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1948). The type and amount of salt added to the solvent water has a negative impact on IWWS, similar to the findings of Grignon for dry paper (Belle et al. 2014a; Grignon and Scallan 1980).
‘‘dangling tails’’ on the fiber surface in Fig. 6 that have a length about 60–80 nm (Neuman 1993).
Internal hydrogen bonds
Internal hydrogen bonds (H-bonds) play a key role with regard to the intermolecular forces within the cellulose. On the one hand the intermolecular H-bonds bind the cellulose chains together and contributes to the lateral strength of the fiber, on the other hand the intramolec- ular H-bonds contributes to the axial stiffness of the cellulose molecules. These bonds are weakened by the adsorption of water and results in swelling of the fibers (Linhart 2005). The correlation between the Young’s modulus of the fibers and their internal hydrogen bonds is described in detail in the literature (Nissan and Batten 1990; Zauscher et al. 1996, 1997). However, these papers mainly focus on correlations to dry paper. Furthermore, the strong influence of water on the fiber– fiber bonds in paper (Hubbe 2006; McKenzie 1984) has led to a thermodynamic examination of fiber–fiber bond formation (Wa˚gberg 2010).
Fiber surface
The hypothesis of ‘‘dissolved fiber surfaces’’ was developed in the middle of the twentieth century. This hypothesis assumes that the surfaces of the cellulose fibers partially solute in water and diffuse into each other during sheet formation (Campbell 1930; Casey 1960; Clark 1978a). This approach was later expanded with the explanation that the reduced end groups of the cellulose form a kind of molecular fibrillations that are solvated or partially soluted in water. As a result, the molecular fibrillation rise up, leading to improved availability for bond formation (Clark 1978a). The assumption is that the wetted fiber is surrounded by water and the fibers and fibrils approaching each other during dewatering to such an extent that at first van der Walls bonds and with further drying hydrogen bonds can form. In this regard, Clarks’ theory emphasizes the high bonding capacity of hemicelluloses. For materi- als with a high percentage of hemicelluloses, there is a relatively high proportion of short molecules, which are more active in bond formation than large molecules when present in an easily accessible, upright state (Clark 1978b). McKenzie developed an additional model based on the adhesion between two plasticized surfaces in respect to Voyutskij’s theory about autohesion and adhesion for high polymers (McKenzie 1984; Voyutskij 1963a). It is assumed that in the intermediate area of two fibers in a plastic state, the micro and macro fibrils are close enough to form molecular alignments (Pelton et al. 2000). Neumans surface force measurements are consistent to these hypothesis and lead to the schematic representation of
Measurement techniques
Various technologies have been used to characterize surfaces and measure surface forces at the nanometer level, such as atomic force microscopy (AFM) (Gustafsson et al. 2003; Huang et al. 2009; Koljonen et al. 2003; Leporatti et al. 2005; Paananen 2007; Stenius and Koljonen 2008) and scanning electron microscopy (SEM) (Belle et al. 2015a, 2016; Heine- mann et al. 2011; Pye et al. 1965; Tejado and van de Ven 2010; Washburn and Buchanan 1964). These results must be evaluated taking into account the fact that the AFM is in contact with the scanned surface. As a result, AFM can disturb the sensitive fiber surface. In contrast, the SEM has limited resolution when imag- ing wet samples due to the vacuum required for
Fig. 6 Outline of Neuman’s dangling tail model (Neuman 1993); (Reprinted with permission of The Pulp and Paper Fundamental Research Society)
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