Relationship between the Microstructure Development and the

Article pubs.acs.org/JPCC

Relationship between the Microstructure Development and the Photoluminescence Efficiency of Electrospun Poly(9,9dioctylfluorene-2,7-diyl) Fibers Chia-Cheng Lee,† Shin-Yi Lai,† Wen-Bin Su,‡ Hsin-Lung Chen,§ Cho-Liang Chung,∥ and Jean-Hong Chen†,* †

Department of Materials Engineering, Kun Shan University, Tainan 71003, Taiwan National Synchrotron Radiation Research Center, HsinChu 30076, Taiwan § Department of Chemical Engineering and Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua University, Hsin-Chu 30013, Taiwan ∥ Department of Materials Science and Engineering, I-Su University, Kaohsiung 82445, Taiwan ‡

ABSTRACT: In this work, we established the relationship between the thermally induced microstructure development and the photoluminescence efficiency (Ep) of the poly(9,9-dioctylfluorene) (PF8) fibers prepared from the semidilute PF8/chloroform solutions by direct electrospinning. The as-spun (AS) fibers were found to be composed of an amorphous phase and a significant fraction of mesomorphic β-phase, which may promote the Ep of PF8. The temperature-dependent wide-angle X-ray scattering (WAXS) measurement revealed that the PF8 fibers exhibited complex structural transformation on heating from the AS state, and there existed a strong correlation between Ep and the microstructure developed. The AS fibers exhibited the highest Ep value. The Ep decreased progressively on heating at temperatures below the cold crystallization temperature (Tcc), where the microstructure remained unperturbed. The β-phase transformed to γ- or α′-phase (Cγ-/Cα′-crystals) above Tcc, and such a structural transformation led to a reduction of Ep. With further heating, the Cγ-/Cα′-crystals transformed into the more stable Cα-crystals, presumably as the premelting−reorganization−remelting proceeded at the temperatures over the crystal transformation temperature (Tct). The crystal transformation process reduced the Ep markedly. Finally, above the melting temperature (Tm), the Cα-crystals were disrupted into the disordered molten state, and the Ep value remained largely unchanged up to 200 °C.

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polymer with another flexible polymer, which may be called the “co-electrospinning”.27 For instance, the fabrication of poly(3hexylthiophene) (P3HT) nanofibers has been accomplished by using a two-component core−shell system, where the P3HT and the flexible polymer (such as PEO and PMMA) formed the core and shell, respectively.20−26 Polyfluorenes constitute a family of conjugated polymers exhibiting a number of attractive features for LED application. Poly(9,9-dioctylfluorene-2,7-diyl) (PF8) is one of the most important members in this family, as a blue-emitting material.28−31 The optoelectronic properties of PF8 were found to depend strongly on its molecular packing and conformation. Chunwaschirasiri et al. have demonstrated that PF8 can exhibit different phases with various degrees of intrachain coplanarities defined by the torsional angle between the monomer units in the backbone. These structures are represented by different conformational isomers denoted as Cα, Cγ, and Cβ (Figure 1),32

INTRODUCTION Conjugated polymers have been used as the active layers in organic electronic devices, such as light emitting diodes (OLEDs), photovoltaics (OPVs) and transistors (OTFTs), due to their unique optoelectronic properties and solution processability.1−4 The optoelectronic properties and device performance are determined not only by the intrinsic chemical nature of the conjugated polymers5,6 but also by the preparation condition of the active layers7,8 It is known that the microstructures of conjugated polymers in the solution and thin-film states are highly sensitive to the processing parameters, including solvent quality, solution concentration,9,10 interchain aggregation,5,6,9−11 and stretching by external field.12 Recently, conjugated polymers have been processed to form nanowires or nanofibers by various methods, including self-assembly,13,14 nanoporous templating,15,16 dippen nanolithography,17,18 and electrospinning.19−26 Among these methods, electrospinning is the simplest to produce polymer fibers with diameters ranging from tens of nanometer to micrometers. The successful electrospinning of conjugated polymer fibers usually involved the blending of the conjugated © 2013 American Chemical Society

Received: May 2, 2013 Revised: September 11, 2013 Published: September 11, 2013 20387

dx.doi.org/10.1021/jp4043478 | J. Phys. Chem. C 2013, 117, 20387−20396

The Journal of Physical Chemistry C

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Figure 1. Schematic illustration of three distinct conformational structures and their characteristic intrachain torsional angles of PF8: (a) Cα, (b) Cγ, and (c) Cβ.

efficiency of PF8 in the electrospun fibers is still largely unexplored. In this study, we report the direct electrosponning of PF8 fibers without resorting to the blending with a flexible homopolymer. More importantly, we demonstrate that the conformational structure characterized by the intrachain torsional angle in the PF8 backbone is affected significantly by the dragging force associated with the electrospinning by showing the formation of a significant amount of β-phase (in which the polymer chains are more extended) in the as-spun fibers. Using the combination of temperature-dependent wideangle X-ray scattering (WAXS) and PL spectroscopy, we further examine the transformation of the microstructure induced by heating and its corresponding effect on the PL efficiency. The result may provide the fundamental understanding on modulating the photophysical properties of electrospun PF8 fibers by thermal treatment.

