Uniform branched polyethylenes produced by metallocene catalysts can be subjected to a limited degree of intramolecular fractionation by crystallization or annealing at pressures of 495 MPa. Longer stem lengths in lamellae are more achievable by this process than by treatments at atmospheric pressure. Although the intervention of the hexagonal phase allows greater lamellar stem lengths to be achieved, the small proportion of longer methylene sequence lengths in the melt imposes limitations on the degree of isothermal thickening achievable. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1986–1996, 2005
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