Structure of bridging polymers

A four-component clay-polymer-salt-water system, consisting of n-butylammon ium vermiculite, poly-(ethylene oxide) (PEO), n-butylammonium chloride, and heavy water was studied by wide-angle neutron diffraction. The one-dimensi onal nature of the system makes it ideal for model studies of colloids. Pre vious results had shown that if the size of the polymer, given as the end-t o-end distance, is larger than the spacing between the clay platelets the p olymer chains bridge between the platelets and draw them together. In order to further investigate the bridging mechanism we have studied the structur e of the polymer chains using H/D isotope substitution of the polymer. The volume fractions of clay r and PEO nu were r=0.05 and nu=0.04, respectively , and the salt concentration c was 0.1 M. The results indicate that a signi ficant part of the polymer chains are directly bound to the clay surfaces, and that the remaining (at least 50%) polymer segments have a Gaussian-like distribution in the middle between the clay layers. Previous studies of th e corresponding three-component system without added polymer had shown the surface to be covered with two layers of water molecules. The new results s how that the ethylene oxide segments displace water molecules immediately a djacent to the clay surfaces, bonding directly to them by physical adsorpti on. Due to the polymer induced reduction of the interlayer spacing it is li kely that each polymer chain is adsorbed on both clay surfaces. The finding s are consistent with our newly proposed model for polymer bridging floccul ation. (C) 1999 American Institute of Physics. [S0021-9606(99)51118-5].