Molecular weight dependence of crystal pattern transitions of poly(ethylene oxide)

Crystal patterns in ultrathin films of six poly(ethylene oxide) fractions with molecular weights from 25000 to 932000 g/mol were characterized within crystallization temperature range from 20 °C to 60 °C. Labyrinthine, dendritic and faceted crystal patterns were observed in different temperature ranges, and then labyrinthine-to-dendritic and dendritic-tofaceted transition temperatures T _L-D and T _D-F were quantitatively identified. Their molecular weight dependences are T _L-D(M _w) = T _L-D(∞) ? K _L-D/M _w, where T _L-D(∞) = 38.2 °C and K _L-D = 253000 °C·g/mol and T _D-F(M _w) = T _D-F(∞) ? K _D-F/M _w, where T _D-F(∞) = 54.7 °C and K _D-F = 27000 °C·g/mol. Quasi two-dimensional blob models were proposed to provide empirical explanations of the molecular weight dependences. The labyrinthine-to-dendritic transition is attributed to a molecular diffusion process change from a local-diffusion to diffusion-limited-aggregation (DLA) and a polymer chain with M _w ≈ 253000 g/mol within a blob can join crystals independently. The dendritic-to-faceted transition is attributed to a turnover of the pattern formation mechanism from DLA to crystallization control, and a polymer chain with a M _w ≈ 27000 g/mol as an independent blob crosses to a depletion zone to join crystals. These molecular weight dependences reveal a macromolecular effect on the crystal pattern formation and selection of crystalline polymers.