which are found in the crystalline α-phase, crystalline γ-phase, and metastable mesomorphic β-phase, respectively. Chen et al. showed that PF8 exhibited the crystalline α-phase with a helical conformation,33 while the mesomorphic β-phase was characterized by a larger intrachain torsional angle, which resulted in a more extended conformation and a longer conjugation length (as schematically illustrated in Figure 1).34 The structural characteristic of the β-phase is favorable for promoting the carrier mobility of the optoelectronic devices;35,36 therefore, controlling the content of β-phase in PFs is an important task for enhancing the device performance. Recently, Kuo et al. have successfully prepared the electrospun nanofibers of polyfluorene derivatives using their binary blends with PMMA.28 They showed the presence of discontinuous fibril-like domains in the PF8 fibers at low PF8/ PMMA blending ratio, but a continuous core−shell structure was formed at high PF8 content. The higher photoluminescence (PL) efficiency in electrospun fibers was attributed to the much smaller PF8 aggregate domains in the fibers than those in the corresponding thin films. Yang et al. reported that a well-dispersed conjugated polymer in an optical matrix manifested dramatically improved photophysical properties by a stress effect because the individual molecular strands were fully stretched in the diluent sample.12 Indeed, many studies have indicated that the strong stretching forces in an electrospinning process may induce orientation of polymer chains along the long axis of the fiber.37,38 Such a stretching effect could also contribute to the enhancement of the PL efficiency. Therefore, the relationship between the conformational order and the photophysical properties in the electrospun fibers is of great interest for controlling or enhancing the optoelectronic performance of conjugated polymers. However, the influence of the microstructure development on the PL

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EXPERIMENTAL SECTION Materials and Solution Preparation. Poly(9,9-dioctylfluorene-2,7-diyl) (MW approximately 65000g/mol) was purchased from American Dye Source, Inc. (ADS129BE). High-purity chloroform (HPLC grade) was used to prepare conjugated polymer solutions. The solutions were prepared by stirring appropriate quantities of PF8 in chloroform at approximately 60 °C for 2 h until macroscopically homogeneous solutions were observed by the naked eyes. The solutions were used for electrospinning to prepare the PF8 as-spun (AS) fibers and for obtaining the films by drop-casting. Electrospinning and Film Casting. PF8/chloroform solutions with the concentration of 20.0 wt % were used to prepare the AS fibers by electrospinning. The electrospinning setup included a Glassman PS/FC60P high-voltage power 20388



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supply. During electrospinning, a positive high voltage was applied via a copper wire to the spinning solution inside a glass pipet with a capillary tip with the diameter of 0.21 mm. An electrically grounded ITO glass slide or a stainless steel net were placed approximately 30 cm from the pipet tip. The PF8 solution flow rate was set to 0.4 mL/h. As the applied voltage was increased to 30 kV, a solution jet was created. The jet followed a complex stretching and looping path as it dried and solidified to form AS fibers. During spinning, the PF8 chains could be more extended as the average intrachain torsional angle along the PF backbones may be increased by the dragging force. The AS fibers in the mat had a very high aspect ratio, and so few fiber ends were identifiable. The drop-cast PF8 thin films were prepared by dropping the solutions onto a microscope coverslip at ca. 30 °C under a purge of nitrogen gas. Morphological Characterizations. The morphology of PF8 AS fibers was investigated by polarized optical microscopy (POM) and fluorescence optical microscopy (FOM) using a Zeiss Axioskop-40 optical microscope (OM). The detailed morphology of the PF8 as-spun fibers was observed with a Hitachi S-4700 field-emission scanning electron microscope (FE-SEM) operated at an accelerating voltage of 1.0 kV. Spectroscopic Measurements. The absorption measurements for the π−π* and β-phase absorption of PF8 were performed using a Hitachi U-3010 spectrophotometer. The fiber mats, approximately 100 μm thick, were placed on the surface of ITO glass slides for UV−vis and photoluminescence (PL) spectroscopy measurement. The UV−vis spectra were corrected for the ITO glass background. The PL spectra were recorded with a HMTECH MFS-630 Multi spectrophotometer, using an excitation wavelength of 235 nm. The temperaturedependent PL spectra were obtained by heating the specimens using a Linkam TH600 heating stag under nitrogen atmosphere at a rate of 10 °C/min. Thermal Properties. The thermal transitions of PF8 AS fibers and thin films were detected using a Perkin-Elmer Pyris Diamond differential scanning calorimeter (DSC) equipped with an intracooler. The samples of approximately 3 mg in weight were placed into the sample pans and heated under nitrogen to 280 °C at a heating rate of 10 °C/min. The samples were then annealed at 280 °C for 5 min to eliminate the thermal history followed by cooling to 30 °C at a rate of 10 °C/ min to detect the thermal transitions during cooling. Microstructure Characterization. The transformation of the microstructure of the PF8 AS fibers induced by heating was investigated by the temperature-dependent WAXS experiment conducted at Beamine BL17B3 of the National Synchrotron Radiation Research Center (NSRRC), Taiwan. The X-ray beam with the wavelength (λ) of 1.33 Å was collimated into a 0.5 mm × 0.5 mm beam size. The WAXS profiles over the scatting vector (q = 4π sin(θ/2)/λ, with θ being the scattering angle) range of 0.8 to 48 nm−1 were collected under the sample-todetector distance of 0.307 m. The detector used was an imaging plate (Fuji BAS III, area = 20 × 40 cm2) having 100 μm pixel resolution. The scattering intensities were corrected for the absorption and the air scattering. The temperature-dependent scattering profiles were probed in situ from 30 to 180 °C at a heating rate of 2.0 °C/min under a nitrogen atmosphere.

