Melting

n-Alkanes

n-alkanes such as C₄₄H₉₀ and C₉₄H₁₉₀ can form perfect crystals.

• Melting produces a sharp increase in volume, as illustrated by Mandelkern's work, PC Fig. 7.19.

• The melting temperature is higher for the sample with the larger molecular weight.

• The melting range is extremely sharp (a fraction of a degree for C₄₄H₉₀).

Polyethylene

The results for a semicrystalline polymer are more complicated, depending on details of structure, prior history, and the experiment itself. A semicrystalline polymer contains amorphous material along with small crystals with a variety of sizes. Kinetic effects prevent the sample from attaining a true state of equilibrium.

• The melting range for polyethylene is broader (1-2 deg) than for C₄₄H₉₀.

• A polydisperse sample melts over a larger range than a fractionated sample with a narrow molecular weight distribution (PC Fig. 7.20, reproduced from Chiang, R.; Flory, P. J. "Equilibrium between Crystalline and Amorphous Phase in Polyethylene" J. Am. Chem. Soc. 1961, 83, 2857-2862).

• Crystals have a variety of sizes and imperfections.

• The lower molecular weight chains crystallize more rapidly than the higher molecular weight chains. There are kinetic effects. The melting temperature and range will also depend on how carefully the sample was crystallized, i. e., the size of the undercooling.

• A narrow molecular weight distribution sample has a narrower melting range than the unfractionated sample, but a broader range than C₄₄H₉₀.