Abstract
Morphological features of melt crystallized poly(9,9-di-n-octyl-2,7-fluorene) (PFO) were examined by means of polarized light microscopy, scanning electron microscopy, and transmission electron microscopy. Upon step-change of temperature from a nematic state at 250 °C to a supercooled melt at Tc = 140 or 145 °C, crystallization of PFO proceeds in 3 stages: (1) nucleation to form micrometer-sized leaf-like entities comprising fibrils of slender lamellae, (2) preferred longitudinal fibrillar growth and branching to give axialites, and (3) filling of the “eyes” (formed due to hindered transverse growth) via nucleation of new patches of locally aligned lamellae that are distinctively oriented from the existing axialitic structure. Results of selected-area electron diffraction indicate generally that PFO backbones tend to orient in-plane and perpendicular to the fibrillar axis as a result of preferred growth along the crystallographic b-axis. However, with decreasing film thickness, axialitic growth is inhibited; formation of facetted flat-on single crystals (to avoid increasingly unfavorable exposure of lateral surfaces) becomes a more competitive mode of crystallinity development.
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