J. Phys. Chem. C 2008, 112, 14343–14347
14343
Nanostructuration via Solvent Vapor Exposure of Poly(2-vinyl pyridine-b-methyl methacrylate) Nanocomposites Using Modified Magnetic Nanoparticles I. Garcia, A. Tercjak, J. Gutierrez, L. Rueda, and I. Mondragon* Materials and Technologies Group, Departamento Ingenierı´a Quı´mica y del Medio Ambiente, Escuela Polite´cnica, UniVersity of the Basque Country, Plaza Europa 1, 20018 Donostia-San Sebastia´n, Spain ReceiVed: March 18, 2008; ReVised Manuscript ReceiVed: July 8, 2008
The structural polymorphism exhibited by a thin polymer film of symmetric poly(2-vinyl pyridine-b-methyl methacrylate) (P2VP-b-PMMA), cast by spin-coating, in the presence of selective solvent CCl4 has been investigated. Morphological features can be switched upon exposure to selective vapor. On the basis of this method, typical self-assembled nanostructures of block copolymers such as hexagonal, lamellar, and micellar are reported. Iron oxide magnetic nanoparticles have been modified with poly(methyl methacrylate) (PMMA) brushes by atomic transfer radical polymerization (ATRP) in order to improve dispersion and the affinity of nanoparticles to poly(methyl methacrylate) block of poly(2-vinyl pyridine-b-methyl methacrylate) block copolymer. This way of preparation of nanocomposites opens new ways on strategy to the generation of magnetic nanomaterials. Tapping-mode atomic force microscopy (TM-AFM) and magnetic force microscopy (MFM) have been employed to characterize ordered nanostructures of P2VP-b-PMMA thin films. The magnetic field generated by interactions between iron oxide nanoparticles in P2VP-b-PMMA block copolymer matrix with the magnetized tip of the MFM provokes the disappearance of hexagonal morphology. Introduction Self-assembly of block copolymers (BC) has received increasing interest due to the rich variety of nanostructures that can be generated depending on the nature of the blocks, the molecular weight and composition, and the processing characteristics. Microphase separation at the mesoscopic scale is generated by the repulsion between the different blocks linked by covalent attachment. This characteristic feature of BC is necessary to applications where a regular periodicity is required. Medical,1,2 surfactant,3 quantum dots,4,5 nanowires,6-8 and magnetic storage media9,10 are some applications where BC can be used. As a consequence of the short interval temperature between the glass transition temperature and thermal degradation temperature, for some BC it is not possible or can be difficult to obtain the thermodynamic equilibrium by annealing above the glass transition temperature. Also, complex metastable morphologies may form during sample preparation that could constitute barrier to achieve thermodynamic equilibrium. Different techniques and parameters of preparation of films as electric field, mechanical strain, controlled solvent evaporation, varying film thickness, or chemically tailoring the film/substrate interface allow manipulating the nanostructures. Swelling in nonselective or selective solvents has received increasing interest as it could be an alternative route for obtaining different nanostructures for BC.11,12 This technique is based in wellestablished physicochemical properties of polymers. Solvent affects the morphology and, particularly, the orientation of nanostructures of BC.13,14 There are many experimental parameters that make it difficult to control the reproducibility of the procedure.15 Some of these experimental parameters are the partial pressure of solvent, amount of solvent, and uptake and rate of solvent extraction. * To whom correspondence should be addressed. E-mail: inaki.mondragon@ ehu.es.
Diblock16 and triblock17 copolymers, poly(2-vinyl pyridine)18 being one of the blocks, have been traditionally self-assembled into different nanostructures, and also different nanoparticles19-23 have been introduced into polymer matrixes. In order to generate new nanocomposites, one of the most important problems is the dispersion of nanoparticles inside an organic medium such as block copolymers. Functionalization of the nanoparticles24-28 before introduction into polymeric matrixes is one of the ways to overcome this problem. Recently, we have reported29 that PMMA brushes can be grown onto the surface of magnetic iron oxide nanoparticles (PMMA-MN) by atomic transfer radical polymerization (ATRP). In this study, we focused on the investigation of the effect of carbon tetrachloride (CCl4) vapor swelling and drying on nanostructures of thin films of a diblock copolymer of poly(2-vinyl pyridine-b-methyl methacrylate) (P2VP-b-PMMA) with nearly symmetric composition. A variety of morphologies obtained by this technique have been analyzed by atomic force microscopy (AFM) and magnetic force microscopy (MFM). Experimental Part Materials. Poly(2-vinyl