A chain of glucose
Take a glucose molecule, remove water and attach it to the next one. Repeat thousands of times and you have cellulose. Each unit is turned by 180°, so the chain is straight, like a ribbon.
Anhydroglucose units are joined by β-(1→4) glycosidic bonds. Each unit carries three hydroxyl groups: at C2 and C3 (secondary) and C6 (primary). The repeating unit, cellobiose, is 1.03 nm long. One chain end carries the reducing unit (hemiacetal at C1), the other the non-reducing end.
Chain length
In wood a cellulose chain holds around ten thousand glucose units. Pulping and bleaching break chains, so in paper and viscose they are much shorter.
The degree of polymerisation (DP) of native wood cellulose is estimated at around 10 000. Chain molar mass is 162.14·DP + 18.02 g/mol, where 162.14 is the mass of an anhydroglucose unit. Cellulose is polydisperse, so DP is always an average, and the method (viscometry, size exclusion chromatography) affects the result. Chemical processing lowers DP several-fold.
Hydrogen bonds and crystallinity
Hydroxyl groups of neighbouring chains grab each other with hydrogen bonds. Chains pack into tight bundles that water cannot pull apart. That is why a cotton shirt gets wet but does not dissolve.
The intramolecular O3-H···O5 bond stiffens the chain; intermolecular bonds (notably O6-H···O3) join chains into sheets, and sheets stack through hydrophobic interaction. Native cellulose has the crystalline form cellulose I; Iβ dominates in higher plants, Iα in algae and bacteria. Mercerisation or dissolution and regeneration give the thermodynamically more stable cellulose II. Measured crystallinity of wood cellulose depends strongly on the method.
Microfibrils
Chains never lie alone. About two dozen parallel chains form a thin thread, a microfibril, about 3 nm across. The cell wall is woven from these threads.
Current evidence puts a wood microfibril at roughly 18-24 chains and about 3 nm across, with more and less ordered regions along it. Microfibrils aggregate into macrofibrils, with hemicelluloses and lignin between them.
Reactions and derivatives
Acid or enzymes can break cellulose down to glucose. It can be dissolved and spun into a thread again, which is how viscose is made. Hang new groups onto the hydroxyls and you get acetate for films, nitrate for smokeless powder and lacquers, and thickeners for food.
The glycosidic bond is hydrolysed by acids and enzymes (cellulases). Hydroxyl groups undergo esterification (cellulose acetates, nitrates) and etherification (carboxymethyl cellulose, methyl cellulose), with a degree of substitution of at most 3 per unit. Cellulose is insoluble in water and common solvents but dissolves in cuprammonium solution, N-methylmorpholine N-oxide (the lyocell process) and ionic liquids; the viscose process turns it into a soluble xanthate.
Key facts
- bond
- β-(1→4)
- unit
- C6H10O5, 162.14 g/mol
- cellobiose
- 1.03 nm
- DP in wood
- around 10 000
- share of wood
- 40-45%
- crystal form
- cellulose Iβ
Sources for this chapter
- Азаров В.И., Буров А.В., Оболенская А.В. Химия древесины и синтетических полимеров: учебник. 2-е изд. СПб.: Лань, 2010.
- Евстигнеев Э.И. и др. Химия древесины и синтетических полимеров. Ч. 1. Строение, свойства, химические реакции и производные целлюлозы: учебно-методическое пособие. СПб.: СПбГТУРП, 2010.
- Sjöström E. Wood Chemistry: Fundamentals and Applications. 2nd ed. San Diego: Academic Press, 1993.
- Klemm D., Heublein B., Fink H.-P., Bohn A. Cellulose: Fascinating Biopolymer and Sustainable Raw Material. Angewandte Chemie International Edition 44, 3358-3393 (2005).