change this picture fundamentally. We report collective many-polymer effects, wherein for each polymer, the surrounding ones generate a dynamical restoring force, characterized by a power-law in time: ∼ t, with α = 0.78± 0.03. Further analysis provides mη = 2 + 1/α = 3.28± 0.05. This new understanding reconciles four decades of theory-experiment discrepancy for mη, a major unsolved problem in polymer physics.