Figure 2. Images of as-spun PF8 fibers observed by various microscopes: (a) optical microscopy (OM); (b) polarized optical microscopy (POM); (c) fluorescence optical microscopy (FOM); (d) SEM; (e) SEM with greater magnification; and (f) TEM. The scale bars in all OM images represent 50 μm. The OM and SEM images show the formation of long continuous uniform fibers with diameters in the range of 0.3−1.5 μm. The POM and FOM images indicate that the as-spun fibers contain a mesophase that promotes the intensity of the deep-blue emission. The SEM image in part e and the TEM image reveal the presence of many nanoscale pellets or pores.

concentration was well above the general overlap concentration (c*) of conjugated polymers, such that the polymer chains should entangle significantly.9 The entanglement and aggregation of the PF8 chains in the solution are required to develop the viscoelasticity, which is essential for successful electrospinning. Indeed, at the lower concentration (c < 15.0 wt %), the fiber mats are found to contain many droplets or spindles. Uniform AS fibers are obtained by increasing the polymer concentration to 20 wt % at 45 °C. As can be seen in Figure 2a and d, the AS fibers show a uniform morphology with average diameters of ca. 0.3−1.5 μm. The fibers in mats exhibit a very high aspect ratio (i.e., almost no fiber end was identified). It is important to note that a clear birefringence phenomenon is observed in the AS fibers through POM (Figure 2b), showing the presence of anisotropic microstructure. The anisotropic structure may be associated with the well-defined mesophase, such as the β-phase,39−41 with larger intrachain torsional34,35 and/or the highly extended and well-aligned chains along the fiber axes induced by the electrostatic stretching force.22,28,37,38 The FOM image in Figure 2c reveals that the AS fibers display strong deep-blue emission, which can be ascribed to the presence of a significant amount of the β-phase.34−36 The greater magnifications of the fibers by SEM and TEM observations in Figure 2e and f reveal many nanoscale pellets or pores on the surface of the AS fibers. The presence of these entities implies that a liquid−liquid phase separation or a very rapid solvent evaporation occurred during the electrospinning.28

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RESULTS AND DISCUSSION Parts a−c of Figure 2 show the OM, POM, and FOM micrographs of PF8 AS fibers prepared from 20.0 wt % PF8/ chloroform solution. According to our previous work, this 20389



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To corroborate the formation of the β-phase in the AS fibers, UV−vis and PL spectra, which are strongly dependent on the molecular packing and conjugation length of the polymer,9,10,32,35,36,39−43 were measured. Figures 3 and 4 show

Figure 4. (a) Normalized PL spectra of PF8 in freshly prepared solution with chloroform, cast film, and as-spun fibers. For the solution, the emission peaks at 420 and 440 nm are attributed to the well-dissolved segments. For the cast film, the emission peaks 470 and 500 nm are ascribed to the formation of Cγ-/Cα′-crystals and β-phase, respectively, in the film. The PL spectrum of the as-spun fibers is broad and shifts to higher energy relative to that of the film, due to the presence of higher content of the β-phase. The FOM images shown in the photographs reveal that the emitting colors of the fibers and film are deep-blue and green-blue, respectively. The scale bars in OM images represent 50 μm.

Figure 3. Normalized UV−vis absorption spectra of PF8 in freshly prepared solution with chloroform, cast film, and as-spun fibers. For the solution, the absorption peak at ca. 385 nm is attributed to characteristic absorption of the well-dissolved segments. The absorption peaks at 400 and 430 nm observed for the film correspond to the formation of Cγ-/Cα′-crystals and β-phase, respectively. The asspun fibers display a broad absorption spectrum due to the inhomogeneous structure.

force during electrospinning.28,37,38 This effect has also been observed for the mechanically stretched MEH-PPV/polyethylene films45 and also in other as-spun fibers.46 As to the PL spectrum of the solution shown in Figure 4, the emission peaks at 420 and 440 nm are contributed by the welldissolved segments, while the weak shoulders at 470 and 500 nm are attributed to the emission from the β-phase domains.9−11,47 The PL spectrum of the cast film is obviously red-shifted relative to that of the solution. The spectrum now is characterized by two peaks at 470 and 500 nm and a shoulder at 540 nm. The former two peaks are associated with the Cα′-/ Cγ-conformer and β-phase, respectively, whereas the 540 nm peak is attributed to the emission from the disordered domains. The PL spectrum of the PF8 AS fiber is very broad and blueshifted relative to that of the cast film. The change of the PL spectrum may be due to the higher β-phase content in the fibers. In fact, the photoexcitons of conjugated polymer chains with various energies should relate to different intrachain torsional angles.32 The emission peaks at 450 and 490 nm displayed by the AS fibers may be ascribed to ordered and disordered conformational states, respectively, as will be discussed later. Parts b and c of Figure 4 show the photographs of PF8 AS fibers and cast films, respectively. The images demonstrate that the emitting color of the fibers is deep-blue compared to green-blue for the film. This observation verifies that the AS fiber contains a higher population of blue-sifted lumophores (e.g., β-phase) than the film. Figure 5 displays the DSC thermograms of the PF8 AS fibers and cast film obtained in a heating and subsequent cooling

the normalized UV−vis and PL spectra of the PF8 AS fiber mats, respectively. The spectra of the cast film and the freshly prepared PF8/chloroform solution are also displayed for comparison. It can be seen in Figure 3 that only a single absorption maximum (λmax) at around 385 nm and a weak peak at 430 nm are observed in the absorption spectrum of the freshly prepared solution. The main peak at 385 nm arises from the well-dissolved PF8 segments, while the weak absorption at 430 nm is associated with the β-phase domains (e.g., sheetlike or membrane type aggregates organized by the PF8 chains39−41) in the semidilute solution.9,10 For the cast film, the absorption peak arising from the crystalline α′- or γ-phase and the β-phase is identified at 400 and 430 nm, respectively. In addition, a high-energy peak is observed at ca. 380 nm. The presence of the α′- or γ-phase peak has been attributed to the formation of Cα′ or Cγ conformational isomers9,10,31,41 and to the transformation of randomly twisted local PF backbone motifs into a P52 helix and other unresolved local backbone motifs.32,41,44 It is interesting to see that the electrospinning process produces a broad distribution of conformations, as the absorption spectrum of the AS fibers shows the overlap of a number of peaks in the range of 300−550 nm. Compared to the cast film, the fact that fibers exhibit a much stronger β-phase absorption at 430 nm indicates that the AS fibers contain a greater amount of PF8 chains adopting the more extended Cβ conformation as a result of the application of strong stretching 20390



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Figure 6 displays the WAXS profiles of the AS fibers and cast film for resolving the microstructue of PF8 in these two states.

