Cellulose Chain stiffness

Less organized (amorphous) cellulose is also present along with the crystalline cellulose. The crystalline forms la and 1(3 differ by their crystalline unit cell structure and overall hydrogen bonding pattern, but the main intermolecular hydrogen bond is the same for both, i.e., 06-H 03 (Fig. 21.2). The intramolecular hydrogen bond of 03-H 05, which is partly responsible for the cellulose chain stiffness and contributes to load transfer along the chain, is also shown in Fig. 21.2. Other crystalline forms of cellulose include cellulose 11, cellulose 111, and cellulose... 

Glucose molecules can link together into chains, with each ring tethered to the next by a bridging oxygen atom. In one form, this is cellulose, the stiff material that gives the stalks of plants and the trunks of trees their structural strength. Chitin, a variation on cellulose, is an even stiffen material that forms the exoskeletons of crustaceans such as crabs and lobsters. 

Since Robinson [1] discovered cholesteric liquid-crystal phases in concentrated a-helical polypeptide solutions, lyotropic liquid crystallinity has been reported for such polymers as aromatic polyamides, heterocyclic polymers, DNA, cellulose and its derivatives, and some helical polysaccharides. These polymers have a structural feature in common, which is elongated (or asymmetric) shape or chain stiffness characterized by a relatively large persistence length. The minimum persistence length required for lyotropic liquid crystallinity is several nanometers1. 

The primary factors governing mesophase formation for cellulose derivatives is not only chain stiffness, but also the type and degree of substitution, the molar mass of the polymer, as well as the solvent and the temperature [103]. Among the water-soluble cellulose biopolymers, HPC is still the most investigated derivative (it forms stable and easy to handle mesophases) and as such will... 

It is proposed in this paper that the molecular flexibility, or in the case of cellulose, the lack thereof, is one of the fundamental factors that determines the suitability of a polymer for use as a fiber. Another factor is the set of interactions between molecules in the crystalline material. Those interactions, which depend on the accessible molecular shapes, diminish solubility, especially if both hydrogen and hydrophobic bonds are formed, as in the case for the common conformation of cellulose. The relatively limited range of shapes (a definition of stiffness) not only keeps the cellulose chains in a conformation that retains the interchain attractions but also minimizes increases in entropy in solution. 

Sophisticated experimental methods allow the development of models for polymers in dilute and semi-dilute solutions. Chain stiffness may be represented by the Kuhn segment lengths and determined in dilute solution. Models for cellulose and cellulose derivatives have recently been published whose main features are the irreversible aggregation of chains, if hydrogen bonding is possible even in dilute solutions. Trisubstituted cellulose derivatives or cellulose in hydrogen bond breaking solvents exist as molecular dispersed chains. How-... 

The formation of liquid-crystalline phases of cellulosics, especially of the lyotropic kind, has yet to be explained. Certainly the chain stiffness may have to be taken into account as one of the factors in question, but the solvent-polymer interaetion may have to be considered as well. In the next section, models for the description of the pitch as a chiral property and models to... 

Cellulose and its derivatives have o values of about 2, i.e., thermodynamically they are about as flexible as poly(isobutylene). Thus, cellulose chains are not extraordinarily stiff, although they are often assumed to be so on the basis of their high exponents in the intrinsic viscosity-molar mass relationship (see Section 9.9.7). These high exponents are interpreted as arising from the particular (high) draining properties of the cellulose molecule. 

Raman spectroscopy has been applied to determine the stiffness (modulus) of CNCs and stress-transfer in CNCs-reinforced composites or biocomposites where reinforcing phase is too small to be characterized by using standard mechanical techniques. This technique involves the measurement of deformation (a shift in the carbonyl (C-O) mode of the cellulose chain) [96], Originally these shifts of Raman bands were reported for single crystals of polydiacetylene [97] and composites [98] followed by shifts reported for stressed regenerated cellulose fibers [99]. The Raman bands shift is the indication of molecular deformation and determine the extent of stress-transfer between reinforcing CNCs and matrix. The intensity of Raman band measures the orientation distribution of the nanocrystals in composites [96]. Recently, some researchers measured the stress-transfer behavior in microfibrillated cellulose-reinforced polylactic acid and cellulose nanowhiskers-reinforced epoxy-resin composites [96,100]. 

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