AbstractThe methods of small systems thermodynamics have been applied to the calculation of the entropy barrier to the growth of polymer crystals. The height of this barrier at the equilibrium melting point of polyethylene molecules of typical lengths (X = 4,000 to 10,000 segments) is estimated to be of the order of 4 to 5 Kcal per molecule. This barrier increases linearly with ln X if the crystal is very large but retains the approximate values mentioned for crystals with the lamella thicknesses usually found. This entropy change causes the free energy to go through a maximum as in a nucleation phenomena but no new surface formation is involved. The existence of such a barrier explains why crystallization (or melting) never occurs at the equilibrium melting temperature.