| 1. |
- Cheun, H., et al.
(author)
-
Polarized optical absorption spectroscopy, NEXAFS, and GIXRD measurements of chain alignment in polyfluorene thin films
- 2008
-
In: Macromolecules. - : American Chemical Society (ACS). - 0024-9297 .- 1520-5835. ; 41:17, s. 6463-6472
-
Journal article (peer-reviewed)abstract
- The uniaxial chain alignment of low molecular weight poly(9,9-bis(ethylhexyl)fluorene-2,7-diyl) (PF2/6) in thin films cast atop rubbed polyimide templates is characterized by polarized optical absorption spectroscopy, carbon K near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and grazing incidence X-ray diffraction (GIXRD) in response to thermal annealing and increasing film thickness (ca. 15-150 nm). The highest overall levels of uniaxial alignment are obtained in the thinnest PF2/6 films. However, the orientation of chains at the top surface after thermal annealing is nearly thickness independent despite a large drop in the maximum optical dichroic ratio as the film thickness increases. The kinetics of chain alignment on heating are strongly film thickness dependent and only weakly correlated with a crossover to a nematic liquid crystal state. All told these data support a structural model in which there is a graded morphology such that the top and bottom surfaces exhibit appreciable planar, uniaxial alignment while the film interior includes a greater proportion of nomplanar (i.e., tilted) chains after thermal cycling. These inhomogeneities are likely to influence technologically important optical and transport properties.
|
|
| 2. |
- Justino, L. L. G., et al.
(author)
-
Structural and electronic properties of poly(9,9-dialkylfluorene)-based alternating copolymers in solution : An NMR spectroscopy and density functional theory study
- 2013
-
In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:35, s. 17969-17982
-
Journal article (peer-reviewed)abstract
- The structural and electronic properties of three alternating poly(9,9-dialkylfluorene) copolymers, poly[2,7-(9,9-bis(octyl)-fluorene)-alt- benzothiadiazole] (F8BT), poly[2,7-(9,9-bis(2′-ethylhexyl)-fluorene)-alt- thiophene S,S-dioxide] (PFTSO2), and poly[2,7-(9,9-bis(2′-ethylhexyl)- fluorene)-alt-1,4-phenylene] (PFP), containing, respectively, benzothiadiazole (BT), thiophene S,S-dioxide, and phenylene groups, have been investigated and compared to those of the homopolymer poly[2,7-(9,9-bis(2′-ethylhexyl)- fluorene)] (PF2/6). The NMR spectra and corresponding shielding tensors of the 1H and 13C nuclei have been studied in solution and are interpreted by density functional theory (DFT) with complete geometry optimization using the B3LYP functional. Particular emphasis is placed on the conformational behavior and electronic properties in the electronic ground state. In addition, time-dependent DFT is applied to obtain detailed insight into the character of selected excited states. A new TDDFT interpretation is presented for optical absorption spectra of singlet and triplet states that have previously been reported for these fluorene-based conjugated copolymers using photoexcitation and pulse radiolysis-energy transfer studies. As well as providing detailed assignment of excited states, the results show that the triplet excitation is slightly more localized than S1 excitation, in agreement with experimental observations. The DFT analysis provides a link between structure and NMR, optical, electronic properties, allowing optimization of the potential of such polymers for photovoltaic and electroluminescence applications.
|
|
| 3. |
- Knaapila, Matti, et al.
(author)
-
An effect of side chain length on the solution structure of poly(9,9-dialkylfluorene)s in toluene
- 2008
-
In: Polymer. - : Elsevier BV. - 0032-3861. ; 49:8, s. 2033-2038
-
Journal article (peer-reviewed)abstract
- The effect of side chain length of pi-conjugated poly(9,9-dialkylfluorene)s has been studied in semi-dilute (10 mg/mL) toluene solutions using small-angle neutron scattering (SANS) and H-1 and H-2 NMR spectroscopies. Under these conditions, SANS data indicate that poly(9,9-dinonylfluorene) and poly(9,9-dioctylfluorene) are dissolved down to the molecule level and appear as elongated one-dimensional chains (length >20-30 nm). In contrast, the shorter side chain polymers exhibit a self-association so that poly(9,9-diheptylfluorene) forms thin sheet-like (similar to 1 nm) and poly(9,9-dihexylfluorene) thin (similar to 1 nm) and thick sheet-like (>6 nm) aggregates. H-1 NMR data, together with the density functional theory (DFT) calculations, however, show that this occurs without changes in the conformation of the polymer backbone.
|
|
| 4. |
- Knaapila, Matti, et al.
