Evidence for structural transition in hairy-rod poly[9,9-bis(2-ethylhexyl)fluorene] under high pressure conditions (original) (raw)
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Physical Review E, 2005
We report on an experimental study of the self-organization and phase behavior of hairy-rod -conjugated branched side-chain polyfluorene, poly͓9,9-bis͑2-ethylhexyl͒-fluorene-2,7-diyl͔-i.e., poly͓2,7-͑9,9-bis͑2-ethylhexyl͒fluorene͔ ͑PF2/6͒-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. 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 n ͑LMW, M n Ͻ M n * and HMW, M n Ͼ 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 ͑XRD͒ 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 ͑frozen͒ nematic phase is observed for M n Ͻ M n * =10 4 g / mol and crystalline hexagonal phase for M n Ͼ 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 Ͼ 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 ജ 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 ͑ab0͒ 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.
Crystalline and Noncrystalline Forms of Poly(9,9-diheptylfluorene)
Macromolecules, 2013
The formation of ordered morphologies in poly(9,9diheptylfluorene) (PF7) was investigated using X-ray diffraction and grazing incidence X-ray diffraction. Two crystalline phases were found. The α-phase is orthorhombic with a = 2.60 nm, b = 2.25 nm, and c = 3.34 nm, and it is structurally very close to the α-phase in poly(9,9-dioctylfluorene) (PF8). The γ-phase is monoclinic with a = 2.88 nm, b = 0.96 nm, and c = 1.68 nm, and the oblique angle is close to 90°. The γ-phase is the stable form in the bulk while the α-phase preferentially forms in thin films. Well-ordered and aligned crystalline films were produced from both good (toluene) and moderate (methylcyclohexane, MCH) solvent. Preparing films from MCH without annealing resulted in mesoscopic crystal with decreased order along the a-axis. This mesoscopic structure differs from the β-phase found in PF8 and is more related to the crystalline γ-phase. This difference may explain why mesoscopic PF8 has a phase transition into the α-phase, whereas the mesoscopic PF7 rather into the γ-phase.
Phase Behavior of Poly(9,9-di-n-hexyl-2,7-fluorene)
The Journal of Physical Chemistry B, 2006
Here we report the phase behavior of poly(9,9-din -hexyl-2,7-fluorene) (PFH), which previously received little attention as compared to its homologues poly(9,9-din -octyl-2,7-fluorene) (PFO) and poly(9,9-di-(2′ethylhexyl)-2,7-fluorene) (PFEH). By means of differential scanning calorimetry, X-ray diffraction, and electron microscopy, we show that there exist four different phases in PFH. The as-cast film is mainly composed of a mesomorphic phase with layer spacing of ca. 1.4 nm. This phase is inherently metastable and, upon heating above 175°C, transforms into a crystalline (R) form that melts into a nematic (N) liquid above 250°C. Upon stepwise cooling, the nematic melt crystallizes into the R phase first, followed by solid-solid transformation into another crystalline (R′) form. Unit cell structure of the R form is monoclinic whereas that of the R′ form is triclinic, but departures from strict orthogonality are slight (by ca. 6°). These observations not only support our previous assignment of two crystalline forms (both orthorhombic in structure) in PFO but also provide insights to the crystalline nature of the polyfluorene series.
Physical Review Letters, 2007
We present optical spectroscopy studies of the conjugated polymer poly(9,9-din -octylfluorene-altbenzothiadiazole) (F8BT) at high pressure. The photoluminescence spectrum of F8BT in a dilute solidstate solution in polystyrene redshifts by 320 meV over 7.4 GPa, while that of a F8BT thin film redshifts 460 meV over a comparable pressure range. We attribute the redshift in solution to intrachain pressure effects, principally conformational planarization. The additional contribution from interchain-electron interactions accounts for the larger redshift of thin films.
Chain Morphologies in Semicrystalline Polyfluorene: Evidence from Raman Scattering
Physical Review Letters, 2006
The phase in di-octyl substituted polyfluorene (PF8) exhibits a surprising level of conformational order at nanometer length scales compared to other phases. We present Raman scattering studies of PF8 as a function of thermal cycling, which establishes a connection between the conformational isomers and the side and main chain morphology. Density-functional calculations of the vibrational spectra of single chain oligomers in conjunction with the experimental results demonstrate the incompatibility of the phase with the overall crystalline phase in PF8.
Measuring structural inhomogeneity of a helical conjugated polymer at high pressure and temperature
Journal of Polymer Science Part B: Polymer Physics, 2019
We report on X-ray scattering measurements of helical poly[9,9-bis(2ethylhexyl)-fluorene-2,7-diyl] by mapping the sample with 10 µm spatial resolution from 0.3 GPa to 36 GPa. We follow the strongest 00l reflection, which moves towards higher scattering angles with pressure indicating planarization of helical polyfluorene. Lateral inhomogeneity is increased for > 10 GPa concomitant with the solidification of the pressure transmitting medium (a 4:1 mixture of methanol and ethanol). We also follow the 00l reflection with increasing temperature at the constant pressure of 4.3 GPa in neon. We observe a sharp shift towards higher scattering angles indicative of a phase transition at 167-176 °C.
Macromolecules, 2004
Model conformations of polytetrafluoroethylene (PTFE) chains containing various amounts of helix reversal defects suitable for the high-temperature form I of PTFE have been modeled using semiempirical methods implemented in the Gaussian package, which makes use of the PM3 Hamiltonian. In these disordered conformations, ordered portions of chains in right-and left-handed 15/7 helical conformation succeed each other statistically along the chain. The Fourier transform of model chains containing various amounts of helix reversal defects obtained through the QM approach are then compared with the experimental X-ray fiber diffraction patterns of PTFE. Straight and slim model chains of PTFE containing helix reversal defects may be obtained at a low cost of internal energy and with small lateral encumbrance. For the minimum energy conformers the defect is always localized into a small region involving only 4 -CF 2-units, with the internal variables placed at the junction between the two enantiomorphic portions of chain, deviating only slightly from their average values in the defect-free portions of chains. Two consecutive helix reversals do not interact with each other if they are separated by more than three dihedral angles, and their contributions to internal energy can be considered as additive. The energy cost is ∼2.3 kcal/mol for each reversal. The disordered conformations of PTFE chains account for the X-ray fiber diffraction patterns of form I of PTFE at temperatures higher than 30°C. With increasing temperature an increase of the frequency of helix reversal defects is observed according to predictions of QM energy and Fourier transform calculations.