- Email: [email protected]
Tensile drawing induced symmetry in poly(p-phenylene vinylene) films☆
MOLSTR 11239
Journal of Molecular Structure 521 (2000) 315–323 www.elsevier.nl/locate/molstruc
Tensile drawing induced symmetry in poly(p-phenylene vinylene) films 夽 C.Y. Yang a,b, K. Lee c, A.J. Heeger a,b,* a
Institute for Polymers and Organic Solids, University of California, Santa Barbara, Santa Barbara, CA 93106-509, USA b Materials Research Laboratory, University of California, Santa Barbara, Santa Barbara, CA 93106-5090, USA c Department of Physics, Pusan National University, Pusan 609-735, South Korea Received 5 March 1999; accepted 29 June 1999
Abstract The microstructure of poly(p-phenylene vinylene), PPV, films with draw ratios l 7; l 3; and l 1 (non-stretched) were investigated by X-ray diffraction. In the PPV film with the draw ratio l 7; tensile drawing causes the crystalline domains (the “crystallites”) to assemble in a hexagonal symmetry. This hexagonal symmetry does not form when the draw ratio is lower; at l 3; the crystallites assemble in cylindrical symmetry instead. For non-stretched films the crystallites are oriented without any symmetry. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: X-ray diffraction; Microstructure; Symmetry; PPV films
1. Introduction The photophysics of poly(phenylene vinylene), PPV, has been studied intensely since the discovery of the electroluminescence in this conjugated polymer [1–7]. Tensile drawn, oriented samples of PPV have been studied [3,4,8] and work on the crystalline phase and structural anisotropy been reported [9–12]. In general, the microstructure of solid polymer films is heterogeneous, including a crystalline phase (ordered domains and/or small crystallites) and regions which are amorphous. The microstructure depends on the details of sample preparation; e.g. 夽
In honour of Professor Giuseppe Zerbi on the occasion of his 65th birthday. * Corresponding author. Tel.: ⫹1-805-893-2001; fax: ⫹1-805893-4755. E-mail address: [email protected] (A.J. Heeger).
the draw ratio after tensile drawing, the temperature at which the processing was done, the solvents used in the processing, etc. Since different microstructures result in different optical properties [13], it is important to characterize the structure of the crystalline phase, the degree of crystallinity, the size of crystallites and the orientation of the crystallites with respect to the film plane and the draw direction. In this paper we describe a novel hexagonal symmetry induced into PPV films with draw ratio l 7: In addition, we studied the microstructure of tensile drawn films with l 3 and of non-oriented films. The hexagonal symmetry is absent in films with l 3 and in non-stretched films. The observation of a diffraction pattern with six-fold symmetry was recently reported for l 7 by Shah et al. [12] We interpret the diffraction pattern in greater detail and provide a clear description of the origin of the six-fold symmetry in terms of an assembly of crystallites in the PPV film.
0022-2860/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00450-0
316
C.Y. Yang et al. / Journal of Molecular Structure 521 (2000) 315–323
Fig. 1. The geometry of the settings of the X-ray beam with respect to the stretched PPV film. A, B and C are the directions of the X-ray beam; A is perpendicular to the plane of the film. B is parallel to the film plane and perpendicular to the draw direction. C is along the draw direction.
2. Experimental Free standing neat PPV films (non-stretched) were prepared from a precursor polymer by Ohnishi et al. [14]. The samples were stretched by tensile drawing to a draw ratio of l 7: The corresponding thickness is around 7 mm. A sample with lower draw ratio l 3 was also prepared with thickness approximately 17 mm. The thickness of the non-stretched samples was approximately 22 mm. X-ray diffraction experiments used a Siemens SmartCCD diffractometer equipped with a normal focus, 2.4 kW sealed tube X-ray source (Mo Ka radiation) and a rotation stage operating at 50 kV (40 mA) for analyzing the structure. As shown in Fig. 1, the films were examined in three physically important directions chosen to correspond to directions that are important for photo-physical measurements; e.g. the configuration used in photoinduced gain measurements with the polarized pump pulses [3]. In the first setting, the X-ray beam is perpendicular to the film plane, i.e. along the A direction. In the second, the beam is parallel to the film plane and perpendicular to the draw direction, i.e. along the B direction. In the third, the beam is parallel to draw direction, i.e. along the C direction. From these three settings, one can obtain a complete picture of the microstructure of the film.
3. Results and discussion 3.1. Crystal structure of PPV films with l 7 Fig. 2 is a map of the X-ray diffraction patterns
obtained from oriented PPV with l 7: Fig. 2a–c are patterns obtained by directing the X-ray beam along A, B and C, respectively, as shown in Fig. 1. One can see diffraction spots (reflections) in the equatorial direction and elongated spots, or layer lines, in the meridian direction in Fig. 2a and b, respectively. The hexagonal symmetry of the diffraction spots is shown in Fig. 2c. The structure of the crystalline phase (or the crystallites), as determined from these X-ray diffraction patterns, is orthorhombic with lattice parameters: a 0:790 nm; b 0:504 nm; c 0:624 nm: A schematic drawing of the projection of the unit cell of PPV along the a-axis of the crystallites is shown in Fig. 3. Initially, the orthorhombic crystal structure appears to be different from that determined by Granier et al obtained from electron diffraction [9]. They found the structure of the PPV crystalline phase to be monoclinic with lattice parameters ag 0:790 nm; bg 0:605 nm; cg 0:658 nm and ag 123⬚ ((where g stands for the lattice parameters from Granier et al.). There is, however, no essential difference between the two structures. If we take b bg sin 123⬚; then the baxis is orthogonal to the c-axis, resulting in an orthorhombic structure. Actually, the 0k0 reflections have not been observed either by Granier et al. or in our work, probably due to fluctuations (disorder) of the interchain spacing in b direction. All the reflections (not only in the equatorial; but also in the meridian directions, i.e. the centers of the elongated reflections), can be indexed with the orthorhombic structure, as shown in Table 1. This orthorhombic structure is correct for the crystalline phase in films with l 7; for the crystalline phase in the films with l 3 and for the crystalline phase in non-stretched films. This was proved by indexing all reflections in the corresponding diffraction patterns (see Sections 3.3 and 3.4). Tensile drawing changes the alignment of the crystallites in the film without any change in the structure within the crystallites. As shown in schematic drawing of the crystal structure (Fig. 3) the chains are stacked in the a-axis direction, resulting in a d-spacing of 0.395 nm between benzene ring layers (the {200} planes). The 0.624 nm repeat length along the c-axis is the length of one repeat unit of the PPV chain. The 0.504 nm periodicity along the b-axis corresponds to the interchain distance.
