In this work we report our investigation on the crystal patterns of a 50/50 blend of two polyethylene oxide (PEO) fractions with different molecular weights ((M) over bar (W)= 5040 and 35000 g/mol) in ultra-thin films. Using AFM with a hot stage the samples on the surface of silicon wafer were isothermally crystallized at 20.0 <= T-c <= 60.0 degrees C. The crystal patterns are different from those of the two pure fractions. Co-crystallization, partial segregation and full segregation have been observed. Especially, within 47.0 <= T-c <= 54.0 degrees C dual thickness crystals formed. The thickness of the middle part of the crystal corresponds to the lamella thickness of 35k-PEO fraction with multiple folds, while the thickness of the edge part is nearly equal to an extended-chain lamella thickness of 5k-PEO fraction. We suppose that the appearance of the dual thickness crystals is due to molecular partial segregation. Utilizing in-situ AFM, the growth of the crystal with dual thicknesses as a function of time was monitored at T-c = 54.0 degrees C. Two growth processes were observed and a pattern formation mechanism was suggested on the basis of specified molecular motion and chain-fold crystallization of polymers. (C) 2009 Elsevier Ltd. All rights reserved.