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ARTICLES Chain End Distribution of Block Copolymer in Two-Dimensional Microphase-Separated Structure Studied by Scanning Near-Field Optical Microscopy Ryojun Sekine, Hiroyuki Aoki,* and Shinzaburo Ito Department of Polymer Chemistry, Graduate School of Engineering, Kyoto UniVersity, Nishikyo, Kyoto 615-8510, Japan ReceiVed: April 7, 2009; ReVised Manuscript ReceiVed: August 7, 2009
The chain end distribution of a block copolymer in a two-dimensional microphase-separated structure was studied by scanning near-field optical microscopy (SNOM). In the monolayer of poly(octadecyl methacrylate)block-poly(isobutyl methacrylate) (PODMA-b-PiBMA), the free end of the PiBMA subchain was directly observed by SNOM, and the spatial distributions of the whole block and the chain end are examined and compared with the convolution of the point spread function of the microscope and distribution function of the model structures. It was found that the chain end distribution of the block copolymer confined in two dimensions has a peak near the domain center, being concentrated in the narrower region, as compared with three-dimensional systems. Introduction Block copolymers are well-known to form a variety of ordered nanostructures via self-organization.1,2 Since such nanostructures have the potential for practical application,3 the morphology of block copolymers has been extensively explored in thin films as well as in bulk.4-9 With a decrease in the film thickness, the effect of the spatial confinement on polymer chains increases. Therefore, the behavior of polymers in thin films differs appreciably from that in bulk.10-12 A polymer monolayer prepared by the Langmuir-Blodgett technique has a molecularly ultimate thickness because the main chain consisting of amphiphilic groups is adsorbed on the water surface. Thus, the polymer monolayer can be regarded as a model of a two-dimensional system of polymers. The twodimensional microphase-separated structures deposited on a substrate are reported by several researchers.13-15 The domain size in two dimensions was significantly larger than that in threedimensional systems, suggesting that the chain conformation in the domains is different from a three-dimensional one. To understand the chain conformation in more detail, the segment distribution of the specific parts of the block copolymers forming a phase-separated structure is helpful information. Therefore, the localization of block segments has been investigated by small-angle neutron scattering,16 neutron reflectivity,17-19 and the energy transfer technique.20 Several research groups have reported that the partial segments near the free end of block copolymers were localized at the center of lamellae with a fairly wide distribution, whereas the segments of block chain adjacent to the chemical junction point are strongly localized at the lamellar interface.17-20 Computational studies have also showed similar results.21,22 In contrast to such techniques, direct observation is expected to provide clear evidence on the polymer nanostructure. In the * Corresponding author. Phone: +81-75-383-2613. Fax: +81-75-3832617. E-mail:
[email protected].
past, direct observation of the two-dimensional morphology of polymer monolayers was carried out by Brewster angle microscopy23,24 and atomic force microscopy.25,26 Recently, scanning near-field optical microscopy (SNOM) has attracted much attention from researchers in various fields. SNOM is an emerging scanning probe technique that allows optical measurement with high resolution beyond the diffraction limit of light.27-29 SNOM is equipped with a probe tip having an aperture smaller than the wavelength. The light incidence to the aperture generates an optical near field restricted in the space of the aperture size, which allows one to illuminate the nanometric space under the sample surface. The introduction of a small amount of fluorescent chromophores into the specific segments in a polymer chain allows us to obtain the information on the location of the chain segment on a nanometer scale.30-36 Thus, SNOM is expected to be a suitable method for investigating the segment distribution of block copolymer in a twodimensional phase-separated structure. In the current study, the spatial distribution of the chain end of the poly(isobutyl methacrylate) (PiBMA) in a two-dimensional microphase-separated structure of poly(octadecyl methacrylate)-block-poly(isobutyl methacrylate) (PODMA-b-PiBMA) diblock copolymer was observed by SNOM. Through comparison with the distribution of the PiBMA block of PODMA-b-PiBMA, the PiBMA chain end distribution is discussed. Experimental Section Synthesis of Diblock Copolymers. Two kinds of dye-labeled PODMA-b-PiBMA diblock copolymers were synthesized: a single perylene diimide derivative (PDI) molecule is tagged at the free end of the PiBMA subchain (PODMA-b-PiBMA-PDI), and the dye moiety is randomly introduced along the contour of the PiBMA subchain (PODMA-b-P(iBMA/PDI)). The chemical structures of the labeled PODMA-b-PiBMAs are shown in Figure 1.
10.1021/jp903227y CCC: $40.75 2009 American Chemical Society Published on Web 09/08/2009
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Figure 2. Topographic image of phase-separated PODMA-b-PiBMAPDI monolayer. The dimension is 5 µm × 5 µm.