Figure 5. DSC thermograms of PF8 as-spun fibers and cast film obtained in (a) heating and (b) subsequent cooling cycles. Tg, Tcc, Tct, Tmc, and Tm represent glass transition temperature, cold crystallization temperature (on heating), crystal transition temperature, melt crystallization temperature (on cooling), and melting temperature, respectively.

Figure 6. WAXS profiles of the PF8 cast film and as-spun fibers. The cast film displays scattering peaks at q = 4.78, 9.1, 13.95, and 14.82 nm−1, which correspond to (200), (300), (250), and (530)/(008) diffraction of Cγ-/Cα′-crystals of PF8, respectively. The WAXS profile of PF8 AS fiber exhibits a relatively broad peak at ca. 4 nm−1, which is attributed to the (200) diffraction of the β-phase.

cycle at the rate of 10 °C/min. The film exhibits a glass transition (Tg) at ca. 58 °C and a crystal transformation peak (Tct) at ca. 127 °C, followed by two melting peaks, Tm1 and Tm2, at 140.7 and 158.4 °C, respectively, with the total heat of melting of ca. 23.6 J/g (Figure 5a). Recently, Chen et al. indicated that the Tm1 endotherm corresponded to the initial melting of the imperfect cold-crystallized crystals, while the Tm2 peak was associated with the final melting of well-organized crystals, and the multiple melting behavior implied the occurrence of significant premelting, reorganization, and remelting upon heating in the thin film.48 The crystallinity of the cast film is estimated to be ca. 32.3%, based on the heat of melting (ΔHm = 73 J/g) of 100% crystalline PF8.49 The AS fibers show more complex thermal transition compared with the film. The DSC curve now displays a Tg followed by a stress relaxation peak at ca. 55 °C, which leads to a thermal shrinkage of the fibers due to the relaxation of the oriented backbone to the orientation-free chains with smaller average torsional angle. It is interesting to note that the AS fibers exhibit a clear cold crystallization exotherm (Tcc) at ca. 90 °C, with the heat of crystallization of 21.8 J/g (crystallinity = ca. 30.0%). The presence of the cold crystallization peak evidences the occurrence of crystallization from the mesomorphic β-phase into the α′- or γ-phase crystals on heating the fibers.50 The AS fibers also exhibit a crystal transformation peak (Tct) at 120.2 °C, followed by a melting endotherm at 152.3 °C (with the corresponding crystallinity of ca. 23.4%). The cooling traces of both the cast film and AS fibers are shown in Figure 5b. The film shows a melt crystallization temperature (Tmc) at 84.7 °C, with the corresponding heat of crystallization of 19.5 J/g (26.5% in crystallinity). On the other hand, the AS fibers show a higher Tmc at 99.2 °C, and the corresponding heat of crystallization is 27.9 J/g (crystallinity = 38.2%). The higher Tmc indicates the faster crystallization kinetics of PF8 in the fibers because the residue of the more extended segments may provide effective nucleation sites for the subsequent cold crystallization process on cooling.

It can be seen that the cast film displays scattering peaks at q = 4.78, 9.1, 13.95, and 14.82 nm−1, which correspond to (200), (300), (250), and (530)/(008) diffraction of Cγ-/Cα′-crystals of PF8, respectively.48 The WAXS profile also shows an amorphous halo at q = 12.0 nm−1. The WAXS profile of the PF8 AS fiber exhibits a relatively broad peak at ca. 4 nm−1, which is attributed to the (200) diffraction of the β-phase. The broad halo at ca. 14 nm−1 is due to the β-phase nanodomain and/or amorphous phase present in the fibers. The WAXS profile reveals that there is negligible crystallinity in the AS fibers. Thus, the AS fibers compose the amorphous region and the mesomorphic β-phase. The amount of these two characteristic structures estimated from the scattering profile is ca. 60% and 40%, respectively (cf. Figure 9). The β-phase, although it constitutes the minor component, may still dominate the photophysical properties of the AS fibers. The DSC heating scans of both AS fibers and cast film demonstrate the occurrences of various thermal transitions, including glass transition, cold crystallization, crystal transformation, and crystal melting, on heating. In the light of this finding, we also carry out a temperature-dependent WAXS experiment to probe the characteristic structure developed during the heating process, and the structure evolution is then correlated with the temperature-dependent PL spectra to draw a clear understanding on the effect of microstructure on the photoemission behavior and the evolutions of structure and PL efficiency upon heating the PF8 AS fibers. Figure 7a displays the temperature-dependent WAXS profiles of the fibers collected with the heating rate of 2 °C/min from 30 to 170 °C. The WAXS profiles below 85 °C shows the presence of amorphous phase and β-phase in the fiber, as discussed above. The WAXS profile changes significantly as the temperature is raised beyond 85 °C (which is close to Tcc observed in DSC 20391



