Structure and Properties of Cellulose

David Wang’s Wood Chemistry Class

Wood Polysaccharides Biosynthesis

• Cellulose is synthesized from UDP-D-glucose, the energy

content of which is used for the formation of glucosidic

bonds in the growing polymer:

UDP-D-glucose + [(1 → 4)-β-D-glucosyl]n →

[(1 → 4)-β-D-glucosyl]n+1 + UDP

Structure of Two Nucleosides

Uridine Guanosine

Formation of uridine diphosphate glucose (UDP-D-glucose)

Hypothetical Model of the Mechanism of Cellulose Synthesis in Plants

sucrose synthesis

UDP glucose pyrophosphorylase

phosphoglucomutase

hexokinase

Polysaccharides Biosynthesis • In the synthesis of other wood polysaccharides both UDP-D-

glucose and GDP-D-glucose are involved.

• GDP-D-glucose is the principal nucleotide as concerns the

formation of mannose-containing hemicellulose, including galacto-

glucomannans and glucomannans.

• Monomeric sugar components needed are formed from the

nucleotides by complex enzymic reactions involving epimerization,

dehydrogenation, and decarboxylation.

Simplified Representation of the formation of Hemicellulose Precursors from UDP-D-glucose

Cellulose Cellulose is the main constituent of wood.

Approximately 40-45% of the dry substance in most wood

species is cellulose, located predominantly in the

secondary cell wall.

1.03 nm

Cellulose Linear polymer made up of β-D- glucopyranose units linked

with 1→4 glycosidic bonds. Repeating unit = cellobiose Glucopyranose units in chair form - most thermodynaically

stable. Only 2% in other forms CH2OH and OH groups in equatorial positions → stability

Cellulose: Reducing End Groups

• Each cellulose chain has 1 reducing end group at the C1 position of the terminal glucopyranose unit

• The C4 position of the other terminal unit is an alcohol and therefore not reducing.

• Does the reducing end mutarotate?

– In fibers, probably not because of hydrogen bonding, etc.

– In solution, probably

Distribution of the Carbon Bound in Organic Matter

40% in cellulose

30% in Lignin

26% in other polysaccharides

Animals + menOther plant substances

•In the biosphere, 27 × 1010

ton of carbon are bound in

living organism, more than

99% of which are plant

Cellulose Content of Various Plant Materials

Plant Material Cellulose (%)Cotton 95-99Ramie 80-90Bamboo 40-50Wood 40-50Bark 20-30Mosses 25-30Horse-tail 20-25Bacterial 20-30

Cellulose

Although the chemical structure of cellulose is

understood in detail, its supermolecular state

(crystalline and febrillar) is still open to discuss.

The extract molecular weight

Polydispersity of native cellulose

Dimensions of the microfibrils

Cellulose• Cellulose is a homopolysaccharide composed of β-D-

glucopyranose units which are linked together by

(1→4)-glycosidic bonds.

• Cellulose molecules are completely linear and have a

strong tendency to form intra and intermolecular

hydrogen bonds.

Hydrogen Bonds in Cellulose

OH-groups as well as NH-groups are able to interact with other or with O-, N- and S- groups forming a particular linkage.

The functional groups of the cellulose chains are the hydroxyl groups, three of them being linked to each glucose unit. These OH-groups are not only responsible for the supramolecular structure but also for the chemical and physical behavior of the cellulose.

Two types of hydrogen bonds formed in cellulose Intramolecular linkage (H-bond) Intermolecular linkage (H-bond)

Intramolecular linkage (H-bond): Hydrogen bonds between OH-groups of adjacent glucose units in the same cellulose molecule. These linkages give a certain stuffiness to the single chain.

O(3)H to ring oxygen (or O(3)H to O5’; O(6) to O(2)

Intermolecular linkage (H-bond): Hydrogen bonds between OH-groups of adjacent cellulose molecules. These linkages are responsible to the formation of supramolecular structures

O(6)H to O(3)

Cellulose Hydrogen Bonds

• Hydrogen bonds do not only exist between cellulose OH-groups but also between cellulose-OH and water-OH.

• The absorption of water by a cellulose sample depends on the number of free OH-groups or rather on the cellulose OH-groups not linked with each other.

• It has been proven that isolated wood cellulose absorbs more water than cotton cellulose at the same relative humidity, indication the presence of fewer free-OH groups in cotton than in wood cellulose.

Hydrogen Bonds in Cellulose I Native Cellulose Intramolecular Bonds

O(6) to O(2)HO(3) to ring oxygen

Intermolecular BondsO(3) to O(6)H

These Bonds in the AC Plane. Bonding in the b plane through van der Waals forces*

c

a

Cellulose Hydrogen Bonds

Cellulose & Water

Structure of Cellulose. β-D-glucopyranose chain

units are in chair conformation (4C1) and the

substituents HO-2, HO-3, and CH2OH are oriented

equatorially.

