7The isothermal crystallization and subsequent melting of a narrow composition distribution polyethylene (PE) copolymer (containing a few mole percent of an a-olefin) was observed by hot-stage atomic force microscopy (AIM). The observations revealed that a thicker lamellae population formed first, followed by a thinner population. This indicated that the crystallization process occurred in two sequential steps. Initially, a single population of uniform lamellar thickness appeared rapidly and progressively occupied the surface. These skeletal lamellae typically ranged in thickness from about 10-15 nm. Progressively, upon isothermal storage, a second, bridging population of lamellae filled in between this initial skeletal population. This second population was thinner with thicknesses of about 4-6 nm. Upon heating, the second, thinner, bridging population of lamellae melted first, leaving the initial, skeletal population, which melted at higher temperatures. These results indicate that PE copolymers crystallize in a stepwise fashion; this was envisioned to begin with a first step in which the thickest lamellae crystallize from the folding of the longest ethylene sequences, followed by a second step in which shorter sequences, constrained between thicker lamellae, crystallize into a population of thinner crystals. The melting of this presumed bimodal population of crystal thicknesses would produce a broad and bimodal endotherm; the bimodal melting endotherm was observed by differential scanning calorimetry. Furthermore, the lamellar thickness dimensions measured by AIM were in approximately the same range as those calculated from the melting endotherm by means of Gibbs-Thompson theory. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 108: 987-994, 2008.