Crystal growth pattern changes in low molecular weight poly(ethylene oxide) ultrathin films

A low molecular weight (MW) poly(ethylene oxide) (PEO) crystallized in ultrathin films displays various crystal growth patterns in a crystallization temperature (Tx) range from 20.0 °C to 50.0 °C. In succession, the following patterns are found: nearly one-dimensional (1D) dendrite-like crystal patterns at Tx ≤ 38.0 °C, two-dimensional (2D) seaweed-like patterns between 39.0 °C ≤ Tx ≤ 42.0 °C and again, nearly 1D dendrite-like patterns at Tx ≥ 43.0 °C. These transitions result from a complex interplay of varying growth rates along different growth directions and preservation of growth planes. Structural analysis carried out via electron diffraction indicates that the dendrite-like crystals formed at the low and high Tx values differ by their fast growth directions: along the {120} normal at the low Tx values and along the (100) and (010) normal at the high Tx values. In the later case however, the major growth faces are still the {120}, this time tilted at 45° and indicating the a∗ and b axes growth tips. In the intermediate Tx range (39.0 °C–42.0 °C), three growth directions coexist giving rise to the seaweed morphology. The crystal growth rates at the low and high Tx values are constant versus time. For the seaweed, a square-root dependence is obtained. These differences are probably due to 1D and 2D growth in the ultrathin films and are associated with different growth patterns of the dendrites and the seaweed, respectively.