Figure 1. Chemical structures of PODMA-b-PiBMA-PDI (a) and PODMA-b-P(iBMA/PDI) (b).
TABLE 1: Characterization of Sample Polymers DPn/× 103 PODMA-b-PiBMA-PDI PODMA-b-P(iBMA/PDI) a
PODMA 1.98 1.71
PiBMA 2.26 1.86
Nfa 1 15
The number of the fluorescence moiety in the PiBMA part.
The end-labeled PiBMA was synthesized by atom transfer radical polymerization of isobutyl methacrylate (iBMA) with CuCl(I)/4,4′-dinonyl-2,2′-bipyridyl at 70 °C from the initiating agent having the PDI moiety. The raw polymer was obtained after the polymerization was reprecipitated from toluene into methanol three times and dried in vacuo. The degree of polymerization (DP) of the PiBMA part was determined by sizeexclusion chromatography (D-7000F, Hitachi) with THF as the eluent calibrated by polystyrene (American Polymer Standards) and poly(isobutyl methacrylate) secondary standards (Aldrich). The block copolymerization of octadecyl methacrylate (ODMA) was carried out using CuCl(I) and 4,4′-dinonyl-2,2′-bipyridyl from the end-labeled PiBMA in anisole at 90 °C, yielding PODMA-b-PiBMA-PDI. PODMA-b-P(iBMA/PDI) was prepared by the random copolymerization of iBMA and PDIlabeled methacrylate followed by the block copolymerization of ODMA, for which the polymerization conditions were the same as PODMA-b-PiBMA-PDI. The number of the fluorescence moiety, Nf, introduced into the polymer chain was evaluated by UV-vis absorption (U3500, Hitachi) before block copolymerization of ODMA. The obtained block copolymer was dissolved in chloroform and passed through an alumina column to remove the catalyst. The block copolymer was reprecipitated from chloroform into methanol three times and was dried in vacuo. The degree of polymerization of the PODMA block was evaluated from the molar ratio of ODMA and iBMA by 400 MHz 1H NMR (JNM-EX400, JEOL) measurements. Characterization of the sample polymer is summarized in Table 1. Monolayer Preparation. A microphase-separated monolayer of PODMA-b-PiBMA was prepared by the Langmuir-Blodgett method. A benzene solution of the block copolymers at a concentration of 0.05 g L-1 was spread on ultrapure water (NANOpure II, Barnstead) at 20 °C to form a monolayer on the water surface. After the solvent was evaporated, the temperature of the subphase was raised to 40 °C and kept
constant for 3 h for promoting the microphase separation on the water surface. After cooling the temperature of the subphase to 20 °C, the phase-separated monolayer was compressed by Teflon bars up to a surface pressure of 5 mN m-1 at a speed of 10 mm min-1. The phase-separated monolayer of PODMA-bPiBMAs was transferred by vertical dipping onto a glass substrate. SNOM Measurement. SNOM imaging was performed using a cantilever probe with a 60-nm aperture (R-SNOM, WITec). All SNOM measurements were carried out by the same cantilever under ambient conditions. The sample film was scanned in the contact mode. A 532-nm laser beam (GSHG3015, Kochi Toyonaka Giken) was coupled to the subwavelength aperture to generate the optical near field for the excitation of the PDI moiety in the samples. The signal light from the sample was collected by a microscope objective (0.80NA, 60×, Nikon) from the backside of the substrate and guided to a photomultiplier (H8631, Hamamatsu Photonics) after passing through a long-pass filter (LP02-532RS-25, Semrock) to acquire the fluorescence image. Results and Discussion Figure 2 shows a topography image of a phase-separated PODMA-b-PiBMA-PDI monolayer. The two-dimensional microphase-separated structure was observed in the surface topography because there is a ∼2 nm thickness difference between PiBMA and PODMA monolayers.37,38 The bright area corresponds to the PODMA domain, which has a larger thickness than PiBMA, and the disordered lamellar structure with a regular spacing can be seen in Figure 2. The fast Fourier transform (FFT) of an image showed a peak corresponding to the domain spacing of 360 nm. For the phase separation structure of PODMA-b-P(iBMA/PDI), the domain size was evaluated to be 300 nm by FFT analysis of the AFM image. The lamellar spacing for the PODMA-b-P(PiBMA/PDI) monolayer was slightly smaller than that for PODMA-b-PiBMA-PDI because of the difference in the molecular weight (the total degrees of polymerization of PODMA-b-P(PiBMA/PDI) and PODMA-bPiBMA-PDI were 3.57 × 103 and 4.24 × 103, respectively). Figures 3a and b show the fluorescence SNOM images of the phase-separated monolayers of PODMA-b-P(iBMA/PDI) and PODMA-b-PiBMA-PDI, respectively. In Figure 3, the bright area corresponds to the spatial distribution of the PDI molecules. Figure 3a indicates the whole segment of the PiBMA chain because the dye moiety is incorporated along the main chain of the PiBMA block. On the other hand, Figure 3b depicts the spatial distribution of the PiBMA chain ends. The PiBMA domain appears as large as the PODMA domain in Figures 3a, indicating the microphase-separated structure of the symmetric
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Figure 3. SNOM images of phase-separated PODMA-b-P(iBMA/PDI) (a) and PODMA-b-PiBMA-PDI monolayers (b). The dimensions of both images are 5 µm × 5 µm. The fluorescence intensity profiles shown in lower panels c and d are obtained from SNOM images a and b, respectively.
