Crystallization and melting of a poly(ethylene oxide) (PEO) diblock copolymer containing a tablet-like block of poly {2,5-bis[(4-methoxyphenyl)oxycarbonyl] styrene} (PMPCS) in ultrathin films have been studied using atomic force microscopy (AFM) coupled with a hot stage. The PEO and PMPCS block possess the number-average molecular weights (M-n) of 5300 and 2 100 g/mol, respectively. The ultrathin films on the mica and glow-discharged carbon surfaces were obtained by static dilute solution casting at room temperature. Isothermal melt crystallization from ultrathin films always leads to flat-on lamellae. Selective area electron diffraction (SAED) experiments have demonstrated that the PEO blocks crystallize with a monoclinic structure identical to that of homo-PEO and the chain direction is perpendicular to the substrate. At T-c < 44 degrees C, the monolayer crystals are dendrites. At T-c > 48 degrees C, square-shaped crystals are formed with the (100) and (020) planes as the crystal edges. At 44 degrees C <= T-c <= 48 degrees C, an intermediate monolayer morphology is observed. The monolayer thickness increases monotonically with increasing T-c. At the same T-c, the monolayer lamellae with the top and bottom amorphous layers contacting with the atmosphere and the substrate possess a significantly larger overall thickness than the long period of the crystals in bulk. For the spiral terraces induced by screw dislocation, the thickness of each terrace is close to that of the monolayer formed at the same T, and their melting is mainly determined by the terrace thickness. (c) 2005 Elsevier Ltd. All rights reserved.