This account highlights recent progress towards understanding the complex hierarchical levels of solid-state structure in a prototypical helical hairy-rod polyfluorene, poly[9,9-bis(2-ethylhexy)fluorene-2,7-diyl] (or PF2/6). This branched-side-chain containing polyfluorene undergoes a systematic intermolecular self-assembly and liquid-crystalline phase behavior in combination with uniaxial and biaxial alignment. The latter processes yield full three-dimensional orientation of the crystallites and polymer chains. Also reviewed are the impact of the molecular structure and phase behavior on surface morphology, anisotropic film formation, and, ultimately, the overall impact of these physical attributes on optical constants. This particular polyfluorene also represents a model system for demonstrating the applicability of mean-field theory in detailing the self-organization of aligned hairy-rod block-copolymer systems. These results of PF2/6 are compared to those of other archetypical pi-conjugated hairy-rod polymers. General guidelines of how molecular weight influences nanostructure, phase behavior alignment, and surface morphology are given.

We report on the impact on the equatorial and meridional orientation in hairy-rod poly [9,9-bis(2-ethylhexyl)-fluorene-2,7-diyl] thin films when blended with tris-cyclometalated oligo-fluorenylpyridine iridium (III) complex. The presence of this transition-metal complex does not alter the underlying helical motif of the polymer chain or the hexagonal unit-cell intermolecular packing. However, systematic changes in the resulting texture patterning of the crystallites are observed. Crystallites with two preferred a-b orientations, so called type I and II and differing by 30 degrees, are superimposed on an isotropic background that is designated as type III. The extent of meridional alignment for all three is found to be similar.

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(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) < 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 degrees 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 degrees 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) >or=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.

We report on the influence of the molecular weight (M) on the alignment and structure of poly(9,9-bis(ethylhexyl)fluorene-2,7-diyl) (PF2/6) in thin films on rubbed polyimide in the equilibrium. The degree of alignment has been studied using optical spectroscopy and compared to theoretical arguments. The structure of PF2/6 has been studied using grazing-incidence X-ray diffraction. PF2/6 realizes a threshold molecular weight M-n* = 10(4) g/mol separating a low M-n (LMW, M-n < M-n*) and a high M-n (HMW, M-n > M-n*) region. LMW materials show only the nematic (Nem) phase while HMW compounds show hexagonal (Hex) and Nem phases at low and high temperatures, respectively. LMW samples align equally well at any temperature above the glass transition, and the dichroic ratio, R, increases with M-n. HMW material aligns well in the Nem regime, and R drops exponentially with M-n, which is in agreement with the theory. The orientational order is maximized near M-n*, and the orientational. order parameter is > 0.9. HMW samples, whether aligned or not, reveal two kinds of coexistent Hex crystallites distributed in the sample plane having one crystal axis a perpendicular or parallel to the surface. Local order within the triaxially aligned Hex phase surpasses that of the in-plane aligned Hex phase.

The surface structure of uniaxially aligned poly(9,9bis(ethylhexyl)-fluorene-2,7-diyl) films on rubbed polymide has been studied as a function of molecular weight (M-n = 3-150 kg mol(-1), number-average molecular weight) using polarized microscopy, atomic force microscopy (AFM), X-ray reflectivity, and grazing-incidences X-ray diffraction. At the threshold M-n, M-n* -10 g mol(-1), there is a prominant transition in morphology from featureless (M-n < M-n*) to rough (M-n > M-n*), corresponding to the nematic-hexagonal phase transition. The hexagonal phase reveals two coexistant crystallite types in the whole film and at least one crystallite type has been observed at the surface by AFM. The combined optimization of alignment and surface smoothness is achieved slightly below M-n* while the combined optimization of orientational and local order and moderately smooth surface is achieved slightly above Mn*.

Singlet exciton migration has been studied in films of the conjugated polymer polyfluorene (PF) by doping the samples with a fluorescent probe molecule, tetraphenyl porphyrin (TPP). Energy transfer in such systems has often been described in terms of Forster transfer, a dipole-dipole mechanism. TPP emission from the films was measured in steady-state, as a function of temperature and dopant concentration. The intensity of the TPP emission was found to be constant up to 150 K, and then to increase with temperature. Therefore, the energy transfer cannot be occurring solely by Forster transfer as that process is temperature-independent. Instead, energy transfer between PF and TPP is considered to take place via thermally-activated exciton diffusion through the polymer followed by Forster transfer between the polymer and dopant. Moreover, TPP emission as a function of dopant concentration could not be described by Forster transfer alone, but could be well fitted at low temperature (15 K) and room temperature by the Yokota-Tanimoto model, which combines diffusion and Forster transfer. Diffusion lengths of 11 +/- 2 nm (15 K) and 20 +/- 2 nm (room temperature) were found. The nonzero exciton diffusion at low temperature is believed to be due to downhill migration to low energy polymer segments.

The optical constants of aligned and unaligned thin polyfluorene films have been investigated using variable angle spectroscopic ellipsometry over the spectral range 250–1000nm. Films were prepared by spin coating, with alignment achieved by deposition on a rubbed polyimide substrate followed by annealing. The films could be well modeled by layers with uniaxial anisotropy. The unaligned film had its optical axis oriented normal to the film surface and was optically negative. There was large absorption and dispersion of the refractive index perpendicular to the optical axis and small absorption and dispersion of the refractive index parallel to it. The aligned film had its optical axis oriented in plane, normal to the rubbing direction, and was optically positive. There was large absorption and dispersion of the refractive index parallel to the optical axis and small absorption and dispersion of the refractive index perpendicular to it. These results will be useful in the design of polarized light-emitting devices using polyfluorene as an active layer.

