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

Circular Intensity Differential Scattering Reveals the Internal Structure of Polymer Fibrils

The optical and electronic properties of π-conjugated polymers in organic electronic devices depend on their intra- and interchain interactions, dictated by the internal arrangement of the polymer chains in an amorphous or semicrystalline aggregated state. Here, we discuss the utility of circular intensity differential scattering (CIDS) of circularly polarized light as a sensitive probe to identify the internal arrangement of the polymer chains in helical polymer aggregates. We advance existing theoretical models to utilize the CIDS response and extract structural properties such as the size, orientation, and periodicity of a polymer aggregate. As an example, we analyze the CIDS signatures of helically assembled fibrillar aggregates of a chiral polymer poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzothiadiazole)] (PFBT) in solution and reveal that PFBT fibrils incorporate at least five intertwined polymer chains. We anticipate our approach can be extended more generally to investigate the internal arrangement of supramolecular assemblies of a wide range of fibrillar aggregates of π-conjugated polymers.

Blue emitting organic semiconductors under high pressure: status and outlook

This review describes essential optical and emerging structural experiments that use high GPa range hydrostatic pressure to probe physical phenomena in blue-emitting organic semiconductors including π-conjugated polyfluorene and related compounds. The work emphasizes molecular structure and intermolecular self-organization that typically determine transport and optical emission in π-conjugated oligomers and polymers. In this context, hydrostatic pressure through diamond anvil cells has proven to be an elegant tool to control structure and interactions without chemical intervention. This has been highlighted by high pressure optical spectroscopy whilst analogous x-ray diffraction experiments remain less frequent. By focusing on a class of blue-emitting π-conjugated polymers, polyfluorenes, this article reviews optical spectroscopic studies under hydrostatic pressure, addressing the impact of molecular and intermolecular interactions on optical excitations, electron-phonon interaction, and changes in backbone conformations. This picture is connected to the optical high pressure studies of other π-conjugated systems and emerging x-ray scattering experiments from polyfluorenes which provides a structure-property map of pressure-driven intra- and interchain interactions. Key obstacles to obtain further advances are identified and experimental methods to resolve them are suggested.

Effect of side-chain asymmetry on the intermolecular structure and order-disorder transition in alkyl-substituted polyfluorenes

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 T^{ODT}(PF6) and T_{bi}^{ODT}(PF1-8). We interpret polymers in terms of monodisperse and bidisperse brushes and predict scenarios T^{ODT}<T_{bi}^{ODT} and T^{ODT}∼T_{bi}^{ODT} for high and low grafting densities (the side-chain length above or below the average grafting distance). Calorimetry and x-ray scattering indicate the condition T^{ODT}(PF6)∼T_{bi}^{ODT}(PF1-8) following the low grafting prediction. PF6 side chains coming from the alternating backbone layers appear as two separate layers with thickness H(PF6), whereas PF1-8 side chains appear as an indistinguishable bilayer with a half thickness H_{bilayer}(PF1-8)/2≈H(PF6). The low grafting density region is structurally possible but not certain for PF6 and confirmed for PF1-8.

Methods for controlling structure and photophysical properties in polyfluorene solutions and gels

Knowledge of the phase behavior of polyfluorene solutions and gels has expanded tremendously in recent years. The relationship between the structure formation and photophysics is known at the quantitative level. The factors which we understand control these relationships include virtually all important materials parameters such as solvent quality, side chain branching, side chain length, molecular weight, thermal history and myriad functionalizations. This review describes advances in controlling structure and photophysical properties in polyfluorene solutions and gels. It discusses the demarcation lines between solutions, gels, and macrophase separation in conjugated polymers and reviews essential solid state properties needed for understanding of solutions. It gives an insight into polyfluorene and polyfluorene beta phase in solutions and gels and describes all the structural levels in solvent matrices, ranging from intramolecular structures to the diverse aggregate classes and network structures and agglomerates of these units. It goes on to describe the kinetics and thermodynamics of these structures. It details the manifold molecular parameters used in their control and continues with the molecular confinement and touches on permanently cross-linked networks. Particular focus is placed on the experimental results of archetypical polyfluorenes and solvent matrices and connection between structure and photonics. A connection is also made to the mean field type theories of hairy-rod like polymers. This altogether allows generalizations and provides a guideline for materials scientists, synthetic chemists and device engineers as well, for this important class of semiconductor, luminescent polymers.

