Water in wood material supramolecular nano-composite

1. Robert Franich, Roger Newman & Stefan Hill Water in wood material supramolecular nano-composite

2. 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

3. 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

4. Wood moisture content and MoE property Radiata pine sapwood Age  Green Dry kg/m3 MoE GPa MoE GPa 30 550 6.42 9.5 500 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

5. Cell wall structure and chemistry S 1-3: Lignocellulose layers P: Primary wall ML: Lignin-rich middle lamella M

6. 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

7. Cell wall chemistry, structure and water 30%-50% water

8. 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

9. 1H NMR imaging of water in wood Green wood 200% mc Proton density (arb NMR units) 40% mc Wood specimen transect

10. Cell wall chemistry, structure and water Hierarchy of wood-water relationships: • Tree • Timber • Sapwood / heartwood • Earlywood / latewood • Cell wall • Supramolecular structure / polymers

11. 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

12. 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

13. 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

14. 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 ?

15. 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

16. 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

17. Velcro mechanics in wood Conceptual supramolecular models Lignin Hemicellulose

18. Velcro mechanics in wood Conceptual supramolecular models Wood supramolecular nano-composite Bound water layer dispersered between nano-composite assemblies Hydrogen bond scission between structural water molecules Hydrogen bonds re-formed

19. Velcro mechanics in wood Conceptual supramolecular models Hydration layers between hemicellulose-lignin and cellulose 1 phase

20. 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

21. Velcro mechanics in wood Testing hydration layer theory by NMR relaxation experiments 13C NMR spectrum of dry wood specimen

22. 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

23. Supramolecular conceptual model for green (wet) wood cell wall nano-composite Hydration layer Ligno-hemicellulose composite Hydration layer Cellulose polymer aggregate

24. 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

25. Wood modification exploiting Velcro mechanics chemistry Cell wall modifcation using chitosan oligomers in water El ~ 65 GPa

26. Secondary wall modification – enhanced modulus composite Radiata pine low, medium & high density specimens Individual specimen modifications

27. 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 .

28. Acknowledgements Dr Kirk Torr - chemistry, spectroscopy Dr Adya Singh - microscopy Mr Barry Penellum - MoE measurements