Cellulose
Axial stiffness of the cellulose crystal
have only one intramolecular H-bond per glucose (Hayakawa et al. 2017), although this conformation does permit a bifurcated O3H3 O6 H-bond as reported for Me-cellobioside (French 2012). Based on literature values, cellulose II and III I are generally 10-40% less stiff than cellulose I, athough experimen- tal variability makes direct comparisons difficult. This variation was reproduced in MD simulations of all three allomorphs (Djahedi et al. 2015). However, in the only experimental study that compared elastic moduli of all three allomorphs using similar condi- tions it was concluded that the stiffnesses of cellulose I andIII I were similar, while it was lower for cellulose II (Ishikawa et al. 1997).
Cellulose has an axial elastic modulus which is often compared to that of steel or Kevlar (Moon et al. 2011). The many hydrogen bonds within and between cellulose chains have been proposed to be important for the mechanical properties of cellulose crystals and highly crystalline cellulose materials. Specifically, it has been suggested that the intramolecular trans- glycosidic H-bonds (O3HO3 O5 and O2HO2 O6 in cellulose I b ) contributes to the axial stiffness of cellulose, since they have a large component in the chain direction. Quantum mechanics (Santiago Cin- tro´n et al. 2011), molecular mechanics modeling(Eich- horn and Davies 2006) and MD simulations(Wohlert et al. 2012; Wu et al. 2013) are generally fairly successful in reproducing the experimental axial crystal modulus of 138 GPa (Nishino et al. 1995). Thus, computer modeling is relevant and suitable for investigating the contribution of H-bonds to the stiffness, both regarding nature and extent. As mentioned above, the intramolecular O2H2 O6 H-bond is on average broken in glucan chains residing in surfaces exposed to water. Owing to the 2 1 -fold symmetry, this means that surface chains have, on average, 1.5 intramolecular H-bonds per glucose unit, as opposed to 2 in the crystalline core. Thus, if H-bonds contributed significantly to the axial modulus, one would expect a lateral size dependence of the stiffness, since the proportion of surface to core becomes smaller as the fibril cross sections become larger. A nano-scale three-point-bending experiment using atomic force microscopy (AFM) on bacterial cellulose fibrils between 35 and 90 nm thick did not detect any difference (Guhados et al. 2005), but that range was probably too small to see any such effect. Moreover, the measured moduli were low, around 76 GPa, which could indicate large contributions from non-crystalline material within their samples. How- ever, also in MD simulations using fibrils of lateral size between 2.3 and 6.8 nm (Wohlert et al. 2012), no such size dependence was found. Cellulose II and cellulose III I are both different from native cellulose with respect to their dominating H-bonding pattern. Due to that the hydroxymethyl group has rotated with respect to the conformation in cellulose I (from tg to gt , see Fig. 2), the trans- glycosidic hydrogen bond to the hydroxyl in position two is no longer possible. Therefore, these allomorphs
Fig. 4 A molecular scale leverage effect, proposed by Altaner et al. (Altaner et al. 2014), amplifies the relative contribution of the O3H3 O5 H-bond to the axial stiffness of cellulose (top). MD simulations show that the O3H3 O5 bond is stretched during axial deformation, whereas the O2H2 O6 bond remains unchanged (bottom, from Djahedi et al. (2016))
123
Made with FlippingBook Online document maker