C.Y. Yang et al. / Journal of Molecular Structure 521 (2000) 315–323
317
Fig. 2. Map of X-ray diffraction patterns of a stretched PPV film with draw ratio l 7 showing hexagonal symmetry: (a) obtained with X-ray beam along A; (b) along B; and (c) along C, respectively, as shown in Fig. 1. (d) A schematic drawing of the planes of (a), (b) and (c) in reciprocal space.
The degree of crystallinity was estimated from the ratio of the integrated scattering intensity under the crystalline peaks to the total scattered intensity [15]. The crystallinity of the film is quite high (⬃70%). Using the Sherrer formula [16], the coherence length can be estimated from the FWHM of the 110 diffraction peaks to be
approximately 20 nm, i.e. the typical crystallite size is approximately 20 nm. 3.2. Hexagonal symmetry of PPV films with l 7 Perhaps the most interesting observation is that the X-ray diffraction spots exhibit hexagonal symmetry
318
C.Y. Yang et al. / Journal of Molecular Structure 521 (2000) 315–323
Fig. 3. Schematic drawing of the projection of the unit cell of the crystalline phase of the PPV film along a-axis.
about the draw direction, as shown in Fig. 2c. The film is not a single crystal; it is a composite containing a high concentration (⬃70%) of ⬃20 nm size of crystallites. Thus, the hexagonal symmetry of the film originates from the alignment of the crystallites in the film by tensile drawing. Tensile drawing simultaneously aligns the crystallites and the macromolecular chains along the draw direction, as illustrated in Fig. 4 (the blocks refer the crystallites in the film). The question is how are these crystallites aligned and ordered with respect to one another into a microstructure with hexagonal symmetry during tensile drawing? By definition, the chain direction within the crystallites is the c-axis of the unit cell (note that the meridian direction in Fig. 2a and b is the draw direction). Thus, the c-axis of the crystallites is parallel to the draw direction, as indicated in Fig. 4b. By projecting one unit cell (or one crystallite) along the c-axis we Table 1 d-spacings observed from X-ray diffraction, and calculated by using lattice parameters determined in this work hkl 100 010 200 110 210 310 220 410 001 101 103 313
dobs. (nm)
0.395 0.424 0.309 0.231 0.208 0.176 0.500 0.204 0.153
dcal. (nm) 0.790 0.504 0.350 0.425 0.310 0.233 0.212 0.198 0.624 0.497 0.206 0.157
Fig. 4. Schematic drawing of the: (a) non-stretched; and (b) stretched PPV films, where the blocks refer to the crystallites and the lines the molecular chains, forming amorphous boundaries.
obtain the (a, b) plane or (001) plane, as shown in Fig. 5a, where the (001) plane is illustrated by a rectangle (the lengths of the edges are proportional to lattice parameters). The intersections of the (110) and (200) planes with the (001) plane are indicated by dashed and broken lines, respectively. As indicated in Fig. 5a, the angle between (110) and (200) planes is calculated from the lattice parameters to be 57.5⬚, i.e. the angle between the [110] direction and the a-axis is close to 60⬚. Assume there is a second crystallite which is aligned with c-axis parallel with the first one but turned around the c-axis by ⬃60⬚ with respect to the first crystallite. As shown in Fig. 5b, the (110) plane of first crystallite is closely parallel to the (200) plane of the second crystallite, i.e. (110)1 // (200)2 (the subscripts 1 and 2 denote the two crystallites). According to diffraction theory, the (110) plane results in a 110 diffraction spot; similarly, the (200) plane results in the 200 diffraction spot. Since (110)1 // (200)2, the 110 and 200 diffraction spots should sit on the same array. This is consistent with the experimental result, as indicated by the indices in Fig. 2c in the meridian direction. If six crystallites are aligned in such way that the second crystallite is turned ⬃60⬚ with respect to the a-axis of the first crystallite, the third is turned ⬃60⬚ with respect to the a-axis of the second, the fourth is turned ⬃60⬚ with respect to the a-axis of the to third, and so on, then the crystalline planes are oriented as follows: (110)1 // (200)2, (110)2 // (200)3, (110)3 // (200)4, (110)4 // (200)5, (110)5 //
C.Y. Yang et al. / Journal of Molecular Structure 521 (2000) 315–323
319
Fig. 5. Schematic drawing of the geometry of the crystallite alignment (projection along the c-axis) in the stretched film with draw ratio l 7: The projection of: (a) one crystallite; (b) two crystallites; and (c) six crystallites. In (c), the crystalline (110) and (200) planes, indicated by dashed and broken lines, respectively, are assembled in hexagonal symmetry, at 0, ⫹60 and ⫺60⬚ with respect to the film plane (shown on the right).
