Morphology and Structure of the β Phase Crystals of Monodisperse

Article pubs.acs.org/Macromolecules

Morphology and Structure of the β Phase Crystals of Monodisperse Polyfluorenes Chengfang Liu,†,‡ Qilin Wang,†,‡ Hongkun Tian,† Jian Liu,†,‡ Yanhou Geng,† and Donghang Yan*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China ABSTRACT: Lenticular crystals of the β phase of monodisperse poly(9,9-dioctylfluorene)s (PFOs) have been prepared from o-dichlorobenzene (ODCB) solution. The diffraction analysis combined with X-ray and electron diffraction diagrams indicates that PFOs can crystallize in the orthorhombic system which have the lattice parameters of a = 1.32 nm, b = 2.10 nm, and c = 3.36 nm. In these single crystals, the backbone chains are packed parallel to the long axis of crystals. Furthermore, a variation of the annealing temperature from 90 to 130 °C demonstrates that there exists morphology and structure evolution, which has been systematically investigated by optical microscopy and electron microscopy.

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INTRODUCTION Poly(9,9-dioctylfluorene)s (PFOs) have become a building block of conjugated polymers, owing to their wide application in low-cost optoelectronic devices,1 such as light-emitting diodes (LEDs)2,3 and electrically pumped organic lasers.4 Apart from device applications, PFOs have also been investigated as an impressive model system5−10 that can adopt various polymorphisms,11−13 such as the nematic (N) phase, the α phase, the α′ phase, and the β phase, which can be selectively obtained by different treatments. For spin-coated thin films, extended treatment of the amorphous film with thermal treatment predominantly resulted in the α phase. The α′ phase12 which is closely related to the α phase is kinetically favored at a lower crystallization temperature. The yield of the β phase can be optimized by spin-casting from relatively poor solvents14,15 or exposure to the solvent vapor.16−19 These structural variations are concomitant with distinctive photophysical differences. Among these, the β phase forms the most planar conformation with an extended conjugation length which can be probed by the characteristic peak of around 430 nm in UV− vis absorption spectra. Therefore, it has significant photophysical differences with other phases of PFOs. A small concentration of the β phase can greatly dominate the luminescence.17,20 The increase of the β phase has also been related to an increased ratio of singlet to triplet excitons and improved hole mobility.21−23 Thus, PFOs containing the β phase exhibit the excellent performance in devices. To the best of our knowledge, the performance of devices based on conjugated polymers strongly depends on their solid-state structure. From this viewpoint, it is imperative to control the morphology in the β phase. Although several studies of the β © 2013 American Chemical Society

phase have been reported from spectroscopic points of view,13,24−26 the structure of the β phase has not yet been resolved, and understanding the relationship between structure, morphology, and properties still remains a challenge. While early efforts in the studies of the β phase have been concentrated in thin crystalline films,12,27 less has been done to describe the exact ways of molecular packing in the β phase. All these are understandably due to the heterogeneous chain lengths. In this context, preparing single crystals that have clear and well-defined structure can provide an extraordinary investigation tool and help assess the major characteristics of the β phase of PFOs, which is crucial for elucidating its crystal structure.28−35 However, the formation of large single crystals is still difficult and time-consuming because of the rigorous conditions required for single crystal growth. In this work, we use monodisperse PFOs36,37 to eliminate the structural obstacles caused by the chain length distribution. Thus, lenticular crystals are obtained by crystallization of PFOs from the solution. A combination of the X-ray diffraction (XRD) and the selectedarea electron diffraction (SAED) pattern has indicated that the β phase crystals have orthorhombic lattice parameters of a = 1.32 nm, b = 2.10 nm, and c = 3.36 nm. These results allow for correlating the crystal morphology with the structure of PFOs in the β phase. In this case, the backbone chains are parallel to the long axis of the crystal. Moreover, we will be particularly interested in morphology evolution and structural transformation of crystals under thermal annealing.38 Received: January 3, 2013 Revised: March 30, 2013 Published: April 12, 2013 3025

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samples for TEM were floated away from the substrate in 10% HF solution and then picked up with a copper grid. The camera length was calibrated with Au to calculate the d-spacing of the observed electron diffractions. The X-ray diffraction (XRD) measurement was performed using a Bruker D8 Discover diffractometer (Cu Kα, λ = 1.540 56 Å) with generation power of 40 kV tube voltages and 40 mA tube current. With the slow evaporation of the solvent, lenticular crystals were obtained. Then, we performed out-of-plane XRD characterization on this sample, and the XRD pattern showed the presence of preferred orientation. Specimens for powder XRD were crystals we collected from the substrate.

