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研究生:張啟聖
研究生(外文):Chi-Sheng Chang
論文名稱:StructuresandAggregationofElectroluminescentMEH-PPVinSolutionStateProbedbySmallAngleNeutronScatteringandViscometry
指導教授:陳信龍陳信龍引用關係
指導教授(外文):Hsin-Lung Chen
學位類別:碩士
校院名稱:國立清華大學
系所名稱:化學工程學系
學門:工程學門
學類:化學工程學類
論文種類:學術論文
畢業學年度:92
語文別:英文
論文頁數:72
中文關鍵詞:MEH-PPVSemi-conducting polymerConjugated polymerhairy-rod polymerNanoscale nematic aggregateSmall angle neutron scatteringPhysical 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.
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