REPTATION MODEL
MSD Regimes:
t < τ_e: ⟨r²⟩ ~ t^½ (free Rouse)
τ_e < t < τ_R: ~ t^¼ (confined)
τ_R < t < τ_d: ~ t^½ (reptation)
t > τ_d: ~ t (diffusive)
t < τ_e: ⟨r²⟩ ~ t^½ (free Rouse)
τ_e < t < τ_R: ~ t^¼ (confined)
τ_R < t < τ_d: ~ t^½ (reptation)
t > τ_d: ~ t (diffusive)