Article pubs.acs.org/Macromolecules
Crystalline and Noncrystalline Forms of Poly(9,9-diheptylfluorene) Matti Knaapila,† Mika Torkkeli,*,‡ Frank Galbrecht,§ and Ullrich Scherf§ †
Institute for Energy Technology, P.O. Box 40, NO-2027 Kjeller, Norway Department of Physics, University of Helsinki, POB 64, FI-00014 Helsinki, Finland § Fachbereich Chemie, Bergische Universität Wuppertal, Gauss-Strasse 20, DE-42097 Wuppertal, Germany ‡
S Supporting Information *
ABSTRACT: 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.
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INTRODUCTION Much of the generic phase behavior and self-organized structure formation of polyfluorenes (PFs)1,2 is illustrated by linear side chain poly(9,9-dioctylfluorene) (PF8).3,4 In the single molecule level,5 the structure of PF8 should be understood against the fact that the fluorene repeat units are rigid but interconnected by a single bond, which allows them to revolve with respect to each others. Energetically favored torsion angles between repeat units allow classification of conformational isomer families Cα, Cβ, and Cγ.6,7 The family Cβ contains only a single member with significant chain planarization (torsion angle ∼165°), whereas many isomers are possible for the families Cα (all about 135°) and Cγ (all about 155°).6 In the solid state level these isomers manifest diverse crystalline8,9 and noncrystalline10,11 phases with distinctive order−disorder transitions. Most well-known are crystalline orthorhombic α and α′ polymorphs and lamellar β mesomorph (best known as the β-phase) at room temperature as well as a nematic phase at elevated temperature. Also known is the glassy g-phase.12,13 These structural variations are concomitant with distinctive photophysical differences (for a comprehensive review see ref 14). Among these, the β-phase is distinctive and associated with a unique equilibrium absorption peak at ∼437 nm and strong changes in the emission spectra. An apparent question emerges how these structural variations are changed with a slight change in the side chain length. This is interesting for two reasons: First, PFs offer a good model system for fundamental studies of phase behavior and intermolecular self-organization in hairyrod polymers.15 For example, after a heating−cooling cycle in the moderate solvent methylcyclohexane (MCH), PF8 and poly(9,9-dinonylfluorene) (PF9) form 10−100 nm sized © 2013 American Chemical Society
membrane-like structures. These contain (but are not entirely) mesomorphic material corresponding to the solid-state β-phase. Poly(9,9-diheptylfluorene) (PF7) and poly(9,9-dihexylfluorene) (PF6) also exhibit sheetlike aggregates and comparable levels of structuring but without the β-phase characteristics. In the same conditions poly(9,9-didecylfluorene) (PF10) is nematic.16 Second, this system is a good model for fundamental photophysical studies.17 In MCH the optical fingerprints of the solidstate β-phase are observed for PF7, PF8, and PF9. However, β-phase is found to occur most favorably in PF8, which is attributed to a balance between two factors: the dimer/aggregate formation efficiency, poorer for longer alkyl chain lengths, and the van der Waals bond energy available to overcome steric repulsion and planarize the conjugated backbone, which is clearly insufficient in PF10. In the case of PF6 a preferred crystallization may inhibit the formation of the β-phase.17,18 Leclère et al.19,20 have calculated that in PF8 a specific side-chain conformation “Y” is needed for the most efficient π-stacking of the polymer backbone into sheets, which may be correlated with its solid-state photoluminescence properties. The role of side chain length to this is unclear. While earlier efforts have been dedicated to the noncrystalline β-phase, solutions, and gelling phenomena, less has been done to describe the effect of side-chain length on crystalline structures. A thorough computational investigation on the fluorene− benzothiadiazole copolymer has indicated that the sidechain interaction invokes a variety of crystalline phases with Received: November 7, 2012 Revised: December 31, 2012 Published: January 24, 2013 836
dx.doi.org/10.1021/ma3023124 | Macromolecules 2013, 46, 836−843
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microphase-separated lamellar morphology for side-chain length n > 4.27 These results are not directly transferable to PFs since benzothiadiazole is less bulky, thus allowing better formation of π-stacks. The PF8 crystal morphology was first demonstrated by Grell et al.21,22 and later systematically characterized by Chen et al.8,9 and by Brinkmann.23 Of the other PFs, a single exception is PF6, for which a monoclinic α and triclinic α′-phase were identified.24 PF6 and PF8 have also been characterized in thin films where similarities in the crystal structure and liquid crystal (LC) morphologies were identified.25 The intermediate case PF7 (see Scheme 1), on the other hand, has not been studied previously in the solid state. Bright et al.17 Scheme 1. Chemical Structure of PF7
Figure 1. Powder XRD curves of PF7 during slow (0.3 °C/min) heating. Displayed curves are between 62 and 220 °C at ca. 6 deg intervals. The sample was prepared from 1% PF7/MCH gel, and the solvent was evaporated at 190 °C for ca. 10 min. After this the sample was quickly cooled to room temperature (by placing on cool metal surface), resulting in a γ-phase with a significant portion of nematic. The thicker curves represent transition stages from (frozen-in) nematic to α (determined transition temperature 83 °C), from α to γ (122 °C), and from γ-phase to high-T nematic (193 °C).
