Effect of Matrix Resin Properties on Activity of a Mechanochromic Fluorescent Probe for use in a novel Non-Destructive Inspection Technique for Aerospace Polymer Matrices

Natalie M. Larson, Alex Jen, Ryan E. Toivola, Zhengwei Shi, Sei-Hum Jang, Gary Georgeson* and Brian D. FlinnUniversity of Washington, *The Boeing Company

13th International Symposium on Nondestructive Characterization of Materials Palais des Congrès et de la Culture du Mans, FranceMay 20, 2013

2

Background

Materials

▪ Matrix▪ Probe

Methods

▪ Sample Characterization▪ Probe Response Testing

Results

▪ Sample Characterization▪ Probe Response Testing

Conclusions

Composite Impact Damage

CompositesBarely Visible Impact Damage (BVID): Subsurface damage produced by impact [2]

Aluminum aircraft fuselage shows dents after impact [1]

Research Goal: Improve Visual Inspection Techniques for Composites

[1] Photo of the day: “Miracle on the Hudson” plane, on I-77, Available Online:http://davidsonnews.net/blog/tag/miracle-on-the-hudson/ [2] Iwahori, Y., JAXA Res. Rep. 14 , www.apg.jaxa.jp/eng/publication

3

Background

Materials

▪ Matrix▪ Probe

Methods

▪ Sample Characterization▪ Probe Response Testing

Results

▪ Sample Characterization▪ Probe Response Testing

Conclusions

NDI with Fluorescent Probes

Fluorescencebased inspection *Under UV Light

Probe

ProbeProbe

ProbeProbe

Block of Functionalized Epoxy

Probe

ProbeProbe

ProbeProbe

Block of Functionalized Epoxy

4

Background

Materials

▪ Matrix▪ Probe

Methods

▪ Sample Characterization▪ Probe Response Testing

Results

▪ Sample Characterization▪ Probe Response Testing

Conclusions

Research Question

How do the mechanical properties of the matrix affect

the fluorescent probe response?

Motivations for Research Question:• Polymeric materials used in aerospace fall

within a large range of mechanical properties• Research may reveal information about

molecular mechanism of probe

Research Approach: Reduce Elastic Modulus of Functionalized Epoxy with Diluent

5

Background

Materials

▪ Matrix▪ Probe

Methods

▪ Sample Characterization▪Probe Response Testing

Results

▪ Sample Characterization▪ Probe Response Testing

Conclusions

Matrix Materials

Epoxy: Diglycidyl ether of bisphenol-A (DGEBA)

Curing Agent: Diethylenetriamine (DETA)

Epoxide Functionalized Reactive Diluent: Diglycidyl ether (polypropylene glycol) (DGE(PPG))

Neat Epoxy System: (RT Cure, Optically Clear)

6

Background

Materials

▪ Probe

Methods

▪ Sample Characterization▪Probe Response Testing

Results

▪ Sample Characterization▪ Probe Response Testing

Conclusions

Mechanochromic Fluorescent Probe

Donor Acceptor

ON State-conjugated backbone

OFF State-unconjugated backbone

Force Rips Probe off of epoxy network

YX

OFF State

White Light UV Light

ON State

White Light UV Light

▪ Matrix

7

Background

Materials

▪ Matrix▪ Probe

Methods

▪ Sample Characterization▪Probe Response Testing

Results

▪ Sample Characterization▪ Probe Response Testing

Conclusions

Mechanochromic Fluorescent Probe

350 400 450 500 550 600 650 700 750 800 8500

0.05

0.1

0.15

0.2 UV Vis Absorbance Spectra

Wavelength (nm)

Absorbance Intensity

"ON" Peak at ~535 nm

"OFF" Absorbance: no peak

460 510 560 610 660 710 7600

0.2

0.4

0.6

0.8

1

Fluorescence Spectra

"ON" State"OFF" State

Wavelength (nm)

Fluorescence Intensity

~610 nm~505 nm

8

Background

Materials

▪ Matrix▪ Probe

Methods

▪ Sample Characterization▪Probe Response Testing

Results

▪ Sample Characterization▪ Probe Response Testing

Conclusions

Mixtures FabricatedMixtures Fabricated:• Ranged from 0-100wt% diluent DGE(PPG)• Named by wt% DGE(PPG)

ie. 40wt% DGE(PPG) + 60wt% DGEBA referred to as 40wt% DGE(PPG)

Mixing Procedure

Neat Epoxy (DGEBA)

DiluentDGE(PPG)

DETA + Probe

Light pink solid

Mix, vac, pour

Cure 24h @ RT

yellow liquid

9

Background

Materials

▪ Matrix▪ Probe

Methods

▪ Sample Characterization▪Probe Response Testing

Results

▪ Sample Characterization▪ Probe Response Testing

Conclusions

Sample Characterization1. Elastic Modulus Measurement• Compression test• Used Unloading

