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Cellulose (2018) 25:3781–3795
Adsorption experiments with QCM-D have previously shown that polyelectrolytes adsorb differently onto model surfaces depending on their charge density and molar mass (Tammelin et al. 2004; Kontturi et al. 2008). Adsorbed polyelectrolyte layers consist of the polymer, electrolyte (if present), and solvent that is imbibed inside the layer (Kontturi et al. 2008). The amount of water moving with the polyelectrolyte can be quite substantial, if the polyelectrolyte forms extended loops, trains, or tails. It was previously shown that adsorbed layers of CMC and these cationic starches onto nanocellulose surfaces contained between 52 and 66% of coupled water by weight at adsorption equilibrium (Strand et al. 2017). Cationic starches with high DS onto cellulose surfaces results in a significantly more rigid layer, compared to cationic starches with a low DS, because polyelectrolytes with high DS are more strongly attracted to surfaces of opposite charge. There are some similarities between adsorption of polyelectrolytes onto model surfaces and the creation of PECs, since both are driven by attraction of oppositely charged groups. Instead of adsorbing the polyelectrolyte onto an existing surface, it is adsorbed onto another polyelectrolyte in order to create a completely new colloidal phase. The FCM results indicate that similar rules apply in the formation of PECs as in the adsorption studies; A polyelectrolyte with higher charge density will have higher affinity to the oppositely charged polyelectrolyte, creating a more dense structure. In these experiments, both polyDADAMAC and CMC consisted of linear and highly charged polyelectrolyte chains, that resulted in FSC/SSC values as low as 1.1. The dense PEC structure was able to scatter light more effectively in SSC, and the increasing complexity of the particle structure caused less effective light scattering in FSC. A determined FSC/SSC value approaching 1.0 indi- cated that quite dense PEC particles were formed. It has previously been reported that higher structure density of PECs was obtained when CMC of DS 1.3 was mixed with amine-epichlorhydrin resin (PAE), compared to when CMC of DS 0.7 was used (Gernandt et al. 2003). Different models for explaining the degree of PEC particle swelling/density as a function of polyelectrolyte chain length and charge density have been theorized upon, and it was shown that the density of the PEC core is influenced by
polyelectrolyte charge density (Dautzenberg 1997; Mende et al. 2002). In mixtures of cationic starches and CMC, the FSC/ SSC values indicated that the PECs were less dense than the PECs formed with polyDADMAC (Fig. 6). The determined FSC/SSC values increased with decreasing charge density of the polycations, indicat- ing that less dense PECs were formed with the cationic starches of lower charge density. It has been reported that large differences in charge density between the polycation and polyanion may lead to less dense, and more swollen PEC structures (Dautzenberg 1997). The cationic starches originated either from potato or waxy maize, that contain about 77 and 98% of branched amylopectin respectively (Fredriksson et al. 1998). Less well-ordered packing of the cationic starches in the PEC particles would be expected, compared with the linear and highly charged polyDADMAC. FCM analysis of various particle types was per- formed in order to gain more insight about FSC/SSC values. The available particles were kaolin, scaleno- hedral and prismatic precipitated calcium carbonates (PCC s, PCC p), ground calcium carbonate (GCC), three different bentonites, cellulose nanocrystals (CNC), and cellulose nanofibrils (CNF). The FCM analyses were performed in similar fashion, and the calculations were performed in the same way as for the PECs. Theoretically, solid particles should have FSC/ SSC values close to 1.0 in the FCM setup. This was seen for both of the PCC particle types, GCC, and kaolin (Table 4). The bentonites, which are known to swell in water, had FSC/SSC values between 2.8 and 4.6. The FSC/SSC of cellulose nanocrystals, which are known to be dense and crystalline cellulose particles,
Table 4 Determined FSC/ SSC values for a variety of different inorganic and organic particles analyzed by FCM
Particle type
FSC/SSC
PCC p
1.0
PCC s
1.1
GCC
1.3
Kaolin 1.4 Bentonite A 2.8 Bentonite B 3.2 Bentonite C 4.6 CNC 1.4 CNF 6.6 PECs
The reported FSC/SSC values are not absolute for each particle type, and may vary with various particle properties to some extent
1.1–8.7
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