Tube-constrained diffusion of an entangled polymer chain
De Gennes (1971) proposed that a polymer in an entangled melt diffuses by reptation: snake-like motion along a confining "tube" formed by surrounding chains. The mean-square displacement has four regimes: MSD ~ t^(1/4) (Rouse within tube), t^(1/2) (tube curvilinear), t^1 (Fickian). The reptation time τ_d ~ N³ and diffusion constant D ~ N^(-2).