have studied PF7 in dilute (99%, Merck) at 80−90 °C until completely transparent solution was formed and subsequently cooled down to −21 °C for 30 min, leading to the gelation. Isotropic samples were prepared both from pure polymer and the gel by annealing/slow cooling. There was no apparent difference in the structures obtained by these two routes. Thin films were prepared by dissolving PF7 either in MCH (at 80 °C) or toluene (at RT) at various concentrations (1−10 mg/mL) and spincoated (1600 rpm, 3 min) on polished silicon substrates, annealed for 3 h in N2, and cooled to RT at the rate 1 °C/min. All these resulted in well-aligned crystalline films. X-ray Measurements. XRD. The measurement were made from 1 mm thick samples in direct transmission geometry. The diffractometer consisted of a sealed Cu-anode X-ray tube, Montel/pinhole optics for obtaining a monochromatic (E = 8.04 keV) beam, and MAR345 image plate detector at 454 mm distance. The q-scale (defined as q = 4π sin θ/λ) was carefully calibrated using Si and silver behenate standards and the exact sample-to-detector distance adjusted using a cross-hair magnifier. The measurement as a function of temperature was made with a sample-todetector distance 145 mm. GIXRD. GIXRD measurements from thin films were conducted at the Beamline W1.1 at the Hamburger Synchrotronstrahlungslabor of the Deutsches Electronen Synchrotron (DESY). The beam was monochromatized with a double crystal Si(111) monochromator, and X-ray energy 10.5 keV was used. The beam was narrowed with slits to 0.2 × 1 mm, and the diffraction pattern was recorded with a flat image plate at 297 mm distance. The angle of incidence was ω = 0.12°. X-ray Diffraction from Shear Oriented Samples. The polymer/ MCH gels were spread on silicon substrates and measured as such with the X-ray beam parallel to substrate. Samples were then annealed with the usual heating/slow cooling cycle and measurements from the crystalline samples repeated. The measurements were conducted at the Beamline BW4 at DESY. The X-ray energy was 9.0 keV, and image was recorded with MAR165 detector at a 196 mm distance. 842
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(24) Chen, S. H.; Su, A. C.; Su, C. H.; Chen, S. A. J. Phys. Chem. B 2006, 110, 4007−4013. (25) Kawana, S.; Durrell, M.; Lu, J.; Macdonald, J. E.; Grell, M.; Bradley, D. D. C.; Jukes, P. C.; Jones, R. A. L.; Bennett, S. L. Polymer 2002, 43, 1907−1913. (26) Knaapila, M.; Bright, D. W.; Stepanyan, R.; Torkkeli, M.; Almásy, L.; Schweins, R.; Vainio, U.; Preis, E.; Galbrecht, F.; Scherf, U.; Monkman, A. P. Phys. Rev. E 2011, 83, 051803. (27) Eslamibidgoli, M. J.; Lagowski, J. B. J. Phys. Chem. A 2012, 116, 10597−10606. (28) Rajakumar, P.; Kanagalatha, R. Tetrahedron Lett. 2007, 48, 2761− 2764. (29) Rathnayake, H. P.; Cirpan, A.; Delen, Z.; Lahti, P. M.; Karasz, F. E. Adv. Funct. Mater. 2007, 17, 115−122. (30) Knaapila, M.; Torkkeli, M.; Monkman, A. P. Macromolecules 2007, 40, 3610−3614. (31) Cadby, A. J.; Lane, P. A.; Mellor, H.; Martin, S. J.; Grell, M.; Giebeler, C.; Bradley, D. D. C.; Wohlgenannt, M.; An, C.; Vardeny, Z. V. Phys. Rev. B 2000, 62, 15604−15609. (32) Lee, J. L.; Pearce, E. M.; Kwei, T. K. Macromolecules 1997, 30, 6877−6883.
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