Hyper-branched quaternary ammonium salts (HQAS) were synthesized by condensation polymerization between quaternary ammonium salt (QAS) and the monomer we prepared. The chemical structure, molecular weight, and thermal properties of these salts were determined by Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), ultraviolet spectra (UV), nuclear magnetic resonance (NMR), and thermogravimetric analysis (TGA). The onset and center temperatures of decomposition (7(-5%) and T-max) were improved from 113 degrees C and 270 degrees C, to 191 degrees C and 275 degrees C, respectively. Sodium-montmorillonite (Na+-MMT) was modified with these two types of intercalation agents, QAS and HQAS, via ion-exchange reactions. The interlayer space, interlamellar structure, thermal stability, and surface morphology of these organic montmorillonites (OMMTs) were investigated by FUR, TGA, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The HQAS modified montmorillonites had expanded interlayer space and possessed good thermal stabilities due to steric hindering and thermal stability of the hyper-branched polymers. The basal spacing of QAS modified samples was 1.6-4.9 nm, indicating a monolayer, bilayer, and pseudo-trilayer arrangement of quaternary ammonium ions in the interlayer space. The HQAS modified samples had basal spacing ranging from 1.9 to 2.5 nm, and the intensive degree was much decreased. These experimental results and phenomena indicated bilayer and pseudo-trilayer arrangements of hyper-branched molecular chains, together with some exfoliated silicate layers. (C) 2011 Published by Elsevier B.V.