We demonstrate filtering characteristics of a polymer band-rejection filter (PBRF) with highly-ordered microphase-separated structure of block copolymers (BCPs). This PBRF is characterized by an Optical Density > 5 blocking at the center wavelength and narrow blocking full bandwidth of 8 nm. Moreover, the wavelength is easily tuned by blending two BCPs with different molecular-weight. A low frequency Raman shift of 200 cm−1 are, in fact, detected with a sufficient resolution by using this filter in Raman spectroscopy.
© 2011 OSA
Bottom-up processes with self-assembled organic soft materials have been attracting much attention for the last decade as low cost methods for nano-structure fabrication in large area. A block copolymer (BCP) is one of typical self-assembled materials. BCPs organize nanoscale periodic structures (microphase-separated structures) such as a lamellar structure, a hexagonal-packed cylinder, a double-gyroid structure, and a body-centered-cubic lattice. Photonic crystals (PCs) with these periodic structures have been investigated by several research groups [1–10]. Thomas and associates provided a thin-film organic laser cavity  and gels with tunable photonic bandgap [6,7], and Parnell and associates optical filters of the solution containing two BCPs with different molecular-weight [8,9]. However, if PCs based on BCPs are used for practical optical devices, a few serious problems still exist, i.e. fluidity and structural distortion, which badly affect optical characteristic. A BCPs solution, which is affected by temperature, fluidity and solvent evaporation, has highly-ordered structure. On the other hand, a PC film fabricated by solution casting process has solidity. However, this film has many structural distortions such as defects, broad distribution of domain spacing and so on. Even if it is annealed at above a glass transition temperature for a long time, it is very difficult to eliminate these distortions, because of the high viscosity of BCPs. Now, we propose a novel fabrication method of a solid state film with a highly-ordered microphase-separated lamellar structure. As mentioned above, BCPs in a solution form highly-ordered structure. Therefore, we use photo-polymerizable acrylate monomer as a solvent, and the BCP solution is solidified by ultraviolet (UV) cure in order to retain the ordered structure. This multilayered film is suitable for an optical band-rejection filter. In this paper, we demonstrate the filtering characteristics of our polymer band-rejection filter (PBRF), and its effectiveness for Raman spectroscopy.
Preparation for the PBRF is as follows. The polystyrene-b-poly(tert-butyl methacrylate) (PS-b-P(tBuMA)) was synthesized by a living anionic polymerization. The refractive indices of PS and P(tBuMA) are 1.590 and 1.464, respectively. The PS-b-P(tBuMA) was dissolved in a photo-polymerizable acrylate monomer at 10 wt.% with a photo-polymerization initiator. The microphase-separated lamellar structure was formed spontaneously in the solution. This solution was subjected to shear flow field by sandwiched in between two glass plates with a spacer of 230 μm. The shearing process is well-known as an appropriate preparation of a highly-ordered microphase-separated structure [11–13]. After that, the solution between the glass plates was annealed for 24h in the dark at 28 °C, higher than the glass transition temperature, in order to form a more ordered periodic structure. The viscosity of this solution at 28 °C is low enough to form a highly-ordered structure. The solution was kept at the constant temperature during annealing, because the domain spacing varies with temperature change . Subsequently, an ultraviolet (UV) cure was performed for 5 min., and then, the solid state film with highly-ordered multilayered lamellar structure same as in the solution was obtained. The multilayered structure in the PBRF was investigated by transmission electron microscopy (TEM) and by small-angle X-ray scattering (SAXS) measurements. The SAXS measurements were performed at BL19B2 beam line at Super Photon ring-8 (SPring-8). The incident direction of X-ray was normal to the cross section of the film. Spectral transmission characteristics of the PBRFs were observed by spectrophotometer in the range of 400 to 900 nm. Raman spectroscopy using 532 nm laser was performed in order to compare our PBRF with a conventional dielectric multilayered band-rejection filter prepared by vapor deposition method.
3. Structure investigation
The photograph of our PBRF is shown in Fig. 1 . The diameter and film thickness are 100 mm and 230 μm, respectively. The reflected colors due to periodic structure with a refractive index difference are observed. The problems such as solution fluidity and evaporation are completely overcome by the aid of UV cure. The film shows rich formability such as bending and cutting, and it is able to be handled as general polymer films.
TEM image of the PBRF is shown in Fig. 2 . The dark and bright regions correspond to PS domains stained with RuO4 and to P(tBuMA) domains, respectively. The domain spacing is approximately 180 nm. The periodicity of the lamellar structure is well oriented parallel to the film plane over large area without defects. Same images as Fig. 2 were observed at any portions of the film.
