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Crystal orientation in poly(ethylene 2,6-naphthalate) ultrathin films revealed by reflection–absorption infrared spectroscopy and grazing incidence X-ray diffraction
Surface Science 600 (2006) 1559–1564 www.elsevier.com/locate/susc
Crystal orientation in poly(ethylene 2,6-naphthalate) ultrathin films revealed by reflection–absorption infrared spectroscopy and grazing incidence X-ray diffraction Ying Zhang a, Shota Mukoyama a, Katsuhito Mori a, Deyan Shen b, Shouke Yan b, Yukihiro Ozaki c, Isao Takahashi a,* b
a Department of Physics, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan State Key Laboratory of Polymer Physics and Chemistry, Joint Laboratory of Polymer Science and Material, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China c Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan
Received 22 December 2005; accepted for publication 9 February 2006 Available online 28 February 2006
Abstract Crystal orientation of poly(ethylene 2, 6-naphthalate) (PEN) ultrathin films was investigated by the combination of reflection–absorption infrared spectroscopy (RAIR) and grazing incidence X-ray diffraction (GIXD) techniques. It is concluded that the main-chain of PEN molecule in ultrathin film is prone to alignment parallel to the substrate when compared with thicker films. During the formation of a form crystalline, the naphthalene ring, the C@O group in molecular chain of PEN as well as the b axis in crystalline tend to take orientation more parallel to the substrate due to the surface-induced effect. However, such an anisotropic structure could not be observed in the bulk PEN. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Orientation; Poly(ethylene 2,6-naphthalate) (PEN); Ultrathin films; Reflection–absorption infrared spectroscopy (RAIR); Grazing incidence X-ray diffraction (GIXD)
1. Introduction The physical behavior of polymers in confinement, such as in ultrathin films, block copolymers, or nanocomposites, differs considerably from the behavior observed in the bulk. In the case of ultrathin films, the glass transition temperature, [1,2] crystallization behavior, [3,4] morphology [5] and electrical properties [6] etc. are known to depend on their thickness. In addition, the polymeric systems in ultrathin films usually show preferred orientation different from that of bulks. For example, Frank et al. [4,7] disclosed that while the backbones of poly (di-n-hexylsilane) deposited on quartz substrate are aligned preferentially in the plane of the
*
Corresponding author. E-mail address: [email protected] (I. Takahashi).
0039-6028/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.02.012
film, the mean orientation of poly (ethylene oxides) molecules on an oxidized silicon substrate is in the surfacenormal direction. The poly(ethylene oxides) lamellar crystal also shows an interesting film thickness dependent morphology and orientation. The characteristic crystal structure and orientation were furthermore observed near the polymeric surface, which are highly different from those deeply buried in the bulk. Factor et al. [8] studied the near surface structure of an aromatic polyimide film. It was found that within ˚ from the surface the ordering of the polymer the first 90 A molecules is markedly enhanced. Such surface-related effects can also be found in other polymeric systems [9–19]. Several experimental techniques can meet the demand to detect the crystalline structure and orientation in ultrathin films. Among them, Fourier transform infrared (FTIR) spectroscopy is a promising technique and has great advantage: it can probe directly to such subtle details as
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intermolecular interactions, localized molecular conformations and orientations. As one of various kinds of infrared techniques, reflection–absorption infrared (RAIR) spectroscopy fits sufficiently to the characterization of ultrathin films with the thickness of nanometer scale. The characteristic [20] of RAIR is that the resultant electric field vector is perpendicular to the metal-based surface. Therefore, if molecules are adsorbed onto the substrate with a preferred orientation, vibration modes having transition moments perpendicular to the surface will appear with greater intensity than modes having transition moments parallel to the surface. Thus, RAIR is especially useful for determining the orientation of adsorbed molecular species. A technique of grazing incidence X-ray diffraction (GIXD) also enables us to gain direct access to information upon the structure at the film surface by choosing an appropriate condition [8–19]. By collecting Bragg reflections from the crystals under the in-plane and out-of-plane scanning conditions, the corresponding information on crystalline structure along normal and parallel to the sample surface can be obtained. Due to the unmatched advantage on exploring the orientational structure in confined geometry for the RAIR and GIXD methods, the two techniques have been used in the investigations of many polymers. However, there is still no such work reported for the combination of both RAIR and GIXD so far, even though it can offer us more detailed information on the crystal structure or microstructure at molecular level for the thin polymer specimens. In the present study, the RAIR and GIXD were employed to detect the crystal orientation in the ultrathin film of poly(ethylene 2, 6-naphthalate) (PEN), which is a high performance thermoplastic polyester with a rigid naphthalene ring and a flexible aliphatic diol unit. Moreover, the difference in the orientation between the ultrathin film and bulk has also been compared.
