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Nanoreactors based on self-assembled amphiphilic diblock copolymers for the preparation of ZnO nanoparticles
European Polymer Journal xxx (2013) xxx–xxx
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
Nanoreactors based on self-assembled amphiphilic diblock opolymers for the preparation of ZnO nanoparticles q Guadalupe del C. Pizarro a,⇑, Oscar G. Marambio a, C.M. González Henríquez a, M. Sarabia Vallejos b, Kurt E. Geckeler c,d a
Departamento de Química, Universidad Tecnológica Metropolitana, J.P. Alessandri 1242, Santiago, Chile Facultad de Física, Pontificia Universidad Católica de Chile, Vicuña Mackenna 4860, Santiago 7820436, Chile Department of Nanobio Materials and Electronics, World-Class University (WCU), Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Republic of Korea d Laboratory of Applied Macromolecular Chemistry, School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 22 April 2013 Received in revised form 26 July 2013 Accepted 4 August 2013 Available online xxxx Keywords: Amphiphilic block copolymers ZnO Nanoparticles Nanoreactor Optical Thermal
a b s t r a c t A self-assembled diblock copolymer containing styrene (S), methyl methacrylate and a certain percentage of hydrophilic segment of poly(methacrylic acid) (i.e., poly(styrene)-blockpoly(methyl methacrylate/methacrylic acid) was synthesized via the ATRP method in two steps. This was followed by a partial hydrolysis of the methyl ester linkages of the methyl methacrylate block under acidic conditions. The resultant block copolymer had a narrow molecular weight dispersity (Ð < 1.3) and was characterized using FT-IR and Raman spectroscopy as well as size exclusion chromatography. The block copolymer was used as a nanoreactor for inorganic nanoparticles (ZnO). The incorporation of a single source precursor, such as ZnCl2, into the self-assembled copolymer matrix and the conversion into ZnO nanostructures was carried out in the liquid phase using wet chemical processing techniques. We report the synthesis and characterization of nanocomposites with dual characteristics due to the functionalities incorporated into the matrix. The optical properties were determined by UV–Vis and fluorescence, the crystallinity was studied using X-ray diffraction, and the thermal stability and studies of the cyclic voltammetry were obtained by thermogravimetric analyzes and potentiodynamic electrochemical measurements, respectively. The structural, topographical and morphological characterizations of the ZnO composite in relation to the precursor block copolymer were analyzed via scanning electron microscopy, transmission electron microscopy and atomic force microscopy. According to the results, the block copolymer shows good transparency in the visible region (when containing 20, 30 and 50 wt.% nanoparticles) and was able to absorb UV irradiation below 350 nm, indicating good UV-screening effects. The thermo gravimetric analysis data showed that the block copolymer composite is more thermally stable compared to the pure block copolymer. The self-assembled nanoscale morphology of the diblock copolymer results in the formation of uniformly distributed spherical nano particles within the polymer matrix. Ó 2013 The Authors. Published by Elsevier Ltd. All rights reserved.
1. Introduction q
This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. ⇑ Corresponding author. E-mail address: [email protected] (G. del C. Pizarro).
