RHEOLOGICAL MEASUREMENTS

BY AFM OF THE FORMATION

OF POLYMER NANOFIBERS

M. M. Yazdanpanah, M. Hosseini, S. Pabba, S. M. Berry, V. V. Dobrokhotov,

A. Safir, R. S. Keynton and R. W. Cohn

ElectroOptics Research Institute & Nanotechnology Center

University of Louisville

The Processing Parameter P: Key material parameter that predicts nanofiber dimensions

Measurements of material constants that comprise P:Surface tension, evaporation rate, viscosity

Using constant diameter nanoneedles for measuring P and drawing of nanofibers

RHEOLOGICAL MEASUREMENTS BY AFM OF THE FORMATION OF POLYMER NANOFIBERS

PROCESSING PARAMETER AND FIBER FORMATION

Fiber diameter depends on

as well as droplet diameter

(Tripathi and McKinley)

P

P = 0.01

P

P = 0.05

Rm

id/R

0R

mid

/R0

P

P = 0.01

P

P = 0.05

P

P = 0.01

P = 0.05

Rm

id/R

0

= 0 P = 0.005P

P = 0.01

P

P = 0.05

Rm

id/R

0

P

P = 0.01

P

P = 0.05

P

P = 0.01

P

P = 0.05

Rm

id/R

0R

mid

/R0

P

P = 0.01

P

P = 0.05

P

P = 0.01

P = 0.05

Rm

id/R

0

= 0 P = 0.005

Evaporation rate x viscosityP =

Surface tension

Harfenist et al. Nanoletter 2004

CONSTANT DIAMETER NANONEEDLE AFM TIPS FOR PULLING POLYMERIC NANOFIBER

Thinnest fibers are drawn with thinnest tips

Needles on AFM tips point to interactive measurementof processing parameter while drawing fibers

5 m

100 nm

Needle

Polymerfiber

Yazdanpanah et. al., JAP 2005

AFM MEASUREMENT OF

Surface tension

Evaporation rate

ViscosityTo get processing parameter

using constant diameter nanoneedles

Circumference needed to extract surface tension

On tapered tips, wetting force grows unstably with depth

Viscous drag force is directly related to insertion depth

ADVANTAGES OF CONSTANT DIAMETER NEEDLES FOR AFM ANALYSIS OF LIQUIDS

cos2 rFe

10m

SELECTIVE GROWTH OF LONG NEEDLES

Time lapse images show growth of a 70 micron long needle

Fs

Fe < FS

Fe = FS

Fe = FS cos(

Fs

Fe < FS

Fs

F < FS

Fr = FS

Fe = FS cos(

Fs

Fe < FS

Fs

Fe < FS

Fe = FS

Fe = FS cos(

Fs

Fe < FS

Fs

F < FS

Fr = FS

Fe = FS cos(

MEASUREMENT OF SURFACE TENSION

PLUS CONTACT ANGLE

Surface tension is found from

Contact angle is found from

cosre FF

SEM images of needle being retracted from vacuum oil

-50

0

4 3 2 1 0

Distance (µm)

-50

0

-50

0

-50

0

F/

D(m

N/m

)

Water

Chlorobenzene

Vacuum Oil

Dibasic ester

Distance (µm)

-50

0

2 1 0

-50

0

-50

0

-50

0

-50

0

4 3 2 1 0

Distance (µm)

-50

0

-50

0

-50

0

F/

D(m

N/m

)

Water

Chlorobenzene

Vacuum Oil

Dibasic ester

-50

0

4 3 2 1 0

Distance (µm)

-50

0

-50

0

-50

0

F/

D(m

N/m

)

-50

0

4 3 2 1 0

Distance (µm)

-50

0

-50

0

-50

0

F/

D(m

N/m

)

Water

Chlorobenzene

Vacuum Oil

Dibasic ester

Distance (µm)

-50

0

2 1 0

-50

0

-50

0

-50

0

-50

0

2 1 0

-50

0

-50

0

-50

0

FrFe

-50

0

4 3 2 1 0

Distance (µm)

-50

0

-50

0

-50

0

F/

D(m

N/m

)

Water

Chlorobenzene

Vacuum Oil

Dibasic ester

Distance (µm)

-50

0

2 1 0

-50

0

-50

0

-50

0

-50

0

4 3 2 1 0

Distance (µm)

-50

0

-50

0

-50

0

F/

D(m

N/m

)

Water

Chlorobenzene

Vacuum Oil

Dibasic ester

-50

0

4 3 2 1 0

Distance (µm)

-50

0

-50

0

-50

0

F/

D(m

N/m

)

-50

0

4 3 2 1 0

Distance (µm)

-50

0

-50

0

-50

0

F/

D(m

N/m

)

Water

Chlorobenzene

Vacuum Oil

Dibasic ester

Distance (µm)

-50

0

2 1 0

-50

0

-50

0

-50

0

-50

0

2 1 0

-50

0

-50

0

-50

0

rFF Sr 2

-50

0

4 3 2 1 0

Distance (µm)

-50

0

-50

0

-50

0

F/

D(m

N/m

)

Water

Chlorobenzene

Vacuum Oil

Dibasic ester

Distance (µm)

-50

0

2 1 0

-50

0

-50

0

-50

0

-50

0

4 3 2 1 0

Distance (µm)

-50

0

-50

0

-50

0

F/

D(m

N/m

)

Water

Chlorobenzene

Vacuum Oil

Dibasic ester

-50

0

4 3 2 1 0

Distance (µm)

-50

0

-50

0

-50

0

F/

D(m

N/m

)

-50

0

4 3 2 1 0

Distance (µm)

-50

0

-50

0

-50

0

F/

D(m

N/m

)

