### 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}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 |
---|---|

Article number | 041802 |

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

Volume | 71 |

Issue number | 4 |

DOIs | |

Publication status | Published - Apr 2005 |

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### ASJC Scopus subject areas

- Physics and Astronomy(all)
- Condensed Matter Physics
- Statistical and Nonlinear Physics
- Mathematical Physics

### Cite this

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

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

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

Research output: Contribution to journal › Article

*Physical Review E - Statistical, Nonlinear, and Soft Matter Physics*, vol. 71, no. 4, 041802. https://doi.org/10.1103/PhysRevE.71.041802

}

TY - JOUR

T1 - Influence of molecular weight on the phase behavior and structure formation of branched side-chain hairy-rod polyfluorene in bulk phase

AU - Knaapila, M.

AU - Stepanyan, R.

AU - Torkkeli, M.

AU - Lyons, B. P.

AU - Ikonen, T. P.

AU - Almásy, L.

AU - Foreman, J. P.

AU - Serimaa, R.

AU - Güntner, R.

AU - Scherf, U.

AU - Monkman, A. P.

PY - 2005/4

Y1 - 2005/4

N2 - 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 nn*=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.

AB - 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 nn*=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.

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U2 - 10.1103/PhysRevE.71.041802

DO - 10.1103/PhysRevE.71.041802

M3 - Article

AN - SCOPUS:41349100007

VL - 71

JO - Physical review. E

JF - Physical review. E

SN - 2470-0045

IS - 4

M1 - 041802

ER -