Structures and Aggregation of Electroluminescent MEH-PPV in Solution State Probed by Small Angle Neutron Scattering and Viscometry

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研究生：	張啟聖 Chi-Sheng Chang
論文名稱：	Structures and Aggregation of Electroluminescent MEH-PPV in Solution State Probed by Small Angle Neutron Scattering and Viscometry
指導教授：	陳信龍 Hsin-Lung Chen
口試委員:
學位類別：	碩士 Master
系所名稱：	工學院 - 化學工程學系 Department of Chemical Engineering
論文出版年：	2004
畢業學年度：	92
語文別：	英文
論文頁數：	72
中文關鍵詞：	MEH-PPV 、Semi-conducting polymer 、Conjugated polymer 、hairy-rod polymer 、Nanoscale nematic aggregate 、Small angle neutron scattering 、Physical cross-links
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The hairy-rod segments of conjugated polymers MEH-PPV undergo aggregation in the solutions of low concentration and the resultant aggregates with unknown structure have strong impacts on the photophysical properties of the polymers for opto-electronic applications. Using small angle neutron scattering (SANS), we have systematically investigated the conformational structures and aggregation behavior of MEH-PPV dissolved in chloroform and toluene. MEH-PPV dissolved in good solvent chloroform exhibited the expanded Gaussian chain behavior, which can be represented by the successive connection of rod-like segments with the length of ca. 157Å. The rod segments aggregates prevalently in the poorer solvent, toluene. We have also demonstrated that these aggregates were nanoscale nematic domains characterized by the mass fractal dimension of ca. 1.7 and radius of gyration of ca. 90Å. These aggregates accounted for about 10% of the total polymer volume fraction in the freshly prepared MEH-PPV/toluene solution. This kind of nematic aggregate distinguished from the nematic phase found in the classical lyotropic liquid crystalline systems in the sense that they are microphase-separated domains appearing at the concentration well below the threshold lyotropic concentration prescribed by the mean-field theory. The anomalous segmental aggregation is proposed to stem from the enrichment of segmental concentration around the inter-chain overlap points in the solution coupled with the attractive force between the hairy-rod segments induced by the relatively poor solvent quality. This nanoscale aggregation could be prevalent among semi-rigid polymers in general as a process preceding the formal formation of macroscopic nematic phase along the concentration coordinate.

Prolonged aging of the freshly prepared MEH-PPV/toluene solution at low temperature induced the development of another type of aggregates which functioned as physical cross-links among the dissolved chains and consequently transformed the initially viscous solution into the soft gel. The viscosity of the solution increased upon aging, indicating the formation of this kind of aggregates caused the agglomerations of several clusters or growth of clusters from the dissolved chains. The high activation barrier involved in the formation of these aggregates during the aging process implied that they were most likely nanocrystallite. Improvement of solvent quality through heating could melt these nanocrystalline aggregates. The subsequent cooling recovered the nematic aggregates but not nanocrystallite aggregates.

Abstract………………………………………………………………………………Ⅰ
Contents………………………………………………………………………………Ⅱ
Figure contents…………………………………………………………………….…Ⅲ

Chapter 1 Literature review……………………………………………………………1
1-1 The history of semiconducting polymer…………………………………………..1
1-2 The history of electroluminescence devices………………………………………3
1-3 Mechanism of light emission………………………………………………….…..5
    1-3-1 Electronic structure of conjugated polymer………………………………..5
    1-3-2 Fluorescence and phosphorescence………………………………………..9
    1-3-3 Photoluminescence and electroluminescence………………………….…11
1-3-4 Aggregate, Excited dimmer, and Energy transfer……………………………...15
    1-3-4-1 Aggregate………………………………………………………………15
    1-3-4-2 Excited dimmer………………………………………………………...16
    1-3-4-3 Energy transfer…………………………………………………………20
1-4 Relationship between photo-physics and physical structure of MEH-PPV……...23
1-5 Gelation behavior of semiconducting polymer…………………………………..25

Chapter 2 Research motivation and objective………………………………………..27

Chapter 3 Experimental section……………………………………………………...28
3-1 Material and sample preparation…………………………………………………28
3-2 Viscosity measurement……..……………………………………………………29
3-3 Small angle neutron scattering (SANS) experiment……………………………..29

Chapter 4 Results and discussions…………………………………..……………….30
4-1 Structure of MEH-PPV in dilute solution with toluene studied by Viscometry…30
4-2 Structure and aggregation behavior of MEH-PPV in the solution state studied by small angle neutron scattering…………………………………………………..39
4-3 MEH-PPV aggregation upon prolonged aging effect………………..…………..61