pyridine-b-methyl methacrylate) with number average molecular masses (Mn) of poly(2-vinyl pyridine) (P2VP) 56 000 g/mol and poly(methyl methacrylate) (PMMA) 57 000 g/mol, respectively, polydispersity index (Mw/Mn) 1.09 for both blocks, was purchased from Polymer Source. Carbon tetrachloride and toluene were purchased from Prolabo. Magnetic iron oxide nanoparticles (MN), with a nominal size of 9 nm and a polydispersity index of 1.34, purchased from Integran Technologies, were surface-modified with poly(methyl methacrylate) brushes by ATRP.29 The grafting density was approximately equal to 0.1 chains/nm2, and molecular weight of polymer brushes was around 30 000 g/mol.29 Preparation of Films. Poly(2-vinyl pyridine-b-methyl methacrylate) was dissolved in toluene and deposited onto a glass wafer by spin-coating using toluene solutions of 1 wt % P2VP-
10.1021/jp802345q CCC: $40.75 2008 American Chemical Society Published on Web 08/20/2008
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TABLE 1: Values of Solubility and Flory-Huggins Parameters
δt
χ12
PMMA (MPa1/2)
P2VP (MPa1/2)
toluene (MPa1/2)
CCl4 (MPa1/2)
19.4
20.4
18.2
17.6
PMMA/toluene
P2VP/toluene
0.46
0.57
PMMA/CCl4
P2VP/CCl4
0.44
0.53
χ12
b-PMMA. Nanocomposites containing 2 and 4 wt % PMMA-MN with respect to P2VP-b-PMMA were deposited on the glass wafer. The samples were exposed to saturated vapors of CCl4 in a closed vessel kept at room temperature for several times. After solvent treatment, the samples were investigated by AFM and MFM. Techniques. Atomic force and magnetic force microscopy images were recorded in tapping mode (TM) at room temperature by using a scanning probe microscope (Nanoscope IVA, from Digital Instruments) and Co/Cr-coated high-moment MESP tips. Every single-beam cantilever (225 µm length) silicon nitride probes having a tip nominal radius of curvature of 25 nm and resonance frequency around 75 kHz were used. Scan rates ranged from 0.8 to 1.9 Hz s-1. Typical lift height was on the order of 60 nm and was adjusted to give better contrast. Analysis of AFM images was performed with WSxM software (Nanotech Electronica). Results and Discussion In the first step, the solubility parameters were analyzed to estimate the affinity between each block of polymer and solvent. The miscibility between polymer and solvent is governed by polymer-solvent interaction parameter (χ12). χ12 were calculated for polymer blocks and solvents, as shown in Table 1.30-32 Results agree with the literature survey33-43 showing that CCl4 is a better solvent for PMMA than for P2VP.
Figure 1. Schematic model of time evolution of nanostructure formation of thin symmetric P2VP-b-PMMA film. P2VPsblack regions, bright regionssPMMA. (a) Cross section and plane view of thin film exposed to carbon tetrachloride vapor for a certain time; PMMA begins to go to the surface. (b) Cross section and plane view of the lamellar formation after increment of exposure to carbon tetrachloride vapor. (c) After a long time of exposure, PMMA is in the free surface. PMMA block takes the stretched conformation, and P2VP block takes the collapsed conformation.
Figure 2. TM-AFM topography (left) and phase (right) images of thin film of neat P2VP-b-PMMA (a) before exposure at CCl4-saturated atmosphere, (b) after 3, (c) 9, and (d) 14 h of exposure at CCl4-saturated atmosphere.
After analysis of solubility and interaction parameters, the effect of exposure time of P2VP-b-PMMA films to saturated CCl4 vapors was analyzed by AFM (Figure 2). Figure 2a shows AFM images of thin film of P2VP-b-PMMA before CCl4 vapor treatment. A soft microphase separation can be observed when film was analyzed by AFM. Due to toluene being a good solvent to PMMA and bad solvent to P2VP, a weak hexagonal morphology can be observed after deposition. In AFM images, it is expected that P2VP appears dark and PMMA bright. In order to improve microphase separation, the film was exposed to saturated CCl4 atmosphere. In Figure 2b, the AFM image of P2VP-b-PMMA exposed to saturated CCl4 for 3 h is shown, where a well-defined hexagonal morphology can be observed. As shown in Figure 2c, increasing the exposure time of P2VPb-PMMA thin film to saturated CCl4 vapors caused a change in the film structure. After 9 h of treatment in CCl4-saturated atmosphere, a quasi-lamellar morphology normal to the substrate can be observed. This change in the microstructure can be attributed to CCl4 being a bad solvent of P2VP block. It provokes migration of P2VP from the free surface to the substrate. It means that PMMA migrates to the free surface due
Nanostructuration of P2VP-b-PMMA Nanocomposites
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Figure 3. TM-AFM topography (left), phase (right) images, and corresponding height and phase profiles for (a) P2VP-b-PMMA/2 wt % PMMA-MN and (b) neat block copolymer.
