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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
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)