Structure and Properties of Cellulose

Transcription

1 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

2 Structure of Two Nucleosides Uridine Guanosine Formation of uridine diphosphate glucose (UDP-Dglucose)

3 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-Dglucose and GDP-D-glucose are involved. GDP-D-glucose is the principal nucleotide as concerns the formation of mannose-containing hemicellulose, including galactoglucomannans and glucomannans. Monomeric sugar components needed are formed from the nucleotides by complex enzymic reactions involving epimerization, dehydrogenation, and decarboxylation.

4 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 nm

5 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 CH 2 OH 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

6 Distribution of the Carbon Bound in Organic Matter Animals + men Other plant substances In the biosphere, ton of carbon are bound in 26% in other polysaccharides 40% in cellulose living organism, more than 99% of which are plant 30% in Lignin Cellulose Content of Various Plant Materials Plant Material Cellulose (%) Cotton Ramie Bamboo Wood Bark Mosses Horse-tail Bacterial 20-30

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8 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

9 Cellulose Cellulose is a homopolysaccharide composed of β-dglucopyranose 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)

10 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 OHgroups 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.

11 Hydrogen Bonds in Cellulose I Native Cellulose Intramolecular Bonds O(6) to O(2)H O(3) to ring oxygen Intermolecular Bonds O(3) to O(6)H These Bonds in the AC Plane. Bonding in the b plane through van der Waals forces* a c Cellulose Hydrogen Bonds

12 Cellulose & Water

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15 Structure of Cellulose. β-d-glucopyranose chain units are in chair conformation (4C1) and the substituents HO-2, HO-3, and CH 2 OH are oriented equatorially.

16 Principal Paratropic Planes in Cellulose І The important lattice planes of the space unit of cellulose Ⅰ

17 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

18 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

19 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

20 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

21 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 ( ) Viscosity Molecular Weight ( ) Weight Average Molecular Weight ( ) Z-average Molecular Weigh (Z )

22 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)

23 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

24 Z (a = 0.6) M n = (10 x x 1000) / (10+5) = 400 M w = (10 x x ) / (10 x x 1000) = 850 M z = (10 x x ) / (10 x x ) = 982 M v = (10 x x ) / (10 x x 1000) (1/0.6) = 811 Molecular Weight Distribution and Average Molecular Weights of a Typical Polymer

25 Methods of Molecular Weight Measurement Methods 1. Osmometry 2. Determining the number of reducing end groups Light scattering Ultracentrifugation Viscosity measurement Type of Molecular Weight Number average molecular weight (M n ) Weight average molecular weight (M w ) Z average molecular weight (M z ) Viscosity average molecular weight (M v ) Degree of Polymerization of Cellulose DP = molecular weight of cellulose molecular weight of one glucose unit 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.

26 Polydispersity of Cellulose The polydispersity index is the ratio of the weight average molecular weight to the number average molecular weight. ( M w /M n ) 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.

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

28 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 = αr 0, where α is the expansion coefficient. At a certain temperature in a given solvent an idea state (R = R 0 ) can be reached. Theta solvent and theta temperature (Flory temperature)

29 Intrinsic Viscosity of a Polymer [η] = K M v...mark-houwink equation