Direct Measurement of Interaction Forces between Adsorbed Polymer Layers

Abstract

Abstract The interaction forces between adsorbed layers of two graft copolymers were directly measured using surface force apparatus and atomic force microscopy. Two types of graft copolymers that were adsorbed on hydrophobic surfaces were used: (i) a graft copolymer consisting of poly(methyl methacrylate)/poly(methacrylic acid) backbone (the B chain) on which several poly(ethylene oxide) chains are grafted (to be referred to as PMMA/PEOn) and (ii) a graft copolymer consisting of inulin (linear polyfructose with degree of polymerization greater than 23) (the A chain) on which several C12 chains are grafted (INUTEC SP1). In the first case, adsorbed layers of the graft copolymer were obtained on mica sheets and the interaction forces were measured using the surface force apparatus. In the second case, the interaction forces were measured using atomic force microscopy (AFM). For this purpose, a hydrophobically modified glass sphere was attached to the tip of the cantilever of the AFM and the glass plate was also made hydrophobic. Both the sphere and the glass plate contained an adsorbed layer of INUTEC SP1. The curves of energy E(D) versus distance D for the graft copolymer of PMMA/PEOn between mica surfaces bearing the graft copolymer could be used to estimate the interaction energy between flat surfaces, by using Derjaguin approximation for cross cylinders. The results were compared with the theoretical calculations using de Gennes scaling theory. The agreement between experimental results and theoretical calculations was satisfactory. The same graft copolymer was used in latex dispersions, and the high frequency modulus G′∝ was measured as a function of the volume fraction φ of the dispersion. This high-frequency modulus could be related to the potential of mean force. In this way, one could compare the results obtained from rheology and those obtained from direct measurement of interaction forces. In the AFM method, the interaction forces are measured in the contact area between two surfaces, i.e., the surfaces of a spherical glass particle and a glass plate. The glass spheres and plates were hydrophobized using dichlorodimethylsilane. Results were obtained for adsorbed layers of INUTEC SP1 in water and in the presence of various concentrations of Na2SO4 (0.3, 0.8, 1.0, and 1.5 mol dm−3). All results showed a rapid increase in force with a decrease in the separation distance, and the forces were repulsive up to the highest Na2SO4 concentration. This explains the high stability of dispersions when using INUTEC SP1 as the stabilizer.