Influence of molecular weight on the phase behavior and structure formation of branched side-chain hairy-rod polyfluorene in bulk phase
- Department of Physics, University of Durham, South Road, Durham DH1 3LE (United Kingdom)
- Department of Applied Physics, University of Twente, P.O. Box 217, 7500 AE Enschede (Netherlands)
- Department of Physical Sciences, P.O. Box 64, FI-00014, University of Helsinki (Finland)
- Research Institute for Solid State Physics and Optics, P.O. Box 49, Budapest-1525 (Hungary)
- Makromolekulare Chemie und Institut fuer Polymertechnologie, Bergische Universitaet Wuppertal, D-42097, Wuppertal (Germany)
We report on an experimental study of the self-organization and phase behavior of hairy-rod {pi}-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{sub n}). The results have been compared to those of phenomenological theory. Samples for which M{sub 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. Thermogravimetry 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{sub n} (LMW, M{sub n}<M{sub n}* and HMW, M{sub n}>M{sub n}{sup *}) 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{sub 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{sub n} and a phase diagram has been formed. Below T{sub g} of 80 deg. C only (frozen) nematic phase is observed for M{sub n}<M{sub n}*=10{sup 4} g/mol and crystalline hexagonal phase for M{sub n}>M{sub n}*. The nematic-hexagonal transition upon heating is observed for the HMW regime depending weakly on M{sub n}, being at 140-165 deg. C for M{sub n}>M{sub n}*. Third, the phase behavior and structure formation as a function of M{sub n} have been probed using powder and fiber XRD and differential scanning calorimetry and reasonable semiquantitative agreement with theory has been found for M{sub 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.
- OSTI ID:
- 20641416
- Journal Information:
- Physical Review. E, Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, Journal Name: Physical Review. E, Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics Journal Issue: 4 Vol. 71; ISSN PLEEE8; ISSN 1063-651X
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
CALORIMETRY
FLEXIBILITY
FLUORENE
FREE ENERGY
HEATING
LATTICE PARAMETERS
LIQUID CRYSTALS
MOLECULAR STRUCTURE
MOLECULAR WEIGHT
NEUTRON DIFFRACTION
PHASE DIAGRAMS
PHASE TRANSFORMATIONS
POLYMERS
POWDERS
SMALL ANGLE SCATTERING
SOLUTIONS
TEMPERATURE DEPENDENCE
TEMPERATURE RANGE 0273-0400 K
THERMAL GRAVIMETRIC ANALYSIS
X-RAY DIFFRACTION