Interfacial reaction of highly-branched polyethyleneimine (PEI) with octadecanoic acid (OA) was performed to prepare a series of supramolecular complexes (PEI(OA)(x)). The complexes at solid state have typical lamellar structure, and the interlamellar distance can be modulated by the OA content. The long period values of the supramolecular complexes measured by SAXS were found to be dependent on the compositions, which are in good consistency with those measured by TEM. For example, the long period values from SAXS for PEI(OA)(0.76), PEI(OA)(1.03), and PEI(OA)(1.67) were 46.5, 62.7, and 56.2 Angstrom, respectively. The corresponding data from TEM were 45.7, 60.7, and 56.6 Angstrom, respectively. A model was proposed for the construction mode of the side alkyl chains (crystallization region) associated with PEI backbone (amorphous region), in which the side alkyl chains were arranged to be 'end-to-end' packing for the x = 1 complex, while an 'interdigitated structure' of the side alkyl chains was deduced for the x > 1 and x < 1 complexes. Temperature variable FT-IR combination investigation of the scissoring band, rocking band, and stretching band of methylene (CH2) and vibrational band of carbonyl group (C=O) indicated that the crystalline form of the crystallization region in the lamellae can be transformed from orthorhombic to hexagonal with the temperature increasing, and vice versa. (C) 2004 Elsevier Ltd. All rights reserved.