Carboxymethyl cellulose on a fiber substrate: The interactions with cationic polyelectrolytes

High and low molecular weight (M _w) carboxymethyl celluloses (CMC) were adsorbed on a well-characterized fiber substrate (long fibers of a commercial bleached birch kraft pulp with the carboxylic acid groups in Na-form) to increase the charge of the fibers in a controlled fashion. The M _w played a role in the utilization of CMCs as a strength additive in paper sheets nearly doubling the tensile strength with the high M _w CMC. Swelling properties of the CMC treated fibers were measured with water retention value (WRV). The WRV increased more with the high M _w CMC. The swelling was further tuned by two highly cationic polyelectrolytes; high M _w poly(diallyldimethyl ammonium chloride) (PDADMAC) and low M _w polybrene (hexadimethrine bromide, [3,6]-ionene). They were chosen because of their known ability to neutralize the anionic charge either exclusively on the surface or in the whole fiber, respectively. Adsorption of PDADMAC could reduce WRV of the CMC pre-treated fibers to the level of the untreated reference, while polybrene adsorbed pulps with 3-10 times more cationic polyelectrolyte deswelled the fibers only slightly more than the surface neutralized fibers. These results indicated surface conformation differences with low and high M _w CMCs. While the conformation did play a role after physical alteration (drying and rewetting) of the fibers, the paper sheets produced from these fibers showed remarkable differences. In extreme cases, the strength of the paper could be retained after drying (low M _w CMC + PDADMAC) or paper, resistant to disintegration, could be achieved (CMC + polybrene).