Crystal Pattern Formation and Transitions of PEO Monolayers on Solid Substrates from Nonequilibrium to near Equilibrium

Abstract

The crystal pattern formation and transition of the monolayers of a low-molecular-weight poly(ethylene oxide) ( M̄ n = 5000 g/mol) on the surface of silicon wafer were studied using atomic force microscopy with a hot stage in the temperature range 25 ≤ T c ≤ 59 °C. Our observations show that the crystal patterns are greatly dependent on the crystallization temperature, T c . At 25 ≤ T c < 39 °C a dendrite pattern with a fractal dimension D p = 1.672 ± 0.027 was found. At 39 < T c ≤ 48 °C the crystal pattern become seaweed with the same fractal dimension. The difference between dendrite and seaweed patterns can be quantitatively described by the angle between main and side branches. The dendrite-to-seaweed transition occurred at T c = 39 °C. At 54 ≤ T c ≤ 59 °C the crystals had a square shape. A dramatic change occurred at the transition temperature from 49 to 52 °C. In this temperature range the crystal pattern alters from seaweed to compact structure and finally to the square shape of single crystals. The possible formation mechanisms are discussed in the text.