Influence of molecular weight on the phase behavior and structure formation of branched side-chain hairy-rod polyfluorene in bulk phase

M. Knaapila, R. Stepanyan, M. Torkkeli, B. P. Lyons, T. P. Ikonen, L. Almásy, J. P. Foreman, R. Serimaa, R. Güntner, U. Scherf, A. P. Monkman

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Abstract

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.

Original languageEnglish
Article number041802
JournalPhysical Review E - Statistical, Nonlinear, and Soft Matter Physics
Volume71
Issue number4
DOIs
Publication statusPublished - ápr. 1 2005

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ASJC Scopus subject areas

  • Statistical and Nonlinear Physics
  • Statistics and Probability
  • Condensed Matter Physics

Cite this

Knaapila, M., Stepanyan, R., Torkkeli, M., Lyons, B. P., Ikonen, T. P., Almásy, L., Foreman, J. P., Serimaa, R., Güntner, R., Scherf, U., & Monkman, A. P. (2005). Influence of molecular weight on the phase behavior and structure formation of branched side-chain hairy-rod polyfluorene in bulk phase. Physical Review E - Statistical, Nonlinear, and Soft Matter Physics, 71(4), [041802]. https://doi.org/10.1103/PhysRevE.71.041802