Figure 3 shows two-dimensional SAXS image of the PBRF. The scale bar is equal to q defined by
The SAXS image shows a small spotlike scattering pattern based on lamellar structure oriented parallel to the film plane. The domain spacing is estimated at 180 nm by Eq. (2), which is the same value as domain spacing shown in Fig. 2. The scattering with recourse to periodic lamellar structures in any other direction, is not observed as in Fig. 3. The results of TEM and SAXS measurements indicate that the PBRFs have highly-ordered multilayered structure similar to a single crystal. This structure cannot be formed by solution casting process owing to structural distortions in solution evaporation. Moreover, these structural distortions are not eliminated by annealing because of a high viscosity of BCPs. In the polymerizable solution having low viscosity, the structural distortions are easily eliminated. Therefore, the film with highly-ordered structure similar to single crystal is obtained by 24h annealing and by UV cure. If BCPs solution is solidified just after applying shear flow field, the structure has some structural distortions. The annealing effect on their optical properties will be showed in the next section.
4. Optical properties
Spectral transmission characteristics of the PBRFs are shown in Fig. 4 . The vertical axis is an Optical Density (OD) defined by14]. Each filter has highly blocking characteristic as OD>5 and as a narrow blocking bandwidth of 8 nm at 50% transmittance with the steep blocking band edge. The optical properties are superior to a conventional band-rejection filter prepared by vapor deposition method. The inset in Fig. 4 shows the transmission spectrum of the film solidified just after applying shear flow field. The structural distortions arising from a shearing process are remained, and, as a result, it provides a poor optical property (low OD of 1.6 and a dull blocking band edge). The spectral transmission characteristic strongly depends on a regularity of a microphase-separated structure.
The filter with a narrow blocking bandwidth is brought about by the low refractive index difference. For a high OD value together with a narrow blocking bandwidth, a large number of layers are necessary. Refractive indices of PS and P(tBuMA) are 1.590 and 1.464, respectively, and the difference is 0.126. However, in this study, each PS and P(tBuMA) layers in microphase-separated lamellar structure includes photo-polymerized acrylate polymer with the refractive index as 1.467; therefore, the refractive index difference is less than the difference between neat PS and P(tBuMA) layers. In this case, refractive index difference estimated by the ratio of BCPs and photo-polymerized acrylate polymer is quite low to be 0.014. The number of layers is 1277, which is estimated from the total film thickness of 230 μm and domain spacing of 180 nm (each layer thickness of 90 nm). Therefore, our PBRFs have the high OD value with narrow blocking bandwidth.
Here, we calculate transmission spectra by transfer matrix method for a finite 1D periodic structure in order to confirm quantitatively the above discussions. The computed spectra are shown in Fig. 5 , together with the measured spectrum at 532 nm. As expected, the full widths at half maximum (FWHMs) of the structure including acrylate polymers are narrower than the estimated FWHM of neat PS and P(tBuMA) layers. Needless to say, this is because the refractive index difference of the former is decreased by dilution effect. The comparison of the computed spectrum of including acrylate polymer with measured spectrum is as follows. The computed spectrum has higher OD value of 9 and narrower FWHM of 4 nm than the values of the measured spectrum, 6.8 and 8 nm, respectively. The difference between them would be caused by the fact that the actual film still contains some distortions. The computed transmission spectra in Fig. 5 have many ripples due to a refractive index profile shaped as a square function. However, the microphase-separated structure of BCPs has interfaces between each layer where a refractive index continuously varies, which result in no ripple on the measured spectrum in Fig. 5 .
Raman spectra of chlorobenzene pumped at 532 nm are measured by using a conventional dielectric multilayered filter (FWHM = 20 nm) and our PBRF (FWHM = 8 nm). The results are shown in Fig. 6 . The Raman signal at 200 cm−1, which cannot be detected by conventional filter, can be clearly shown in the case of using the PBRF. Weak Rayleigh scattering is observed at 0 cm−1. This is because the OD value of our PBRF is slightly lower than the conventional one. From the each value of FWHM, a blind width of Raman spectroscopy pumping at 532 nm estimated at less than 350 cm−1 for the conventional filter and less than 140 cm−1 for the PBRF, respectively. The results of Raman spectroscopy suggest that the PBRF based on BCP with the narrow band-rejection width is more advantageous for fluorescence and Raman spectroscopy.
