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Characterization of Cell Walls Treated with Ethylene Glycol
using Nanoindentation
Joseph JakesUniversity of Wisconsin-Madison
Materials Science ProgramUSDA Forest Products Laboratory
Advisor at UW: Don StoneAdvisors at FPL: Chuck Frihart, Robert Moon, Jim
Beecher
Outline• Nanoindentation
– Accounting for edge effects– Broadband Nanoindentation Creep
(strain rate sensitivity)• Experiments
– PMMA edge effects– Characterization of tracheid walls before
and after ethylene glycol treatment• Tracheid wall ultrastructure and
Norimoto’s model• Conclusions/Future Work
Ideal Nanoindentation Experiment
0 50 100 150 200 250 300Depth, h (nm)
0
200
400
600
800
1000
Load
.L(μ
N)
0 50 100 150 200 250 300Depth, h (nm)
0
200
400
600
800
1000
Load
.L(μ
N)
0 50 100 150 200 250 300Depth, h (nm)
0
200
400
600
800
1000
Load
.L(μ
N)
0 50 100 150 200 250 300Depth, h (nm)
0
200
400
600
800
1000
Load
.L(μ
N)
( )ceff hA
SE =⎟⎟⎠
⎞⎜⎜⎝
⎛ −+
−=
d
d
s
s
eff EEE
22 1111 ννβ
S1
( )chALH max=
Lmax
hc
hc
SLhhc
maxmax ε−=
Infinite half-spaceDepth, h
Load
, L
Real Nanoindentation Experiment
0 50 100 150 200 250 300Depth, h (nm)
0
200
400
600
800
1000
Load
.L(μ
N)
0 50 100 150 200 250 300Depth, h (nm)
0
200
400
600
800
1000
Load
.L(μ
N)
0 50 100 150 200 250 300Depth, h (nm)
0
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400
600
800
1000
Load
.L(μ
N)
0 50 100 150 200 250 300Depth, h (nm)
0
200
400
600
800
1000
Load
.L(μ
N)
Cm
0 50 100 150 200 250 300Depth, h (nm)
0
200
400
600
800
1000
Load
.L(μ
N)
Cm not accounted for
( ) ( ) ( ) ( )cmtcpceff hACChAChA
SE−
=== 11
Infinite half-spaceDepth, h
Load
, L
Nanoindentation of Wood
Cellular Structure
Use methods we have established to account for additional displacements from cellular
structure and edge
Edge
Nanoindentation of HeterophaseInterphase (Substrate-Adhesive)
Will the same method work for a heterophase interface?
Adhesive, Es,adh Substrate, Es,sub
Es,sub > Es,adhBehave similar to
free edge
Nanoindentation of Substrate-Adhesive Interphase
Will the same method work for a heterophase interface?
Adhesive, Es,adh
Constraining effect from stiffer phaseEs,sub > Es,adh
Substrate, Es,sub
Nanoindentation of Substrate-Adhesive Interphase
Will the same method work for a heterophase interface?
Adhesive, Es,adh
Constraining effect from stiffer phaseEs,sub > Es,adh
Substrate, Es,sub
Nanoindentation of Substrate-Adhesive Interphase
Will the same method work for a heterophase interface?
Adhesive, Es,adh
Constraining effect from stiffer phaseEs,sub > Es,adh
Substrate, Es,sub
measALH max=
( )( )smtmeaseff CCCA
E+−
= 1
( ) ( )( )smtceff CCChA
E+−
= 1
( )chALH max=( )( )mtc
eff CChAE
−= 1
Corrected Nanoindentation Methods
Measure areasfrom AFM images
Stone, Yoder and Sproul (SYS) Correlation
( )eff
sm EHLCCLC ++= maxmax
Plot C√Lmax vs √Lmax
Assume a structural compliance (Cs) is present
measALH max=
Jakes et al. (2008) J. Mat. Res. 23(4) pp. 1113-1127
No dependence on area!
( )( )smtmeaseff CCCA
E+−
= 1
External Compliances in Real Nanoindentation Experiments
• Machine Compliance (Cm)– Property of nanoindenter
• Constant for every indent performed– Standard methods established to account for Cm
• Structural Compliance (Cs)– Property of specimen and location of indent
• Cellular structure• Edge effects
– Use methods of Jakes et al. to measure Cs• Jakes et al. (2008) J. Mater. Res. 23(4) pp. 1113.
( )smtP CCCC +−=
Strain Rate Sensitivity Analysis
• Investigate deformation mechanisms by determining hardness as a function of strain rate during hold segment– Use Stone and Yoder creep analysis
• Stone and Yoder (1994) J. Mater. Res. 9(10) pp. 2524
• Determine strain rate sensitivity parameter:
)ln()ln(
HdHdε
υ&
=Ηdt
AdH
)ln(=ε&
Preparing PMMA specimen
• Specimen cut from an extruded PMMA rod
Nanoindentation surface prepared with diamond knife in ultramicrotome
3 mm
Extruded Edge
Preparing PMMA specimen
• Specimen cut from an extruded PMMA rod
Nanoindentation surface prepared with diamond knife in ultramicrotome
Extruded Edge
3 mm
MicrotomedEdge
Preparing UnembeddedSpecimens for Nanoindentation
• Current literature embed wood specimens in epoxy– Diffusion of epoxy components may be altering tracheid wall
• Developed microtoming techniques– Custom built diamond knife holder for sled microtome– Rotary ultramicrotome with diamond knife
C
AC
B
A
5 µm
Clearance and cutting angle of
diamond knife were both set to
approximately 5º for sled microtome
(SEM courtesy of Jim Beecher, FPL.)
