### 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 language | English |
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Article number | 041802 |

Journal | Physical Review E - Statistical, Nonlinear, and Soft Matter Physics |

Volume | 71 |

Issue number | 4 |

DOIs | |

Publication status | Published - Apr 2005 |

### ASJC Scopus subject areas

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

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## Cite this

*Physical Review E - Statistical, Nonlinear, and Soft Matter Physics*,

*71*(4), [041802]. https://doi.org/10.1103/PhysRevE.71.041802