(200)6, (110)6 // (200)1. The hexagonal symmetry of the planes is formed as a result of this packing of the planes; see Fig. 5c in which the dashed and broken lines are (110) and (200) planes, respectively. This description is valid for all equivalent {110} and {200} crystal planes. Note that very weak and very strong 110 and 200 reflections occur in the equatorial directions of Fig. 2a and b, respectively, and that the X-ray beam is
incident perpendicular and parallel to the film plane, respectively. This means that the (110) and (200) planes are primarily parallel to the film plane, (and give rise to very strong 110 and 200 reflections in Fig. 2b). In Fig. 2c there are very weak 110 and 200 reflections, similar to that in Fig. 2a, in the equatorial direction indicated by the horizontal bar, and very strong 110 and 200 reflections, similar to that in Fig. 2b, in the meridian direction, indicated by vertical bar. In
320
C.Y. Yang et al. / Journal of Molecular Structure 521 (2000) 315–323
Fig. 6. Map of the X-ray diffraction pattern of stretched PPV film with draw ratio l 3 showing cylindrical symmetry. (a)–(c) are obtained by setting X-ray beam along A, B and C, respectively (see in Fig. 1).
addition, there are two other series of very strong 110 and 200 reflections in directions ⫹60 and ⫺60⬚ from the meridian (vertical bar), in Fig. 2c. This means that there are two other sets of (110) and (200) planes at ⫹60 or ⫺60⬚ with respect to the film plane. Therefore, it is clear that in the PPV film with draw ratio of l 7; the crystallites are aligned with c axes (or chain axes) parallel to the draw direction. Moreover, there
are one set of {110} and {200} planes parallel, and two sets of {110} and {200} planes at approximately ⫹60 and ⫺60⬚ with respect to the film plane, as shown in Fig. 5c (see the relationship of the hexagonal dashed and broken lines with respect to the ‘film surface’ of the strip, indicated at the right side of the figure). Thus, the chosen unit cell is correct in that it leads to the correct geometry between crystalline
C.Y. Yang et al. / Journal of Molecular Structure 521 (2000) 315–323
planes of the crystallites; i.e. the geometry that leads to the hexagonal symmetry of the diffraction pattern of Fig. 2c. Moreover, according to the geometry of the experimental setting (the diffraction geometry, see Fig. 1), Fig. 2a is a cross-section in reciprocal space along the normal (c ⴱ-axis) and the equatorial direction (indicated by a horizontal bar) of Fig. 2c. Fig. 2b is a cross-section along the normal (c ⴱ-axis) and the meridian direction (indicated by vertical bar) of Fig. 2c. The relationship among Fig. 2a–c is illustrated in Fig. 2d. In Fig. 2c, the intensities of the 110 and 200 reflections in the equatorial and meridian directions are very weak and very strong, respectively. Ideally, along the equatorial direction the intensity of the 110 and 200 reflections should be zero if all the crystallites in the film are aligned perfectly as shown in Fig. 5c. Thus, there are a few misaligned crystallites, which give weak diffraction. From Fig. 2c, one can predict that the diffraction intensity of the 110 and 200 reflections should be very weak in the cross-section of Fig. 2a, and very strong in the cross-section of Fig. 2b, in agreement with the observed patterns. The three diffraction patterns in Fig. 2a–c are fully self-consistent. This implies once again that the microstructure, including the crystal structure and the alignment of the crystallites in the film (in real space), are correct. Otherwise, it would not be possible to obtain either the hexagonal symmetry of the diffraction pattern of Fig. 2c or the self-consistency of Fig. 2c with Fig. 2a and b in reciprocal space. Shah et al. [12] recently reported a hexagonal diffraction pattern for a PPV film with draw ratio l 7 (Fig. 12 in Ref. [12]). However, the three diffraction patterns in Ref. [12] for the three principal directions (transmission mode, edge mode, and a second edge mode, similar to the A, B, C directions in Fig. 1) are not self-consistent. The intensities of equatorial 110 and 200 reflections in Figs. 9 and 10 for the transmission and edge modes, respectively, in Ref. [12] are comparably strong, whereas based on the diffraction geometry they should be very weak and very strong (in Figs. 9 and 10, respectively). Apparently, there was some misalignment with transmission mode in Ref. [12]. As a result, the authors interpreted the microstructure of stretched film with l 7 simply as ‘uniaxial orientation with nematic ordering of PPV chains assuming three preferred orientations in the plane of the film’.
321
Finally, we note that the hexagonal reflections of Fig. 2c are not spots, but arcs spanning 15–20⬚. This implies that there is misalignment of the [110] and aaxes of the crystallites. The corresponding order parameter is approximately 0.93–0.97. In addition, the 110 and 200 reflections appear to be superposed. This is probably because the tensile drawing results in some strain in the crystallites that blurs the diffraction spots. Without strain the spots are separated although very close, as can be seen more clearly in the patterns obtained from a non-stretched film (see Fig. 7b below in Section 3.4). In summary, tensile drawing with draw ratio of l 7 causes the crystallites in the PPV film to assemble with apparent uniaxial hexagonal symmetry about the draw direction.
