Systematic Control of Self-Seeding Crystallization Patterns of Poly(ethylene oxide) in Thin Films

Using optical microscopy and atomic force microscopy, we studied systematically crystallization patterns in thin films of a low molecular weight polyethylene oxide (PEO) resulting from a kinetically controlled self-seeding approach. In particular, the influence of seeding temperature (Ts) and heating rate (Vh) on the various resulting crystallization patterns was investigated. Crystallization at 49 °C resulted in dendritic PEO crystals consisting of almost exclusively twice-folded chains. Upon heating these crystals, we observed crystal thickening due to a reduction in the average number of chain folds. Based on the detected morphology, we deduced that the density of seeded PEO crystals decreased when increasing Ts from 54 to 57 °C. At the highest Vh (i.e., 100 °C/min), only a few well-separated faceted single crystals of PEO were grown from individual seeds. In contrast to such random distribution of crystals, because of a faster reduction of chain folds at the edges of PEO lamellae, an almost continuous sequence of seeded crystals was formed at the periphery of the original crystals at significantly lower Vh (i.e., 10 °C/min). Interestingly, reflecting the different metastable states within the initial crystal resulting from seeding at Ts = 54 °C, the seeding probability for crystals at the diagonals was higher than for the major side branches. In addition, we estimated activation energies (213-376 kJ/mol) for thickening of PEO lamellar crystal from an Arrhenius-type behavior of the lateral spreading rates as a function of Vh. Our findings suggest that the interplay between thickening and melting of metastable states within the initial crystals are considered is responsible for the resulting nucleation density and crystal morphology induced by self-seeding.