Thickness measurements of thin films on the gold-coated glass wafers substrates were performed with NanoScope IIIA MultiMode atomic force microscope (AFM) (Digital Instrument) in tapping mode. The film thickness was determined by AFM height profile after partially removing the thin film from the glass wafers. The thickness of gold film and PEN-gold film were measured separately. They were defined as the distance between the glass surface and the average of all height values taken in a line scan across the sample surface [22]. The thickness of thin film on the silicon wafer was determined by a fit of the X-ray reflectivity data with a self-made software in which a recursive method based on the dynamical scattering theory was employed [23]. In order to investigate the corresponding bulk behavior of PEN, some thick PEN films were also prepared by dropping concentrated solution on KBr wafers (for RAIR) or silicon (1 0 0) wafers (for GIXD), and then evaporated in a vacuum oven at 40 °C for 24 h. Thickness of the thick film was estimated to be 22 lm from the dropping volume of solution, deposit area and the density of PEN. The crystallized PEN films (ultrathin films and thick films) were obtained by annealing the amorphous samples from the glassy state with a hot plate at predetermined temperatures. 2.2. Reflection–absorption FTIR Reflection–absorption FTIR spectra in the region of 4000–400 cm1 were collected with a Bruker EQUINOX 55 FT-IR spectrometer equipped with an MCT detector. The measurements were obtained by average 32 scans and at a resolution of 4 cm1. The incidence angle was fixed at 83° for the best signal recording. The polarization of the incoming beam was parallel to the plane of incidence (p-polarized). 2.3. X-ray diffraction
2. Experimental part 2.1. Sample preparation PEN sample with an intrinsic viscosity of 0.50 dL/g was kindly supplied by Prof. Zhongyong Fan of Fudan University. The PEN pellets were dissolved in chloroform–trifluoroacetic acid (vol. ratio 1:1). Substrates of silicon (1 0 0) wafers and gold-coated glass wafers were used to prepare thin PEN films for GIXD and RAIR measurements, respectively (see Ref. [21] for detailed procedures of cleaning the substrates and making the gold-coated glass wafers). Thin films with different thickness were prepared by spin-coating PEN solutions of various concentrations at several speeds for about 60 s onto the gold-coated glass wafers and silicon (1 0 0) wafers. The films were kept under vacuum at 40 °C for 24 h to remove the residual solvent. All the prepared samples were then annealed at 135 °C (about 20 °C above the Tg of bulk value 117 °C) for 1 h to relax internal stresses. The films after annealing at 135 °C were proved to be amorphous by their X-ray diffraction profiles.
The crystalline structure of PEN thin films was investigated by GIXD measurements. The measurements were carried out using a diffractometer (TTR-450, Rigaku Co. Ltd.) with a rotating Cu anode X-ray source ˚ The size of the incident beam was defined (k ¼ 1:5406 A). by a collimator with diameter of 1 mm. A soller slit was placed before a scintillation detector to collimate the scattered beam. The samples were mounted on a four-circle goniometer with the film plane nearly horizontal. The GIXD measurements were performed with in-plane and out-of-plane configuration to detect the crystalline structure of perpendicular and parallel to the substrate surface, respectively. The schematic pictures of in-plane and out-ofplane GIXD measurements are shown in Fig. 1. 3. Results and discussion The preferred orientation of the main-chain of PEN molecule in the ultrathin film on gold substrate was interrogated by comparing the transmission mode and the
Y. Zhang et al. / Surface Science 600 (2006) 1559–1564
(A)
Polymer coating
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(A)
Diffracted lattice planes
IR Transmission RAIR
1729
Incident beam
2θ Diffracted beam
Absorbance
αi
(B)
Polymer coating
1338 1376
1406
1602
Diffracted beam
Incident beam
αi
1800
2θ
1700
1600
1500
1400
-1
Wavenumber (cm ) Diffracted lattice planes
(B)
Fig. 1. Schematic pictures of (A) in-plane and (B) out-of-plane GIXD measurements.
1091
Absorbance
p-polarized RAIR mode. Fig. 2 shows the transmission spectrum of a PEN film drop-coated on a KBr wafer and the RAIR spectrum of a 60 nm PEN ultrathin film spincoated on gold surface. Both films we investigated are amorphous. Fig. 2(A) and (B) depict the spectra in the different wavenumber regions. It is known that reflectance spectra clearly show large changes in both peak position and shape compared to transmission spectra [24,25]. The phenomenon is indeed observed from the band shift of C@O stretching at 1729 cm1 and ring CH out-of-plane deformation at 769 cm1. To obtain the qualitative information about the molecular orientation, we avoid the disturbance of the band distortions by comparing the relative intensity of bands in the same spectra. Comparison with the corresponding IR transmission spectrum indicates the differences in the RAIR spectrum of PEN ultrathin film. In Fig. 2(A), the intensity of 1729 cm1 band increases with respect to the bands at 1602, 1406, 1376 and 1338 cm1. As summarized in Table 1 [26], the band at 1729 cm1 is assigned to C@O stretching mode, the bands at 1602 and 1406 cm1 are assigned to naphthalene ring vibration, and the bands at 1376, 1338 cm1 are attributed to CH2 wagging vibration. The transition moment of 1729 cm1 band is perpendicular to the main-chain axis of PEN molecules, whereas it is parallel to the main-chain axis of PEN molecules for the transition moments of 1602, 1406, 1376 and 1338 cm1 bands. The similar relative intensity changes are also shown in Fig. 2(B). The bands near 1135 and 1091 cm1, which are assigned to naphthalene ring vibration and C–O symmetry stretching vibration respectively, are parallel bands, while the ring CH out-of-plane deformation vibration at 769 cm1 is a perpendicular band [26]. The ratio of the intensity of 769 cm1 relative to those of the parallel bands at 1135 and 1091 cm1 also increases. This indicates that
769
IR Transmission RAIR
1135
1100
1000
900
800
Wavenumber (cm-1) Fig. 2. The RAIR spectrum of a 60 nm PEN ultrathin film spin-coated on gold and IR transmission spectrum of a PEN film drop-coated on the KBr pellet, (A) and (B) are in different wavenumber region.
there is a selective orientation for the PEN backbone on the gold substrate. On the basis of the mutually perpendicular direction of the electric field vector for transmission vs. p-polarized RAIR modes, we can conclude that the mainchain of PEN in the ultrathin film shows some alignment parallel to the substrate. The orientation during crystallization of the PEN ultrathin film was also investigated. Fig. 3 gives the RAIR spectrum of a 60 nm PEN film after crystallized at 210 °C for 2 h from glassy state. The spectrum of the film before crystallization is also given for comparison. It is known that PEN can crystallize into at least two different crystal modifications, i.e., the a and b forms [27,28]. The a crystal form can be obtained by annealing amorphous PEN in the solid state, while the crystal form b can be obtained directly from the melt. It can be concluded from the appearance of characteristic bands at 814, 839, 931, 1004 and 1104 cm1 in Fig. 3 that the a-form crystalline structure is formed.
