Experimental and Theoretical Investigations on Dilute

MEH-PPV Solutions

謝盛昌、陳建龍、張志偉、林信宏、華繼中

國立中正大學化學工程系

Polymer Rheology and Molecular Simulation Lab., National Chung Cheng University, Taiwan

Conjugated polymers

S. A. Chen, http://www.che.nthu.edu.tw/jacky/010422NYHU-CHE.pdf

Polyacetylene (PA)

Conducting Polymers

Progression of conducting polymer

S. A. Chen, http://www.che.nthu.edu.tw/jacky/010422NYHU-CHE.pdf

PLED: Schematic Diagram

Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 347, 539 (1990).

LUMO

HOMO

hv

Cathode

Anode

e-

e-

Preparation of CP Thin Film

Spin-coating

Casting

Ink-jet printing

PLED DevicePerformance

FilmProperty

PolymerAggregation

Schwartz et al. J. Phys. Chem., 104, 237 (2000).

MEH-PPV thin film

Effect of Polymer Aggregationon Light-Emitting Properties

Aggregation

ITO

The polymer structure in solution can apparently affect the film morphology.

SolutionAggregation

S.-A. Chen et al. J. Phys. Chem., 103, 2375 (1999).

FilmAggregation

Red Shift

Memory Effect

Macroscopic

properties

Microscopic

properties

SANS, SAXS

RheologicalMeasurement

MolecularDynamics

SANS Study of MEH-PPV Solutions

MEH-PPV chains in chloroform are well separated and MEH-PPV chains in toluene are tied together by aggregates yielding clusters.

The aggregates present in the solution may serve as the nuclei from which the nematic domains will grow at high polymer conc. during the solvent evaporation in the spin coating process.

H. L. Chen

MEH-PPV/toluenec = 10 mg/ml (fresh)

sol

MEH-PPV/toluenec = 10 mg/ml

(aged at RT for 60 days)gel

heating to ca. 60 oC

Gel Formation at Constant-Temperature Annealing

H. L. Chen

MEH-PPV/chloroform

c = 5 mg/ml, T = 25 0C

MEH-PPV/chloroform

c = 5 mg/ml, T = 40 0C

Sample MEHPPV (Mw:86000, Aldrich); Chloro

form and Toluene

Instrument Paar Physica MCR500; CC28.7 (Concentric cylinder)

Rheological Experiments

Strategies for Probing Polymer Aggregation in Solutions

Measuring Samplespolymer viscosity

vs. stationary time

Precursor Solution ultrasonic

& isothermal condition for a week

Keeping Stationary

& Isothermal Condition

Annealing at const temperature

Constant temperature Annealing

PS in cyclohexane T : 313K

t (hr)20 40 60 80 100 120 140 160 180 200 220 240

po (cP)

0.00

0.02

0.04

0.06

0.08

0.546 mg/ml0.780 mg/ml2.344 mg/ml

5 mg/ml

annealing time (hr)

0 20 40 60 80 100 120 140 160 180

poly

mer

vis

cosi

ty (

cP)

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

313 k ; Annealing

Viscosity decrease with time

MEH-PPV in chloroform MEH-PPV in toluene

annealing time (hr)

0 200 400 600 800 1000 1200 1400 1600

Zer

o sh

ear

rate

vis

cosi

ty (

cp)

0.10

0.15

0.20

0.25

0.30

0.35

0.40

313 K298 K

0.1 mg/ml

The time scale for aggregate formation is about days or months)

t (polymer-chain orientation relaxation < ms ??)<< t (experimental measurements ~ hrs) << t (formation of polymer aggregates ~ days)

Measuring Samplespolymer viscosity

vs. varied temperature

Precursor Solution ultrasonic

& isothermal condition for a week

Heating

Cooling DownMeasuring Samples

polymer viscosityvs.

varied temperature

Annealing at varied temperatures

Annealing at Varied Temperatures

PS in cyclohexane

Mw : 2.8*105 C:1.56 mg/ml

1/T(K)0.00295 0.00300 0.00305 0.00310 0.00315 0.00320 0.00325 0.00330 0.00335

p(M/cRT) (s)

1e-8

1e-7

1e-6

1e-5

heatingannealing

303308313318323328333 298338

1/T

0.0028 0.0030 0.0032 0.0034 0.0036 0.0038

M

/ cR

T (s

)

