Nanomechanical Properties of Biologically Significant Microstructural Elements of Wood

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1 Nanomechanical Properties of Biologically Significant Microstructural Elements of Wood Valery Bliznyuk, Roman Rabiej, Ondrej Pekarovic Western Michigan University, Kalamazoo, MI 49008

2 Objectives To reveal morphology of particular layers (S2) of wood cell with the Atomic Force Microscopy (AFM) technique in order to understand the micro-structural organization of the main wood elements: earlywood vessels, latewood fibers, and wood rays. To determine the magnitude of nano-modules of elasticity (NME), microfibril angle and topography of each studied layer in the above listed micro-structural elements. To study the effect of specific gravity on the wood nanostructure and nano-modulus of elasticity. The broad goals: 1) to establish nano-mechanical characteristics of micro-structural elements of wood with relation to the naturally-occurring orthotropic wood structure; 2) to elucidate possibilities of practical applications of unique nanomechanical properties of biologically significant micro-structural elements of wood.

3 RESEARCH DESIGN Sugar Maple (Acer saccharum) - three orthotropic wood sections: the transverse plane, longitudinal-radial plane and longitudinaltangential plane. The effect of specific gravity on the morphology, microfibril angle, and nano-modulus is determined at low, medium, high range of the specific gravity. Range of the specific gravity variation from low (0.50 to 0.55 g/cm 3 ) to medium (0.63 to 067 g/cm 3 ) and high range (0.75 to 0.80 g/cm 3 ). The address nano-mechanical properties of secondary wall layers (S2) and middle lamella (ML).

Multidimensional Approach Comparison between summer wood and spring wood Comparison between tangential, radial and end-grain sections Density

4 Multidimensional Approach Comparison between summer wood and spring wood Comparison between tangential, radial and end-grain sections Density Location Top location 3.6 m Low density 0.65 Medium density 0.72 High density 0.81 Radial Spring wood Tangential End grain Bottom location 0 m Summer wood

5 Orthotropy and Hierarchy of Structural Organization of Wood. Composite Viewpoint

6 Materials and preparation of specimens The Sugar Maple (Acer saccharum) specimens for tests were prepared through several stages procedure: First 20 mm x 20 mm x 650 mm samples were cut from 50 mm thick planks. They were brought to an equilibrium moisture content at relative humidity of 40% +/- 5% and temperature of 20 o C +/- 2 o C. Macro-mechanical properties have been studied for these samples under three point bending test for radial and tangential sections. Specific gravity was also measured and controlled. Microscopic density distribution was addressed with X-ray densitometer. Samples for X-ray densitometry were 2 mm x 20 mm x 60 mm in size (radial and end-grain sections only) Samples of wood for optical, transmission electron microscopy (TEM), and atomic force microscopy (AFM) characterization were prepared through microtoming of thin sections (~ 150 nm in thickness). First, shaving of 80 to 120 um thick chips was done. Then, these chips were immersed into epoxy glue and cured for 7 days. Ultrathin slices of wood samples were prepared with the help of Ultramicrotom equipment. To preserve the natural structure of wood during such preparation we used a diamond knife (MicroStar Technology Incorporated, Huntsville, TX). For TEM ultramicrotommed sections were put on a copper grids coated with formvar. For AFM studies they were put on glass slides.

7 Sample Preparation

8 Bulk Characterization Wood samples of sugar maple (Acer saccharum) were prepared from 7 planks of 51 mm thick, 200+ mm wide and 3.60 m long stock. Pieces of stock 20 mm x 20 mm x 650 mm long were selected for straightness of grain and freedom of defects. These represented, as nearly as possible, ideally quartersawn (radial section) and flatsawn (tangential section) pieces. The equilibrium moisture content was 7.0 %. The samples were cut from the stump end of planks and top end of planks. The planks were 12 feet long (3.60 m). Table 1. Planks of Sugar Maple. Planks Number #1 #2 #3 #4 #5 #6 #7 Specific Gravity g/cm 3 Top end samples (T) Bottom end samples (B) Comments: All top samples had slightly lower specific gravity in comparison to the bottom samples

9 Quintek Measurement Systems, Inc.; Tree Ring System - QTRS Radial End grain Thanks to Mr. Chuck Dawson, President of QMS Inc.

10 Bulk Modulus of Elasticity Bulk Modulus of Elasticity vs. Specific Gravity y = 15495x R 2 = Tangential section MPa y = 11273x R 2 = Top Tangential Bottom Tangential Apr 22 Linear (Top Tangential ) Linear (Bottom Tangential Apr 22) Bulk M odulus of Elasticity vs. Specific Gravity y = 10971x R 2 = Radial section Top Radial MPa y = 15179x R 2 = Bottom Radial Apr

11 Electron Microscopy Morphology of Sugar Maple Vessels, wood-rays, fibers End grain section Summer wood area

12 Tangential section Multilayer wood rays Electron Microscopy Morphology of Sugar Maple

13 Electron Microscopy Morphology of Sugar Maple Tangential section with wood ray

14 Measuring nanomechanical properties with AFM Laser Photo-detector PC y z x Tip Cantilever Sample Anatomy of Force-Distance Curve Piezo F, nn AFM setup Distance