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Table 1 Assignment and polarization of IR band of PEN Wavenumber (cm1)a
Band assignmentb
Polarizationc
1729 (1715) 1602 1406 1376 1338 1135 1104 1091 1004 931 839 814 769 (765)
m(C@O) Naphthalene ring vibration Naphthalene ring vibration x(CH2) gauche x(CH2) trans Naphthalene ring vibration Crystalline ms(C–O) gauche Crystalline d(ring CH out-of-plane) Crystalline Crystalline d(ring CH out-of-plane)
?p k k k k k k ? ? k ?r
a
The wavenumbers in bracket represent the peak positions in transmission IR spectra. b m = stretching, x = wagging, ms = symmetry stretching, d = deformation. c ? and k denote perpendicular and parallel polarization, respectively, with respect to the main-chain axis of PEN molecules. p and r denote parallel and perpendicular to the plane of the naphthalene ring respectively.
after crystallization before crystallization
Absorbance
1729
769
1104 931 1004
1800
1600
1400
1200
1000
839
814
The crystal structure of PEN ultrathin film was also investigated by X-ray diffraction method. Fig. 4 shows the out-of-plane and in-plane GIXD profiles of a 73 nm PEN ultrathin film after isothermal crystallization at 210 °C for 2 h. The incident angle was kept at 0.175°, which is above the critical angle of 0.15°, thus the resulting profiles may reflect the structural information deep in the films. As mentioned above, PEN has been reported to have at least two modifications, i.e. the a and b forms. The unit cell of the a form was determined by Mencik [27] as triclinic with a = 0.651 nm, b = 0.575 nm, c = 1.32 nm, a = 81.33°, b = 144°, c = 100°. A triclinic unit cell was also proposed by Zachmann et al. [28] for the b crystal form with a = 0.926 nm, b = 1.559 nm, c = 1.273 nm, a = 121.6°, b = 95.57°, and c = 122.52°. The present GIXD result shows the crystalline reflections of (1 0 0), ð 110Þ, and (0 1 0) plane (2h are 23.3, 27.0, and 15.6°, respectively), which are major reflections of the a-form crystal. This indicates the formation of a-form crystal and agrees with that obtained from RAIR measurement. Furthermore, out-ofplane and in-plane profiles show quite different the relative intensity of each diffraction peak. While the out-of-plane scattering is dominated by the (1 0 0) and ð 1 1 0Þ peaks, the in-plane scattering is dominated by a unique Bragg peak: (0 1 0) peak. These results clearly indicate that the crystals formed during the present crystallization process are highly anisotropic and hence preferential orientation occurs throughout the PEN ultrathin film. Since the ð1 1 0Þ lattice plane is approximately parallel to the naphthalene ring in PEN molecule (making an angle of about 4.7° with the naphthalene ring) [29], it can be inferred that the naphthalene ring plane is almost parallel to the film surface in the crystalline region from the presence of ð 1 1 0Þ Bragg peak only in the out-of-of plane scan. This is consistent with the result derived from the RAIR spectra. The c axis can also be proved to align along the direction parallel
800
Wavenumber (cm-1)
(100)
Fig. 3 further indicates that the bands at 1729 and 769 cm1 display difference in the relative intensities. After crystallization, the intensity of 1729 cm1 band decreases while that of 769 cm1 band increases. As mentioned above, both of these two bands are perpendicular polarization. However, the transition moment of 1729 cm1 band is parallel to the plane of naphthalene ring while it is perpendicular to the plane for the transition moment of 769 cm1 band. Considering that all the atoms of the polymer chain including O@C–O and naphthalene ring (except the hydrogens of CH2) are coplanar in PEN [26], it is thus concluded from the relative change in intensity of the above two bands during crystallization that both the naphthalene ring and carbonyl group in PEN molecular chain tend to be aligned preferentially parallel to the substrate.
Intensity (arb. units)
Fig. 3. The RAIR spectra of a 60 nm PEN ultrathin film before and after isothermal crystallization at 210 °C for 2 h.
out-of-plane in-plane
(110)
(010)
10
15
20
25
30
35
2θ (degree) Fig. 4. Out-of-plane and in-plane GIXD profiles of a 73 nm PEN ultrathin film after isothermal crystallization at 210 °C for 2 h (incident angle a = 0.175°).