3.3. Cylindrical symmetry of the PPV film with draw ratio l 3 Fig. 6 is a map of the diffraction patterns from a PPV film with draw ratio l 3: Again, the diffraction patterns of Fig. 6a–c are obtained by directing the Xray beam along A, B and C directions, respectively, as shown in Fig. 1. There is no evidence of hexagonal symmetry of the diffraction spots in Fig. 6. Instead, in Fig. 6c, the diffraction rings are clearly seen to have cylindrical symmetry. These rings can be indexed as 110, 200 and 210 reflections. Fig. 6a and b are identical. This is consistent with the cylindrical symmetry of Fig. 6c, since they are the cross-sections along the equator and meridian, together with the normal directions of Fig. 6c, respectively. Thus, Fig. 6 indicates that the film with draw ratio l 3 has cylindrical symmetry about the draw direction, i.e. the order parameter of the [110] and a axes of the crystallites, discussed above, is 0. Evidently, tensile drawing to l 3 is not sufficient to arrange the majority of the crystalline planes of {110} and {200} into the hexagonal order observed with l 7: Granier et al noted cylindrical symmetry from transmission electron microscope, TEM, on sample with draw ratio l 14: The absence of hexagonal symmetry in this TEM experiment might result from radiation damage which destroys the hexagonal symmetry in the very thin TEM film sample (typically ⬃100 nm). Alternatively, it is also possible that the
322
C.Y. Yang et al. / Journal of Molecular Structure 521 (2000) 315–323
210, 103, etc., the crystal structure of the crystallites in the non-stretched film is identical to that discussed above for stretched film. Second, these reflections are big arcs or rings, implying random orientation of the {110} and {200}, and {210} crystalline planes as well. The arcs and non-uniform rings in Fig. 7a and b show some kind of anisotropy of the diffraction. This kind of anisotropy results from the method of sample preparation [14]. In addition the same 110, 200 and 210 reflections from different crystallites are shown in Fig. 7a and b, respectively. And Fig. 7a and b, obtained by perpendicular and parallel settings of the X-ray beam, are orthogonal. This means that the {110}, {200} and {210} crystalline planes of some crystallites in the film are aligned orthogonal to those of other crystallites. Therefore, in the non-stretched PPV films, the crystallites are self-assembled in multi-axial system. Thus, the nonstretched film has no overall symmetry. Acknowledgements This work was supported by the Office of Naval Research (N00014-91-J-1235) and the MRL Program of the National Science Foundation under Award No DMR96-32716. References
Fig. 7. X-ray diffraction patterns of non-stretched PPV film, obtained by setting the X-ray beam: (a) perpendicular; and (b) parallel to the film. There is no evidence of uniaxial symmetry.
hexagonal symmetry forms only under specific tensile drawing conditions (e.g. around l 7). 3.4. No symmetry in non-stretched film, l 1 Since the non-stretched film has no preferred direction, the complete X-ray diffraction map was obtained with two settings; with the X-ray beam perpendicular and parallel to the film. The corresponding diffraction patterns are shown in Fig. 7a and b, respectively. Since the reflections can be indexed as 110, 200,
[1] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackey, R.H. Friend, P.L. Burns, A. Holmes, Nature 347 (1990) 539. [2] M. Yan, L.J. Rothberg, F. Papadimitrakopoulos, M.E. Gavin, T.M. Miller, Phys. Rev. Lett. 72 (1994) 1104. [3] M. Yan, L. Rothberg, B.R. Hsieh, R.R. Ajfano, Phys. Rev. B 49 (1994) 9419. [4] U. Lemmer, D. Vacar, D. Moses, A.J. Heeger, T. Ohnishi, T. Noguchi, Appl. Phys. Lett. 68 (1996) 3007. [5] L.J. Rothberg, M. Yan, F. Papadimitrakopoulos, M.E. Galvin, E.W. Kwock, T.M. Miller, Synth. Met. 80 (1996) 4158. [6] N. Tessler, G.J. Denton, R.H. Friend, Nature 382 (1996) 695. [7] G.J. Denton, N. Tessler, N.T. Harrison, R.H. Friend, Phys. Rev. Lett. 78 (1997) 733. [8] M.A. Diaz-Garcia, C.Y. Yang, et al., not published. [9] T. Granier, E.L. Thomas, D.R. Gagnon, F.E. Karasz, R.W. Lenz, J. Polym. Sci. Part B: Polym. Phys. 24 (1986) 2793. [10] X.B. Zhang, G. Van Tendeloo, J. Van Landuyt, D. Van Dijck, J. Briers, Y. Bao, H.J. Geise, Macromolecules 29 (1996) 1554. [11] D. Chen, M.J. Winokur, M.A. Masse, F.E. Karasz, Polymer 33 (1992) 3116.
C.Y. Yang et al. / Journal of Molecular Structure 521 (2000) 315–323 [12] H.V. Shah, J.I. Scheinbeim, G.A. Arbuckle, J. Polym. Sci. Part B: Polym. Phys. 37 (1999) 605. [13] C.Y. Yang, F. Hide, M.A. Diaz-Garcia, A.J. Heeger, Y. Cao, Polymer 39 (1998) 2299. [14] T. Ohnishi, T. Noguchi, T. Nakano, M. Hirooka, I. Murase, Synth. Met. 4143 (1991) 309.
323
[15] A.M. Hindeleh, D. Johnson, Polymer 19 (1978) 27. [16] A. Guinier, X-Ray Diffraction in Crystals, Imperfect Crystals, and Amorphous Bodies, W.H. Freeman, San Francisco, CA, 1963 Translated by P. Lorrain, D.S.-M. Lorrain.