Structural study of helical polyfluorene under high quasihydrostatic pressure

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 π-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 π-conjugated polymers.

Suzuki Polycondensation: Polyarylenes à la Carte

This review draws a rather comprehensive picture of how Suzuki polycondensation was discovered in 1989 and how it was subsequently developed into the most powerful polymerization method for polyarylenes during the last 20 years. It combines insights into synthetic issues with classes of polymers prepared and touches upon aspects of this method's technological importance. Because a significant part of the developmental work was carried out in industry, the present review makes reference to an unusually large number of patents.

Evidence for structural transition in hairy-rod poly[9,9-bis(2-ethylhexyl)fluorene] under high pressure conditions

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.

Aggregation of conjugated polymers in aromatic solvent

Segments of conjugated polymers display the propensity to aggregate in solutions with common organic solvents. Here we revealed that the segmental aggregation of a conjugated polymer, poly(9,9-bis(2-ethylhexyl)fluorene-2,7-diyl), (PF2/6), in toluene was stabilized by the polymer-solvent complex formation through pi-pi stacking induction of solvent molecules and polymer segments. In this case, a portion of the solvent was trapped inside the aggregate domains upon bringing the system to the subambient temperatures. The residence time of these associated solvent molecules became long enough to yield a separate upfield-shifted NMR resonance. The line-shape of this resonance revealed alignment of the polymer segments in the aggregates. A portion of the solvent was frozen in the compact structure due to the formation of strong polymer-solvent complex.

Control over phase behavior and solution structure of hairy-rod polyfluorene by means of side-chain length and branching

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{ *} approximately 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 > or = 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 > or = 7 and T<T_{mem}{ *} the tendency for membrane formation becomes stronger with increasing temperature.

X-ray Scattering Study of Thin Films of Poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene)

Poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene), PBTTT, is a semiconducting polymer that forms thin film transistors (TFTs) with high field effect mobility on silicon dioxide dielectrics that are treated with alkyltrichlorosilanes ( approximately 0.2 to 0.5 cm2/V s) but forms TFTs with poor mobility on bare silicon dioxide (<0.005 cm2/V s). The microstructure of spin-coated thin films of PBTTT on these surfaces was studied using synchrotron X-ray diffraction and atomic force microscopy. PBTTT crystallizes with lamellae of pi-stacked polymer chains on both surfaces. The crystalline domains are well-oriented relative to the substrate in the as-spun state and become highly oriented and more ordered with thermal annealing in the liquid crystalline mesophase. Although the X-ray scattering from PBTTT is nearly identical on both surfaces, atomic force microscopy showed that the domain size of the crystalline regions depends on the substrate surface. These results suggest that electrical transport in PBTTT films is strongly affected by the domain size of the crystalline regions and the disordered regions between them.

Conformations in dioctyl substituted polyfluorene: A combined theoretical and experimental Raman scattering study

The structural properties of polyfluorenes (PF) are extremely sensitive to the choice of functionalizing side chains. Dioctyl substituted PF (PF8) adopts metastable structures that depend upon the thermal history and choice of solvents used in film forming conditions. We present a detailed study of the changes in the backbone and side chain morphology in PF8, induced by the various crystallographic phases, using Raman scattering techniques. The vibrational frequencies and intensities of fluorene oligomers are calculated using hybrid density-functional theory with a 3-21G(*) basis set. The alkyl side chains are modeled as limiting conformations: all anti, anti-gauche-gauche, and end gauche representations. The calculated vibrational spectra of single chain oligomers in conjunction with our experimental results demonstrate the beta phase, which is known to originate in regions of enhanced chain planarity as a direct consequence of the alkyl side chain conformation.

