Selective Distribution of Surface-Modified TiO 2 Nanoparticles in Polystyrene-b-poly (Methyl Methacrylate) Diblock Copolymer Chin-Cheng Weng and Kung-Hwa Wei* Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan 30049 Republic of China Received April 10, 2003 Ordered aggregates of surfactant-modified TiO 2 nanoparticles in the selective block of lamellar assemblies of the diblock copolymer PS-b-PMMA have been prepared. The hydrophobic or hydrophilic nature of the tethered surfactant determines the location of TiO 2 nanoparticles in the corresponding block, as confirmed by transmission electron microscopy, differential scanning calorimetry, and Fourier transform infrared spectroscopy. The modes of dispersion of TiO 2 in the blocks depend on the type of bonding between the surfactant and TiO 2 . Photoluminescence studies of these nanocomposites demonstrate that the location of TiO 2 nanoparticles affect the block copolymer’s luminescence at different wavelengths. Introduction Owing to their optical and electrical properties, semiconductor nanoparticles or clusters are emerging materials and have the potential to be used in a wide range of applications. 1,2 For semiconductor or metal oxide nanoparticles with sizes close to their Bohr radius (typically between 1 and 10 nm), the size-dependent band gap results in tunable optical properties. 1 Nano- particles that are not treated with a surfactant or bonded to polymer chains will, however, form large aggregates. Furthermore, optoelectronic devices require nanoparticles to form ordered, one- to three-dimensional structures. 3 Block copolymers (BCPs) are a versatile platform material because they can self-assemble into various periodic structures for proper compositions and under adequate conditions, owing to the microphase separation between dissimilar blocks. 4,5 A diblock copolymer, the simplest case, self-assembles into various equilibrium morphologies, such as alternating layers, complex to- pologically connected cubic structures, cylinders on hexagonal lattices, and spheres on a body-centered lattice. Self-assembly of BCPs can therefore serve as templates for the spatial arrangement of nanoparticles in thin films or in bulk samples and can provide an effective means to manipulate their positions. In recent years, much effort has been directed toward the synthesis of semiconductor or metal oxide nanopar- ticles within block copolymer matrix materials. 6-16 For instance, BCPs/semiconductor nanoparticle nanocom- posites have been synthesized for applications involving photonic band gap devices. 17,18 Studies using ZnS, 7,10,11 PbS, 6,8,9 and CdS 7,12-14 within BCPs and CdS in salt- induced BCPs micelles 15,16 have also been reported. Among these studies, the common approach has been to synthesize the nanocrystal clusters within microphase- separated diblock copolymer films by attaching metal complexes to the functionalized block of the copolymer before microdomain formation. Then, the composite block copolymers are treated with hydrogen sulfur gases for obtaining nanoparticles in situ. Although the func- tional groups in the monomer that are used to bind the metals can be designed appropriately for one block of the copolymer, variations within the nanocrystal cannot be easily controlled within the microdomains of the block copolymers. Furthermore, these functionalized block copolymers are not suitable for use as large area templates, as opposed to the more readily available block copolymers such as polystyrene-b-poly (methyl methacrylate) (PS-b-PMMA) or polystyrene-b-poly (eth- ylene oxide) (PS-b-PEO). In the present study we have adopted an approach of synthesizing nanoparticles with modified surfactants. The surfactant can be either hydrophilic or hydrophobic, with one of its ends tethered * To whom correspondence should be addressed. Tel: 886-35- 731871. Fax: 886-35-724727. E-mail: khwei@cc.nctu.edu.tw. (1) Henglein, A. Chem. Rev. 1989, 89, 1861. (2) Wang, Y.; Herron, N. J. Phys. Chem. 1991, 95, 525. (3) Murry, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270. (4) Bates, F. S. Science 1991, 251, 898. (5) Thomas, E. L. Science 1999, 286, 1307. (6) Sankaran, V.; Cummins, C. C.; Schrock, R. R.; Cohen, R. E.; Silbey, R. J. J. Am. Chem. Soc. 1990, 112, 6858. (7) Cummins, C. C.; Schrock, R. R.; Cohen, R. E. Chem. Mater. 1992, 4, 27. (8) Kane, R. S.; Cohen, R. E.; Silbey, R. Chem. Mater. 1996, 8, 1919. (9) Tassoni, R.; Schrock, R. R. Chem. Mater. 1994, 6, 744. (10) Yue, J.; Sankaran, V.; Cohen, R. E.; Schrock, R. R. J. Am. Chem. Soc. 1993, 115, 4409. (11) Sankaran, V.; Yue, J.; Cohen, R. E. Chem. Mater. 1993, 5, 1133. (12) Moffitt, M.; Eisenberg, A. Chem. Mater. 1995, 7, 1178 (13) Moffitt, M.; McMahon, L.; Pessel, V.; Eisenberg, A. Chem. Mater. 1995, 7, 1185. (14) Moffitt, M.; Vali, H.; Eisenberg, A. Chem. Mater. 1998, 10, 1021. (15) Zhao, H.; Douglas, E. P.; Harrison, B. S.; Schanze, K. S. Langmuir 2001, 17, 8428. (16) Zhao, H.; Douglas, E. P. Chem. Mater. 2002, 14, 1418. (17) Fink, Y.; Urbas, A. M.; Bawendi, M. G.; Joannopoulos, J. D.; Thomas, E. L. J. Lightwave Technol. 1999, 17, 1963. (18) Edrington, A. C.; Urbas, A. M.; DeRege, P.; Chen, C. X.; Swager, T. M.; Hadjichristidis, N.; Xenidou, M.; Fetters, L. J.; Joannopoulos, J. D.; Fink, Y.; Thomas, E. L. Adv. Mater. 2001, 13, 421. 2936 Chem. Mater. 2003, 15, 2936-2941 10.1021/cm0300617 CCC: $25.00 © 2003 American Chemical Society Published on Web 06/17/2003