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scattering peaks develop at the expense of the Cγ-/Cα′-crystal peaks. These new peaks at q = 3.5, 7.12, 10.52, 12.64, and 14.0 nm−1 are ascribed to the (110), (310), (330)/(140), (520), and (250) diffractions of Cα-crystals, respectively.48,49 The result verifies that the crystal structure of PF8 in the fibers transforms from Cγ-/Cα′-crystals to the more stable Cα- (α-phase) crystals at Tct. A more detailed interpretation of the crystal transformation is examined by the temperature dependences of the (110) and (200) peaks, as shown by the enlarged WAXS profiles in Figure 7b. It is clear that, as temperature exceeds Tct, the position of the (200) diffraction shifts to lower q, and such a shift is accompanied with an increase of the intensity of the (110) peak. This observation attests the occurrence of a crystal transformation from the Cγ-/Cα′-crystals to the more stable Cαcrystals in the fibers. On further heating, Cα-crystal melts above ca. 155 °C (Tm). Considering that the microstructure of PF8 in the AS fibers depends strongly on temperature, it is of great interest to examine the corresponding dependence of PL efficiency on temperature. Here, the PL spectra at various temperatures were recorded in situ by heating the fiber mats of PF8 from 30 to 200 °C with a rate of 10 °C/min under nitrogen atmosphere. The intensities of the PL peaks change markedly as the temperature is raised, as shown in Figure 8a. Here, we illustrate the PL spectra in two parts according to the temperature: (i) the spectra at 30−90 °C (T < Tcc), and (ii) the spectra at 95− 200 °C (T > Tcc), as displayed in Figure 8b and c, respectively. Figure 8b shows that, when the temperature is lower than 90 °C, the intensity of PL decreases progressively with the increase of temperature. This fact may be due to the relaxation of the PF8 backbone from a larger average torsional angle to a smaller one. When the temperature reaches 90 °C (near Tcc), the PL peaks change markedly (Figure 8b), as the intensities of the

Figure 7. (a) Temperature-dependent WAXS profiles of PF8 as-spun fibers collected in a heating cycle with a heating rate of 2.0 °C/min. (b) Enlarged WAXS profiles in the q range of 3 to 6 nm−1 to examine the thermally induced crystal transformation in further detail.

scan), where a series of diffraction peaks develop at 4.78, 9.1, and 14.82 nm−1; they are attributed to the (200), (300), and (530) (008) diffractions of the Cγ-/Cα′-crystals formed from the mesophase in AS fibers. It is interesting that, at temperatures over 120 °C (close to Tct in the DSC scan), several new

Figure 8. (a) Temperature-dependent PL spectra of PF8 as-spun fibers obtained in a heating cycle from 30 to 200 °C with a rate of 10 °C/min under nitrogen atmosphere. It can be seen that the intensities of the PL peaks vary markedly as the temperature is raised. The PL spectra are further separated into two parts according to the temperature, as shown in parts b and c for the temperature range of 30 to 90 °C (T < Tcc) and 95 to 200 °C (T > Tcc), respectively. The FOM images shown in the insets of parts b and c demonstrate the change of the morphology of the PF8 fibers at different temperatures. 20392



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peaks at 450 and 490 nm are enhanced significantly, due to the cold crystallization of PF8 from the β-phase to the Cγ-/Cα′crystalline phase. Upon further heating, Figure 8c shows that the intensities of the PL peaks change abruptly above 95 °C (T > Tcc), where the 0−2, 0−3, and 0−4 vibronic peaks grew significantly with the increase of temperature from 95 to 105 °C32,42 due to the further development of Cγ-/Cα′-crystals. Subsequent heating is found to reduce the PL peak intensities, where the 0−2 vibronic peak decreases slightly in intensity and shifts to shorter wavelength, while the intensity of the 0−3 peak drops significantly as the temperature exceeds Tct. This observation suggests that the 0−3 vibronic peak is associated with the Cγ-/Cα′-crystals. The PL spectra remain largely unperturbed above the melting temperature. The FOM images shown in the insets of Figure 8b and c demonstrate the change of the morphology of the PF8 fibers at different temperatures. A relaxation of the fibers is observed on heating over Tg. Moreover, the FOM images show a red-shift and an increase in fluorescence intensity upon heating to 100 °C. At 170 °C (>Tm), the molten PF8 fibers exhibit weak fluorescence intensity, consistent with the result of PL spectra. The relationship between the thermally induced microstructure development and the PL efficiency (Ep) of the PF8 fibers is of particular interest in this study. Therefore, we deconvoluted the WAXS profiles collected at different temperatures to determine the relative fractions of the various types of microstructure and then drew their correlation with the Ep of the PF8 fibers. Figure 9 displays the representative deconvolutions for the AS fibers and the fibers heated to 130 °C. For the AS fibers, the peaks at 3.5, 4.82, and 14.12 nm−1 are attributed to the β-phase, while the broad peaks at 12 nm−1 are ascribed to the amorphous phase. The fractions of the β-phase and amorphous phase estimated from the corresponding peak areas are approximately 39 and 61%, respectively, in the AS fibers. The heated PF8 fibers show a series of scattering peaks at 3.5, 4.82, 6.4, 10.52, 12.64, and 14.1 nm−1, corresponding to (110), (200), (310), (330)/(140), (520), and (250) diffraction of the α-phase crystal, respectively.48 The diffuse peaks at 4.24 and 14.12 nm−1 are attributed to the β-phase, while that at 12.0 nm−1 is associated with the amorphous phase. The fractions of the α-phase crystal, the β-phase, and the amorphous phase are 31.1%, 8.5%, and 60.4%, respectively, in this heated fiber. Similar analyses were carried out for other experimental temperatures. The PL efficiency (Ep) of the PF8 fibers was calculated by Ep =

APL at‐T × 100% APL at‐30

Figure 9. WAXS profiles of (a) PF8 as-spun fibers and (b) the heated fiber at 130 °C for demonstrating the deconvolution of the observed scattering profiles into the combination of various peaks. The areas of the characteristic peaks were used to calculate the relative fractions of the corresponding microstructures in the fibers at different temperatures.