EXPERIMENTAL SECTION

Materials. Monodisperse PFOs used here were synthesized according to the previous report.39 The molecular weight obtained from MALDI-TOF mass spectrometry is 6220 g/mol, and the polydispersity index PDI = 1.09 as determined via GPC using polystyrene standards. o-Dichlorobenzene (ODCB, anhydrous, 99%) was purchased from Aldrich Co. and used without further purification. Crystallization of PFO. The PFOs were dissolved in the solution with concentration of 0.5 mg/mL. The solution was heated up to 70 °C for complete dissolution. After cooling to room temperature, the solution was standing in vibration-free environment over 24 h to obtain homogeneous samples. The substrate was put inside a cylinder container with a radius and height of 1.5 and 3.0 cm, respectively. A small amount of solution was dropped onto the glass substrate. At the same time, solvent vapor could only escape through the small gap between the container and its lid. It should be noted that slow evaporation of the solvent guaranteed polymer chains had sufficient time to adjust themselves and induce them grow into crystals. Because of the good solubility of PFO molecules, o-dichlorobenzene (ODCB), toluene, and chlorobenzene, etc., had been used as the solvent, and the results demonstrated that ODCB was the optimum choice for the preparation of crystals due to its slow evaporation. Thus, lenticular crystals were obtained, and the whole experimental procedures were depicted in Figure 1.

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RESULTS AND DISCUSSION Optical Absorption. To investigate the optical properties of the PFO crystals, their UV−vis absorption and PL spectra have been characterized. Given in Figure 2a is the UV−vis absorption spectrum of the sample drop-casting from ODCB solution. The specimen has a major absorption maximum at 397 nm, which is associated with π−π* transitions. In addition, it exhibits a clearly developed peak around 430 nm, which has been considered characteristic of the β phase in the literature.13 In contrast, the main absorption of the specimen composed of the α phase locates near 385 nm, resulting in a red-shifted absorption edge at 440 nm. The phase identification of samples composed of lenticular crystals can also be determined from their PL spectra. Figure 2b shows the PL spectrum of the same sample. The excitation wavelength is 380 nm. The PL spectrum shows a well-resolved vibronic progression with peaks at 439, 466, and 499 nm, which are unique to the β phase. Morphology and Structure of Lenticular Crystals. The solution concentration could change the size of crystals. With the decrease of the solution concentration, the size of crystals would become smaller accordingly. Alternatively, the density of crystals on the substrate would also decrease. After a series of experiments, we chose the concentration as 0.5 mg/mL to carry out the studies. Crystals of PFOs are mainly lenticular crystals with a length of 2−30 μm and a width of 0.1−2 μm, and the optical anisotropy is clearly observed under polarized light (Figure 3a). The SEM image of several crystals is shown in Figure 3b, and the detailed morphology of a typical crystal is depicted in Figure 3c. The crystal is sharp pointed and the surface is very smooth. AFM is further performed to characterize lenticular crystals. As shown in Figure 4, there are lenticular crystals of different sizes. Figure 5 illustrates that the height images are measured along the length axis and cross section, where apparent changes are noted in the height profiles. In Figure 5a, the thickness demonstrates a gradual decrease from the center to the tip. The variation of thicknesses

Figure 1. Apparatus for the growth of lenticular crystals. Thermal annealing was performed by using a THMS-600 hot stage (Linkam) connected to a TMS-94 temperature controller. Prior to annealing, the chamber of the hot stage was purged several times with nitrogen. Characterizations. The UV−vis spectra of samples of PFOs were measured using a Shimadzu UV 2450 UV−vis spectrophotometer, and their photoluminescence (PL) spectra were measured using a Shimadzu RF-5301PC spectrofluorophotometer. The optical microscopy and SEM images were obtained using Zeiss Axio Imager A2m equipped with the polarizer and FEL XL 30, respectively. Atomic force microscopy (AFM) images were obtained using an SPA-300HV instrument with an SPI3800N controller (Seiko Instruments Inc., Japan) in tapping mode. A silicon microcantilevel (spring constant = 15 N/m, resonant frequency ≈ 130 kHz, Olympus, Japan) was used for the scanning. Transmission electron microscopy (TEM) experiments were performed using a JEOL JEM-1011 with an accelerating voltage of 100 kV and selected area electron diffraction (SAED) modes. The

Figure 2. (a) UV−vis absorption and (b) PL spectrum of the sample composed of lenticular crystals. 3026

