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, Polyacetylene (PA)

Conducting Polymers

Progression of conducting polymer S. A. Chen,

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- e-e-

Preparation of CP Thin Film Spin-coating Casting Ink-jet printing

PLED Device Performance Film Property Polymer Aggregation Schwartz et al. J. Phys. Chem., 104, 237 (2000). MEH-PPV thin film Effect of Polymer Aggregation on Light-Emitting Properties Aggregation ITO

The polymer structure in solution can apparently affect the film morphology. Solution Aggregation S.-A. Chen et al. J. Phys. Chem., 103, 2375 (1999). Film Aggregation Red Shift Memory Effect Macroscopic properties Microscopic properties SANS, SAXS Rheological Measurement Molecular Dynamics

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/toluene c = 10 mg/ml (fresh) sol MEH-PPV/toluene c = 10 mg/ml (aged at RT for 60 days) gel heating to ca. 60 o C Gel Formation at Constant-Temperature Annealing H. L. Chen MEH-PPV/chloroform c = 5 mg/ml, T = 25 0 C MEH-PPV/chloroform c = 5 mg/ml, T = 40 0 C

 Sample MEHPPV (Mw:86000, Aldrich); Chloroform and Toluene  Instrument Paar Physica MCR500; CC28.7 (Concentric cylinder) Rheological Experiments

Strategies for Probing Polymer Aggregation in Solutions Measuring Samples polymer viscosity vs. stationary time Precursor Solution ultrasonic & isothermal condition for a week Keeping Stationary & Isothermal Condition  Annealing at const temperature

Constant temperature Annealing

Viscosity decrease with time MEH-PPV in chloroformMEH-PPV in toluene 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 Samples polymer viscosity vs. varied temperature Precursor Solution ultrasonic & isothermal condition for a week Heating Cooling Down Measuring Samples polymer viscosity vs. varied temperature  Annealing at varied temperatures

Annealing at Varied Temperatures

MEH-PPV in toluene MEH-PPV in chloroform heating 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 dramatically Segregation 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 Arbitrary Origin rcrc +1 Attraction

Forces Acting on A Rigid Dumbbell

Linear Response Theory for Computing Zero-Shear Viscosity

The Simulation Ensemble

Realizations at Different Times

Annealing at Constant Temperature

Heating and Cooling Processes

Attractive Force

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