Figure 4. A fluorescence intensity profile of the PODMA-b-P(iBMA/ PDI) block copolymer (solid circles) and the calculated profile (solid line) (a) from a density profile of the PiBMA block (b).
PODMA-b-PiBMA. However, Figure 3b clearly shows that the bright areas are smaller than the dark areas. This result suggests that the PiBMA block chain end is not homogeneously distributed in the PiBMA domain. In addition, comparison with the simultaneously obtained topographic and fluorescence SNOM images indicates that the PiBMA chain end was localized near the PiBMA domain center (see the Supporting Information). Figures 3c and d show the signal intensity profiles along the line in Figures 3a and b, respectively, which were drawn perpendicular to the domain interface. The peak-to-peak distance in the SNOM image was evaluated from 50 fluorescence
intensity profiles, which corresponds to the sum of the size of the lamellar domains of PODMA and PiBMA. The domain size was evaluated to be 300 ( 25 and 360 ( 49 nm for PODMAb-P(PiBMA/PDI) and PODMA-b-PiBMA-PDI, respectively (the values before and after the plus/minus sign represent the mean value and the standard deviation, respectively). These values were in good agreement with the results of the FFT analysis, indicating that the peak-to-peak distance corresponds to the periodicity of the lamellar spacing of the block copolymers. The SNOM image corresponds to the convolution of the spatial distribution of the fluorescent object and the point spread function (PSF) of the microscope. Therefore, given the PSF of the microscope and the appropriate spatial distribution function of the fluorophores, we can reconstruct the fluorescence intensity profiles of the SNOM images. To determine the PSF of the microscope, the nanocrystal of CdSe, the so-called quantum dot (QD), was observed.39 The size of the QD is less than 10 nm, which is far smaller than the aperture size of the SNOM probe. Therefore, the observed fluorescence image of a QD corresponds to the PSF of the imaging system. The one-dimensional fluorescence intensity profile of a single QD in SNOM images was fitted to a modeled function of a sum of two Gaussian profiles,
( )
I(x) ) p exp -
( )
x2 x2 + (1 - p)exp 2 2σ1 2σ22
(1)
and we obtained the best fit parameters, p ) 0.57, σ1 ) 43 nm, and σ2 ) 140 nm. Before the discussion of the PiBMA chain end distribution, the fluorescence intensity profile of the PODMA-b-P(iBMA/ PDI) system is discussed, in which the PDI moiety is homogeneously distributed in the PiBMA domain of the microphaseseparated structure. Since PiBMA and PODMA are incompatible with each other,40,41 the distribution of the PiBMA block chains can be modeled as Figure 4b. At first, we defined the x-axis in
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the direction perpendicular to the lamellar interface and set the origin at the middle point of the PiBMA domain. When the PiBMA domain has a width of W nm and the x-coordinate of the ith PiBMA domain center is Xi nm, the distribution function of the PiBMA block chains along the x-axis is modeled as
∑ F′(x - Xi)
F(x) )
(2)
i
F′(u) )
{
F0 (|u| e W/2) 0 (|u| > W/2)
where F0 is the plane density of the PiBMA domain. The surface occupied areas in a monolayer of iBMA and ODMA units are 0.27 and 0.28 nm2, respectively. Considering the degree of polymerization, the block copolymers show almost a symmetric microphase separation structure. The domain spacing evaluated above corresponds to the sum of the widths of the PODMA and PiBMA domains. Therefore, W for the PODMA-b-P(iBMA/ PDI) and PODMA-b-PiBMA-PDI monolayers is evaluated to be 150 and 180 nm, respectively. Figure 4a shows the fluorescence intensity profile of the PODMA-b-P(iBMA/PDI) block copolymer. The solid circles represent the experimental data, whereas the solid line indicates the calculated profile convoluted by the PSF and the assumed distribution function of the PiBMA chains shown in Figure 4b. The calculated line gave good agreement with the experimental result, indicating that the fluorescence intensity profile of a SNOM image can be described by the convolution of the PSF of the microscope and the distribution of the PiBMA block chain. If the PiBMA chain end distributes homogeneously in the PiBMA domain, the distribution function of the PiBMA chain end along the x-axis can be described by eq 2. Figure 5a shows the fluorescence intensity profile for the PODMA-b-PiBMAPDI copolymer experimentally obtained from a SNOM image
Figure 6. A fluorescence intensity profile of the PODMA-b-PiBMAPDI block copolymer (solid circles) and the calculated profile (solid line) (a) from a density profile of the PiBMA chain end (b). The chain end distribution is assumed to be a Gaussian one.
