Crystallization and melting of a branched polyethylene with precisely controlled chemical structure


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The heat capacity of a linear polyethylene with dimethyl branches, at every 21st backbone atom was analyzed by differential scanning calorimetry (DSC) and quasi-isothermal temperature-modulated DSC. This novel copolyethylene (PE2M) is relatively difficult to crystallize from the melt. On subsequent heating, a first, sharp melting peak is followed by a sharp cold-crystallization and crystal perfection and a smaller endotherm, before reaching the main melting at 315–320 K, close to the melting temperatures of eicosane and tetracontane. The low-temperature melting is sensitive to the cooling rate and disappears below 1.0 K min−1. The cold crystallization can be avoided by heating with rates faster than 80 K min−1. The PE2M exhibits some reversing and reversible melting, which is typical for chain-folded polymers. The glass transition of semicrystalline PE2M is broadened and reaches its upper limit at about 260 K (midpoint at about 0.355 K). Above this temperature, the crystals seem to have a heat capacity similar to that of the liquid. A hypothesis is that the melting transition can be explained by changes in crystal perfection without major alteration of the crystal structure and the lamellar morphology. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3461–3474, 2006