Modulus

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.40

100

200

300

f(x) = 1988.14968185262 x − 427.125041830675

Stress Strain Curve

Strain

Stre

ss (M

Pa)

2. Glass Transition Temperature (Tg) • PerkinElmer 7e DMA: Storage Modulus vs. Temp

3. UV-Vis Absorbance Spectra• ThermoFisher UV-Vis

4. Fluorescence Spectra• Stellarnet Blue-WAVE UVN spectrometer • 390 nm excitation light

10

Background

Materials

▪ Matrix▪ Probe

Methods

▪ Sample Characterization▪Probe Response Testing

Results

▪ Sample Characterization▪ Probe Response Testing

Conclusions

Probe Response TestingFluorescence Before Compression

Compress Specimens

Fluorescence After Compression

11

Background

Materials

▪ Matrix▪ Probe

Methods

▪ Sample Characterization▪Probe Response Testing

Results

▪ Sample Characterization▪ Probe Response Testing

Conclusions

Modulus and Tg Results-as cured

0 10 20 30 40 50 60 70 80 90 1000

0.51

1.52

2.5Elastic Modulus vs. DGE(PPG) wt%

wt% DGE(PPG)

Elastic Modu-lus (GPa)

0 10 20 30 40 50 60 70 80 90 1000

102030405060

Glass Transition Temperature Results

wt% DGE(PPG)

Tg (°C) RT

12

Background

Materials

▪ Matrix▪ Probe

Methods

▪ Sample Characterization▪Probe Response Testing

Results

▪ Sample Characterization▪ Probe Response Testing

Conclusions

UV-Vis Absorbance Spectra—as cured

350 450 550 650 750 8500

0.05

0.1

0.15UV Vis Absorbance Spectra

10wt% DGE(PPG)20wt% DGE(PPG)30wt% DGE(PPG)40wt% DGE(PPG)50wt% DGE(PPG)75wt% DGE(PPG)100wt% DGE(PPG)

Wavelength (nm)

Nor

mal

ized

Inte

nsity

"ON" State

0wt % DGE (PPG)

10wt % 20wt % 30wt % 40wt % 50wt % 75wt % 100wt %

13

Background

Materials

▪ Matrix▪ Probe

Methods

▪ Sample Characterization▪Probe Response Testing

Results

▪ Sample Characterization▪ Probe Response Testing

Conclusions

Fluorescence Spectra—as cured

460 520 580 640 700 7600

0.2

0.4

0.6

0.8

1

1.2Normalized Fluorescence Spectra of Samples

0%DGE(PPG) 10%DGE(PPG) 20%DGE(PPG) 30%DGE(PPG) 40%DGE(PPG) 50%DGE(PPG) 75%DGE(PPG) 100%DGE(PPG)

Wavelength (nm)

Nor

mal

ized

Flu

ores

cenc

e In

tens

ity

"ON" State

"OFF" State~505 nm

~610 nm

14

Background

Materials

▪ Matrix▪ Probe

Methods

▪ Sample Characterization▪Probe Response Testing

Results

▪ Sample Characterization▪ Probe Response Testing

Conclusions

Probe Response Testing: Typical Compression Behavior

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.50

500

1000

1500

2000

2500

True Stress-Strain Curve for Incremental Compression of 10wt% DGE(PPG) Sample

True Strain

True

Str

ess (

MPa

) 12

3456

15

Background

Materials

▪ Matrix▪ Probe

Methods

▪ Sample Characterization▪Probe Response Testing

Results

▪ Sample Characterization▪ Probe Response Testing

Conclusions

Probe Response after Deformation*

460 520 580 640 700 7600

0.2

0.4

0.6

0.8

1

Normalized Fluorescence Spectra of Samples (10wt%DGE(PPG))

Wavelength (nm)

Fluo

resc

ence

Inte

nsity

True Plastic Strain:Specimen 1: 0.39±0.02Specimen 2: 0.41±0.02Specimen 3: 0.35±0.02Specimen 4: 0.26±0.02Specimen 5: 0.15±0.02Specimen 6: 0.06±0.02

Arrow indicates Increasing True Strain

UNDEFORMED

*Typical Response for 0-30wt% diluent samples

16

Background

Materials

▪ Matrix▪ Probe

Methods

▪ Sample Characterization▪Probe Response Testing

Results

▪ Sample Characterization▪ Probe Response Testing

Conclusions

Probe Response after Deformation*

460 510 560 610 660 710 7600

0.2

0.4

0.6

0.8

1

Normalized Fluorescence Spectra of Samples (40wt%DGE(PPG))

Wavelength (nm)

Fluo

resc

ence

Inte

nsity

True Plastic Strain:Specimen 1: 0.23±0.02Specimen 2: 0.26±0.02Specimen 3: 0.21±0.02Specimen 4: 0.21±0.02Specimen 5: 0.18±0.02Specimen 6: 0.13±0.02Specimen 7: 0.07±0.02Specimen 8: 0.02±0.02