We present a structural study of poly(9,9-bis(2-ethylhexyl)-fluorene-2,7-diyl) (PF2/6) in aligned thin films on a rubbed polyimide substrate. PF2/6 forms 5/2-helical hairy-rodlike molecular structure which is self-organized in the hexagonal structure. In thin films, the aligned rigid polymer chains are parallel to the substrate in the direction corresponding to the molecular backbone, c axis. The cells are flattened in the direction of the surface normal and -in particular- reveal a multiple orientation where the greater proportion of the crystallites have one crystal axis a perpendicular to the substrate surface but a small proportion are aligned with the crystal axis a parallel to surface. We find further that by reducing the polymer chain length to approximately twenty repeat units the degree of axial alignment of PF2/6 is considerably improved while the local order changes from hexagonal packing towards nematic phase with no sign of multiple orientation.

We present investigations of the self-organized structure, overall uniaxial alignment, and larger-scale interface morphology in thin films of low-molecular-weight hairy-rodlike π-conjugated poly(9,9-bis(ethylhexyl)-fluorene-2,7-diyl), (LMW-PF2/6) (Mn = 7600 g/mol) and a comparison of high-molecular-weight PF2/6 (HMW-PF2/6) (Mn = 150 000 g/mol). Photoabsorption, grazing-incidence X-ray diffraction, small/wide-angle X-ray scattering, and X-ray reflectivity methods have been used. The experiments have been supported by molecular mechanics-calculated molecular structure and recursively simulated reflectivity curves and discussed in terms of the semiquantitative theoretical analysis of the self-organization of hairy-rodlike polymers. As with HMW-PF2/6, LMW-PF2/6 is found to be a thermotropic liquid crystal consisting of rodlike 5/2 helices. Blue photoluminescence with an absolute photoluminescence quantum yield of 32% in the solid films is observed. After thermotropic alignment on rubbed substrates, considerab...

The energy transfer processes between polyfluorene and tetraphenyl porphyrin have been investigated through steady state and time resolved measurements. Radiative transfer is found to be negligible and so the results are analyzed in terms of Förster resonance energy transfer. Rate equations are fitted to the experimental data to obtain values for the concentration dependent energy transfer rate. From this, we calculate a value of 38 Å for the energy transfer distance. This compares well with the value for the Förster radius we calculate from the spectral overlap, even though Förster theory is not strictly valid for this system.

The structural investigation of poly(9,9-bis(2-ethylhexyl)fluorene-2,7-diyl) (PF2/6) in aligned thin films is presented. Formation of a thickness dependent triaxial texturing is identified in thermotropically aligned films. X-ray reflectivity measurements reveal good macroscopic quality, and polarized photoluminescence and dichroic ratios in absorption indicate clear axial alignment. Grazing-incidence X-ray diffraction shows axially aligned mesomorphic structure with a distinct arrangement of helices and large correlation lengths, indicating a high local lateral order. Theoretical models produced using molecular mechanics methods suggest 5/2-helicity. The polymer chains are parallel to the substrate in the c direction. In particular, the hexagonal-like cells are flattened in the direction of the surface normal and reveal two kinds of coexistent crystallites, a multiple orientation where the greater proportion of the crystallites have one crystal axis a perpendicular to the substrate surface, whereas a smaller proportion is aligned with the crystal axis a parallel to the surface. In thinner films the former class of orientation is usually dominant, while the proportion of the parallel orientation type increases with prolonged annealing.

The energy transfer processes between polyfluorene and tetraphenyl porphyrin have been investigated through steady state and time resolved measurements. Radiative transfer is found to be negligible and so the results are interpreted in terms of Forster resonance energy transfer. Rate equations are fitted to the results to obtain a concentration dependent transfer rate. From this we calculate a value for the Forster radius of 36Angstrom. This agrees well with the values we calculate from the spectral overlap, taking a wavelength dependent refractive index into account.

Samples of each of two high-density polyethylenes with various initial degrees of crystallinity, but otherwise identical, were exposed under a vacuum to moderate doses of gamma irradiation. The results indicate that, for otherwise initially identical polymer samples, the dose required to reach the gel point increases with increase of the initial degree of crystallinity. Above the critical dose for gelation, the gel content decreases with higher degrees of crystallinity at equal radiation doses. The mechanical behavior of the polymers changed progressively from ductile to brittle as the crystallinity was increased. The extensibility of originally ductile samples decreases with increasing radiation dose. The irradiation of samples having intermediate behavior produces a change to ductile behavior. Mechanical behavior is not modified substantially when brittle samples are irradiated. The initial modulus is little altered by irradiation, while the yield stress shows a slight increase with irradiation. The mechanical properties, such as draw ratio at break and ultimate tensile stress, decrease with dose in ductile samples. (C) 1999 John Wiley & Sons, Inc.

A MECHANISM has been recently proposed1 to account for the behaviour of polypropylene on irradiation, in which the formation of a cross-link between molecules involves reaction between an activated centre in the polymer and a chain fragment containing a vinylidene double bond. This suggests that the cross-linking induced by radiation would be enhanced by the incorporation of unsaturated additives into this polymer. Considerable enhancement of cross-linking has been claimed to occur in the irradiation of ‘solid solutions’ of polyvinyl chloride in di- and tri-allyl esters2.