transform to the more stable Cα-crystals at temperatures above Tct. Above the onset of melting, the Cα-crystals are disrupted into the molten state. The transition temperatures identified by DSC are indeed in close agreement with those deduced from the temperature-dependent WAXS measurement. Moreover, the results reveal that the change of microstructure mainly stems from the cold crystallization from the mesomorphic β-phase to form Cγ-/Cα′-crystals, followed by a crystal transformation into the more stable αphase. The relationship between the temperature-dependent structural evolution and the Ep of PF8 fibers can also be established from Figure 10. The Ep of PF8 fibers decreases slightly when T < Tg, and it drops significantly at Tg < T < Tcc. However, Ep increases markedly at temperatures close to Tcc and subsequently decreases gradually with further heating to 200 °C. Figure 10 reveals that the Ep of PF8 AS fibers (which are composed of mainly amorphous and β-phase) is higher than

(1)

where APL at‑30 is the area of the PL spectrum at 30 °C and APL at‑T is the corresponding area at the temperature of interest. Figure 10 presents the fractions of various types of microstructure as a function of temperature along with the corresponding values of Ep. The locations of Tg, Tcc, Tct, and Tm obtained from DSC are indicated by the dashed lines. It is interesting to note that the fraction of amorphous phase in the PF8 fiber remains largely unchanged with increasing temperature until the melting region. The fraction of the β-phase remains unchanged at temperatures below Tg, but it decreases progressively with the increase of temperature above Tg. Accompanied with such a decrease is the increase of the fraction of the γ- or α′-phase (i.e., Cγ-/Cα′-crystals) at temperatures between Tg and Tct. The Cγ-/Cα′-crystals 20393



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Figure 10. Fractions of various types of microstructure in the PF8 fibers as a function of temperature and the corresponding values of PL efficiency (Ep). The fractions are denoted as FA, FCβ, FCγ, and FCα for amorphous phase, β-phase, γ-phase, and α-phase, respectively. The locations of Tg, Tcc, Tct, and Tm obtained from DSC are indicated by the dashed lines.

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that of crystalline PF8 fibers at Tcc by about 12.5%. This fact allows us to conclude that it is the higher content of the βphase in AS fibers that leads to a greater PL efficiency than the crystalline fibers.

ACKNOWLEDGMENTS We acknowledge the financial support from the National Science Council, Taiwan, under Grant Nos. NSC 99-2221-E168-009 and 100-2632-E-168-001-MY3.

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CONCLUSIONS We provided insight into the influence of the thermally induced microstructure development on the photoluminescence efficiency (Ep) of PF8 fibers prepared by direct electrospinning without resorting to polymer blending. Comparing with the cast film, PF8 AS fibers exhibited broad spectra, with a red-shift in UV−vis absorption and a blue-shift in PL, respectively. The microstructure of AS fibers is heterogeneous, composed of the amorphous phase and the β-phase as the major and minor component, respectively. We deconvoluted the WAXS profiles to calculate the fractions of different types of microstructure at various temperatures. We found that the microstructure development of PF8 fibers was complex as the temperature was raised. Four main features were identified: (1) there was a minor fraction of β-phase within the AS fibers that remained essentially unperturbed at temperatures below Tg; (2) the fraction of the β-phase decreased progressively with the increase of temperature above Tg, and accompanied with such a decrease is the increase of the fraction of the γ- or α′phase (i.e., Cγ-/Cα′-crystals) at temperatures between Tg and Tct; (3) the Cγ-/Cα′-crystals transformed into the more stable Cα-crystals over Tct; (4) above Tm, the crystals were disrupted into the disordered molten state. We demonstrated that the Ep was directly influenced by the microstructure of the PF8 fibers. Ep decreased slightly at T < Tg and then dropped markedly at Tg < T < Tcc; however, a significant increase of the Ep was observed near Tcc before it decreased again with further heating up to 200 °C.

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REFERENCES

(1) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Light-Emitted Diodes Based on Conjugated Polymers. Nature 1990, 347, 539−541. (2) Gross, M.; Müller, D. C.; Nothofer, H. G.; Scherf, U.; Neher, D.; Bräuchle, C.; Meerholz, K. Improving the performance of doped pconjugated polymers for use in organic light emitting diodes. Nature 2000, 405, 661−665. (3) Hoppe, H.; Sariciftci, N. S. Organic solar cells: An overview. J. Mater. Res. 2004, 19, 1924−1945. (4) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Two-dimensional charge transport in self-organized, high-mobility conjugated polymers. Nature 1999, 401, 685−688. (5) Rahman, M. H.; Chen, C. Y.; Chen, H. L.; Chen, J. H.; Liao, S. C.; Liao, J. L.; Ivanov, V. A.; Chen, S. A. Aggregation of Conjugated Polymers in Aromatic Solvent. Langmuir 2009, 25, 1667−1674. (6) Li, Y. C.; Chang, Y. X.; Chuang, P. Y.; Chen, H. L.; Chen, J. H.; Hsu, C. S.; Chen, K. B.; Tsao, C. S.; Chen, S. A. Scattering Study of the Conformational Structure and Aggregation Behavior of Conjugated Polymer Solution. Langmuir 2009, 25, 4668−4677. (7) Liu, J.; Shi, Y.; Ma, L.; Yang, Y. Device performance and polymer morphology in polymer light emitting diodes: The control of device electrical properties and metal/polymer contact. J. Appl. Phys. 2000, 88, 605−609. (8) Nguyen, T. Q.; Martini, I. B.; Liu, J.; Schwartz, B. J. Controlling Interchain Interactions in Conjugated Polymers: The Effects of Chain Morphologyon Exciton-Exciton Annihilation and Aggregation in MEH-PPV Films. J. Phys. Chem. B 2000, 104, 237−255. (9) Chen, J. H.; Chang, C. S.; Chang, Y. X.; Chen, C. Y.; Chen, H. L.; Chen, S. A. Gelation and Its Effect on the Photophysical Behavior of Poly(9,9-dioctylfluorene-2,7-diyl) in Toluene. Macromolecules 2009, 42, 1306−1315. (10) Chen, C. Y.; Chang, C. S.; Huang, S. W.; Chen, H. L.; Chen, J. H.; Su, C. I.; Chen, S. A. Phase-Separation-Induced Gelation of Poly(9,9-dioctylfluorene)/Methylcyclohexane Solution. Macromolecules 2010, 43, 4346.

