Cationic polyelectrolytes play an important role in papermaking as they improve the retention of fines and added mineral fillers. They have also been used to increase fiber interactions in fiber/fiber joints to increase paper strength. In earlier investigations, cationic starch (Lindström and Florén
1984; Lindström et al.
2005), polyallylamine (Rathi and Biermann
2000), and polyamideamine epichlorohydrin (Enarsson and Wågberg
2007) resins were used to enhance the paper strength by adsorbing these polyelectrolytes onto the fibers before sheet preparation. The negative charges located on fibers originate from residual hemicelluloses and the oxidation of cellulose or lignin during cooking, and bleaching of the fibers promotes a good interaction between the fibers and cationic polyelectrolytes (Wågberg and Annergren
1997). The adsorption of cationic polyelectrolytes on anionic surfaces is due to an entropically driven ion-exchange process (Fleer et al.
1993; Stuart et al.
1991; Trout
1951; Winter et al.
1986; Wågberg et al.
1987) where the counter-ions to the charges on the polyelectrolytes and the fibers are released upon adsorption. Wågberg et al. investigated the adsorption of cationic polyelectrolytes onto cellulose-rich fibers and the effects of the porous structure of the cell wall (with pores ranging from 1 to 30 nm) (Wågberg and Hägglund
2001), and it has also been shown that the adsorbed amount (i.e., mg/g fiber) is highly dependent on the molecular mass of the polymer (Alince
1990; Haggkvist et al.
1998; Stone et al.
1968; van de Ven and Alince
1997; Wågberg and Hägglund
2001). Earlier works have shown that polyelectrolytes with different molecular weights can penetrate the fiber wall either through a reptation process or through simple diffusion (van de Ven
2000; Wågberg and Hägglund
2001; Wågberg et al.
1988). Strong polyelectrolytes with medium to high molecular mass adsorbed onto the external fiber surface with a flat conformation at low to medium salt concentrations and do not obstruct further adsorption into the fiber wall (Wågberg and Hägglund
2001). It has also been shown that the saturation adsorption of weak cationic polyelectrolytes used in paper making, such as polyethylenimine (PEI), onto cellulose fibers and silica is significantly affected by the molecular weight, concentration of solution, pH and salt concentration used during the adsorption (Alince et al.
1996; van de Ven
2000). HM
w-PEI exhibits a two-step adsorption onto porous cellulose fibers in which PEI is first adsorbed to the external surface of the fiber and then in the second step, the adsorption of the low molecular mass fraction of PEI continues to the internal parts of the fiber wall through diffusion. At high PEI addition level (i.e., 40 mg/g fiber) the amount of LM
w fraction is also high which as a consequence leads to an increased adsorbed amount of PEI. Van de Ven showed that at pH 10 the percentage of PEI that reached into macro pores of the fiber wall significantly increased compared to the adsorption at pH 6 (van de Ven
2000). This behavior can be explained by the decrease in PEI size at high pH and also by the lower interaction between the cellulose and the PEI due to the decreased charge of the PEI at pH 10 (Horvath et al.
2008). The addition of electrolyte reduces the repulsion between the charged groups of PEI and allows for a contraction of the polymer chain which can also allow for a better access to the interior of the fiber for the polyelectrolyte. The increased electrolyte concentration also decreases the repulsion between adsorbed polymers on the fiber surface which hence also allow for an increased adsorbed amount at higher salt concentrations (Xie et al.
2016). It is also well documented that the adsorption of PEI onto nanoporous structure cellulose-rich fibers and microporous substrates decreases with increasing the molar mass of the polymer (Alince
1990). It has been suggested that such behavior is due to the saturation of the available charges throughout the fiber wall and due to the penetration of the low molar mass fraction PEI at the lower addition levels (Alince
1990). It has been shown that the external part of the fiber wall plays a very important role in creating strong joints between adjacent fibers and that treatment of the external part of the fibers is therefore essential for optimizing the use of modifying chemicals (Lindström et al.
2005). In addition to the treatment of fiber surfaces with single layers of polyelectrolytes and chemical modification of fiber wall (e.g., carboxymethylation (Wågberg et al.
2008)), polyelectrolyte multilayers have also been used to alter the chemistry of the external region of the fiber wall. Polyelectrolyte multilayers are formed by a technique called layer-by-layer (LbL) assembly, where the substrate is treated by consecutively alternating the adsorption of cationic and anionic polyelectrolytes with an intermediate rinsing step between each adsorption (Decher
1997). Wågberg et al. (
2002) have studied how such multilayer formation on fibers enhances the dry and wet strength properties of paper made from the modified fibers. Further studies have shown that LbL treatment of fibers can enhance the specific joint strength by increasing the molecular interaction in the fiber–fiber joint (Eriksson et al.
2006; Torgnysdotter and Wågberg
2004). Marais et al. (
2014) investigated the build-up of cationic PEI and colloidally stable anionic cellulose nanofibrils in order to study the mechanical properties of paper sheets prepared from treated fibers and showed that the outer layer of the thin film was an important parameter. The LbL deposition onto fibers has also been exploited to impart functionalities such as hydrophobicity (Gustafsson et al.
2012) and electrical conductivity, (Agarwal et al.
2006) and it has been shown that modified fibers can indeed be used for the preparation of new and advanced products. In addition to the poor wet strength of paper and the moisture sensitivity of the cellulose fibers, the highly flammable character of cellulosic fibers obtained from wood is a significant drawback which limits the application of paper-based materials in advanced applications. Paper-based materials are nevertheless used in a large variety of applications such as packaging products and wallpaper, although their flammable character is a possible risk for ignition and/or propagation of fire. It has however been suggested that multilayer formation of polyelectrolytes and/or nanoparticles using the LbL assembly technique may be an alternative flame-retardant treatment for substrates such as cotton (Li et al.
2009,
2010; Malucelli et al.
2014), polyurethane foam (Laufer et al.
2012), plastic thin films (Alongi et al.
2014; Apaydin et al.
2013), polyethylene terephthalate (PET) fabrics (Carosio et al.
2011,
2012) and recently also for cellulose rich fibers (Koklukaya et al.
2015) and cellulose nanofibril-based wet-stable aerogels (Koklukaya et al.
2017). We have recently demonstrated that it is also possible to use the LbL technique to form multilayers on cellulose-rich fibers and reduce their inherent flammability. The use of intumescent thin films (20BL) consisting of chitosan and poly(vinylphosphonic acid) resulted in self-extinguishing behavior in horizontal flame testing (Koklukaya et al.
2015). However, the large number of layers needed to impart flame-retardant properties to the fibers with this chemical strategy would negatively affect the up-scalability of this strategy. It was, therefore, essential to develop an alternative strategy to achieve similar properties with much fewer layers.