Microtome cut
10 mm
Latewood
Experimental Procedure
• Indentation performed with HysitronTriboindenter– PMMA– Lobllolly pine
• Untreated• Treated with ethylene glycol
– Soaked in ethylene glycol for 3 days
(hysitron.com)
Berkovich Tip
Experimental Procedure
• Multiload indents used
0 100 200 300 400X Axis
0
200
400
600
800
1000
1200
Loa
dDepth
0 50 100 150X Axis
0
20
40
60
80
100
YA
xis
Loa
d
Time
Hold for drift
0.01 s load
Hold for creep
• Areas measured from AFM images
( )eff
sm EHLCCLC ++= maxmax
Results: Edge effect in PMMA
0 20 40 60 80
Distance to Edge, d (μm)
0
2
4
6
8
Mod
ulus
,Es
(GPa
)
Assume Cs = 0Account for Cs
Bulk Es = 5.3±0.2 GPa
2 µm
0 20 40 60 80
Distance to Edge, d (μm)
0
2
4
6
8
Mod
ulus
,Es
(GPa
)
Microtomed edgeExtruded edge
Results: Microtomed vs. Extruded
Bulk Es = 5.3 GPa
0 20 40 60 80
Distance to Edge, d (μm)
050
100150200250300350400
Har
dnes
s,H
(MPa
)
Microtomed edgeExtruded edge
Results: Microtomed vs. Extruded
Bulk H = 270 MPa
Results: Microtomed vs. Extruded
10-4 10-2 100 102
dε/dt (s-1)
103
log(
H)(
MPa
) Bulk indentsMicrotomed edgeMicrotomed 7.5 um from edgeExtruded edgeExtruded 11.1 um from edgeExtruded 5.7 um from edge
ƲH ≈ 0.108ƲH ≈ 0.117
ƲH ≈ 0.146
Results: Unmodified Wood
2 µm
21 1.2 mN indentsEs = 21 ± 3 GPaH = 380 ± 20 MPa
6 0.8 mN indentsEs = 6 ± 1 GPaH = 340 ± 20 MPa
2 µm
Results: Comparison Before and After Ethylene Glycol Treatment
UntreatedEthylene Glycol
Treated
2 µm
2 µm
Results: Comparison Before and After Ethylene Glycol Treatment
2 µm2 µm
UntreatedEthylene Glycol
Treated
Results: Ethylene Glycol Treated Wood
2 µm
18 0.4 mN indentsEs = 6 ± 4 GPaH = 80 ± 10 MPa
5 0.2 mN indentsEs = 2 ± 1 GPaH = 80 ± 40 MPa
2 µm
No residual indent! Used area based on contact depth for calculations
Results: Strain Rate Sensitivity of Wood
10-4 10-3 10-2 10-1 100 101 102
dε/dt (s-1)
102
103
Har
dnes
s(M
Pa)
Untreated Cell WallUntreated Middle LamellaEthylene Glycol Cell WallEthylene Glycol Middle Lamella
ƲH ≈ 0.12
ƲH ≈ 0.07
ƲH ≈ 0.07
ƲH ≈ 0.04Unt ML
Unt CW
EG ML
EG CW
10 µm
Microfibril
(~10 nm)
Discussion: Ultrastructure of S2 Layer
Hemicellulose (red)
Lignin
(blue)
Cellulose
(yellow)
Cellulose
Lignin
Hemicellulose
Discussion: Norimoto’s Model
Lignocellulosic chain Hydroxyl group (-OH)
Microfibril
(~10 nm)
Hemicellulose (red)
Lignin
(blue)
Cellulose
(yellow)
Discussion: Norimoto’s Model for Ethylene Glycol
Microfibril
(~10 nm)
Hemicellulose (red)
Lignin
(blue)
Cellulose
(yellow)
Results confirm ethylene glycol
plasticizes both cell wall and middle lamella
No stable bonds
Summary for Loblolly Pine
Es (GPa) H (MPa) ƲH
Untreated CW 21±3 380±20 0.04
Untreated ML 6±1 340±20 0.07
Ethylene Glycol CW 6±4 80±10 0.07
Ethylene Glycol ML 2±1 80±40 0.12
• Testing both cell walls and middle lamella offers insight to effects of modification on lignin-rich and hemicellulose domains
Conclusion
• Capable of accounting for edge effects in determining Es, H and ƲH
• Extruded edge on PMMA has an effect on properties
• Ethylene glycol plasticizes cell walls and middle lamella– Greater change in ƲH for middle lamella
Future Work
• Develop processing-structure-property relationships– Relate Hardness vs. Strain rate curves to thermodynamic model
• Calculate activation energy and activation volume for deformation events
– Nanoindentation is only one component of characterization• Chemical analyses also important (IR, RAMAN, 2-d NMR)• TEM
• Use single component chemical modifications – Choose chemical probes based on solubility parameters to
selectively modify lignin and hemicellulose sub domains– Create a one stable bond modification (acetylation)– Create IPN’s in sub domains
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