SEC-MALS study on carboxymethyl cellulose (CMC): Relationship between conformation of CMC molecules and their adsorption behavior onto pulp fibers

Despite that CMC has anionic charges as well as pulp fibers, CMC can be adsorbed onto pulp fibers. The CMC adsorption ratio varies, depending on the degree of substitution (DS) of CMC and electrical conductivity of pulp slurries. Because the CMC adsorption introduces new anionic sites onto pulp fiber surfaces, efficiency of wet end additives and the resultant paper properties can be improved. In order to understand the CMC adsorption behavior in terms of CMC molecular conformations in aqueous solutions, the SEC-MALS (size-exclusion chromatography equipped with a multi-angle laser light scattering detector) method was applied to three CMC samples with different DS values. The effects of DS of CMC and electrical conductivity (electrolyte concentration) of aqueous solutions on CMC molecular conformations were investigated. The relation between molecular mass and radius of gyration of CMC obtained by the SEC-MALS analysis revealed that CMC molecules tended to have compressed conformations or to form aggregates in aqueous solutions with decreasing the DS and increasing the electrical conductivity. These conditions are consistent with those where CMC molecules are preferably adsorbed on pulp fibers. Moreover, it was found that CMC molecules had high affinity to each other to form aggregates under such conditions. The obtained results in the fundamental analyses of CMC are explainable for our success in the application of the CMC addition at the wet end in practical papermaking process.