Experimental and Theoretical Investigations on Dilute MEH-PPV Solutions

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

Conjugated polymersS. A. Chen, http://www.che.nthu.edu.tw/jacky/010422NYHU-CHE.pdf Polyacetylene (PA)

Conducting Polymers

Progression of conducting polymerS. A. Chen, http://www.che.nthu.edu.tw/jacky/010422NYHU-CHE.pdf

PLED: Schematic DiagramBurroughes, 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).

Preparation of CP Thin Film

PLED DevicePerformanceFilmPropertyPolymerAggregationSchwartz et al. J. Phys. Chem., 104, 237 (2000).MEH-PPV thin filmEffect of Polymer Aggregationon Light-Emitting PropertiesAggregationITO

The polymer structure in solution can apparently affect the film morphology.SolutionAggregationS.-A. Chen et al. J. Phys. Chem., 103, 2375 (1999).FilmAggregationRed ShiftMemory EffectMacroscopicpropertiesMicroscopicproperties SANS, SAXS RheologicalMeasurement MolecularDynamics

SANS Study of MEH-PPV SolutionsMEH-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

Gel Formation at Constant-Temperature Annealingheating to ca. 60 oCH. L. ChenMEH-PPV/chloroformc = 5 mg/ml, T = 25 0CMEH-PPV/chloroformc = 5 mg/ml, T = 40 0C

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

Strategies for Probing Polymer Aggregation in Solutions Annealing at const temperature

Measuring Samplespolymer viscosityvs. stationary timePrecursor Solution ultrasonic & isothermal condition for a weekKeeping Stationary & Isothermal Condition

Constant temperature Annealing

Viscosity decrease with time MEH-PPV in chloroformMEH-PPV in toluene0.1 mg/mlThe time scale for aggregate formation is about days or months)

t (polymer-chain orientation relaxation < ms ??)

Annealing at Varied Temperatures

Annealing at Varied Temperatures MEH-PPV in tolueneMEH-PPV in chloroformheatingheatingcoolingcooling

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 polymert C Size of polymer coils reduce dramaticallySegregationAfter cooling state less aggregationviscosity t C

two different states of aggregation Heating:Cooling:After the cooling state more aggregation viscosity MEH-PPV in tolueneCompact aggregation

SummaryDiscovery 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 DynamicsSeveral Coarse-Grained Pictures for conjugated polymers

The Rigid-Dumbbell Presentation+1-1Arbitrary Originrc+1-1Attraction

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

ConclusionThe 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