Journal of Molecular Structure 521 (2000) 315–323 www.elsevier.nl/locate/molstruc
Tensile drawing induced symmetry in poly(p-phenylene vinylene) films 夽 C.Y. Yang a,b, K. Lee c, A.J. Heeger a,b,* a
Institute for Polymers and Organic Solids, University of California, Santa Barbara, Santa Barbara, CA 93106-509, USA b Materials Research Laboratory, University of California, Santa Barbara, Santa Barbara, CA 93106-5090, USA c Department of Physics, Pusan National University, Pusan 609-735, South Korea Received 5 March 1999; accepted 29 June 1999
Abstract The microstructure of poly(p-phenylene vinylene), PPV, films with draw ratios l 7; l 3; and l 1 (non-stretched) were investigated by X-ray diffraction. In the PPV film with the draw ratio l 7; tensile drawing causes the crystalline domains (the “crystallites”) to assemble in a hexagonal symmetry. This hexagonal symmetry does not form when the draw ratio is lower; at l 3; the crystallites assemble in cylindrical symmetry instead. For non-stretched films the crystallites are oriented without any symmetry. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: X-ray diffraction; Microstructure; Symmetry; PPV films
1. Introduction The photophysics of poly(phenylene vinylene), PPV, has been studied intensely since the discovery of the electroluminescence in this conjugated polymer [1–7]. Tensile drawn, oriented samples of PPV have been studied [3,4,8] and work on the crystalline phase and structural anisotropy been reported [9–12]. In general, the microstructure of solid polymer films is heterogeneous, including a crystalline phase (ordered domains and/or small crystallites) and regions which are amorphous. The microstructure depends on the details of sample preparation; e.g. 夽
In honour of Professor Giuseppe Zerbi on the occasion of his 65th birthday. * Corresponding author. Tel.: ⫹1-805-893-2001; fax: ⫹1-805893-4755. E-mail address: [email protected] (A.J. Heeger).
the draw ratio after tensile drawing, the temperature at which the processing was done, the solvents used in the processing, etc. Since different microstructures result in different optical properties [13], it is important to characterize the structure of the crystalline phase, the degree of crystallinity, the size of crystallites and the orientation of the crystallites with respect to the film plane and the draw direction. In this paper we describe a novel hexagonal symmetry induced into PPV films with draw ratio l 7: In addition, we studied the microstructure of tensile drawn films with l 3 and of non-oriented films. The hexagonal symmetry is absent in films with l 3 and in non-stretched films. The observation of a diffraction pattern with six-fold symmetry was recently reported for l 7 by Shah et al. [12] We interpret the diffraction pattern in greater detail and provide a clear description of the origin of the six-fold symmetry in terms of an assembly of crystallites in the PPV film.
0022-2860/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00450-0
316
C.Y. Yang et al. / Journal of Molecular Structure 521 (2000) 315–323
Fig. 1. The geometry of the settings of the X-ray beam with respect to the stretched PPV film. A, B and C are the directions of the X-ray beam; A is perpendicular to the plane of the film. B is parallel to the film plane and perpendicular to the draw direction. C is along the draw direction.
2. Experimental Free standing neat PPV films (non-stretched) were prepared from a precursor polymer by Ohnishi et al. [14]. The samples were stretched by tensile drawing to a draw ratio of l 7: The corresponding thickness is around 7 mm. A sample with lower draw ratio l 3 was also prepared with thickness approximately 17 mm. The thickness of the non-stretched samples was approximately 22 mm. X-ray diffraction experiments used a Siemens SmartCCD diffractometer equipped with a normal focus, 2.4 kW sealed tube X-ray source (Mo Ka radiation) and a rotation stage operating at 50 kV (40 mA) for analyzing the structure. As shown in Fig. 1, the films were examined in three physically important directions chosen to correspond to directions that are important for photo-physical measurements; e.g. the configuration used in photoinduced gain measurements with the polarized pump pulses [3]. In the first setting, the X-ray beam is perpendicular to the film plane, i.e. along the A direction. In the second, the beam is parallel to the film plane and perpendicular to the draw direction, i.e. along the B direction. In the third, the beam is parallel to draw direction, i.e. along the C direction. From these three settings, one can obtain a complete picture of the microstructure of the film.
3. Results and discussion 3.1. Crystal structure of PPV films with l 7 Fig. 2 is a map of the X-ray diffraction patterns
obtained from oriented PPV with l 7: Fig. 2a–c are patterns obtained by directing the X-ray beam along A, B and C, respectively, as shown in Fig. 1. One can see diffraction spots (reflections) in the equatorial direction and elongated spots, or layer lines, in the meridian direction in Fig. 2a and b, respectively. The hexagonal symmetry of the diffraction spots is shown in Fig. 2c. The structure of the crystalline phase (or the crystallites), as determined from these X-ray diffraction patterns, is orthorhombic with lattice parameters: a 0:790 nm; b 0:504 nm; c 0:624 nm: A schematic drawing of the projection of the unit cell of PPV along the a-axis of the crystallites is shown in Fig. 3. Initially, the orthorhombic crystal structure appears to be different from that determined by Granier et al obtained from electron diffraction [9]. They found the structure of the PPV crystalline phase to be monoclinic with lattice parameters ag 0:790 nm; bg 0:605 nm; cg 0:658 nm and ag 123⬚ ((where g stands for the lattice parameters from Granier et al.). There is, however, no essential difference between the two structures. If we take b bg sin 123⬚; then the baxis is orthogonal to the c-axis, resulting in an orthorhombic structure. Actually, the 0k0 reflections have not been observed either by Granier et al. or in our work, probably due to fluctuations (disorder) of the interchain spacing in b direction. All the reflections (not only in the equatorial; but also in the meridian directions, i.e. the centers of the elongated reflections), can be indexed with the orthorhombic structure, as shown in Table 1. This orthorhombic structure is correct for the crystalline phase in films with l 7; for the crystalline phase in the films with l 3 and for the crystalline phase in non-stretched films. This was proved by indexing all reflections in the corresponding diffraction patterns (see Sections 3.3 and 3.4). Tensile drawing changes the alignment of the crystallites in the film without any change in the structure within the crystallites. As shown in schematic drawing of the crystal structure (Fig. 3) the chains are stacked in the a-axis direction, resulting in a d-spacing of 0.395 nm between benzene ring layers (the {200} planes). The 0.624 nm repeat length along the c-axis is the length of one repeat unit of the PPV chain. The 0.504 nm periodicity along the b-axis corresponds to the interchain distance.