Chapter 5 Conclusion………………...……………………………………………....68

References……………………………………………………………………………69



Figure contents
Figure 1-1-1. Chemical structure of polyacetylene.-----------------------------------------2
Figure 1-1-2. Chemical structure of polythiophene (PT), polypyrrole (PPy), poly(para-phenylene)(PPP), polyaniline (PAn),and poly(phenylene vinylene) (PPV).-----------------------------------------------------------------2
Figure 1-2-1. Chemical structure of Alq3, TPD, and PBD.---------------------------------4
Figure 1-2-2. Device structure for polymer light emitting diode based on PPV.--------4
Figure 1-3-1-1. Band picture for an insulator, a semiconductor and a metal.------------6
Figure 1-3-1-2. Diagrammatic representation of the energy levels of πMOs with increasing size of the molecule for [CH]x.---------------------------------6
Figure 1-3-1-3. Illustration of radical cation (polaron) formed by removal of one electron and the migration of polaron.--------------------------------------8
Figure 1-3-1-4. Illustration of the formation of bipolaron.---------------------------------8
Figure 1-3-1-5. The energy level of polaron and bipolaron.-------------------------------8
Figure 1-3-2-1. A typical Jabłoński diagram.-----------------------------------------------10
Figure 1-3-3-1. Illustration band diagram of single exciton formation for photoluminescence.----------------------------------------------------------13
Figure 1-3-3-2. Illustration band diagram of single exciton formation for electroluminescence.--------------------------------------------------------13
Figure 1-3-3-3. Schematic picture of a single-layer electroluminescent device …… 13
Figure 1-3-3-4. Schematic energy level diagram for an ITO/PPV/Al LED, showing the ionization potential (IP) and electron affinity (EA) of PPV, the work functions of ITO and Al (□ITO and □Al ), and the barriers to injection of electrons and holes (□Ee and □Eh).------------------------ 14
Figure 1-3-4-2-1. Orbital interactions of MN collision pairs and M---*---N exciplexes.------------------------------------------------------------------19
Figure 1-3-4-2-2. Surface interpretation of excimer emission.---------------------------19
Figure 1-3-4-3-1. Diagram for energy transfer between different extents of aggregations.---------------------------------------------------------------22
Figure 3-1-1. Chemical structure of MEH-PPV.--------------------------------------------28
Figure 4-1-1. The plot of □sp/c and ln□r/c against concentration at 30℃.--------------32
Figure 4-1-2. The effect of annealing time on the specific viscosity of 0.01% and
0.05% MEH-PPV/toluene solutions.---------------------------------------- 35
Figure 4-1-3. The effect of annealing time on the specific viscosity of 0.1% and 0.3%
MEH-PPV/toluene solutions.----------------------------------------------- 35
Figure 4-1-4. The specific viscosity vs. annealing temperature for 0.01% and 0.05%. MEH-PPV/toluene solutions.----------------------------------------------- 36
Figure 4-1-5. The specific viscosity against annealing temperature for 0.1% and 0.3% MEH-PPV/toluene solutions.-------------------------------------------------36
Figure 4-1-6. Proposed model of solution structure with annealing and thermal effect.-------------------------------------------------------------------------37,38
Figure 4-2-1. SANS profiles of MEH-PPV/chloroform solutions at 25℃ in a log-log plot, demonstrating the power law of I(q), i.e. I(q) ~ q-□, in different q regions.--------------------------------------------------------------------------43
Figure 4-2-2. Kratky plots of I(q)q1.7 to demonstrate the power law of I(q) ~ q-1.7 in the middle-q regions.----------------------------------------------------------43
Figure 4-2-3. Kratky plots of I(q)q to demonstrate the power law of I(q) ~ q-1 in the high-q regions.----------------------------------------------------------------44
Figure 4-2-4. Concentration-normalized SANS profiles of MEH-PPV/chloroform solutions.------------------------------------------------------------------------44
Figure 4-2-5. G(q)-1 vs. q plots for the determination of the mass per unit length (ML) of the dissolved MEH-PPV chain segment using the high-q SANS intensity.-------------------------------------------------------------------------45
Figure 4-2-6. The radius of the rod is evaluated from the cross-section Guinier’s plot.------------------------------------------------------------------------------45
Figure 4-2-7. Schematic illustration showing the expanded chain behavior of the hairy-rod polymer MEH-PPV in chloroform solutions.------------------46
Figure 4-2-8. Typical conformations of a 100-segment homopolymer generated by Monte Carlo simulations. They are denoted as: Ⅰ, random coil; Ⅱ, molten globule; Ⅲ, toroid; Ⅳ, rod; Ⅴ, defect-coil; Ⅵ, defect-cylinder.-----------------------------------------------------------------46
