Fluoro-terminated hyperbranched poly(ether ketone) (FHPEK) was synthesized and its end groups were modified with alkyl compounds of different chain lengths, i.e., hexyloxy (C6), dodecyloxy (C12), and octadecyloxy, (C18), to produce alkyl-modified HPEKs (HPEK-C6, HPEK-C12, and HPEK-C18, respectively). Master curves were constructed by using the time-temperature superposition principle. The horizontal shift factors, a(T), used for the construction of the master curves were fit using the William-Landel-Ferry (WLF) equation. From the fitting parameters, the apparent activation energy, Ea, was estimated. With increasing alkyl chain length, the Ea values were found to decrease in the order FHPEK > HPEK-C6 > HPEK-C12, and then increase for HPEK-C18. The average relaxation time, tau(HN), was determined by fitting of the dynamic moduli G'(omega) and G '(omega) to the empirical Havriliak-Negami equation. Similarly, the tau(HN) values decreased in the order of FHPEK > HPEK-C6 > HPEK-C12, and then increased for HPEK-C18. This indicates that the endgroup modification with short alkyl chains (C6, C12) increased the molecular mobility due to the internal plasticization effect of these alkyl chains. Modification with the longer alkyl chain (C18) retarded the molecular motion through an antiplasticization effect caused by summation of nonpolar hydrophobic interactions between long hydrocarbon chains.(C) 2008 Wiley Periodicals, Inc.