Effect of polyelectrolyte morphology and adsorption on the mechanism of nanocellulose flocculation

The effect of polyelectrolyte morphology, charge density, molecular weight and concentration on the adsorption and flocculation of Microfibrillated Cellulose (MFC) were investigated. Linear Cationic Polyacrylamide (CPAM) and Branched Polyethylenimine (PEI) of varying charge density and molecular weight were added at different dosages to MFC suspensions. The flocculation mechanisms were quanti-fied by measuring gel point by sedimentation, and floc size, strength and reflocculation ability through Focussed Beam Reflectance Measurements. Polymer adsorption was quantified through zeta potential
and adsorption measurements using polyelectrolyte titration. The flocculation mechanism of MFC is shown to be dependent on polyelectrolyte morphology. The high molecular weight branched polymer, HPEI formed rigid bridges between the MFC fibres. HPEI had low coverage and negative zeta potential at the optimum flocculation dosage, forming flocs of high strength. After breaking of flocs, total reflocculation was achieved because the high rigidity of polymer did not allow reconformation or flattening of the polyelectrolyte adsorbed on MFC surface. The lower molecular weight branched polymer, LPEI (2 kDa)
showed rapid total deflocculation, complete reflocculation and had maximum flocculation occurring at the point of zero charge. These characteristics correspond to a charge neutralisation mechanism. However, if the flocculation mechanism was purely charge neutralisation mechanism, the minimum gel point would be at the point of zero charge. Since this is not the case, this difference was attributed to the high polydispersity of the commercial LPEI used, allowing some bridges to be formed by the largest molecules, changing the minimum gel point. With the linear 80% charged 4 MDa CPAM, bridging mechanism dominates since maximum flocculation occurred at the minimum gel point, negative zeta potential and low coverage required for maximum flocculation. Reflocculation was not possible as the long linear polymer reconformed on the MFC surface under a flat conformation. Flocculation with the linear 50% charged 13 MDa CPAM happened by bridging with the minimum gel point and maximum flocculation corresponding to roughly half polyelectrolyte surface coverage on cellulose.