(author)
-
Control over phase behavior and solution structure of hairy-rod polyfluorene by means of side-chain length and branching
- 2008
-
In: Physical Review E (Statistical, Nonlinear, and Soft Matter Physics). - 1539-3755. ; 77:5
-
Journal article (peer-reviewed)abstract
- We present guidelines on how the solution structure of pi-conjugated hairy-rod polyfluorenes is controlled by the side-chain length and branching. First, the semiquantitative mean-field theory is formulated to predict the phase behavior of the system as a function of side-chain beads (N). The phase transition at N=N* separates a lyotropic phase with solvent coexistence (N < N*) and a metastable membrane phase (N > N*). The membrane phase transforms into the isotropic phase of dissolved rodlike polymers at the temperature T-mem*(N), which decreases both with N and with the degree of side-chain branching. This picture is complemented by polymer demixing with the transition temperature T-IN*(N), which decreases with N. For N < N*, the lyotropic phase turns isotropic with increasing T at T-IN*. For N > N*, stable membranes are predicted for T-IN*< T < T-mem* and metastable membranes with nematic coexistence for T < T-IN*. Second, in experiment, samples of poly(9,9-dialkylfluorene) with N=6-10 were mixed in methylcyclohexane. For N=8 the side-chain branching was controlled by (9,9-dioctylfluorene)/(9,9-bis(2-ethylhexyl)fluorene) (F8/F2/6) random copolymers. The proportion of F8 to F2/6 repeat units was 100:0, 95:5, 90:10, 50:50, and 0:100. In accordance with the theory, lyotropic, membrane, and isotropic phases with the corresponding phase transitions were observed. For N < N*similar to 6 only the lyotropic phase is present for attainable temperatures. The membrane and isotropic phases are present for N > N*. T-mem*(N) decreases from 340 K to 280 K for N >= 8. For copolymers, the membrane phase is found when the fraction of F8 units is at least 90%, T-mem* decreasing with this fraction. The membrane phase contains three material types: loose sheets of two polymer layers, a better packed beta phase, and dissolved polymer. For N >= 7 and T < T-mem* the tendency for membrane formation becomes stronger with increasing temperature.
|
|
| 5. |
- Knaapila, M., et al.
(author)
-
Effect of side-chain asymmetry on the intermolecular structure and order-disorder transition in alkyl-substituted polyfluorenes
- 2016
-
In: Physical Review E: covering statistical, nonlinear, biological, and soft matter physics. - 2470-0045. ; 93:4
-
Journal article (peer-reviewed)abstract
- We study relations among the side-chain asymmetry, structure, and order-disorder transition (ODT) in hairy-rod-type poly(9,9-dihexylfluorene) (PF6) with two identical side chains and atactic poly(9-octyl-9-methyl-fluorene) (PF1-8) with two different side chains per repeat. PF6 and PF1-8 organize into alternating side-chain and backbone layers that transform into an isotropic phase at TODT(PF6) and TbiODT(PF1-8). We interpret polymers in terms of monodisperse and bidisperse brushes and predict scenarios TODT
|
|
| 6. |
- Knaapila, Matti, et al.
(author)
-
Evidence for structural transition in hairy-rod poly[9,9-bis(2-ethylhexyl)fluorene] under high pressure conditions
- 2010
-
In: Physical Review E (Statistical, Nonlinear, and Soft Matter Physics). - 1539-3755. ; 82:5
-
Journal article (peer-reviewed)abstract
- We report on an x-ray scattering experiment of bulk poly[9,9-bis(2-ethylhexyl)fluorene] under quasihydrostatic pressure from 1 to 11 GPa at room temperature. The scattering pattern of high molecular weight (HMW) polyfluorene (>10 kg/mol) undergoes significant changes between 2 and 4 GPa in the bulk phase. The 110 reflection of the hexagonal unit cell disappears, indicating a change in equatorial intermolecular order. The intensity of the 00 21 reflection drops, with a sudden move toward higher scattering angles. Beyond these pressures, the diminished 00 21 reflection tends to return toward lower angles. These changes may be interpreted as a transition from crystalline hexagonal to glassy nematic phase (perceiving order only in one direction). This transition may be rationalized by density arguments and the underlying theory of phase behavior of hairy-rod polyfluorene. Also the possible alteration of the 21-helical main chain toward more planar main chain conformation is discussed. The scattering of low molecular weight polyfluorene (<10 kg/mol), which is glassy nematic in ambient pressure, is reminiscent with that of HMW polymer above 2–4 GPa.