In this paper, we have demonstrated filtering characteristics of the PBRFs with highly-ordered microphase-separated structure of BCPs. The filters exhibit a highly blocking of OD>5 and a narrow blocking bandwidth of 8 nm, and there is no ripple in pass band. A low frequency Raman shift of 200 cm−1 are detected successfully with a sufficient resolution by using the PBRF in Raman spectroscopy. This is the first report in the world that the filter based on organic PCs can realize high performance enough for practical use. Moreover, the filters can easily be prepared in large area by simple process. We expect this work makes a breakthrough in organic PCs devices.
The synchrotron radiation experiments were performed at BL19B2 of SPring8 as Priority Research Proposal (priority field: Industrial Application) with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2010B1940).
References and links
1. P. A. Urbas, Y. Fink, and E. L. Thomas, “One-Dimensionally Periodic Dielectric Reflectors from Self-Assembled Block Copolymer-Homopolymer Blends,” Macromolecules 32(14), 4748–4750 (1999). [CrossRef]
2. T. Deng, C. Chen, C. Honeker, and E. L. Thomas, “Two-dimensional block copolymer photonic crystals,” Polymer (Guildf.) 44(21), 6549–6553 (2003). [CrossRef]
3. A. M. Urbas, M. Maldovan, P. DeRege, and E. L. Thomas, “Bicontinuous Cubic Block Copolymer Photonic Crystals,” Adv. Mater. (Deerfield Beach Fla.) 14(24), 1850–1853 (2002). [CrossRef]
4. K. Tsuchiya, S. Nagayasu, S. Okamoto, T. Hayakawa, T. Hihara, K. Yamamoto, I. Takumi, S. Hara, H. Hasegawa, S. Akasaka, and N. Kosikawa, “Nonlinear optical properties of gold nanoparticles selectively introduced into the periodic microdomains of block copolymers,” Opt. Express 16(8), 5362–5371 (2008). [CrossRef] [PubMed]
5. J. Yoon, W. Lee, and E. L. Thomas, “Optically pumped surface-emitting lasing using self-assembled block-copolymer-distributed Bragg reflectors,” Nano Lett. 6(10), 2211–2214 (2006). [CrossRef] [PubMed]
7. E. Kim, C. Kang, H. Baek, K. Hwang, D. Kwak, E. Lee, Y. Kang, and E. L. Thomas, “Control of Optical Hysteresis in Block Copolymer Photonic Gels: A Step Towards Wet Photonic Memory Films,” Adv. Funct. Mater. 20(11), 1728–1732 (2010). [CrossRef]
8. A. J. Parnell, N. Tzokova, A. Pryke, J. R. Howse, O. O. Mykhaylyk, A. J. Ryan, P. Panine, and J. P. A. Fairclough, “Shear ordered diblock copolymers with tuneable optical properties,” Phys. Chem. Chem. Phys. 13(8), 3179–3186 (2011). [CrossRef] [PubMed]
9. A. J. Parnell, A. Pryke, O. O. Mykhaylyk, J. R. Howse, A. M. Adawi, N. J. Terrill, and J. P. A. Fairclough, “Continuously tuneable optical filters from self-assembled block copolymer blends,” Soft Matter 7(8), 3721–3725 (2011). [CrossRef]
10. J. Yoon, W. Lee, and E. L. Thomas, “Thermochromic Block Copolymer Photonic Gel,” Macromolecules 41(13), 4582–4584 (2008). [CrossRef]
11. I. W. Hamley, “Structure and flow behaviour of block copolymers,” J. Phys. Condens. Matter 13(33), R643–R671 (2001). [CrossRef]
12. I. Rychkov and K. Yoshikawa, “Structural Changes in Block Copolymer Solutions under Shear Flow as Determined by Non-Equilibrium Molecular Dynamics,” Macromol. Theory and Simul. 13(3), 257–264 (2004). [CrossRef]
13. Y. Takahashi, M. Naruse, Y. Akazawa, A. Takano, and Y. Matsushita, “Comparison between Flow-Induced Alignment Behaviors of Poly(styrene-block-2-vinylpyridine)s and Poly(styrene-block-isoprene)s Solutions near ODT,” Polym. J. 37(12), 900–905 (2005). [CrossRef]
14. T. Hashimoto, K. Yamasaki, S. Koizumi, and H. Hasegawa, “Ordered structure in blends of block copolymers. 1. Miscibility criterion for lamellar block copolymers,” Macromolecules 26(11), 2895–2904 (1993). [CrossRef]
15. H. V. Baghdasaryan and T. M. Knyazyan, “Modelling of strongly nonlinear sinusoidal Bragg gratings by the Method of Single Expression,” Opt. Quantum Electron. 32(6/8), 869–883 (2000). [CrossRef]