Figure 4. TM-AFM topography (left), phase (right) images, and corresponding height and phase profiles of P2VP-b-PMMA containing 4 wt % PMMA-MN.
to the higher affinity between CCl4 and PMMA than that of CCl4 and P2VP. When the exposure time to CCl4 was increased, a new change in the morphology was observed. Figure 2d shows the AFM image of a thin film of P2VP-b-PMMA after CCl4 vapor treatment for 14 h. PMMA migrated to the surface. When the film was analyzed by AFM, P2VP appeared as the discontinuous phase and the continuous phase was PMMA. This microphase separation was generated due to the films having to adopt their thermodynamically more stable state. The nanostructure shown in Figure 2d is the last step before reaching the thermodynamically stable state that finally would lead to PMMA completely filling the top surface of the film, as shown in Figure 1, due to CCl4 being not a good solvent to P2VP. These results suggest that a better interaction between solvent and PMMA did exist that produced the diffusion of polymer chains when these were enough swollen by solvent, thus resulting in changes of morphology. The selectivity of solvent to one of the blocks provoked swelling of microdomains, thus changing their effective volumes. During solvent evaporation the changes in effective volumes induced the variations in
microphase separation. Diffusion of adsorbed solvent from the surface plays a control role in the definition of morphology.12 Figure 3a shows AFM images of nanocomposite containing 2 wt % PMMA-MN exposed to saturated CCl4 atmosphere for 3 h. It shows the same hexagonal nanostructure as that of neat P2VP-b-PMMA and a good distribution of nanoparticles inside PMMA domains. To verify those indications, phase profiles of pure block copolymer and nanocomposites were also studied. In the phase and height profiles of neat block copolymer (Figure 3b) a nearly constant intensity of phase variation between 4 ° and -4 ° indicative of both components of the block copolymer has been observed. The size of cylinders was around 50 nm. In nanocomposites, there is a correspondence between sizes of cylinders and intensity of phase. When the height of phase intensity was analyzed, an increase on the size of cylinders was seen, the size of cylinders being around 61 nm. This size variation can be attributed to confinement of nanoparticles inside PMMA domains. For the low-intensity phase profile, the size of cylinders was around 50 nm, similar to that for the neat block copolymer. In the phase profile of nanocomposites, a variation in intensity across the profile can
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Figure 5. TM-AFM topography (left) and phase (right) images of thin film of (a) P2VP-b-PMMA/2 wt % PMMA-MN and (b) neat P2VP-bPMMA after 3 h of exposure at CCl4-saturated atmosphere and area after magnetic scan inside the white envelope. Image size: 3 µm × 3 µm.
be observed. Indeed, the higher intensity values can be attributed to sites where the nanoparticles were inside of the PMMA phase. In Figure 4, the AFM image of the nanocomposite of P2VPb-PMMA containing 4 wt % PMMA-MN, exposed to saturated CCl4 atmosphere for 3 h, is shown, where a hexagonal morphology can be observed. Some agglomeration onto the surface can be observed. The presence of agglomerates can be due to that the graft density of nanoparticles, around 0.1 PMMA chain/nm2, was not enough to prevent flocculation of nanoparticles. In order to analyze the effect of PMMA-MN on nanostructure evolution of P2VP-b-PMMA/2 wt % PMMA-MN nanocomposite after exposure at CCl4 vapors, a scan whose size is 2 µm × 1 µm was performed in magnetic mode to confirm the magnetic properties of the incorporated nanoparticles. After this measurement, a scan of 3 µm × 3 µm in AFM mode was carried out to both the nanocomposite and neat P2VP-b-PMMA. In Figure 5a, the AFM image of the nanocomposite with positional magnetic Fe3O4 nanoparticles is shown. The area within the white envelope shows that hexagonal morphology vanished after applying the magnetic field to the nanocomposites’ surface by the magnetized MFM tip. In the case of neat P2VP-b-PMMA, the same procedure was also performed. Figure 5b shows the AFM image of block copolymer where the morphology was retained after magnetic scanning. Disappearance of the morphology in the nanocomposite can be attributed to the magnetic field generated by the interactions between magnetic iron oxide nanoparticles and the magnetized tip. These magnetic interactions seem to be enough to move block copolymer chains as a consequence of the weak physicochemical interactions (graft
density in PMMA-MN was 0.1 chains/nm2 19) between nanoparticles and block copolymer PMMA domains. Work is in progress to determine the threshold in grafting density to avoid the disappearance of the microstructural features of the block copolymer matrix in nanocomposites. Conclusions In this study, phase behavior of a thin film of symmetric P2VP-b-PMMA and its nanocomposites containing PMMAfunctionalized magnetic nanoparticles, cast by spin-coating, has been investigated after exposure to a selective solvent CCl4. In the block copolymer, selectivity of copolymer blocks to solvent along with exposure to various treatment times leads to hexagonal and lamellar microphase-separated morphologies. Preferential segregation of PMMA to the free surface provokes different nanostructures through the time of exposure to saturated vapor atmosphere. The different morphological features generated only by playing with the exposure time open new ways to use this block copolymer such as matrix to generate new nanocomposites without modifying the composition ratio between blocks. PMMA modification onto the surface of nanoparticles via ATRP allows their segregation inside PMMA domains. Even if the morphology of the nanocomposites did not change if compared with that for the neat block copolymer, size increase of PMMA domains was generated due to segregation of nanoparticles. Finally, it should be pointed out that the magnetic field originated by interactions between the magnetized AFM tip and
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