Y. Zhang et al. / Surface Science 600 (2006) 1559–1564
to the sample surface in terms of the coplanar structure in a-form crystal of PEN. Moreover, in the case of out-ofplane geometry, the intensity of (1 0 0) reflection is the strongest one, while this peak cannot be observed for the in-plane geometry. This result reveals that the b axis in the PEN ultrathin film is also preferentially oriented parallel to the film surface. The surface-induced effect should play an important role for the anisotropic arrangement of the crystals in the PEN ultrathin film. It has been found in many polymeric systems that chain segments in the proximities to the polymer–air interface crystallize more easily than those from inner structures. Therefore, it is possible that an anisotropic structure originates preferentially at the surface, and this anisotropy will be followed by the crystal growth with the same direction in the interior of PEN film. It can be further imagined that this kind of surface effect diminishes as the distance away from the surface increases. This point is confirmed by comparing the in-plane and out-of-plane GIXD patterns using a crystallized thick film. Fig. 5 gives the out-of-plane and in-plane GIXD profiles of a 22 lm solution-cast PEN film after isothermal crystallization at 210 °C for 2 h. The figure discloses that in both out-ofplane and in-plane profiles the (0 1 0) Bragg diffraction becomes the main peak. Diffraction profile in Fig. 5 exhibits much less preferred orientation, which has a sharp contrast with the data of ultrathin film. The GIXD patterns in Fig. 5 are similar to that reported for bulk PEN [27]. This fact indicates that the preferential crystal orientation hardly occurs in very thick PEN film. It should be noted that the buried polymer–substrate interface could also play a part for the anisotropic crystal orientation in PEN ultrathin films. However, taking into account the coincidence of the crystal orientation obtained from both GIXD and RAIR profiles despite of the different substrates of PEN film used in two methods (silicon wafer
(010)
Intensity (arb. units)
out-of-plane in-plane
(100)
10
15
20
25
(110)
30
35
2θ (degree) Fig. 5. Out-of-plane and in-plane GIXD profiles of a 22 lm solution-cast PEN film after isothermal crystallization at 210 °C for 2 h (incident angle a = 1°).
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for GIXD and gold-coated glass wafer for RAIR), the effect of the polymer–substrate interface on the orientation might be relatively weak. In addition, the influence of other factors such as the epitaxial effect of polymer on substrate and the preparation condition of films on the experimental results should also exist, even though such possibilities are difficult to be testified by the present techniques. In conclusion, the crystal orientation of PEN ultrathin films has been investigated by the combination of RAIR and GIXD techniques. It is concluded that the main-chain of PEN molecule in an ultrathin film is prone to aligning parallel to the substrate although it is not in the thicker film. During the formation of a form crystallites, the naphthalene ring, the C=O group in molecular chain of PEN as well as the b axis of the crystals tend to take the orientation more parallel to the substrate. However, this kind of anisotropic structure cannot be observed in the bulk PEN. It is very likely that the surface is the origin of the preferred orientation observed in the ultrathin films. Acknowledgements We would like to thank Prof. H. Terauchi of Kwansei Gakuin University for fruitful discussion and ceaseless encouragements. Also the financial support from the National Natural Science Foundation of China is gratefully acknowledged. References [1] J.L. Keddie, R.A.L. Jones, R.A. Cory, Faraday Discuss. 98 (1994) 219. [2] J.A. Forrest, K. Dalnoki-Veress, J.R. Stevens, J.R. Dutcher, Phys. Rev. Lett. 77 (1996) 2002. [3] C.W. Frank, V. Rao, M.M. Despotopoulou, R.F.W. Pease, W.D. Hinsberg, R.D. Miller, J.F. Rabolt, Science 273 (1996) 912. [4] H. Schonherr, C.W. Frank, Macromolecules 36 (2003) 1188. [5] S.J. Sutton, K. Izumi, H. Miyaji, Y. Miyamoto, S. Miyatashi, J. Mater. Sci. 32 (1997) 5621. [6] T. Liang, Y. Makita, S. Kimura, Polymer 42 (2001) 4867. [7] M.M. Despotopoulou, R.D. Miller, J.F. Rabolt, C.W. Frank, J. Polym. Sci., Part B, Polym. Phys. 34 (1996) 2335. [8] B.J. Factor, T.P. Russell, M.F. Toney, Phys. Rev. Lett. 66 (1991) 1181. [9] B.J. Factor, T.P. Russell, M.F. Toney, Macromolecules 26 (1993) 2847. [10] H. Yakabe, K. Tanaka, T. Nagamura, S. Sasaki, O. Sakata, A. Takahara, T. Kajiyama, Polym. Bull. 53 (2005) 213. [11] M. Durell, J.E. Macdonald, D. Trolley, A. Wehrum, P.C. Jukes, R.A.L. Jones, C.J. Walker, S. Brown, Europhys. Lett. 58 (2002) 844. [12] R.F. Saraf, C. Dimitrakopoulos, M.F. Toney, S.P. Kowalczyk, Langmuir 12 (1996) 2802. [13] N. Kawamoto, H. Mori, K. Nitta, S. Sasaki, N. Yui, M. Terano, Angew. Macromol. Chem. 256 (1998) 69. [14] N. Kawamoto, H. Mori, K. Nitta, S. Sasaki, N. Yui, M. Terano, Macromol. Chem. Phy. 199 (1998) 261. [15] T. Nishino, T. Matsumoto, K. Nakamae, Polym. Eng. Sci. 40 (2000) 336. [16] M.F. Toney, T.P. Russell, J.A. Logan, H. Kikuchi, J.M. Sands, S.K. Kumar, Nature 374 (1995) 709. [17] M.J. Capitan, D.R. Rueda, T.A. Ezquerra, Macromolecules 37 (2004) 5653. [18] P.C. Jukes, A. Das, M. Durell, D. Trolley, A.M. Higgins, M. Geoghegan, J.E. Macdonald, R.A.L. Jones, S. Brown, P. Thompson, Macromolecules 38 (2005) 2315.
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[19] Y. Liang, H.S. Lee, Macromolecules 38 (2005) 9885. [20] S.A. Francis, A.H. Ellison, J. Opt. Soc. Am. 49 (1959) 131. [21] Y. Zhang, J. Zhang, Y. Lu, Y. Duan, S. Yan, D. Shen, Macromolecules 37 (2004) 2532. [22] J. Kressler, C. Wang, H.W. Kammer, Langmuir 13 (1997) 4407. [23] L.G. Parratt, Phys. Rev. 95 (1954) 359. [24] D.L. Allara, A. Baca, C.A. Pryde, Macromolecules 14 (1978) 1215.
