We report on an experimental study of the self-organization and phase behavior of hairy-rod π-conjugated branched side-chain polyfluorene, poly[9,9-bis(2-ethylhexyl)-fluorene-2,7-diyl] - i.e., poly[2,7-(9,9-bis(2- ethylhexyl)fluorene] (PF2/6) - as a function of molecular weight (M n). The results have been compared to those of phenomenological theory. Samples for which M n=3-147 kg/mol were used. First, the stiffness of PF2/6, the assumption of the theory, has been probed by small-angle neutron scattering in solution. Thermo-gravimetry has been used to show that PF2/6 is thermally stable over the conditions studied. Second, the existence of nematic and hexagonal phases has been phenomenologically identified for lower and higher M n (LMW, M n < M n* and HMW, M n > M n*) regimes, respectively, based on free-energy argument of nematic and hexagonal hairy rods and found to correspond to the experimental x-ray diffraction (XRD) results for PF2/6. By using the lattice parameters of PF2/6 as an experimental input, the nematic-hexagonal transition has been predicted in the vicinity of glassification temperature (T g) of PF2/6. Then, by taking the orientation parts of the free energies into account the nematic-hexagonal transition has been calculated as a function of temperature and M n and a phase diagram has been formed. Below T g of 80°C only (frozen) nematic phase is observed for M n<M n*=10 4 g/mol and crystalline hexagonal phase for M n>M n*. The nematic-hexagonal transition upon heating is observed for the HMW regime depending weakly on M n, being at 140-165°C for M n>M n*. Third, the phase behavior and structure formation as a function of M n have been probed using powder and fiber XRD and differential scanning calorimetry and reasonable semiquantitative agreement with theory has been found for M n ≥ 3 kg/mol. Fourth, structural characteristics are widely discussed. The nematic phase of LMW materials has been observed to be denser than high-temperature nematic phase of HMW compounds. The hexagonal phase has been found to be paracrystalline in the (ab0) plane but a genuine crystal meridionally. We also find that all these materials including the shortest 10-mer possess the formerly observed rigid five-helix hairy-rod molecular structure.
|Journal||Physical Review E - Statistical, Nonlinear, and Soft Matter Physics|
|Publication status||Published - Apr 2005|
ASJC Scopus subject areas
- Statistical and Nonlinear Physics
- Statistics and Probability
- Condensed Matter Physics