Polarized optical spectroscopy applied to investigate two poly(phenylene-vinylene) polymers with different side chain structures

Two related poly(phenylene-vinylene) (PPV) light-emitting polymers have been investigated by means of polarized optical spectroscopy. The purpose of the investigation was to investigate the nature of the interactions in thin films and to examine what impact the difference in side chain structure and molecular weight in poly(2'-methoxy-5-2-ethyl-hexoxy)-1,4-phenylene vinylene (MEH-PPV) and poly(2-(3',7'-dimethyloctyloxy)-5-methoxy-1,4-phenylene-vinylene) (OC1C10-PPV) has on the electronic and optical properties of the two polymers. Aligning the polymers by dispersing them in anisotropic solvents and stretched films shows that the side chains have an impact on the relative orientations of the transition dipole moments. In anisotropic solvents the linear dichroism is larger for MEH-PPV than for the related polymer OC1C10-PPV, while in stretched films the opposite situation prevails. A lower polarization of the luminescence from OC1C10-PPV, relative to MEH-PPV, was also obtained independent of alignment medium used. The data therefore suggest that while mechanical stretching may align the OC1C10-PPV to a greater degree, the emitting species is distinct from the absorbing species. The circular dichroism (CD) spectra of both polymers undergo dramatic changes when the liquid phase and the solid state (film) are compared. The solution CD spectra shows no evidence of interchain interactions; instead the spectra of both systems indicate a helical conformation of the polymers. The CD spectra of films are dramatically different with the strong Cotton effect being observed. This points to the formation of an aggregate in the film, with an associated ground state interaction, an interchain species such as a physical dimer, or a more complex higher aggregate.

Solubilization of Polyelectrolytic Hairy-Rod Polyfluorene in Aqueous Solutions of Nonionic Surfactant

We report on the solubilization, phase behavior, and self-organized colloidal structure of a ternary water-polyfluorene-surfactant (amphiphile) system comprised of polyelectrolytic poly{1,4-phenylene[9,9-bis(4-phenoxybutylsulfonate)]fluorene-2,7-diyl} (PBS-PFP) in nonionic pentaethylene glycol monododecyl ether (C12E5) at 20 degrees C. We show in particular how a high amount (milligrams per milliliter) of polyfluorene can be solubilized by aqueous C12E5 via aggregate formation. The PBS-PFP and C12E5 concentrations of 0.31 x 10(-4)-5 x 10(-4) M and 2.5 x 10(-4)-75 x 10(-4) M, respectively, were used. Under the studied conditions, the photoluminescence (PL), surface tension, static contact angle, and (pi-A) isotherm measurements imply that D2O-PBS-PFP(C12E5)x realizes three phase regimes with an increasing molar ratio of surfactant over monomer unit (x). First, for x < or = 0.5, the mixture is cloudy. In this regime polymer is only partially dissolved. Second, for 1 < or = x < or = 2, the solution is homogeneous. In this regime polymer is dissolved down to the colloidal level. Small-angle neutron scattering (SANS) patterns indicate rigid elongated (polymer-surfactant) aggregates with a diameter of 30 A and mean length of approximately 900 A. The ratio between contour length and persistence length is less than 3. Third, for x > or = 4, the solution is homogeneous and there is cooperative binding between polymer and surfactant. Surface tension, contact angle, and surface pressure remain essentially constant with increasing x. A PL spectrum characteristic of single separated polyfluorene molecules is observed. SANS curves show an interference maximum at q approximately 0.015 A(-1), indicating an ordered phase. This ordering is suggested to be due to the electrostatic repulsion between polymer molecules adsorbed on or incorporated into the C12E5 aggregates (micelles). On dilution the distance between micelles increases via 3-dimensional packing. In this regime the polymer is potentially dissolved down to the molecular level. We show further that the aggregates (x = 2) form a floating layer at the air-water interface and can be transferred onto hydrophilic substrates.

Chain morphologies in semicrystalline polyfluorene: evidence from Raman scattering

The beta 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 beta phase with the overall alpha crystalline phase in PF8.