[25] B. Schneider, J. Stokr, P. Schmidt, M. Mihajlov, S. Dirlikov, N. Peeva, Polymer 20 (1979) 705. [26] I. Ouchi, M. Hosoi, S. Shimotsuma, J. Appl. Polym. Sci. 21 (1997) 3445. [27] Z. Mencik, Chem. Prum. 17 (1976) 78. [28] S. Buchner, D. Wiswe, H.G. Zachmann, Polymer 30 (1989) 480. [29] M. Cakmak, S.W. Lee, Polymer 36 (1995) 4093.
Crystal orientation in poly(ethylene 2,6-naphthalate) ultrathin films revealed by reflection–absorption infrared spectroscopy and grazing incidence X-ray diffraction Ying Zhang a, Shota Mukoyama a, Katsuhito Mori a, Deyan Shen b, Shouke Yan b, Yukihiro Ozaki c, Isao Takahashi a,* b
a Department of Physics, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan State Key Laboratory of Polymer Physics and Chemistry, Joint Laboratory of Polymer Science and Material, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China c Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan
Received 22 December 2005; accepted for publication 9 February 2006 Available online 28 February 2006
Abstract Crystal orientation of poly(ethylene 2, 6-naphthalate) (PEN) ultrathin films was investigated by the combination of reflection–absorption infrared spectroscopy (RAIR) and grazing incidence X-ray diffraction (GIXD) techniques. It is concluded that the main-chain of PEN molecule in ultrathin film is prone to alignment parallel to the substrate when compared with thicker films. During the formation of a form crystalline, the naphthalene ring, the C@O group in molecular chain of PEN as well as the b axis in crystalline tend to take orientation more parallel to the substrate due to the surface-induced effect. However, such an anisotropic structure could not be observed in the bulk PEN. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Orientation; Poly(ethylene 2,6-naphthalate) (PEN); Ultrathin films; Reflection–absorption infrared spectroscopy (RAIR); Grazing incidence X-ray diffraction (GIXD)
1. Introduction The physical behavior of polymers in confinement, such as in ultrathin films, block copolymers, or nanocomposites, differs considerably from the behavior observed in the bulk. In the case of ultrathin films, the glass transition temperature, [1,2] crystallization behavior, [3,4] morphology [5] and electrical properties [6] etc. are known to depend on their thickness. In addition, the polymeric systems in ultrathin films usually show preferred orientation different from that of bulks. For example, Frank et al. [4,7] disclosed that while the backbones of poly (di-n-hexylsilane) deposited on quartz substrate are aligned preferentially in the plane of the
*
Corresponding author. E-mail address: [email protected] (I. Takahashi).
0039-6028/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.02.012
film, the mean orientation of poly (ethylene oxides) molecules on an oxidized silicon substrate is in the surfacenormal direction. The poly(ethylene oxides) lamellar crystal also shows an interesting film thickness dependent morphology and orientation. The characteristic crystal structure and orientation were furthermore observed near the polymeric surface, which are highly different from those deeply buried in the bulk. Factor et al. [8] studied the near surface structure of an aromatic polyimide film. It was found that within ˚ from the surface the ordering of the polymer the first 90 A molecules is markedly enhanced. Such surface-related effects can also be found in other polymeric systems [9–19]. Several experimental techniques can meet the demand to detect the crystalline structure and orientation in ultrathin films. Among them, Fourier transform infrared (FTIR) spectroscopy is a promising technique and has great advantage: it can probe directly to such subtle details as
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Y. Zhang et al. / Surface Science 600 (2006) 1559–1564
intermolecular interactions, localized molecular conformations and orientations. As one of various kinds of infrared techniques, reflection–absorption infrared (RAIR) spectroscopy fits sufficiently to the characterization of ultrathin films with the thickness of nanometer scale. The characteristic [20] of RAIR is that the resultant electric field vector is perpendicular to the metal-based surface. Therefore, if molecules are adsorbed onto the substrate with a preferred orientation, vibration modes having transition moments perpendicular to the surface will appear with greater intensity than modes having transition moments parallel to the surface. Thus, RAIR is especially useful for determining the orientation of adsorbed molecular species. A technique of grazing incidence X-ray diffraction (GIXD) also enables us to gain direct access to information upon the structure at the film surface by choosing an appropriate condition [8–19]. By collecting Bragg reflections from the crystals under the in-plane and out-of-plane scanning conditions, the corresponding information on crystalline structure along normal and parallel to the sample surface can be obtained. Due to the unmatched advantage on exploring the orientational structure in confined geometry for the RAIR and GIXD methods, the two techniques have been used in the investigations of many polymers. However, there is still no such work reported for the combination of both RAIR and GIXD so far, even though it can offer us more detailed information on the crystal structure or microstructure at molecular level for the thin polymer specimens. In the present study, the RAIR and GIXD were employed to detect the crystal orientation in the ultrathin film of poly(ethylene 2, 6-naphthalate) (PEN), which is a high performance thermoplastic polyester with a rigid naphthalene ring and a flexible aliphatic diol unit. Moreover, the difference in the orientation between the ultrathin film and bulk has also been compared.