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(11) Chen, C. Y.; Chan, S. H.; Li, J. Y.; Wu, K. H.; Chen, H. L.; Chen, J. H.; Huang, W. Y.; Chen, S. A. Formation and ThermallyInduced Disruption of Nanowhiskers in Poly(3-hexylthiophene)/ Xylene Gel Studied by Small-Angle X-ray Scattering. Macromolecules 2010, 43, 7305−7311. (12) Tung, K. P.; Chen, C. C.; Lee, P. W.; Liu, Y. W.; Hong, T. M.; Hwang, K. C.; Hsu, J. H.; White, J. D.; Yang, A. C.-M. Large Enhancements in Optoelectronic Efficiencies of Nano-plastically Stressed Conjugated Polymer Strands. ACS Nano 2011, 9, 7296− 7302. (13) Ihn, K. J.; Moulton, J.; Smith, P. Whiskers of poly(3alkylthiophene). J. Polym. Sci., B: Polym. Phys. 1993, 31, 735−742. (14) Liu, J. S.; Sheina, E.; Kowalewski, T.; McCullough, R. D. Tuning the Electrical Conductivity and Self-Assembly of Regioregular Polythiophene by Block Copolymerization: Nanowire Morphologies in New Di- and Triblock Copolymers. Angew. Chem., Int. Ed. 2002, 41, 329−332. (15) Wu, C. G.; Bein, T. Conducting Polyaniline Filaments in a Mesoporous Channel Host. Science 1994, 264, 1757−1759. (16) Martin, C. R. NanomaterialsA Membrane-Based Synthetic Approach. Science 1994, 266, 1961−1966. (17) Noy, A.; Miller, A. E.; Klare, J. E.; Weeks, B. L.; Woods, B. W.; DeYoreo, J. J. Fabrication of Luminescent Nanostructures and Polymer Nanowires Using Dip-Pen Nanolithography. Nano Lett. 2002, 2, 109−112. (18) Maynor, B. W.; Filocamo, S. F.; Grinstaff, M. W.; Liu, J. DirectWriting of Polymer Nanostructures: Poly(thiophene) Nanowires on Semiconducting and Insulating Surfaces. J. Am. Chem. Soc. 2002, 124, 522−523. (19) MacDiarmid, A. G.; Jones, W. E.; Norris, I. D.; Gao, J.; Johnson, A. T.; Pinto, N. J.; Hone, J.; Han, B.; Ko, F. K.; Okuzaki, H.; Llaguno, M. Electrostatically-Generated Nanofibers of Electronic Polymers. Synth. Met. 2001, 119, 27−30. (20) Sun, Z.; Zussman, E.; Yarin, A. L.; Wendorff, J. H.; Greiner, A. Compound Core−Shell Polymer Nanofibers by Co-Electrospinning. Adv. Mater. 2003, 15, 1929−1932. (21) Yu, J. H.; Fridrikh, S. V.; Rutledge, G. C. Production of Submicrometer Diameter Fibers by Two-Fluid Electrospinning. Adv. Mater. 2004, 16, 1562−1566. (22) Dan, Li; Babel, A.; Jenekhe, S. A.; Xia, Y. Nanofibers of Conjugated Polymers Prepared by Electrospinning with a TwoCapillary Spinneret. Adv. Mater. 2004, 16, 2062−2066. (23) Sundarrajan, S.; Murugan, R.; Nair, A. S.; Ramakrishna, S. Fabrication of P3HT-PCBM solar cloths by electrospinning technique. Mater. Lett. 2010, 64, 2369−2372. (24) Norris, I. D.; Shaker, M. M.; Ko, F. K.; Macdiaramid, A. G. Electrostatic fabrication of ultrafine conducting fibers: polyanilinerpolyethylene oxide blends. Synth. Met. 2000, 114, 109−114. (25) Chen, J. Y.; Kuo, C. C.; Lai, C. S.; Chen, W. C.; Chen, H. L. Manipulation on The Morphology and Electrical Properties of Poly(3hexylthiophene) Aligned Electrospun Nanofibers For High Performance Field-Effect Transistors. Macromolecules 2011, 44, 2883−2892. (26) Chana, K. H. K.; Yamaob, T.; Kotakia, M.; Hotta, S. Unique structural features and electrical properties of electrospun conjugated polymer poly(3-hexylthiophene) (P3HT) fibers. Synth. Met. 2010, 160, 2587−2595. (27) Bazilevsky, A. V.; Yarin, A. L.; Megaridis, C. M. Coelectrospinning of Core−Shell Fibers Using a Single-Nozzle Technique. Langmuir 2007, 23, 2311−2314. (28) Kuo, C. C.; Lin, C. H.; Chen, W. C. Morphology and Photophysical Properties of Light-Emitting Electrospun Nanofibers Prepared from Poly(fluorene) Derivative/PMMA Blends. Macromolecules 2007, 40, 6959−6966. (29) Knaapila, M.; Stepanyan, R.; Lyons, B. P.; Torkkeli, M.; Monkman, A. P. Towards General Guidelines for Aligned, Nanoscale Assemblies of Hairy-Rod Polyfluorene. Adv. Funct. Mater. 2006, 16, 599−609. (30) Akcelrud, L. Electroluminescent polymers. Prog. Polym. Sci. 2003, 28, 875−962.