(solid circles) and the calculated profile (solid curve), which corresponds to the uniform distribution function of the PiBMA chain end with W ) 180 nm shown in Figure 5b. Figure 5a indicates that the uniform distribution cannot reconstruct the SNOM image and that the peak width experimentally obtained from the fluorescence intensity profile is smaller than the calculated ones. This result indicates that the PiBMA chain end localizes near the PiBMA domain center. As a probable distribution of the block chain end, we assume a Gaussian distribution with a peak at the PiBMA domain center. The distribution function of the PiBMA chain end along the x-axis is modeled as
F(x) ) F0
i
Figure 5. A fluorescence intensity profile of the PODMA-b-PiBMAPDI block copolymer (solid circles) and the calculated profile (solid line) (a) from a density profile of the PiBMA chain end (b). The chain end distribution is assumed to be a uniform one.
(
∑ exp -
(x - Xi)2 2σ2
)
(3)
where σ is the standard deviation of the assumed PiBMA chain end distribution. Figure 6a shows the experimentally obtained fluorescence intensity profile duplicated from Figure 5a (solid circles) and the best-fitted profile calculated from the distribution function shown in Figure 6b (solid curve). The standard deviation of the Gaussian distribution in Figure 6b was 25 nm. Considering that the PiBMA domain width is 180 nm, it is found that about 70% of the PiBMA chain ends are contained within the 28% (2σ) region of the PiBMA domain. In three dimensions, on the other hand, 70% of the chain ends were estimated to be contained within the 55-70% of the domain thickness.17,19,42 Our result shows that the PiBMA chain end in a twodimensional microphase-separated structure is confined in the spatially narrow region near the middle of the PiBMA domain, although the chain end distribution of block copolymers has a concentration maximum at the domain center in both two- and three-dimensional systems.17-22,42 In the three-dimensional system, the chain ends reach back to fill the space to maintain uniform density and interlace with other chains. However, in the two-dimensional system, the polymer chain is not allowed
Segment Distribution of Block Copolymers to have the crossover with the other chains, resulting in few interlaces among the polymer chains. Therefore, the steric hindrance among the neighboring chains would be relaxed only by stretching in the normal direction to the interface. In addition, the steric hindrance in the block chain near the domain interface is larger than that near the domain center because the block chains are densely aligned at the domain boundary. Consequently, the PiBMA block chain end in a two-dimensional microphase-separated structure are not homogeneously distributed in the PiBMA domain and tend to localize near the PiBMA domain center. Conclusions The chain end distribution of the poly(isobutyl methacrylate) (PiBMA) block in a two-dimensional microphase-separated structure formed by poly(octadecyl methacrylate)-block-poly(isobutyl methacrylate) (PODMA-b-PiBMA) diblock copolymer was studied by SNOM. The real space imaging by SNOM allows discussion of the distribution of the specific parts of the block chains in the two-dimensional lamellar structure. Our results showed that the chain end distribution of the block copolymer in two dimensions has a peak near the domain center, and the chain ends are confined in the narrower region near the domain center, as compared with the three-dimensional bulk state. Acknowledgment. This work was supported by Grants-inAid from the Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The authors also thank the Global COE Program, “International Center for Integrated Research and Advanced Education in Material Science”, and the Innovative Techno-Hub for Integrated Medical Bioimaging Project of the Special Coordination Funds for Promoting Science and Technology from MEXT. Supporting Information Available: Topographic and fluorescence SNOM images for PODMA-b-PiBMA-PDI are shown in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: New York, 1998. (2) Bates, F. S.; Fredrickson, G. H. Annu. ReV. Phys. Chem. 1990, 41, 525–557. (3) Lazzari, M.; Liu, G.; Lecommandoux, S. Eds. Block Copolymers in Nanoscience; Wiley-VCH: Weinheim, 2006. (4) Fukunaga, K.; Hashimoto, T.; Elbs, H.; Krausch, G. Macromolecules 2002, 35, 4406–4413. (5) Sohn, B. H.; Yun, S. H. Polymer 2002, 43, 2507–2512. (6) Xu, T.; Xhu, Y.; Gido, S. P.; Russell, T. P. Macromolecules 2004, 37, 2625–2629.
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