Thickness measurements of thin films on the gold-coated glass wafers substrates were performed with NanoScope IIIA MultiMode atomic force microscope (AFM) (Digital Instrument) in tapping mode. The film thickness was determined by AFM height profile after partially removing the thin film from the glass wafers. The thickness of gold film and PEN-gold film were measured separately. They were defined as the distance between the glass surface and the average of all height values taken in a line scan across the sample surface [22]. The thickness of thin film on the silicon wafer was determined by a fit of the X-ray reflectivity data with a self-made software in which a recursive method based on the dynamical scattering theory was employed [23]. In order to investigate the corresponding bulk behavior of PEN, some thick PEN films were also prepared by dropping concentrated solution on KBr wafers (for RAIR) or silicon (1 0 0) wafers (for GIXD), and then evaporated in a vacuum oven at 40 °C for 24 h. Thickness of the thick film was estimated to be 22 lm from the dropping volume of solution, deposit area and the density of PEN. The crystallized PEN films (ultrathin films and thick films) were obtained by annealing the amorphous samples from the glassy state with a hot plate at predetermined temperatures. 2.2. Reflection–absorption FTIR Reflection–absorption FTIR spectra in the region of 4000–400 cm1 were collected with a Bruker EQUINOX 55 FT-IR spectrometer equipped with an MCT detector. The measurements were obtained by average 32 scans and at a resolution of 4 cm1. The incidence angle was fixed at 83° for the best signal recording. The polarization of the incoming beam was parallel to the plane of incidence (p-polarized). 2.3. X-ray diffraction
2. Experimental part 2.1. Sample preparation PEN sample with an intrinsic viscosity of 0.50 dL/g was kindly supplied by Prof. Zhongyong Fan of Fudan University. The PEN pellets were dissolved in chloroform–trifluoroacetic acid (vol. ratio 1:1). Substrates of silicon (1 0 0) wafers and gold-coated glass wafers were used to prepare thin PEN films for GIXD and RAIR measurements, respectively (see Ref. [21] for detailed procedures of cleaning the substrates and making the gold-coated glass wafers). Thin films with different thickness were prepared by spin-coating PEN solutions of various concentrations at several speeds for about 60 s onto the gold-coated glass wafers and silicon (1 0 0) wafers. The films were kept under vacuum at 40 °C for 24 h to remove the residual solvent. All the prepared samples were then annealed at 135 °C (about 20 °C above the Tg of bulk value 117 °C) for 1 h to relax internal stresses. The films after annealing at 135 °C were proved to be amorphous by their X-ray diffraction profiles.
The crystalline structure of PEN thin films was investigated by GIXD measurements. The measurements were carried out using a diffractometer (TTR-450, Rigaku Co. Ltd.) with a rotating Cu anode X-ray source ˚ The size of the incident beam was defined (k ¼ 1:5406 A). by a collimator with diameter of 1 mm. A soller slit was placed before a scintillation detector to collimate the scattered beam. The samples were mounted on a four-circle goniometer with the film plane nearly horizontal. The GIXD measurements were performed with in-plane and out-of-plane configuration to detect the crystalline structure of perpendicular and parallel to the substrate surface, respectively. The schematic pictures of in-plane and out-ofplane GIXD measurements are shown in Fig. 1. 3. Results and discussion The preferred orientation of the main-chain of PEN molecule in the ultrathin film on gold substrate was interrogated by comparing the transmission mode and the
Y. Zhang et al. / Surface Science 600 (2006) 1559–1564
(A)
Polymer coating
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(A)
Diffracted lattice planes
IR Transmission RAIR
1729
Incident beam
2θ Diffracted beam
Absorbance
αi
(B)
Polymer coating
1338 1376
1406
1602
Diffracted beam
Incident beam
αi
1800
2θ
1700
1600
1500
1400
-1
Wavenumber (cm ) Diffracted lattice planes
(B)
Fig. 1. Schematic pictures of (A) in-plane and (B) out-of-plane GIXD measurements.
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Absorbance
p-polarized RAIR mode. Fig. 2 shows the transmission spectrum of a PEN film drop-coated on a KBr wafer and the RAIR spectrum of a 60 nm PEN ultrathin film spincoated on gold surface. Both films we investigated are amorphous. Fig. 2(A) and (B) depict the spectra in the different wavenumber regions. It is known that reflectance spectra clearly show large changes in both peak position and shape compared to transmission spectra [24,25]. The phenomenon is indeed observed from the band shift of C@O stretching at 1729 cm1 and ring CH out-of-plane deformation at 769 cm1. To obtain the qualitative information about the molecular orientation, we avoid the disturbance of the band distortions by comparing the relative intensity of bands in the same spectra. Comparison with the corresponding IR transmission spectrum indicates the differences in the RAIR spectrum of PEN ultrathin film. In Fig. 2(A), the intensity of 1729 cm1 band increases with respect to the bands at 1602, 1406, 1376 and 1338 cm1. As summarized in Table 1 [26], the band at 1729 cm1 is assigned to C@O stretching mode, the bands at 1602 and 1406 cm1 are assigned to naphthalene ring vibration, and the bands at 1376, 1338 cm1 are attributed to CH2 wagging vibration. The transition moment of 1729 cm1 band is perpendicular to the main-chain axis of PEN molecules, whereas it is parallel to the main-chain axis of PEN molecules for the transition moments of 1602, 1406, 1376 and 1338 cm1 bands. The similar relative intensity changes are also shown in Fig. 2(B). The bands near 1135 and 1091 cm1, which are assigned to naphthalene ring vibration and C–O symmetry stretching vibration respectively, are parallel bands, while the ring CH out-of-plane deformation vibration at 769 cm1 is a perpendicular band [26]. The ratio of the intensity of 769 cm1 relative to those of the parallel bands at 1135 and 1091 cm1 also increases. This indicates that
769
IR Transmission RAIR
1135
1100
1000
900
800
Wavenumber (cm-1) Fig. 2. The RAIR spectrum of a 60 nm PEN ultrathin film spin-coated on gold and IR transmission spectrum of a PEN film drop-coated on the KBr pellet, (A) and (B) are in different wavenumber region.
