Morphology, lamellar thickening, and the transition between the crystal and the hexagonal columnar phase were studied in 1,4-trans-polybutadiene (1,4-t-PBD) by optical and electron microscopy, time-resolved synchrotron X-ray diffraction, and DSC. It is shown for the first time that 1,4-t-PBD can form several thousand angstrom thick lamellae of the extended or nearly extended chain type. These may form either by direct growth from the melt or through annealing of thinner chain-folded lamellae, provided they are in the columnar phase. Lamellar thickness was found to increase by several hundred angstroms in solution-crystallized mats immediately upon heating above the crystal-columnar transition, which contrasts with some previous reports on single crystals of 1,4-t-PBD. In addition, a unique phase reversal was observed: the low-temperature crystal phase reappears spontaneously on annealing above the temperature of the initial transition to the high-temperature columnar phase. © 1992, American Chemical Society. All rights reserved.