15 Mapping of surface stiffness distribution. Ultrasonic Force Microscopy (UFM) technique UFM piezo AFM piezo Acos(wt) UFM response in modulated ultrasonic vibration reference signal LOCK-IN AMPLIFIER FUNCTION GENERATOR out SPM ADC Ultrasonic Force Microscopy Topography

16 AFM Topography UFM End grain Radial 2 um

AFM tip Nano-Mechanics with AFM Scheme of AFM tip surface mechanical interaction Zdefl kn Zpos For the simplest case of Hertzian model the surface stiffness (modulus of elasticity) can be expressed

17 AFM tip Nano-Mechanics with AFM Scheme of AFM tip surface mechanical interaction Zdefl kn Zpos For the simplest case of Hertzian model the surface stiffness (modulus of elasticity) can be expressed as: E = 3 4 (1 ν 2 ) k R n 1/ 2 z h defl 3/ 2 Force program by Dr. V.Gorbunov, Veeco Inc. E? h = Zpos - Zdefl k n is cantilever s spring constant Z defl is deflection of the cantilever R is radius of the curvature of the tip h is explained in scheme above ν is the Poisson s ratio

18 Nanomechanical properties of wood. Surface stiffness. 4B/RL end grain summer 4B/RL end grain spring 4B/TL 9.72 Summer 4B/RL 6.72 spring TAN Z, nm Spring wood E= GPa Summer wood E= GPa End grain section E= GPa

19 Nano-Modulus of Elasticity of Fibers and Vessels of Sugar Maple Sample Fibers and Vessels from Sample 4B Bottom End Sections Tangential Radial Transverse Tangential Radial Summer/Spring Summer Summer Summer Spring Spring Spring Units GPa Nano-Modulus of elasticity (NME)

20 Effect of Sample Location on NME of Fibers and Vessels of Sugar Maple Sections Tangential Radial Transverse Tangential Radial Summer/ Spring Summer Summer Summer Spring Spring Spring Units GPa Sample Fibers and vessels from sample 4B (Bottom End) NME Sample Fibers and vessels from sample 4T (Top End) NME

21 Effect of Bulk Specific Gravity on NME of Fibers and Vessels of Sugar Maple Sample 7B-Bottom End 4B-Bottom End 1B-Bottom End Sections Tangential Radial Tangential Radial Tangential Radial Summer/ Spring Summer Summer Summer Summer Summer Summer Bulk SG 0.65 g/cm g/cm g/cm 3 Units GPa NME

22 AFM imaging of end grain section

23 Structure of the cell wall and Nanomechanical properties Z, nm

24 AFM imaging of Microstructure Elements of Wood Fibers S3 S2 S1 Middle lamella (ML)

25 Nanostructure of wood cell walls 4B-RL_10.72 end grain section S2 micro-fibrils

26 AFM imaging: Tangential Section of summer wood at various scales

27 Tangential vs Radial and Spring vs Summer Wood Sections 4trl_3.68_Tan/summer 4btl872Radial/early 4brl772 Tan/summer10um 4btl972summer/radial2

28 AFM Imaging: Entanglement of wood fibers

29 CONCLUSIONS: Macro level Characterization of samples: X-ray densitometry, electron microscopy and atomic force microscopy techniques were applied to address structure- mechanical properties relationships for sugar maple at macroscopic (bulk) and nano levels. The micro-structural organization of the main wood elements: early-wood vessels, latewood fibers, and wood rays has been studied Macro-test results: Bulk specific gravity of the sample from bottom planks location was slightly higher than the samples from top planks. The magnitude of the Modules of Elasticity was higher for radial and tangential sections for the top located samples than for the samples taken from the bottom planks location. Bulk specific gravity positively affected the magnitude of Modules of Elasticity.

30 CONCLUSIONS: Micro- and Nano-Levels S2 of Spring wood (vessels) is characterized with lower NME in comparison to S2 summer wood (fibers). Location: samples cut from the top portion have generally lower NME than samples cut from the bottom end of a plank. Orthotropy: end grain section is characterized with the highest NME (15-25 GPa) while tangential and radial sections have approximately the same NME values which are much lower (~1 to 5 GPa). Specific gravity: bulk SG shows no correlation with NME (?) Our results establish a new approach to mechanical characterization at different levels starting from macroscopically (~ cm, annual rings), microscopically (several um walls of fibers and vessels,) down to nanoscale (~100 nm, microfibrils). Future research will address microfibril angle vs. specific gravity and topography of each studied layers on the above listed micro-structural elements. Relationship between the nano-mechanical properties of structural elements of wood and macro-mechanical properties of solid wood still has to be revealed.

31 Acknowledgements The Authors would like to express heartfelt gratitude to Western Michigan University Faculty Research and Creative Activities Support Fund (FRACASF No: ) for financial support.

The Structure of Wood

The Structure of Wood WOOD Wood Wood is the oldest and still most widely used of structural materials. In the 17th and 18th centuries the demand for wood is so great that much of Europe was deforested. Today, about 10 9 tons