C.Y. Yang et al. / Journal of Molecular Structure 521 (2000) 315–323
317
Fig. 2. Map of X-ray diffraction patterns of a stretched PPV film with draw ratio l 7 showing hexagonal symmetry: (a) obtained with X-ray beam along A; (b) along B; and (c) along C, respectively, as shown in Fig. 1. (d) A schematic drawing of the planes of (a), (b) and (c) in reciprocal space.
The degree of crystallinity was estimated from the ratio of the integrated scattering intensity under the crystalline peaks to the total scattered intensity [15]. The crystallinity of the film is quite high (⬃70%). Using the Sherrer formula [16], the coherence length can be estimated from the FWHM of the 110 diffraction peaks to be
approximately 20 nm, i.e. the typical crystallite size is approximately 20 nm. 3.2. Hexagonal symmetry of PPV films with l 7 Perhaps the most interesting observation is that the X-ray diffraction spots exhibit hexagonal symmetry
318
C.Y. Yang et al. / Journal of Molecular Structure 521 (2000) 315–323
Fig. 3. Schematic drawing of the projection of the unit cell of the crystalline phase of the PPV film along a-axis.
about the draw direction, as shown in Fig. 2c. The film is not a single crystal; it is a composite containing a high concentration (⬃70%) of ⬃20 nm size of crystallites. Thus, the hexagonal symmetry of the film originates from the alignment of the crystallites in the film by tensile drawing. Tensile drawing simultaneously aligns the crystallites and the macromolecular chains along the draw direction, as illustrated in Fig. 4 (the blocks refer the crystallites in the film). The question is how are these crystallites aligned and ordered with respect to one another into a microstructure with hexagonal symmetry during tensile drawing? By definition, the chain direction within the crystallites is the c-axis of the unit cell (note that the meridian direction in Fig. 2a and b is the draw direction). Thus, the c-axis of the crystallites is parallel to the draw direction, as indicated in Fig. 4b. By projecting one unit cell (or one crystallite) along the c-axis we Table 1 d-spacings observed from X-ray diffraction, and calculated by using lattice parameters determined in this work hkl 100 010 200 110 210 310 220 410 001 101 103 313
dobs. (nm)
0.395 0.424 0.309 0.231 0.208 0.176 0.500 0.204 0.153
dcal. (nm) 0.790 0.504 0.350 0.425 0.310 0.233 0.212 0.198 0.624 0.497 0.206 0.157
Fig. 4. Schematic drawing of the: (a) non-stretched; and (b) stretched PPV films, where the blocks refer to the crystallites and the lines the molecular chains, forming amorphous boundaries.
obtain the (a, b) plane or (001) plane, as shown in Fig. 5a, where the (001) plane is illustrated by a rectangle (the lengths of the edges are proportional to lattice parameters). The intersections of the (110) and (200) planes with the (001) plane are indicated by dashed and broken lines, respectively. As indicated in Fig. 5a, the angle between (110) and (200) planes is calculated from the lattice parameters to be 57.5⬚, i.e. the angle between the [110] direction and the a-axis is close to 60⬚. Assume there is a second crystallite which is aligned with c-axis parallel with the first one but turned around the c-axis by ⬃60⬚ with respect to the first crystallite. As shown in Fig. 5b, the (110) plane of first crystallite is closely parallel to the (200) plane of the second crystallite, i.e. (110)1 // (200)2 (the subscripts 1 and 2 denote the two crystallites). According to diffraction theory, the (110) plane results in a 110 diffraction spot; similarly, the (200) plane results in the 200 diffraction spot. Since (110)1 // (200)2, the 110 and 200 diffraction spots should sit on the same array. This is consistent with the experimental result, as indicated by the indices in Fig. 2c in the meridian direction. If six crystallites are aligned in such way that the second crystallite is turned ⬃60⬚ with respect to the a-axis of the first crystallite, the third is turned ⬃60⬚ with respect to the a-axis of the second, the fourth is turned ⬃60⬚ with respect to the a-axis of the to third, and so on, then the crystalline planes are oriented as follows: (110)1 // (200)2, (110)2 // (200)3, (110)3 // (200)4, (110)4 // (200)5, (110)5 //
C.Y. Yang et al. / Journal of Molecular Structure 521 (2000) 315–323
319
Fig. 5. Schematic drawing of the geometry of the crystallite alignment (projection along the c-axis) in the stretched film with draw ratio l 7: The projection of: (a) one crystallite; (b) two crystallites; and (c) six crystallites. In (c), the crystalline (110) and (200) planes, indicated by dashed and broken lines, respectively, are assembled in hexagonal symmetry, at 0, ⫹60 and ⫺60⬚ with respect to the film plane (shown on the right).