Principal Paratropic Planes in Cellulose І

The important lattice planes of the space unit of cellulose Ⅰ

Crystalline Structure of Cellulose

Crystalline structure of

cellulose has been

characterized by X-ray

diffraction analysis and by

methods based on the

absorption of polarized

infrared radiation

Absorption Spectrum of Cellulose Using Polarized Infrared Radiation

35

Crystalline Structure of Cellulose

The unit cell of native cellulose (cellulose Ⅰ ) consists of four D-

glucose residues.

In the chain direction (c), the repeating unit is a cellbiose residue (1.03 nm), and every glucose residue is accordingly displaced 180° with respect to its neighbors, giving cellulose a 2-fold crew axis.

Axial projections of the structures of native cellulose

Projection of the chains in

cellulose І perpendicular to the

a-c plane

Projection of the (O2O) plane in cellulose І, showing the hydrogen bonding network and the numbering of the atoms

Crystalline Structure of Cellulose

Regenerated cellulose (cellulose Ⅱ) has antiparallel chains.

The hydrogen bonds within the chains and the between the chains in the a-c plane are the same as celluloseⅠ. In addition, there are two H-bonds between a corner chain and a center chain, namely from O(2) in one chain to O(2) H in the other and also from O(3)H to O(6)

Axial projections of the

structure of regenerated

cellulose

•Cellulose Ⅱ is formed whenever

the lattice of celluloseⅠ is

destroyed, for example on

swelling with strong alkali or on

dissolution of cellulose.

•Cellulose Ⅱ is thermodynamically

more stable than cellulose Ⅰ.•Cellulose Ⅱ can not

reconverted into cellulose Ⅰ.

Transformation of Cellulose into its Various Lattice Modification

Polymer Properties of Cellulose

The polymer properties of cellulose are usually studied in solution, using solvents, such as CED or Cadoxen. Average molecular weight

Polydispersity

Conformation of the polymer

Definitions of Molecular Weight

Number Average Molecular Weight (數量平均分子量)

Viscosity Molecular Weight (黏度平均分子量)

Weight Average Molecular Weight (重量平均分子量)

Z-average Molecular Weigh (Z平均分子量)

Number Average Molecular Weight

Number average molecular weight is determined by:

Osmotic Pressure

End-group titration

Weight Average Molecular Weight

Weight average molecular weight is determined by:

Light scattering

Small Angle Neutron Scattering (SANS)

Viscosity Average Molecular Weight

Viscosity average molecular weight is determined by

intrinsic viscosity and the Mark Houwink equation.

Z-Average Molecular Weight

Z-average molecular weight is determined by

Sedimentation equilibrium

設有一高分子化合物，由10個分子量100之分子與5個分子量1000的分子所混合組成，試求此高分子化合物的數量、重量、Z以及粘度(a = 0.6)平均分子量。

Mn = (10 x 100 + 5 x 1000) / (10+5) = 400

Mw = (10 x 1002 + 5 x 10002) / (10 x 100 + 5 x 1000) = 850

Mz = (10 x 1003 + 5 x 10003) / (10 x 1002 + 5 x 10002) = 982

Mv = (10 x 1001.6 + 5 x 10001.6) / (10 x 100 + 5 x 1000)(1/0.6) = 811

Molecular Weight Distribution and Average Molecular Weights of a Typical Polymer

Methods of Molecular Weight Measurement

Methods Type of Molecular Weight

1. Osmometry2. Determining the

number of reducing end groups

Number average molecular weight(Mn)

Light scattering Weight average molecular weight(Mw)

Ultracentrifugation Z average molecular weight(Mz)

Viscosity measurement Viscosity average molecular weight(Mv)

Degree of Polymerization of Cellulose

Cellulose: the relationship between molecular weight and DP is DP =

M/162, where 162 is the molecular weight of anhydroglucose unit.

The DP of cellulose in wood is reduced during aging of a living tree,

i.e. the DP is highest in cells adjacent to the cambium and decrease

towards the pith.

DP = molecular weight of cellulosemolecular weight of one glucose unit

Polydispersity of Cellulose The polydispersity index is the ratio of the weight average molecular weight to

the number average molecular weight. ( Mw/Mn)

It indicates the distribution of individual molecular weights in a batch of

polymers.

Polydispersity of Cellulose

M.W. measurements have shown that cotton cellulose in

its native state consists of about 15,000 and wood

cellulose of about 10,000 glucose residues.

It has been suggested in the literature that the native

cellulose present in the secondary cell wall of plants in

monodisperse.

Based on properties in solution such as intrinsic viscosity and sedimentation and diffusion rates, conclusions can be drawn concerning the polymer conformation

Schematic Representation of Randomly Coiling

Macromolecules in Solution

Flory’s Equation

The expansion tendency of a polymer molecule is

characterized by Flory’s equation R = αR0, where α is the

expansion coefficient.

At a certain temperature in a given solvent an ”idea” state

(R = R0) can be reached.

Theta solvent and theta temperature (Flory temperature)

Intrinsic Viscosity of a Polymer

[η] = K Mv ….. …….Mark-Houwink equation