We report the observation of a labyrinthine crystal pattern with a periodic structure in the crystal width direction in ultrathin films of poly(ethylene oxide) fractions with molecular weight ranging from 25,000 to 932,000 g/mol. The polymer thin films are crystallized at temperatures well below the bulk melting temperature, thus the system is characterized by limited diffusion of the polymer chains and rapid growth of the crystal fronts. The competition of these two competing factors leads to the formation of the labyrinthine pattern. This mechanism is supported by a scaling relation between the long period and molecular weight, (L) over bar proportional to(M) over bar (-0.24 +/- 0.03)(W), indicating the importance of chain diffusion. Furthermore, a linear relationship between ln((L) over bar/(M) over bar (-0.24)(W))-Delta E/RTC and T-m(0)/T-C Delta T is observed, implying that the crystal growth is dominated by a secondary nucleation. (C) 2009 Elsevier Ltd. All rights reserved.