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of the unit cell in the plane normal to the electron beam are then calculated to be b = 2.10 nm and c = 3.36 nm. Combining the 2D reciprocal lattice from the SAED pattern and the reciprocal vector from the XRD pattern, an orthogonal unit cell with a = 1.32 nm, b = 2.10 nm, and c = 3.36 nm is assumed. Moreover, powder X-ray diffraction of collected crystals shows 12 peaks (Figure 7). The calculated data of XRD (Table 1) coincide with the observed data very well using the above set of crystal unit cell parameters. Therefore, the crystal unit cell parameters are reasonable. According to the structural analysis, it is shown that polymers in a single crystal pack with the backbone chains parallel to the long axis of the crystal with side chains in thickness direction. Hence, the molecular packing in the β phase crystal could be depicted in Figure 6d. This packing is obtained in a combination of several factors such as π−π stacking, sidechain effects, and solvent effects. In this case, the anisotropy of self-assembly of polymer chains into crystals can be attributed to the very rigid molecular backbone that enables the polymer chains to form intermolecular interactions with extraordinary strength. Moreover, some groups have reported that the backbones are also along the fiber axis in the crystalline films.11 Therefore, this packing behavior is not uncommon in the system of PFOs. A similar molecular stacking motif has also been observed for thioacetate substituted poly(p-phenylene ethynylene) (TA-PPE),34 cyclopentadithiophene−benzothiadiazole (CDT−BTZ) copolymers,41 poly(ferrocenyldimethylsilane-block-2-vinylpyridine) (PFS-b-P2VP) diblock copolymers,42 and poly(benzobisimidazobenzophenanthroline) (BBL),43 wherein the molecules are oriented with the backbones packing along the long axis of the crystals. In regards to the β phase, direct structural studies on it are limited in the literature. According to the previous studies, the β phase is viewed as an intermediate stage of transformation midway from a solvent-induced clathrate structure toward the solvent-free crystalline phase. From the structural analysis, d spacing values in commensuration of lamellar structure with layer spacing of 1.23 nm may be identified. Additional yet diffuse equatorial arcs with d spacing values around 0.64 and 0.43 nm may also be identified, indicating additional in-plane order.12 Nevertheless, details of the chain packing in the β phase remain unclear. In contrast, lenticular crystals we obtained possess higher order than that of the β phase films. Thus, lenticular crystals can help us elucidate the chain alignment in the β phase. In this way, the formation of crystals has offered us an opportunity to decipher the main structural aspects of the β phase of PFOs. Growth of Lenticular Crystals. When polymer chains are dissolved in ODCB solution, the chains have enough time to adjust themselves into a more thermodynamically favorable conformation, resulting in a decreased extent of chains entanglement. This chain motion plays as a trigger to cause the conformational ordering to generate a nucleus to continue the crystallization. As we know, solvent evaporation is a strong, highly directional field. When the solution is deposited onto the substrate and the solvent is evaporated at a slow speed, the molecules could diffuse over a certain distance, exclude defects, and optimize chain packing. We should mention that the most significant factor contributing to this molecular stacking is the very rigid molecular backbone, which enables the polymer chains to form strong intermolecular interactions. Lateral crystal growth of the seed crystals occurs by adding chains onto their lateral surfaces and longitudinal crystal growth by

Figure 3. (a) Polarized light microscopy (PLM) image showing the anisotropy of lenticular crystals. (b, c) SEM images of the crystals.

Figure 4. AFM images of lenticular crystals, demonstrating the typical morphology.

in Figure 5b corresponds to the thicknesses of several layers of stacked polymer chains with the side alkyl chain interdigitated with other layers. Such a lenticular morphology of the PFO crystal is analogous to that of the P3OT crystals induced by solvent vapor annealing.40 To acquire a better understanding of the crystal structure of lenticular crystals, the out-of-plane XRD measurement is carried out. As shown in Figure 6a, the XRD pattern exhibits a series of (h00) reflection peaks as (100), (200), and (300), respectively. For a preliminary determination of molecular organization in the β phase, we performed electron diffraction studies. It is well-known that an electron beam destroys the diffracting power of polymer crystals without altering their shape. The lenticular crystals appear to be particularly sensitive to the electron beam even at very low intensities, which is common in polymer single crystals. Therefore, special care is exercised via prefocusing at a neighboring region, followed by immediate taking of the SAED pattern. Figure 6b,c shows the typical TEM image and corresponding ED pattern of the lenticular crystal. There are two principal reflections which have been indexed as (010) and (004), the corresponding spacing being 2.10 and 0.84 nm, respectively. It is apparent that all the diffraction spots correspond to 0kl diffraction. The dimensions 3027

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Figure 5. (a, b) AFM images of the crystals and height profiles along the lines marked in the images.

Figure 7. Powder X-ray diagram of crystals from solution crystallization.

Table 1. Powder XRD Results of Crystals from Solution Crystallization

Figure 6. Out-of-plane XRD curve of crystals on the substrate. (b, c) The TEM image and the SAED pattern of the lenticular crystal. (d) Simplified schematic illustrations of the parallel alignments of backbones to the substrate. The size of the unit cell is indicated by the broken lines.