(31) Cadby, J.; Lane, P. A.; Mellor, H.; Martin, S. J.; Grell, M.; Giebeler, C.; Bradley, D. D. C. Film morphology and photophysics of polyfluorene. Phys. Rev. B 2000, 62, 15604−15609. (32) Chunwaschirasiri, W.; Tanto, B.; Huber, D. L.; Winokur, M. J. Chain conformations and photoluminescence of poly(di-n-octylfluorene). Phys. Rev. Lett. 2005, 94, 107402 (1−4). (33) Chen, S. H.; Chou, H. L.; Su, A. C.; Chen, S. A. Molecular Packing in Crystalline Poly(9,9-di-n-octyl-2,7-fluorene). Macromolecules 2004, 37, 6833−6838. (34) Knaapila, M.; Bright, D. W.; Stepanyan, R.; Torkkeli, M.; Almasy, L.; Scheweins, R.; Vainio, U.; Preis, E.; Galbrecht, F.; Scherf, U.; Monkman, A. P. Phys. Rev. E 2011, 83, 051803−051814. (35) Lu, H. H.; Liu, C. Y.; Chang, C, H.; Chen, S. A. Self-Dopant Formation in Poly(9,9-di-n-octylfluorene) Via Dipping Method for Efficient and Stable Pure-Blue Electroluminescence. Adv. Mater. 2007, 19, 2574−2579. (36) Peet, J.; Brocker, E.; Xu, Y. H.; Bazan, G. C. Controlled β-Phase Formation in Poly(9,9-di-n-octylfluorene) by Processing with Alkyl Additives. Adv. Mater. 2008, 20, 1882−1885. (37) Pedicine, A.; Farris, R. J. Mechanical behavior of electrospun polyurethane. Polymer 2003, 44, 6857−6862. (38) Tosaka, M.; Yamaguchi, K.; Tsuji, M. Latent orientation in the skin layer of electrospun isotactic polystyrene ultrafine fibers. Polymer 2010, 51, 547−553. (39) Knaapila, M.; Garamus, V. M.; Dias, F. B.; Almãsy, L.; Galbrecht, F.; Charas, A.; Morgado, J.; Burrows, H. D.; Scherf, U.; Monkman, A. P. Influence of solvent quality on the self-organization of archetypical hairy rodsBranched and linear side chain polyfluorenes: Rodlike chains versus “beta-sheets” in solution. Macromolecules 2006, 39, 6505−6512. (40) Knaapila, M.; Stepanyan, R.; Torkkeli, M.; Garamus, V. M.; Galbrecht, F.; Nehls, B. S.; Preis, E.; Scherf, U.; Monkman, A. P. Network structure of polyfluorene sheets as a function of alkyl side chain length. Phys. Rev. E 2008, 77, 051803. (41) Knaapila, M.; Dias, F. B.; Garamus, V. M.; Almasy, L.; Torkkeli, M.; Leppanen, K.; Galbrecht, F.; Preis, E.; Burrows, H. D.; Scherf, U.; Monkman, A. P. Influence of side chain length on the self-assembly of hairy-rod poly(9,9-dialkylfluorene)s in the poor solvent methylcyclohexane. Macromolecules 2007, 40, 9398−9405. (42) Zhu, R.; Lin, J. M.; Wang, W. Z.; Zheng, C.; Wei, W.; Huang, W.; Xu, Y. H.; Peng, J. B.; Cao, Y. Use of the beta-phase of poly(9,9dioctylfluorene) as a probe into the interfacial interplay for the mixed bilayer films formed by sequential spin-coating. J. Phys. Chem. B 2008, 112, 1611−1618. (43) Winokur, M. J.; Slinker, J.; Huber, D. L. Structure, photophysics, and the order-disorder transition to the b phase in poly.9,9-.di-n,noctyl.fluorene. Phys. Rev. B 2003, 67, 184106. (44) Grell, M.; Bradley, D. D. C.; Long, X.; Chamberlain, T.; Inbasekaran, M.; Woo, E. P.; Soliman, M. Chain geometry, solution aggregation and enhanced dichroism in the liquid crystalline conjugated polymer poly(9,9-dioctylfluorene). Acta Polym. 1998, 79, 439−444. (45) Nguyen, T. Q.; Wu, J.; Doan, V.; Schwartz, B. J.; Tolbert, S. H. Control of energy transfer in oriented conjugated polymermesoporous silica composites. Science 2000, 288, 652−656. (46) Hagler, T. W.; Pakbaz, K.; Voss, K. F.; Heeger, A. J. Enhanced order and electronic derealization in conjugated polymers oriented by gel processing in polyethylene. Phys. Rev. B 1991, 44, 8652−8666. (47) Neher, D. Macromol. Polyfluorene Homopolymers: Conjugated Liquid-Crystalline Polymers for Bright Blue Emission and Polarized Electroluminescence. Rapid Commun. 2001, 22, 1365−1385. (48) Chen, S. H.; Wu, Y. H.; Su, C. H.; Jeng, U.; Hsieh, C. C.; Su, A. C.; Chen, S. A. Cold Crystallization of Poly(9,9-di-n-octyl-2,7fluorene). Macromolecules 2007, 40, 5353−5359. (49) Chen, S. H.; Su, C. H.; Su, A. C.; Sun, Y. S.; Jeng, U.; Chen, S. A. Gibbs-thomson Analysis of Crystalline Poly(9,9-di-n-octyl-2,7fluorene). J. Appl. Crystallogr. 2007, 40, s573−s576. 20395



Article

(50) Shimizu, J.; Okui, N.; Kikutani, T. High-Speed Fiber Spinning: Science and Engineering Aspects; Ziabicki, A., Kawai, H., Eds.; WileyInterscience: Krieger, 1991; Chapter 15, pp 448−454.

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