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Libin Wu1,2, Takahiro Sago1,2, Hongzhi Tang2 and Michiya Fujiki2,31Department of Macromolecular Science, Osaka University, 1-1 Machikaneyama-cho, Osaka 560-0043, Japan, 2CREST-JST (Japan Science and Technology Corperation), 4-1-8 tton-cho, Kawaguchi, Saitama 332-0012, Japan, 3Advanced Polymer Science Laboratory, Graduate School of Material Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikorea, Nar a 630-0101, Japan tsato@chem.sci.osaka-u.ac.jp
Light-emitting diodes (LEDs) fabricated from polyfluorene (PF) derivatives exhibit bright blue emission with much higher effieieneies than other π-conjugated polymers. Thus, PF derivatives are potential candidates for emitting materials for display and light sources with high energy efficiencies, which may contribute to the global energy problem. For chiral PF derivatives, we can expect a novel function as circularly polarized LED, and the present study has been undertaken to elucidate the molecular origin of this function. Poly(2,7-(9,9-bis((S)-3,7-dimethyloctyl)))fluorene (PF2) was synthesized using a zero-valent nickel reagent, and fraetionated by SEC into several fractions, and the intrinsic viscosity and circular diehroism (CD) were measured as functions of the molecular weight and temperature T. CD measurements were made also for poly(2,7-(9,9-citronellyl))fluorene (PF3) synthesized in a similar procedure. Figure 1 shows by dots the molecular weight dependence of [η] of PF2 in THF at 20. In the double-logarithmic plot, the data points obey a curve convex downward, whose slopes are 0.97 and 0.82 below and above Mw = 7.0 x 104, respectively. The latter slope indicates that the chain of PF2 is not so stiff in THF. The solid curve in Figure 1 indicates theoretical values of [η] calculated by the theory of Yoshizaki, Nitta, and Yamakawa for the touched-bead wormlike chain model, using q (the persistence length) = 9.5 nm, ML (the molar arnss per unit contour length) = 530 nm? , and db (the bead diameter) = 1.45 nm, which perfectly fits to the experimental data points. The Kuhn segment number N of the highest molecular weight fraction is about 20. It is known that the intramoleeular excluded-volume effect is not appreciable at N = 50 for stiff and semiflexible polymers. Therefore, this effect may not affect the above determinations of q, ML, and db. We also determined q at different temperatures from [η] data for the PF2 fraction of Mw = 1.23 x 105, assuming the temperature independence of ML and db.
Figure 2 compares the T dependence of q for PF2 in THF with the theories of the rotational isomer model, the broken helical wormlike chain model and the freely rotation chain model indicated by curves in the figure. From the comparison, the broken helical wormlike model and freely rotation chain model overestimates and underestimates the experimental q at different T, respectively. A good fitting to the experimental q is obtained for the rotational isomer model in which every monomer unit takes independently, four rotational states, fight- and left-handed 5/2 and 5/1 helical states, and the bond angles fluctuate slightly. This comparison results turn out that the isolated PF2 chain in dilute THF solution does not take any regular helical conformation in spite of relatively high q values. In accordance with this result, molecularly dispersed PF2 in dilute THF solution exhibit no CD even at low T, as shown in Fig.3 (the curve 0min.). However, CD gradually increases with the quenching time at ? 0℃, which indicates the formation of chiral aggregates of PF2.
[Reference] Tang, H.; Fujiki, M.; Sato, T., Macromolecules 35, 6439-6445 (2002)论文来源:PP’2004 Dail international symposium on Polymer Physics Preprints,june 1-5,2004,Dail,china |
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