*Typical Response for 40-100wt% diluent samples

UNDEFORMED

17

Background

Materials

▪ Matrix▪ Probe

Methods

▪ Sample Characterization▪Probe Response Testing

Results

▪ Sample Characterization▪ Probe Response Testing

Conclusions

Probe Response Index (PRI)

𝑃𝑅𝐼=𝐼610𝑛𝑚𝐴𝐶 / 𝐼 505𝑛𝑚

𝐴𝐶 − 𝐼610𝑛𝑚𝐵𝐶 /𝐼 505𝑛𝑚

𝐵𝐶

𝐼610𝑛𝑚𝐵𝐶 / 𝐼 505𝑛𝑚

𝐵𝐶 ∗100%

460 520 580 640 700 7600

0.2

0.4

0.6

0.8

1

Fluorescence Spectra (10wt%DGE(PPG))

Wavelength (nm)

Fluo

resc

ence

Inte

nsity

“OFF” Peak(505nm)

“ON” Peak(~610nm)

HIGH PRI

LOW PRI

• is the fluorescence emission intensity of the peak of wavelength x under condition y,

• BC=Before Compression, AC=After Compression

18

Background

Materials

▪ Matrix▪ Probe

Methods

▪ Sample Characterization▪Probe Response Testing

Results

▪ Sample Characterization▪ Probe Response Testing

Conclusions

PRI vs. Mechanical Deformation

ΔPRI = slope of PRI vs. True Plastic Strain curve

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45-250

255075

100125150175 PRI vs. True Plastic Strain

0wt% DGE(PPG)Linear (0wt% DGE(PPG))10wt%DGE(PPG)Linear (10wt%DGE(PPG))20wt%DGE(PPG)

True Plastic Strain

PRI

Linear Fit?• PRI vs. strain energy• PRI vs. maximum stress on sample• PRI vs. true plastic strain

19

Background

Materials

▪ Matrix▪ Probe

Methods

▪ Sample Characterization▪Probe Response Testing

Results

▪ Sample Characterization▪ Probe Response Testing

Conclusions

ΔPRI vs. wt% DGE(PPG) and Elastic Modulus

0 10 20 30 40-10

90

190

290

ΔPRI vs. wt% DGE(PPG)

wt% DGE(PPG)

ΔPRI

1.2 1.45 1.7 1.95 2.2-10

90

190

290

ΔPRI vs. Elastic Modulus

Elastic Modulus (GPa)

ΔPRI

=Probe Responded to Compression =Probe did not respond to compression

20

Background

Materials

▪ Matrix▪ Probe

Methods

▪ Sample Characterization▪Probe Response Testing

Results

▪ Sample Characterization▪ Probe Response Testing

Conclusions

Conclusions

Conclusion #2:• ΔPRI does not vary monotonically with elastic modulus• ΔPRI decreases monotonically as diluent wt% increases• Effect of elastic modulus on the probe response was not

isolated• Diluent changed mechanical properties and local chemical

environment (polarity, dielectric strength, free volume) –possible effect on probe integration into network

• ΔPRI is more significantly affected by the changes in the local chemical environment than the matrix mechanical properties

Conclusion #1:• Fluorescent probe responds to mechanical deformation for

samples with 0-30wt% diluent• PRI varies linearly with true plastic strain

21

Background

Materials

▪ Matrix▪ Probe

Methods

▪ Sample Characterization▪Probe Response Testing

Results

▪ Sample Characterization▪ Probe Response Testing

Conclusions

Plans for Future Work1. Determine how the local chemical environment

affects the interactions between the probe and host polymers.

2. Separate the effects of matrix stiffness from the effects of the local chemical environment

3. Study effects of DGEBA derivatives with varying molecular weights as reactive diluents.

4. In General, a further understanding of • Temporal stability• Thermal stability• Effect of fiber reinforcements

22

Acknowledgements

Flinn Group:• Dr. Brian Flinn• Dr. Ryan Toivola• Students: Dana Rosenbladt, Ashley Tracey, Curtis Hickmott, Tucker

Howie, Kevin Braun, Ali Dillon, Gary Weber, Jonathan Morasch, Dave Pate, David Sapiro

Funding:• The Boeing Company Project Code B8LDL• Washington Research Foundation • UW Office of Research, Mary Gates Endowment, Levinson Emerging

Scholars Program, UW Undergraduate Research Program

Special Thanks (The Boeing Company):• Kelsi Hurley• Grant Zenkner• Mark Wilenski

23

Backup Slides

R2 Values for Linear Fits to PRI vs:wt% DGE(PPG)

True Plastic Strain

Strain Energy

Maximum Stress Applied

0 0.95 0.98 0.8810 0.88 0.82 0.7220 0.88 0.65 0.6130 0.90 0.66 0.56

R2 Values for Linear Fits of PRI vs. Mechanical Testing Values

24

Backup Slides

43 46 49 52 55-10

90

190

290

390ΔPRI vs. Tg

Glass Transition Temperature (°C)

ΔPRI