Young’s Modulus of Polyurethane foam from Force Spectroscopy using AFM

Tejasvi Parupudi

Department of ECET

Purdue University

Motivation

• Polyurethane foams - used in flexible implants.

• Polyurethane in conjunction with Polyethylene-glycol or PEG coatings used for neural electrodes

• Comparison of Young’s Modulus in macroscaleand nanoscale

(Tsukruk et al., 2000)

Project Goals

• To determine Young’s Modulus based on the indentation (strain) produced and force applied per area (stress) by an AFM tip on two different regions- 1)on the cell and 2)in between cells

• To understand the viscoelastic properties of polyurethane (a soft sample because it is a polymer) using contact mode force-distance curves

• To explore adhesion forces from retraction curves

Deflection sensitivity

• On pretreated glass (Ethanol + DI), thrice

Deflection Z curve on glass before (left) and after (right) force curve experiment on polyurethane foam

Spring constant• Thermal tune

• Lorentzian fit

• Average of five trials

K=0.0471 N/m and 0.12 N/m(Soft cantilever)

Tip properties

Bruker Inc.

VEDA Simulations- Approach curves

With Adhesion force 1.2 nNWith Adhesion force 6 nN

Experiment 1: K=0.04708 N/m; JKR model; Z start: 10nm; Z end: -5 nm; Sample YM: 2.323kPa

k=0.0471N/mFadh= 6nN

k=0.0471N/mFadh= 1.2nN

VEDA Simulations- Approach curves

With Adhesion force 1.2 nNWith Adhesion force 6 nN

Experiment 2: K=0.1197 N/m; JKR model; Z start: 10nm; Z end: -5 nm; Sample YM: 2.323kPa

k=0.1197N/mFadh= 6nN

k=0.1197N/mFadh= 1.2nN

Flattened height image and section profile 3D height image

Laser position on cantilever before (left) and after (right) force curve experiment. SUM: 7.24V

Tapping Mode height image of Polyurethane

Force-distance curves and Sneddon fitExperiment 1

Image Data Deflection Error X Data Type Ramp (Z position)Sens. DeflSens 44.62 nm/VPlot Units Force Spring Constant 0.04708 N/mDisplay Mode Deflection Error vs. Sep Plot Invert Normal / Invert

δ

InputsActive Curve Extend (Approach)Fit Method Linearized Model Include AdhesionForce Yes Max Force Boundary 70 %Min Force Boundary 10 %Fit Model Sneddon (Conical)

ResultsR² 0.8195 Young's Modulus 6.79MPa

Force-distance curves and Sneddon fitExperiment 2

Image Data Deflection Error X Data Type Ramp (Z position)Sens. DeflSens 27.69 nm/VPlot Units Force Spring Constant 0.1197 N/mDisplay Mode Defl Error vs. Sep Plot Invert Normal

InputsActive Curve Extend (Approach)Fit Method Linearized Model Include Adhesion Force Yes Max Force Boundary 70 %Min Force Boundary 10 %Fit Model Sneddon (Conical)

ResultsR² 0.9901 Young's Modulus 0.0279 MPa

Nanoindentation experiment using AFM on Polyurethane resulted in vales of Young’s Modulus between 10-20 MPa

(Chizhik et al., 1998)

Comparison of Young’s Modulus from literature

Conical punch

Experiment 1 Experiment 2Y= 6.79 MPa Y= 27.9 kPa

Sneddon model equation

Adhesion force

δ

Conclusions/Recommendations

• Young’s Modulus value is dependent on location of indentation, theoritical model used, tip geometry and shape.

• Value of Young’s Modulus closely relates to the value from other studies but is higher than the macroscalemodulus (=>higher mechanical strength)

• Viscoelastic nature of polyurethane foam (non linear region) is clearly seen

• Adhesion force is greater when a indentation was performed with softer cantilever

• Need for viscoelastic model on VEDA