(200)6, (110)6 // (200)1. The hexagonal symmetry of the planes is formed as a result of this packing of the planes; see Fig. 5c in which the dashed and broken lines are (110) and (200) planes, respectively. This description is valid for all equivalent {110} and {200} crystal planes. Note that very weak and very strong 110 and 200 reflections occur in the equatorial directions of Fig. 2a and b, respectively, and that the X-ray beam is
incident perpendicular and parallel to the film plane, respectively. This means that the (110) and (200) planes are primarily parallel to the film plane, (and give rise to very strong 110 and 200 reflections in Fig. 2b). In Fig. 2c there are very weak 110 and 200 reflections, similar to that in Fig. 2a, in the equatorial direction indicated by the horizontal bar, and very strong 110 and 200 reflections, similar to that in Fig. 2b, in the meridian direction, indicated by vertical bar. In
320
C.Y. Yang et al. / Journal of Molecular Structure 521 (2000) 315–323
Fig. 6. Map of the X-ray diffraction pattern of stretched PPV film with draw ratio l 3 showing cylindrical symmetry. (a)–(c) are obtained by setting X-ray beam along A, B and C, respectively (see in Fig. 1).
addition, there are two other series of very strong 110 and 200 reflections in directions ⫹60 and ⫺60⬚ from the meridian (vertical bar), in Fig. 2c. This means that there are two other sets of (110) and (200) planes at ⫹60 or ⫺60⬚ with respect to the film plane. Therefore, it is clear that in the PPV film with draw ratio of l 7; the crystallites are aligned with c axes (or chain axes) parallel to the draw direction. Moreover, there
are one set of {110} and {200} planes parallel, and two sets of {110} and {200} planes at approximately ⫹60 and ⫺60⬚ with respect to the film plane, as shown in Fig. 5c (see the relationship of the hexagonal dashed and broken lines with respect to the ‘film surface’ of the strip, indicated at the right side of the figure). Thus, the chosen unit cell is correct in that it leads to the correct geometry between crystalline
C.Y. Yang et al. / Journal of Molecular Structure 521 (2000) 315–323
planes of the crystallites; i.e. the geometry that leads to the hexagonal symmetry of the diffraction pattern of Fig. 2c. Moreover, according to the geometry of the experimental setting (the diffraction geometry, see Fig. 1), Fig. 2a is a cross-section in reciprocal space along the normal (c ⴱ-axis) and the equatorial direction (indicated by a horizontal bar) of Fig. 2c. Fig. 2b is a cross-section along the normal (c ⴱ-axis) and the meridian direction (indicated by vertical bar) of Fig. 2c. The relationship among Fig. 2a–c is illustrated in Fig. 2d. In Fig. 2c, the intensities of the 110 and 200 reflections in the equatorial and meridian directions are very weak and very strong, respectively. Ideally, along the equatorial direction the intensity of the 110 and 200 reflections should be zero if all the crystallites in the film are aligned perfectly as shown in Fig. 5c. Thus, there are a few misaligned crystallites, which give weak diffraction. From Fig. 2c, one can predict that the diffraction intensity of the 110 and 200 reflections should be very weak in the cross-section of Fig. 2a, and very strong in the cross-section of Fig. 2b, in agreement with the observed patterns. The three diffraction patterns in Fig. 2a–c are fully self-consistent. This implies once again that the microstructure, including the crystal structure and the alignment of the crystallites in the film (in real space), are correct. Otherwise, it would not be possible to obtain either the hexagonal symmetry of the diffraction pattern of Fig. 2c or the self-consistency of Fig. 2c with Fig. 2a and b in reciprocal space. Shah et al. [12] recently reported a hexagonal diffraction pattern for a PPV film with draw ratio l 7 (Fig. 12 in Ref. [12]). However, the three diffraction patterns in Ref. [12] for the three principal directions (transmission mode, edge mode, and a second edge mode, similar to the A, B, C directions in Fig. 1) are not self-consistent. The intensities of equatorial 110 and 200 reflections in Figs. 9 and 10 for the transmission and edge modes, respectively, in Ref. [12] are comparably strong, whereas based on the diffraction geometry they should be very weak and very strong (in Figs. 9 and 10, respectively). Apparently, there was some misalignment with transmission mode in Ref. [12]. As a result, the authors interpreted the microstructure of stretched film with l 7 simply as ‘uniaxial orientation with nematic ordering of PPV chains assuming three preferred orientations in the plane of the film’.
321
Finally, we note that the hexagonal reflections of Fig. 2c are not spots, but arcs spanning 15–20⬚. This implies that there is misalignment of the [110] and aaxes of the crystallites. The corresponding order parameter is approximately 0.93–0.97. In addition, the 110 and 200 reflections appear to be superposed. This is probably because the tensile drawing results in some strain in the crystallites that blurs the diffraction spots. Without strain the spots are separated although very close, as can be seen more clearly in the patterns obtained from a non-stretched film (see Fig. 7b below in Section 3.4). In summary, tensile drawing with draw ratio of l 7 causes the crystallites in the PPV film to assemble with apparent uniaxial hexagonal symmetry about the draw direction.