1e-6

1e-5

1e-4

0.3 mg, heating0.3 mg, annealing5 mg, heating5 mg, annealing

Annealing at Varied Temperatures

MEH-PPV in tolueneMEH-PPV in chloroform

1 / T0.0031 0.0032 0.0033 0.0034 0.0035 0.0036 0.0037

0 M

/ cR

T (s

)

1e-6

1e-5

1e-4

5 mg, heating5 mg, annealing0.3 mg, heating0.3 mg, annealing

278288298308T (K)

318heating

heating

cooling

cooling

Proposed Aggregation Dynamics under Varied-Temperature Annealing for MEH-PPV in Chloroform

More Aggregation is formed

Heating process Size of polymer coils Kinetic energy of polymer

Less Aggregation is formed

Cooling process Size of polymer coils Kinetic energy of polymer

t °C Size of polymer coils

reduce dramaticallySegregation

After cooling state

less aggregation

viscosity

t °C

two different states of aggregation

Heating:

Cooling:

After the cooling state more aggregation

viscosity

MEH-PPV in toluene

Compact aggregation

Summary

Discovery of gel formation in a ‘good’ solvent (i.e., Chloroform)

Evidence of aggregate formation in both solvents based on rheological measurements in both types of annealing experiments

Molecular Dynamics

Several Coarse-Grained Pictures for conjugated polymers

The Rigid-Dumbbell Presentation

+1

-1

uL

Arbitrary Origin

rc+1

-1

Attraction

Forces Acting on A Rigid Dumbbell

1,2

1

Force Drag

c,f,h,

ii,iii L urrVrF

tTk i

i d

d2

ForceBrownian

,B

b,

WF

,,

,,

,1

a, 1

Force Attractive

ij

ijN

ijji

rr

rrF

Linear Response Theory for Computing Zero-Shear Viscosity

0 ,p,pB

0

e1

e1

hp,

d0

2

13

tttTnk

TkTkTk

ii

iBiBB

ii

ττ

FFuuuδ

FRτ

The Simulation Ensemble

0

5

10

15

20

25

30

0

5

10

15

20

25

30

05

1015

2025

Z

X

Y

/ (kBTref) = 1, t / = 2000000

0

5

10

15

20

25

30

0

5

10

15

20

25

30

05

1015

2025

Z

X

Y

/ (kBTref) = 1, t / = 0.1

0

5

10

15

20

25

30

0

5

10

15

20

25

30

05

1015

2025

Z

X

Y

/ (kBTref) = 1, t / = 1000000

0

5

10

15

20

25

30

0

5

10

15

20

25

30

05

1015

2025

Z

X

Y

/ (kBTref) = 1, t / = 4000000

0

5

10

15

20

25

30

0

5

10

15

20

25

30

05

1015

2025

Z

X

Y

/ (kBTref) = 1, t / = 6000000

0

5

10

15

20

25

30

0

5

10

15

20

25

30

05

1015

2025

Z

X

Y

/ (kBTref) = 1, t / = 8000000

Realizations at Different Times

Annealing at Constant Temperature / (kBTref) = 1

t /

1e-1 1e+0 1e+1 1e+2 1e+3 1e+4 1e+5 1e+6 1e+7Zer

o-S

hear

Vis

cosi

ty,

0 / (

nkBT

)

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

T / Tref = 1

T / Tref = 0.8

T / Tref = 1.2

Heating and Cooling Processes

/ (kBTref) = 1

T / Tref

0.92 0.94 0.96 0.98 1.00 1.02 1.04 1.06 1.08 1.10 1.12Zer

o-S

hear

Vis

cosi

ty,

0 / (

nkBT

ref)

0.3

0.4

0.5

0.6

0.7

0.8

0.9

heatingcoolingfast heating again

Attractive Force

0

5

10

15

20

25

30

0

5

10

15

20

25

30

05

1015

2025

Z

X

Y

/ kBTref = 10, t / = 1000000

0

5

10

15

20

25

30

0

5

10

15

20

25

30

05

1015

2025

Z

X

Y

/ (kBTref) = 1, t / = 8000000

0

5

10

15

20

25

30

0

5

10

15

20

25

30

05

1015

2025

Z

X

Y

/ kBTref = 100, t / = 4000000

Conclusion

The molecular dynamics simulations are able to provide molecular insights into the dynamics of aggregate formation as well as the effect of annealing on low-shear polymer viscosity