Cellulose forces, between chains

The small change in stereochemistry between cellulose and amylose creates a large difference in their overall shape and in their properties. Some of this difference can be seen in the strorcture of a short portion of fflnylose in Figure 25.9. The presence of the a-glycosidic linkages imparts a twist to the fflnylose chain. Where the main chain is roughly linear- in cellulose, it is helical in anylose. Attractive forces between chains are weaker in fflnylose, and fflnylose does not form the same kind of strong fibers that cellulose does. 

Cellulose is a polymer composed of glucose units linked by J-l,4-glycosidic bonds (Fig. 1). Its linear structure is strengthened by hydrogen bonding and van der Waal s forces between chains, resulting in a crystalline structure [27],... 

As a result of these studies it became evident that mechanical and chemical treatments may alter the relative amounts of crystallized and amorphous cellulose in a sample. For example, it was estimated that a normally coagulated viscose filament was about 40 % crystalline and 60 % amorphous while filaments of the same material, after being stretched, appeared to be 70% crystalline and 30% amorphous. This effect was observed to be more pronounced in cellulose derivatives than in unsubstituted cellulose where apparently the blocking of hydroxyl groups reduces the lateral forces of cohesion between chains, facilitates slipping and consequently promotes parallelization of chains. 

The chains form a layer in the a-c crystallographic plane, where they are held together by hydrogen bonds from 0(3) in one chain to 0(6)H in the other. There are no hydrogen bonds in cellulose I between these layers, only weak van der Waal s forces in the direction of the b-axis. Native cellulose therefore has a chain lattice and a layer lattice at the same time. 

The cellulose chains in adjacent layers are in strictly ordered positions relative to one another despite the forces between the layers being comparatively weak. The cellulose chains in adjacent layers are staggered laterally by a distance of a/2 and are shifted axially by about a quarter of the unit cell dimension, 0.275c (Figure 2.3). 

Cellulose esters and ethers also give fibrillar crystal structures of the same type. The cohesive forces between the chains are, however, weaker than in cellulose. Cellulose triacetate and triethyl-cellulose, for instance, show a melting point, but melting can not be accomplished without a marked breakdown of the chains. According as the substituents themselves are larger and their polarity decreases, the melting point of the derivatives becomes lower, and their plasticity increases. This becomes clearly apparent if the properties of the triesters of cellulose of the homologous series of the normal fatty acids are compared. ... 

The recovery of the cellulose surfaces between consecutive force measurements was found to require at least five minutes. This is related to the previous publications (77,78) that reported on the effect of the approach speed and relaxation of the cellulose chains, as well as hydrodynamic effects involved. The slow recovery of the cellulose beads between force runs may also explain why we in this work observed adhesion between cellulose surfaces at pH 10 in contrast to previous observations (77). 

Although limited hydrogen bonding occurs between molecules within acetates, triacetate is incapable of forming hydrogen bonds. Van der Waals forces caused by the interaction of adjacent acetate and triacetate chains are the major associative forces between the acetylated cellulose molecular chains. The average number of acetylated anhydroglucose units in chains of acetates and triacetates usually varies between 250 and 300 units. 

In cellulose II with a chain modulus of 88 GPa the likely shear planes are the 110 and 020 lattice planes, both with a spacing of dc=0.41 nm [26]. The periodic spacing of the force centres in the shear direction along the chain axis is the distance between the interchain hydrogen bonds p=c/2=0.51 nm (c chain axis). There are four monomers in the unit cell with a volume Vcen=68-10-30 m3. The activation energy for creep of rayon yarns has been determined by Halsey et al. [37]. They found at a relative humidity (RH) of 57% that Wa=86.6 kj mole-1, at an RH of 4% Wa =97.5 kj mole 1 and at an RH of <0.5% Wa= 102.5 kj mole-1. Extrapolation to an RH of 65% gives Wa=86 kj mole-1 (the molar volume of cellulose taken by Halsey in his model for creep is equal to the volume of the unit cell instead of one fourth thereof). 

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