Periodic extinction bands composed of all flat-on lamellae in poly(dodecamethylene terephthalate) thin films crystallized at high temperatures

Using in-house synthesized poly(dodecamethylene terephthalate) (P12T) as a model, periodic extinction-banded spherulites melt-crystallized at high T_cs (100–115 °C) are expounded in terms of growth mechanism. The extinction-banded spherulites wildly differing from the usual blue/orange double ring-banded spherulites are composed of all flat-on discrete single-crystalline lamellae packed like roof shingles (or fish scales) along the circularly curved bands and the lamellae in the extinction bands are flat with a lozenge shape with no continuous twisting at all. For P12T films of more than 10 µm crystallized at T_c = 105–115 °C, no periodic bands were seen, and all spherulites were ringless, where periodic growth precipitation of crystals to extinction does not occur until impingement. Extinction bands in the P12T spherulites with the inter-ring spacing steadily decrease with decreasing film thickness, because for thinner films (submicrons to 2 µm), draining or depletion of available molten species takes place more frequently, leading to bands of smaller inter-ring spacing. The petal-like extinction bands are discussed and analyzed in detail using 3D AFM imaging.