|
|
| 7. |
- Knaapila, Matti, et al.
(author)
-
Influence of side chain length on the self-assembly of hairy-rod poly(9,9-dialkylfluorene)s in the poor solvent methylcyclohexane
- 2007
-
In: Macromolecules. - : American Chemical Society (ACS). - 0024-9297 .- 1520-5835. ; 40:26, s. 9398-9405
-
Journal article (peer-reviewed)abstract
- We report on the phase behavior of poly(9,9-dihexylfluorene) (PF6), poly(9,9-diheptylfluorene) (PF7), poly(9,9-diocylfluorene) (PF8) poly(9,9-dinonylfluorene) (PF9), and poly(9,9-didecylfluorene) (PF10) in methylcyclohexane (MCH). After a heating-cooling cycle, in the 10-50 mg/mL. concentration range. PF6/ MCH. PF7/MCH. PF8/MCH, and PF9/MCH systems were found to be gel-like, while PF10/MCH appears less viscous. PF6/MCH, PF7/MCH, PF8/MCH, and PF9/MCH form large (10- 100 nm) sheetlike assemblies (thickness of 2-3 nm). The larger length scale Structures of these sheets show an odd-even dependence on the side chain (spacer) length: the PF6 and PF8 sheets are broader and thinner, whereas PF7 and PF9 sheets are thicker with a Putative double layer Structure. PF10 does not follow this sequence, and only part of the polymer is assembled into a sheetlike structure the rest remaining dissolved at the molecular level. PF8/MCH and PF9/MCH mixtures also have lower length scale crystalline structures with an internal period corresponding to the periodicity observed in the solid-state P phase of PF8. Vestiges of crystalline domains are found for PF6 and PF7 but not for PF10. PF7/MCH. PF8/MCH, and PF9/MCH systems contain the conformational isomer C, of those chains observed in the ss phase, while this is not observed with other polymer/MCH systems.
|
|
| 8. |
- Knaapila, M., et al.
(author)
-
Structural study of helical polyfluorene under high quasihydrostatic pressure
- 2013
-
In: Physical Review E (Statistical, Nonlinear, and Soft Matter Physics). - 1539-3755. ; 87:2
-
Journal article (peer-reviewed)abstract
- We report on an x-ray diffraction (XRD) study of helical poly[9,9-bis(2-ethylhexyl)fluorene] (PF2/6) under high quasihydrostatic pressure and show an effect of pressure on the torsion angle (dihedral angle) between adjunct repeat units and on the hexagonal unit cell. A model for helical backbone conformation is constructed. The theoretical position for the most prominent 00l x-ray reflection is calculated as a function of torsion angle. The XRD of high molecular weight PF2/6 (M-n = 30 kg/mol) is measured through a diamond anvil cell upon pressure increase from 1 to 10 GPa. The theoretically considered 00l reflection is experimentally identified, and its shift with the increasing pressure is found to be consistent with the decreasing torsion angle between 2 and 6 GPa. This indicates partial backbone planarization towards a more open helical structure. The h00 peak is identified, and its shift together with the broadening of 00l implies impairment of the ambient hexagonal order, which begins at or below 2 GPa. Previously collected high-pressure photoluminescence data are reanalyzed and are found to be qualitatively consistent with the XRD data. This paper provides an example of how the helical pi-conjugated backbone structure can be controlled by applying high quasihydrostatic pressure without modifications in its chemical structure. Moreover, it paves the way for wider use of high-pressure x-ray scattering in the research of pi-conjugated polymers. DOI: 10.1103/PhysRevE.87.022602
|
|
