Adsorption of cationic polyacrylamide at the cellulose–liquid interface: A neutron reflectometry study

Abstract

The layer thickness and density of high molecular weight cationic polyacrylamide (CPAM) adsorbed at the cellulose–water interface was quantified by neutron reflectometry. The thickness of a full monolayer of CPAM of constant molecular weight (13MD) but different charge densities, adsorbed with or without NaCl (10−3M), was studied. Thin cellulose films (40±7Å) of roughness <10Å were produced by spin coating a cellulose acetate–acetone solution and regenerating by alkaline hydrolysis. Film smoothness was greatly improved by controlling the concentration of cellulose acetate (0.13wt%) and the hydrolysis time in sodium methoxide. The adsorption thickness of CPAM (40% charge 13MD) at the solid-D2O interface was 43±4Å on cellulose and 13±2Å on silicon, an order of magnitude smaller than the CPAM radius of gyration. At constant molecular weight, the thickness of the CPAM layer adsorbed on cellulose increases with polymer charge density (10±1Å at 5%). Addition of 10−3M NaCl decreased the thickness of CPAM layer already adsorbed on cellulose. However, the adsorption layer on cellulose of a CPAM solution equilibrated in 10−3M NaCl is much thicker (89±11Å for 40% CPAM). For high molecular weight CPAMs adsorbed from solution under constant conditions, the adsorption layer can be varied by 1 order of magnitude via control of the variables affecting electrostatic intra- and inter-polymer chain interactions.