Two kinds of ionic self-assembled complexes of linear or branched poly(ethyleneimine) (lPEI or bPEI) with Percec-type dendrons [(3,4,5)16G1-COOH] were prepared as lPEI-(3,4,5)16G1-x and bPEI-(3,4,5)16G1-x, where x is the mole ratio of the carboxyl groups of the dendritic amphiphile to the amino groups at the PEI chain. The crystal and mesomorphous structures and thermal properties of these complexes were investigated with X-ray diffraction (XRD), Fourier transform infrared (FTIR), differential scanning calorimetry (DSC), and polarized optical microscope (POM). Both the lPEI and bPEI complexes exhibited the same alpha(H) crystal phase and similar lamellar mesomorphous phase, irrespective of the branching of the polymer backbone and the binding degree. The lPEI series complexes lPEI-(3,4,5) 16G1-x, however, had more ordered lamellar stacking than that of the bPEI-(3,4,5) 16G1-x complexes, so the thermotropic liquid crystal phase SmA was formed only in the lPEI-(3,4,5) 16G1-x complexes beyond the melting point of the tail crystal of the dendritic amphiphile. No liquid crystalline phase was found from the bPEI-(3,4,5) 16G1-x complexes. The results suggest that the branching of polymer backbone plays a key role to the formation of thermotropic liquid crystal in the polymer-dendritic amphiphile complex. The present finding is significant for the design of functional nanostructures based on the ionic complexation of polymers and amphiphiles. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1012-1021, 2011