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)

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Load

.L(μ

N)

0 50 100 150 200 250 300Depth, h (nm)

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.L(μ

N)

0 50 100 150 200 250 300Depth, h (nm)

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.L(μ

N)

0 50 100 150 200 250 300Depth, h (nm)

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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)

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1000

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.L(μ

N)

0 50 100 150 200 250 300Depth, h (nm)

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.L(μ

N)

0 50 100 150 200 250 300Depth, h (nm)

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.L(μ

N)

Cm

0 50 100 150 200 250 300Depth, h (nm)

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1000

Load

.L(μ

N)

Cm not accounted for

( ) ( ) ( ) ( )cmtcpceff hACChAChA

SE−

=== 11

Infinite half-spaceDepth, h

Load

, L

Nanoindentation of Wood

Cellular Structure

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

Thank you!

Questions?

2 µm2 µm