there is a selective orientation for the PEN backbone on the gold substrate. On the basis of the mutually perpendicular direction of the electric field vector for transmission vs. p-polarized RAIR modes, we can conclude that the mainchain of PEN in the ultrathin film shows some alignment parallel to the substrate. The orientation during crystallization of the PEN ultrathin film was also investigated. Fig. 3 gives the RAIR spectrum of a 60 nm PEN film after crystallized at 210 °C for 2 h from glassy state. The spectrum of the film before crystallization is also given for comparison. It is known that PEN can crystallize into at least two different crystal modifications, i.e., the a and b forms [27,28]. The a crystal form can be obtained by annealing amorphous PEN in the solid state, while the crystal form b can be obtained directly from the melt. It can be concluded from the appearance of characteristic bands at 814, 839, 931, 1004 and 1104 cm1 in Fig. 3 that the a-form crystalline structure is formed.
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Table 1 Assignment and polarization of IR band of PEN Wavenumber (cm1)a
Band assignmentb
Polarizationc
1729 (1715) 1602 1406 1376 1338 1135 1104 1091 1004 931 839 814 769 (765)
m(C@O) Naphthalene ring vibration Naphthalene ring vibration x(CH2) gauche x(CH2) trans Naphthalene ring vibration Crystalline ms(C–O) gauche Crystalline d(ring CH out-of-plane) Crystalline Crystalline d(ring CH out-of-plane)
?p k k k k k k ? ? k ?r
a
The wavenumbers in bracket represent the peak positions in transmission IR spectra. b m = stretching, x = wagging, ms = symmetry stretching, d = deformation. c ? and k denote perpendicular and parallel polarization, respectively, with respect to the main-chain axis of PEN molecules. p and r denote parallel and perpendicular to the plane of the naphthalene ring respectively.
after crystallization before crystallization
Absorbance
1729
769
1104 931 1004
1800
1600
1400
1200
1000
839
814
The crystal structure of PEN ultrathin film was also investigated by X-ray diffraction method. Fig. 4 shows the out-of-plane and in-plane GIXD profiles of a 73 nm PEN ultrathin film after isothermal crystallization at 210 °C for 2 h. The incident angle was kept at 0.175°, which is above the critical angle of 0.15°, thus the resulting profiles may reflect the structural information deep in the films. As mentioned above, PEN has been reported to have at least two modifications, i.e. the a and b forms. The unit cell of the a form was determined by Mencik [27] as triclinic with a = 0.651 nm, b = 0.575 nm, c = 1.32 nm, a = 81.33°, b = 144°, c = 100°. A triclinic unit cell was also proposed by Zachmann et al. [28] for the b crystal form with a = 0.926 nm, b = 1.559 nm, c = 1.273 nm, a = 121.6°, b = 95.57°, and c = 122.52°. The present GIXD result shows the crystalline reflections of (1 0 0), ð 110Þ, and (0 1 0) plane (2h are 23.3, 27.0, and 15.6°, respectively), which are major reflections of the a-form crystal. This indicates the formation of a-form crystal and agrees with that obtained from RAIR measurement. Furthermore, out-ofplane and in-plane profiles show quite different the relative intensity of each diffraction peak. While the out-of-plane scattering is dominated by the (1 0 0) and ð 1 1 0Þ peaks, the in-plane scattering is dominated by a unique Bragg peak: (0 1 0) peak. These results clearly indicate that the crystals formed during the present crystallization process are highly anisotropic and hence preferential orientation occurs throughout the PEN ultrathin film. Since the ð1 1 0Þ lattice plane is approximately parallel to the naphthalene ring in PEN molecule (making an angle of about 4.7° with the naphthalene ring) [29], it can be inferred that the naphthalene ring plane is almost parallel to the film surface in the crystalline region from the presence of ð 1 1 0Þ Bragg peak only in the out-of-of plane scan. This is consistent with the result derived from the RAIR spectra. The c axis can also be proved to align along the direction parallel
800
Wavenumber (cm-1)
(100)
Fig. 3 further indicates that the bands at 1729 and 769 cm1 display difference in the relative intensities. After crystallization, the intensity of 1729 cm1 band decreases while that of 769 cm1 band increases. As mentioned above, both of these two bands are perpendicular polarization. However, the transition moment of 1729 cm1 band is parallel to the plane of naphthalene ring while it is perpendicular to the plane for the transition moment of 769 cm1 band. Considering that all the atoms of the polymer chain including O@C–O and naphthalene ring (except the hydrogens of CH2) are coplanar in PEN [26], it is thus concluded from the relative change in intensity of the above two bands during crystallization that both the naphthalene ring and carbonyl group in PEN molecular chain tend to be aligned preferentially parallel to the substrate.
Intensity (arb. units)
Fig. 3. The RAIR spectra of a 60 nm PEN ultrathin film before and after isothermal crystallization at 210 °C for 2 h.
out-of-plane in-plane
(110)
(010)
10
15
20
25
30
35
2θ (degree) Fig. 4. Out-of-plane and in-plane GIXD profiles of a 73 nm PEN ultrathin film after isothermal crystallization at 210 °C for 2 h (incident angle a = 0.175°).