Water

Chlorobenzene

Vacuum Oil

Dibasic ester

Distance (µm)

-50

0

2 1 0

-50

0

-50

0

-50

0

-50

0

2 1 0

-50

0

-50

0

-50

0

TYPICAL F-D CURVES FOR MEASURING SURFACE TENSION AND CONTACT ANGLE

5 m

100 nm

5 m

300nm

Ag2Ga alloy, dual-diameter

needle

Parylene coated, single-diameter

needle

Vertical axis scaled by circumference to be in units of surface tension

Surface tension (mN/m)

Surface tension

published (mN/m)

Contact angle(degrees)

Bare Coated Bare Coated

Water 74.6 74.2 72.8 51 ± 2 62 ± 2

Chlorobenzene 33.1 35.4 33.0 27± 3 21 ± 3

Dibasic ester 37.5 37.4 35.6 24 ±1 22 ± 1

Vacuum oil 36.8 36.9 35 22 ± 1 27 ± 1

Isopropanol 24.9 24.8 23 20 ±3 19 ± 3

Toluene - 30.3 27.9 - 32 ± 2

SURFACE TENSION AND

CONTACT ANGLE

Measured and published values of surface tension in agreement

Contact angles differ by up to 11o for bare and coated needles

Each measurement is the average of at least 20 measurements. The deviation reported is peak-to-peak.

MEASURING EVAPORATION RATE

AFM is sensitive to sub-nanometer changes in surface height between repetitive scans

AFM data is in good agreement with TGA data

-20

0

nN

2 1 0µm

l

AFM(m/s)

TGA (m/s)

Acetone 1.45 ± 0.05 1.02

IPA 0.32 ± 0.01 0.178

CLB 0.11 ± 0.01 0.098

DI 0.027 ± 0.001 0.036

DMF 0.0115 ± 0.0005 0.0094

DBE 0.00026 ± 0.0001 0.00022

Vacuum oil ~ 0 ~0

VISCOUS DRAG FORCE ON NEEDLELOWERS Q OF THE VIBRATING CANTILEVER

Measurements of Q-damping vs. needle insertion depth are fit to a model of drag force to give one value of viscosity

Measurements of glycerol-water track literature and our own shear plate viscometer measurements over three orders of magnitude

Needle is rugged: Same needle used for every measurement!!

0 20 40 60 80 100

1

10

100

1000

Vis

cosi

ty (

cP)

Concentration (wt.%)

20o C

30o C

AFM data at 22 oC

Viscometer data 24 oC

0.0 5.0x10-6 1.0x10-5 1.5x10-5

0.0

0.5

1.0

1/Q

Distance (m)

(a) (b)

0 20 40 60 80 100

1

10

100

1000

Vis

cosi

ty (

cP)

Concentration (wt.%)

20o C

30o C

AFM data at 22 oC

Viscometer data 24 oC

0.0 5.0x10-6 1.0x10-5 1.5x10-5

0.0

0.5

1.0

1/Q

Distance (m)

(a) (b)

99.5 wt %

80 wt %

13.4 wt %

0 20 40 60 80 100

1

10

100

1000

Vis

cosi

ty (

cP)

Concentration (wt.%)

20o C

30o C

AFM data at 22 oC

Viscometer data 24 oC

0.0 5.0x10-6 1.0x10-5 1.5x10-5

0.0

0.5

1.0

1/Q

Distance (m)

(a) (b)

0 20 40 60 80 100

1

10

100

1000

Vis

cosi

ty (

cP)

Concentration (wt.%)

20o C

30o C

AFM data at 22 oC

Viscometer data 24 oC

0.0 5.0x10-6 1.0x10-5 1.5x10-5

0.0

0.5

1.0

1/Q

Distance (m)

(a) (b)

Glycerolin water

Fiber length increase over an order of magnitude closely following a log-log trend

Liquid with these low value of P does not form stable fibers or strings, but were used to enable cleaner, more ideal first-time measurements

These results point towards the controlled, interactivedrawing of nanofibers with these needles and higher P liquids

STRONG CORRELATION BETWEEN FIBER LENGTH AND PROCESSING PARAMETER

(a) (b)

10-10 10-8 10-6 1x10-4

102

103

lb

tb

t b (s)

l b (n

m)

P

1 10

0.1

1

Concentration (wt.%)

0 2 4 6 8

-160

-80

0

nN

1.51.00.50.0

Sec

time (S)

Time (s)

Fo

rce

(n

N)

tb

(a) (b)

10-10 10-8 10-6 1x10-4

102

103

lb

tb

t b (s)

l b (n

m)

P

1 10

0.1

1

Concentration (wt.%)

0 2 4 6 8

10-10 10-8 10-6 1x10-4

102

103

lb

tb

t b (s)

l b (n

m)

P

1 10

0.1

1

Concentration (wt.%)

0 2 4 6 8

-160

-80

0

nN

1.51.00.50.0

Sec

time (S)

Time (s)

Fo

rce

(n

N)

tb-160

-80

0

nN

1.51.00.50.0

Sec

time (S)

Time (s)

Fo

rce

(n

N)

tb

P

AFM F-T curves of breakup length

Processing Parameter PLength (m)

lb

Constant diameter nanoneedles were used to measure the Processing Parameter P as well as to show the correlation between P and fiber length

For simple liquids, surface tension, evaporation rate and viscosity were measured with reasonable to high accuracy

For random chain polymeric liquids viscosity was very low, but this may accurately reflect viscosity for the nanoscale

displacements of the thermally-actuated AFM cantilevers

The needles are very rugged and reliable for liquid studies—Only 4 needles were used for the entire study!

SUMMARY

RHEOLOGICAL MEASUREMENTS BY AFM