Water in wood material supramolecular nanocomposite

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Water in wood material supramolecular
nano-composite

• Robert Franich, Roger Newman Stefan Hill

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Overview
Why the need to understand the structure of
water in cell walls ? Cell wall chemistry,
structure Water distribution in cell
walls Velcro mechanics of wood
material Hypothesis water structures Some
initial results Future applications and summary
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Why the need to understand the structure of water
in cell walls ?
Water conduction in xylem necessary for living
tree. Importance to wood utilisation Logging
- wood volumes and weights for transportation
costs Timber drying- mass to be evaporated to a
target moisture content Material stability-
dimensional and conformational change with
relative humidity variation Material
durability- moisture content and wood decay
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Wood moisture content and MoE property
Radiata pine sapwood Age ? Green
Dry kg/m3 MoE GPa
MoE GPa 30 550 6.42 9.5 50
0 5.47 8.23 450 4.95 7.54 15 450 7

Green MC 150-250 range Dry equilibrium MC 12-15
range MC (Wg-Wd) / Wd x 100
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Cell wall structure and chemistry

S 1-3 Lignocellulose layers P Primary
wall ML Lignin-rich middle lamella
M
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Cell wall chemistry, structure

Hemicellulose Cell wall component in which 1,4
linked pyranosyl units with O4 in equatorial
orientation. Conformational homology between
cellulose and hemicellulose strong non-covalent
H-binding
Koshijima Watanabe, 2003
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Cell wall chemistry, structure and water
30-50 water
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Water distribution within wood
Green sapwood Large natural moisture
content gradients between earlywood and latewood
Processed green sapwood Uniform wood moisture
content
Stahl, M. 2004
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1H NMR imaging of water in wood

Green wood 200 mc
Proton density (arb NMR units)
40 mc
Wood specimen transect
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Cell wall chemistry, structure and water

• Hierarchy of wood-water relationships
• Tree
• Timber
• Sapwood / heartwood
• Earlywood / latewood
• Cell wall
• Supramolecular structure / polymers

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Cell wall chemistry, structure and water
Cellulose phases I? triclinic, Alternating
glucose conformers regularly displaced in same
direction I? monoclinic Two conformationally
distinct alternating sheets Change H-bond
pattern 2-OH and 6-OH I? and I? interconvertible
during microfibril formation by bending Altered
H20 layer on I? extends 1nm

I?
I?
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Cell wall chemistry, structure and water
Hydration layers of saccharides Few
monosaccharides form hydrates Oligosaccharides
3- or 4- coordinated water molecules

Jeffrey, G.A. 1992
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Dynamic structure of water

• Dielectric relaxation of free water - ? 8.27 ps
• bound water ? ?1ns
• Water clusters
• Water local structure perturbed by carbohydrates
• Cole-Cole parameter ? from microwave dielectric
  measurement using time domain relectometry method

Hayashi, Y et al, 2004
Jeffrey, G.A. 1992
Hermida-Ramon, J.M Larlstrom, G 2004
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Perturbation of water structure dynamics by
carbohydrates

• Represented by plot of ? vs ?
• Implies a gradient in water dyanamics at
  polysaccharide surfaces

Hayashi, Y et al, 2004
Conformational homology between cellulose and
hemicellulose reflected in bound water ?
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Velcro mechanics in wood
Ductile behaviour qualitatively similar to that
of metal

Wet spruce wood foil
Viscous relaxation
Stress-strain curve
MFA-strain curve with simultaneous synchrotron
XRD
Keckes, J. et al 2003 Kretschemann, D Green, D
1996
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Velcro mechanics in wood

Critical shear stress
Explanatory model invoking inter-fibril
Velco-like stick-and-slip process within the
microfibril supramolecular assembly of
hemicellulose-lignin
Keckes, J. et al 2003
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Velcro mechanics in wood
Conceptual supramolecular models
Lignin

Hemicellulose
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Velcro mechanics in wood
Conceptual supramolecular models
Wood supramolecular nano-composite

Bound water layer dispersered between nano-composi
te assemblies
Hydrogen bond scission between structural
water molecules
Hydrogen bonds re-formed
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Velcro mechanics in wood
Conceptual supramolecular models

Hydration layers between hemicellulose-lignin and
cellulose 1? phase
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Velcro mechanics in wood
Wet (green) wood to dry wood conceptual model
Retention of bound water layer at 12 equilibrium
mc

Tethering of hemicellulose to cellulose fibril
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Velcro mechanics in wood
Testing hydration layer theory by NMR relaxation
experiments

13C NMR spectrum of dry wood specimen
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Velcro mechanics in wood
Spin-echo CP/MAS NMR relaxation experiments with
green (wet) wood Spin-diffusion barrier
detection

T2(13C) focus on segmental motion in nuclear
vicinity
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Supramolecular conceptual model for green (wet)
wood cell wall nano-composite

Hydration layer
Ligno-hemicellulose composite
Hydration layer
Cellulose polymer aggregate
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Role of water in secondary cell wall
supramolecular assembly and wood properties

Green cell wall 30-50 mc cell wall
elements / polymers separated by water
enabling slip and stick Velcro mechanics
between wall elements 5-7 GPa MoE Dry,
12 water self-organised between elements
with tethering of hemicellulose to cellulose
between hydration layers maximum strength
and stiffness - 10-20 mc 7-9 GPa MoE
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Wood modification exploiting Velcro mechanics
chemistry
Cell wall modifcation using chitosan oligomers in
water
El 65 GPa
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Secondary wall modification enhanced modulus
composite
Radiata pine low, medium high density
specimens
Individual specimen modifications

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In summary Cell wall supramolecular structure
conceptual model invokes structural hydration
layers reflecting conformational homology with
cellulose and hemicellulose polymers enabling
Velcro mechanics in wood. Modification of
secondary cell walls with carbohydrates using a
bio-mimicry approach can enhance cell wall and
consequently bulk material properties, such as
MoE. Control of hydration structures within the
wood cell wall supramolecular nano-composite
might offer new 21st C approaches to wood drying
and wood modification .
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Acknowledgements Dr Kirk Torr - chemistry,
spectroscopy Dr Adya Singh - microscopy Mr
Barry Penellum - MoE measurements