Entropic Elasticity — Freely Jointed Chain vs Worm-Like Chain

Stretching a polymer costs entropy. The FJC (freely jointed chain) uses the inverse Langevin function; the WLC (worm-like chain) uses bending rigidity (persistence length Lp). Both show the characteristic upturn near full extension — but WLC diverges as (1−z/L)^−2.

Contour L (nm): 100
Persist. Lp (nm): 50
Segment b (nm): 5
Force (pN): —