Y. Zhang et al. / Surface Science 600 (2006) 1559–1564
to the sample surface in terms of the coplanar structure in a-form crystal of PEN. Moreover, in the case of out-ofplane geometry, the intensity of (1 0 0) reflection is the strongest one, while this peak cannot be observed for the in-plane geometry. This result reveals that the b axis in the PEN ultrathin film is also preferentially oriented parallel to the film surface. The surface-induced effect should play an important role for the anisotropic arrangement of the crystals in the PEN ultrathin film. It has been found in many polymeric systems that chain segments in the proximities to the polymer–air interface crystallize more easily than those from inner structures. Therefore, it is possible that an anisotropic structure originates preferentially at the surface, and this anisotropy will be followed by the crystal growth with the same direction in the interior of PEN film. It can be further imagined that this kind of surface effect diminishes as the distance away from the surface increases. This point is confirmed by comparing the in-plane and out-of-plane GIXD patterns using a crystallized thick film. Fig. 5 gives the out-of-plane and in-plane GIXD profiles of a 22 lm solution-cast PEN film after isothermal crystallization at 210 °C for 2 h. The figure discloses that in both out-ofplane and in-plane profiles the (0 1 0) Bragg diffraction becomes the main peak. Diffraction profile in Fig. 5 exhibits much less preferred orientation, which has a sharp contrast with the data of ultrathin film. The GIXD patterns in Fig. 5 are similar to that reported for bulk PEN [27]. This fact indicates that the preferential crystal orientation hardly occurs in very thick PEN film. It should be noted that the buried polymer–substrate interface could also play a part for the anisotropic crystal orientation in PEN ultrathin films. However, taking into account the coincidence of the crystal orientation obtained from both GIXD and RAIR profiles despite of the different substrates of PEN film used in two methods (silicon wafer
(010)
Intensity (arb. units)
out-of-plane in-plane
(100)
10
15
20
25
(110)
30
35
2θ (degree) Fig. 5. Out-of-plane and in-plane GIXD profiles of a 22 lm solution-cast PEN film after isothermal crystallization at 210 °C for 2 h (incident angle a = 1°).
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for GIXD and gold-coated glass wafer for RAIR), the effect of the polymer–substrate interface on the orientation might be relatively weak. In addition, the influence of other factors such as the epitaxial effect of polymer on substrate and the preparation condition of films on the experimental results should also exist, even though such possibilities are difficult to be testified by the present techniques. In conclusion, the crystal orientation of PEN ultrathin films has been investigated by the combination of RAIR and GIXD techniques. It is concluded that the main-chain of PEN molecule in an ultrathin film is prone to aligning parallel to the substrate although it is not in the thicker film. During the formation of a form crystallites, the naphthalene ring, the C=O group in molecular chain of PEN as well as the b axis of the crystals tend to take the orientation more parallel to the substrate. However, this kind of anisotropic structure cannot be observed in the bulk PEN. It is very likely that the surface is the origin of the preferred orientation observed in the ultrathin films. Acknowledgements We would like to thank Prof. H. Terauchi of Kwansei Gakuin University for fruitful discussion and ceaseless encouragements. Also the financial support from the National Natural Science Foundation of China is gratefully acknowledged. References [1] J.L. Keddie, R.A.L. Jones, R.A. Cory, Faraday Discuss. 98 (1994) 219. [2] J.A. Forrest, K. Dalnoki-Veress, J.R. Stevens, J.R. Dutcher, Phys. Rev. Lett. 77 (1996) 2002. [3] C.W. Frank, V. Rao, M.M. Despotopoulou, R.F.W. Pease, W.D. Hinsberg, R.D. Miller, J.F. Rabolt, Science 273 (1996) 912. [4] H. Schonherr, C.W. Frank, Macromolecules 36 (2003) 1188. [5] S.J. Sutton, K. Izumi, H. Miyaji, Y. Miyamoto, S. Miyatashi, J. Mater. Sci. 32 (1997) 5621. [6] T. Liang, Y. Makita, S. Kimura, Polymer 42 (2001) 4867. [7] M.M. Despotopoulou, R.D. Miller, J.F. Rabolt, C.W. Frank, J. Polym. Sci., Part B, Polym. Phys. 34 (1996) 2335. [8] B.J. Factor, T.P. Russell, M.F. Toney, Phys. Rev. Lett. 66 (1991) 1181. [9] B.J. Factor, T.P. Russell, M.F. Toney, Macromolecules 26 (1993) 2847. [10] H. Yakabe, K. Tanaka, T. Nagamura, S. Sasaki, O. Sakata, A. Takahara, T. Kajiyama, Polym. Bull. 53 (2005) 213. [11] M. Durell, J.E. Macdonald, D. Trolley, A. Wehrum, P.C. Jukes, R.A.L. Jones, C.J. Walker, S. Brown, Europhys. Lett. 58 (2002) 844. [12] R.F. Saraf, C. Dimitrakopoulos, M.F. Toney, S.P. Kowalczyk, Langmuir 12 (1996) 2802. [13] N. Kawamoto, H. Mori, K. Nitta, S. Sasaki, N. Yui, M. Terano, Angew. Macromol. Chem. 256 (1998) 69. [14] N. Kawamoto, H. Mori, K. Nitta, S. Sasaki, N. Yui, M. Terano, Macromol. Chem. Phy. 199 (1998) 261. [15] T. Nishino, T. Matsumoto, K. Nakamae, Polym. Eng. Sci. 40 (2000) 336. [16] M.F. Toney, T.P. Russell, J.A. Logan, H. Kikuchi, J.M. Sands, S.K. Kumar, Nature 374 (1995) 709. [17] M.J. Capitan, D.R. Rueda, T.A. Ezquerra, Macromolecules 37 (2004) 5653. [18] P.C. Jukes, A. Das, M. Durell, D. Trolley, A.M. Higgins, M. Geoghegan, J.E. Macdonald, R.A.L. Jones, S. Brown, P. Thompson, Macromolecules 38 (2005) 2315.
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