Figure 4-2-9. SANS profiles of MEH-PPV/toluene solutions at 25℃ in a log-log plot.-----------------------------------------------------------------------------52
Figure 4-2-10. Concentration-normalized profiles of MEH-PPV/toluene solutions.--52
Figure 4-2-11. Neutron contrast-normalized profiles of 0.5 wt% toluene and chloroform solutions.---------------------------------------------------------53
Figure 4-2-12. Neutron contrast-normalized profiles of 1.0 wt% toluene and chloroform solutions.---------------------------------------------------------53
Figure 4-2-13. A series of SANS profiles of 0.5 wt% MEH-PPV/toluene solution collected in-situ in a heating cycle.-----------------------------------------54
Figure 4-2-14. A series of SANS profiles of 1.0 wt% MEH-PPV/toluene solution collected in-situ in a heating cycle.-----------------------------------------54
Figure 4-2-15. The SANS profile of 0.5 wt% MEH-PPV/toluene solution collected in-situ at 85℃.----------------------------------------------------------------55
Figure 4-2-16. The SANS profile of 1.0 wt% MEH-PPV/toluene solution collected in-situ at 85℃.----------------------------------------------------------------55
Figure 4-2-17. SANS profiles of 0.5 wt% demonstrates how the scaling factor is chosen to obtain Ia(q). The scaling factor is chosen in such a way that the tail region of the intensity profile at 85℃ multiplied by the scaling factor matches that of I(q,T).------------------------------------------------56
Figure 4-2-18. SANS profiles of 1.0 wt% demonstrates how the scaling factor is chosen to obtain Ia(q). The scaling factor is chosen in such a way that the tail region of the intensity profile at 85℃ multiplied by the scaling factor matches that of I(q,T).------------------------------------------------56
Figure 4-2-19. Ia(q) in a log-log plot obtained from the subtraction procedure for 0.5 wt% and 1.0 wt% solutions at 25℃. In the middle- to high-q region where the form factor of the aggregates dominates, the data possesses a slope of ca. -1.7, signifying that the aggregates are fractal domains with a mass fractal dimension of ca. 1.7.----------------------------------57
Figure 4-2-20. Schematic illustration showing the aggregation of hairy-rod polymers in solution. The hairs or side chains attached to the rod segments are not shown for simplicity.   A portion of the segments in different molecules assemble into nanoscale nematic domains (marked by the dashed circles), while the rest remains uniformly dissolved in the solvent.-------------------------------------------------------------------------58
Figure 4-2-21. Temperature dependence of □a determined from the invariant of the aggregate scattering in the heating and cooling cycles. ----------------59
Figure 4-2-22. The SANS profiles of the 0.5 wt% MEH-PPV/toluene solution collected in-situ in the cooling cycle.--------------------------------------60
Figure 4-2-23. The SANS profiles of the 1.0 wt% MEH-PPV/toluene solution collected in-situ in the cooling cycle.--------------------------------------60
Figure 4-3-1. SANS profiles of 1.0 wt% MEH-PPV/toluene solutions in the freshly prepared state and after aging -4℃ for two days.--------------------------63
Figure 4-3-2. The plot displays the intensity curves in the semi-logarithmic plot to
demonstrate more clearly the enhanced low-q intensity upon aging.---63
Figure 4-3-3. A series of SANS profiles collected in-situ in the heating cycle for the aged MEH-PPV gel.-----------------------------------------------------------64
Figure 4-3-4. Comparison between the SANS profile of the aged gel heated to 85℃ and that of the freshly prepared solution heated to 85℃. Both scattering curves match well with each other, indicating that the aggregates developed through the aging are almost washed out at this temperature.---------------------------------------------------------------------64
Figure 4-3-5. A series of SANS profiles collected in-situ in the cooling cycle for the aged MEH-PPVgel.------------------------------------------------------------65
Figure 4-3-6. The SANS profile of the aged gel cooled after the series of heating experiment represented by Figure 4-3-3 was completed.  The scattering profiles of the aged gel heated to 85℃, the aged gel at 25℃ RT, and the freshly prepared solution at 25℃ are also displayed for comparison.--------------------------------------------------------------------66
Figure 4-3-7. The DSC heatng curve of MEH-PPV gel formed by aging at RT for 24 days.------------------------------------------------------------------------------67

                                

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