The rigid polymer poly(p-phenylene benzobisthiazole) (PBZT) was crystallize
d from dilute solution. Electron microscopy showed that, on quenching, flat
fibrils several nanometers thick were produced. Subsequent heat treatment
in a solvent changed the morphology from fibrillar into "segmented ribbon"
structure. Isothermal crystallization at a temperature of about 30 degreesC
below the dissolution temperature, in general, resulted in aggregation of
rod crystals. The polymer chains were oriented normal to the rod crystals.
The width of the rod crystal increased with average molecular length, but s
aturated to a value much smaller than the average molecular length. In the
shorter molecular length range, the rod crystals clustered in a fanned-out
manner, while with a medium molecular length (ca. 70-120 nm), all rods crys
tals in a cluster aligned parallel to each other and were of the same lengt
h. With longer molecular length (more than ca. 180 nm), the rod growth slow
ed because of small diffusion constants of molecular chains to the growing
face. Based on observation of the morphology and the crystallization proces
s, an isothermal crystallization mechanism is proposed. Because of the rigi
dity and wide length distribution of polymer chains, the chain ends were in
evitably included within the crystals, resulting in crystal defects such as
axial shifts, lattice curvatures, and edge dislocations, all of which were
observed directly by lattice imaging.