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

Abstract

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.