3.3. Cylindrical symmetry of the PPV film with draw ratio l 3 Fig. 6 is a map of the diffraction patterns from a PPV film with draw ratio l 3: Again, the diffraction patterns of Fig. 6a–c are obtained by directing the Xray beam along A, B and C directions, respectively, as shown in Fig. 1. There is no evidence of hexagonal symmetry of the diffraction spots in Fig. 6. Instead, in Fig. 6c, the diffraction rings are clearly seen to have cylindrical symmetry. These rings can be indexed as 110, 200 and 210 reflections. Fig. 6a and b are identical. This is consistent with the cylindrical symmetry of Fig. 6c, since they are the cross-sections along the equator and meridian, together with the normal directions of Fig. 6c, respectively. Thus, Fig. 6 indicates that the film with draw ratio l 3 has cylindrical symmetry about the draw direction, i.e. the order parameter of the [110] and a axes of the crystallites, discussed above, is 0. Evidently, tensile drawing to l 3 is not sufficient to arrange the majority of the crystalline planes of {110} and {200} into the hexagonal order observed with l 7: Granier et al noted cylindrical symmetry from transmission electron microscope, TEM, on sample with draw ratio l 14: The absence of hexagonal symmetry in this TEM experiment might result from radiation damage which destroys the hexagonal symmetry in the very thin TEM film sample (typically ⬃100 nm). Alternatively, it is also possible that the
322
C.Y. Yang et al. / Journal of Molecular Structure 521 (2000) 315–323
210, 103, etc., the crystal structure of the crystallites in the non-stretched film is identical to that discussed above for stretched film. Second, these reflections are big arcs or rings, implying random orientation of the {110} and {200}, and {210} crystalline planes as well. The arcs and non-uniform rings in Fig. 7a and b show some kind of anisotropy of the diffraction. This kind of anisotropy results from the method of sample preparation [14]. In addition the same 110, 200 and 210 reflections from different crystallites are shown in Fig. 7a and b, respectively. And Fig. 7a and b, obtained by perpendicular and parallel settings of the X-ray beam, are orthogonal. This means that the {110}, {200} and {210} crystalline planes of some crystallites in the film are aligned orthogonal to those of other crystallites. Therefore, in the non-stretched PPV films, the crystallites are self-assembled in multi-axial system. Thus, the nonstretched film has no overall symmetry. Acknowledgements This work was supported by the Office of Naval Research (N00014-91-J-1235) and the MRL Program of the National Science Foundation under Award No DMR96-32716. References
Fig. 7. X-ray diffraction patterns of non-stretched PPV film, obtained by setting the X-ray beam: (a) perpendicular; and (b) parallel to the film. There is no evidence of uniaxial symmetry.
hexagonal symmetry forms only under specific tensile drawing conditions (e.g. around l 7). 3.4. No symmetry in non-stretched film, l 1 Since the non-stretched film has no preferred direction, the complete X-ray diffraction map was obtained with two settings; with the X-ray beam perpendicular and parallel to the film. The corresponding diffraction patterns are shown in Fig. 7a and b, respectively. Since the reflections can be indexed as 110, 200,
[1] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackey, R.H. Friend, P.L. Burns, A. Holmes, Nature 347 (1990) 539. [2] M. Yan, L.J. Rothberg, F. Papadimitrakopoulos, M.E. Gavin, T.M. Miller, Phys. Rev. Lett. 72 (1994) 1104. [3] M. Yan, L. Rothberg, B.R. Hsieh, R.R. Ajfano, Phys. Rev. B 49 (1994) 9419. [4] U. Lemmer, D. Vacar, D. Moses, A.J. Heeger, T. Ohnishi, T. Noguchi, Appl. Phys. Lett. 68 (1996) 3007. [5] L.J. Rothberg, M. Yan, F. Papadimitrakopoulos, M.E. Galvin, E.W. Kwock, T.M. Miller, Synth. Met. 80 (1996) 4158. [6] N. Tessler, G.J. Denton, R.H. Friend, Nature 382 (1996) 695. [7] G.J. Denton, N. Tessler, N.T. Harrison, R.H. Friend, Phys. Rev. Lett. 78 (1997) 733. [8] M.A. Diaz-Garcia, C.Y. Yang, et al., not published. [9] T. Granier, E.L. Thomas, D.R. Gagnon, F.E. Karasz, R.W. Lenz, J. Polym. Sci. Part B: Polym. Phys. 24 (1986) 2793. [10] X.B. Zhang, G. Van Tendeloo, J. Van Landuyt, D. Van Dijck, J. Briers, Y. Bao, H.J. Geise, Macromolecules 29 (1996) 1554. [11] D. Chen, M.J. Winokur, M.A. Masse, F.E. Karasz, Polymer 33 (1992) 3116.
C.Y. Yang et al. / Journal of Molecular Structure 521 (2000) 315–323 [12] H.V. Shah, J.I. Scheinbeim, G.A. Arbuckle, J. Polym. Sci. Part B: Polym. Phys. 37 (1999) 605. [13] C.Y. Yang, F. Hide, M.A. Diaz-Garcia, A.J. Heeger, Y. Cao, Polymer 39 (1998) 2299. [14] T. Ohnishi, T. Noguchi, T. Nakano, M. Hirooka, I. Murase, Synth. Met. 4143 (1991) 309.
323
[15] A.M. Hindeleh, D. Johnson, Polymer 19 (1978) 27. [16] A. Guinier, X-Ray Diffraction in Crystals, Imperfect Crystals, and Amorphous Bodies, W.H. Freeman, San Francisco, CA, 1963 Translated by P. Lorrain, D.S.-M. Lorrain.