2θ (deg)

dobs (nm)

(hkl)

dcal (nm)

6.70 8.49 11.91 13.24 15.07 15.88 17.18 18.52 19.99 21.05 22.30 26.51

1.318 1.041 0.742 0.668 0.587 0.557 0.516 0.479 0.444 0.422 0.398 0.336

(100) (020) (122) (200) (124) (220) (040) (230) (300) (008) (028) (340)

1.320 1.050 0.738 0.660 0.587 0.559 0.525 0.480 0.440 0.420 0.390 0.337

chain can diffuse over a distance. In a word, the hypothesized growth mechanism is depicted as shown in Figure 8. As the chains still crystallizing onto the longitudinal surface will have different growth rate, a nonstable growth front may be formed

sandwiching crystals on top of each other. The crystal growth in the chain direction is limited by the rate with which a single 3028

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Figure 8. Hypothesized growth mechanism for the formation of lenticular crystals.

sometimes, resulting in a splaying of this front. This growth model is explaining the morphological observations as well as the growth kinetics. Morphology and Structural Development of Lenticular Crystals upon Heating. Thermal behaviors induced by annealing were examined on lenticular crystals, which were held at different temperatures in the hot stage. First, in order to understand how the morphology and structure of lenticular crystals can evolve prior to the melting, we annealed the sample at the lower temperature. Figure 9 shows the SEM images of Figure 10. (a) PLM image of the spherulites. (b, c) TEM images of the crystals in the dark field, indicating the detailed surface morphology.

a center and branch sufficiently to occupy the outward volume. If there are plenty of the primary nuclei in a certain area, some spherulites will inevitably impinge on one another as growth of spherulites proceeds, resulting in grain boundaries (Figure 10c). In a word, the β phase is intrinsically metastable and it can be dissipated when heating, which is consistent with the results in the literature.12 In contrast, the lamellar crystals in our previous study in which chains adopt perpendicular alignment demonstrate good thermal stability.35 Compared with the lenticular crystal, the lamellar crystal is the thermodynamically stable form either at room temperature or high temperature. Therefore, the drastic discrepancy in the thermal stability of the two kinds of crystals mainly results from the different crystalline polymorphisms.

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Figure 9. (a) SEM image of a typical lenticular crystal annealed at 90 °C. (b) Magnified SEM image of part of the crystal showing the rough surface. (c, d) TEM image and the SAED pattern of the crystal annealed at 90 °C.

CONCLUSION The growth of lenticular crystals of PFOs from solution has uncovered several important structural aspects and thermal behaviors, especially concerning the β phase. First, the detailed structure of the β phase crystals of PFOs is studied using absorbance, PL measurements, PLM, TEM, and XRD. From the structural analysis, we can deduce that the β phase crystals have orthorhombic lattice parameters of a = 1.32 nm, b = 2.10 nm, and c = 3.36 nm. In these single crystals, the backbone chains are parallel to the long axis of crystals. Second, the morphology and the structural dependences of crystals upon thermal annealing have been investigated in detail. At 90 °C, the crystals maintain the lenticular morphology. However, the surface becomes rough and holes develop in the crystals, and the b-axis is moderately increased. When the temperature is raised to 130 °C, all lenticular crystals melt and develop into spherulite-like entities by slow cooling. The thermal behaviors of crystals upon heating reveal that the lamellar crystal instead of the lenticular crystal is the thermodynamically stable structure for PFOs.

the crystals after thermal annealing at 90 °C for 8 h. The crystals remained their original morphology, but the surface became rough and holes developed in the crystals. The corresponding electron diffraction pattern of the observed crystals (Figure 9d) has a close resemblance with that of lenticular crystals without annealing. The electron diffraction spots correspond to the equatorial 0kl reflections. Therefore, it indicates that the backbone is still parallel to the long axis of lenticular crystals. However, a closer comparison between the fully indexed SAED patterns in Figures 6c and 9d indicates that there is an increase in the b-axis from 2.10 to 2.28 nm. The result suggests that a certain degree of motion of the chains occurs during this process. Heating of the crystals to 130 °C for 8 h led to the melting of all the lenticular crystals, which were converted into droplets. Cooling the sample from 130 °C to room temperature induced crystallization of the melted overgrowths to spherulites. The PLM micrograph (Figure 10a) demonstrates large (ca. 10−20 μm in diameter) spherulites. As further revealed by TEM in dark field (Figure 10b), the uniformly space filling spherulite is made up of smaller crystalline substructures which radiate from

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. 3029

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21274146 and 51133007).

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