In recent decades, there has been increasing demand for new materials in diverse areas in the technology industry. Organic materials for photovoltaic devices have been investigated because of the increasing demand of energy
0014-3057/$ - see front matter Ó 2013 The Authors. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2013.08.008
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around the world [1]. The confining of diblock copolymers in topographically or chemically patterned structures has frequently been employed to control the order and orientation of the nanostructures of the copolymer [2–4]. The synthesis of zinc oxide (ZnO) semiconductor nanoparticles within a microphase-separated diblock copolymer has also been reported [5–7]. Very small ZnO nanoparticles are well-known as multifunctional inorganic fillers that have unique properties, such as strong UV absorption in combination with good transparency in the visible range, a lowdielectric constant, and a large electromechanical coupling coefficient [8]. ZnO can be synthesized by various synthetic paths in various shapes and particle sizes [9–11]. Advanced applications require nanoparticles with a narrow particle size distribution and defined particle shape. ZnO is a material for semiconductor device applications because it has a direct and wide band gap of 3.37 eV, making it an excellent candidate for use in UV-light-emitting diodes (LEDs), lasers, and transparent transistors [12,13]. A large number of research in recent years has focused on the development of polymer-nanocomposite materials. Demir et al. [14] prepared ZnO/PMMA composite films with strong UV absorption and light transmittance in the visible range by the polymerization of particle dispersions in the monomer. Chae and Kim [15] successfully fabricated PS/ZnO nanocomposite films, which exhibited UV-absorbance without losing transparency at a low ZnO content by solution mixing. However, these films showed a transparency loss at a high concentration due to the aggregation of individual ZnO nanoparticles. ZnO has been incorporated into many other polymers, such as poly(hydroxyethylmethacrylate) [16], poly(amic acid) [17], polyimide [18], nylon [19], and PS-PMMA [20]. It is important to note that the ZnO is considered to be an interesting material for photovoltaic applications because of its unique combination of optical and semiconducting properties [21]. The goal of our work is to form self-assembled ZnO nanoparticles using the poly(S699-block-MMA232/MAA58) diblock copolymer matrix as a nanoreactor of this inorganic nanoparticles [22,23], studying the optical, thermal, topographical and morphological properties of the composites as well as the effect of the nanoparticles (20%, 30% and 50%) on the polymeric matrix. Additionally, with the goal of identifying semiconductor applications, potentiodynamic electrochemical measurements were conducted. The advantage in the use of this block-copolymer is focus in the easy synthetic route and low cost of it, beside we can obtain nanoparticles uniformly distributed in the polymeric matrix, also with a similar morphological shapes. These reasons are supported in the following comments, the diblock copolymer was synthesized via normal ATRP in two steps, followed by partial hydrolysis of the methyl ester linkages of the MMA block under acidic conditions and had a narrow molecular weight dispersity (Ð < 1.28). The amphiphilic diblock copolymer consisting of a majority polymer (styrene) and a minority polymer (methyl methacrylate/methacrylic acid) with a block repeat unit ratio of 699/264, this allows a spherical microphase separation and hence a spherical morphology for the metal oxide nanoparticles, achieved at room temperature in the liquid phase, using ZnCl2 as precursor dopant.
The phase separation occurs on the nanometer scale, as determined by the dimension of the blocks. If the polymer chains have narrow size distribution, the phase separation should produce ordered nanostructures. As result, the amphiphilic diblock copolymer, poly(styrene)-block-poly(methyl methacrylate/methacrylic acid), self-assembled materials, will produce spherical micelles in a continuous phase. The self-assembled nanoscale morphology of the composites results in the formation of uniformly distributed spherical nanoparticles within the polymer matrix, and the amount of ZnO nanoparticles cause changes in the copolymer behavior. This does not mean that the nanoparticles have similar size. 2. Experimental 2.1. Reagents Styrene and methyl methacrylate were purchased from Sigma–Aldrich Chemicals, Germany and Merck-Schuchardt OHG Chemicals, Germany, respectively. Both compounds were distilled under a reduced pressure before being utilized. Benzoyl peroxide (BPO), copper (I) bromide (CuBr), and N,N-bipyridine (Bpy) reagents were purchased from Sigma–Aldrich Chemicals, Merck-Schuchardt, Germany. 2.2. Measurements The number-average (Mn) and weight-average (Mw) for the molecular weight and the molecular weight distribution (dispersity, Ð = Mw/Mn) of the copolymer were determined using size exclusion chromatography (SEC) using a WATERS 600E instrument equipped with UV and RI detectors and using THF as the solvent (flow rate: 1.0 mL/min). The samples were measured at 30 °C with a concentration of 6 mg/mL, and the calibration was performed using polystyrene. FT-IR spectra were recorded on a Bruker Vector 22 spectrometer (Bruker Optics Inc., Ettlingen, Germany). The structural and vibrational properties of the pure block copolymer and the respective composites (20%, 30% and 50% of ZnO) were characterized by Raman spectroscopy with a LabRam 010 instrument from ISA equipped with a 5.5 mW, 632.8 nm He–Ne laser without a filter. The Raman microscope uses a back-scattering geometry, where the incident beam is linearly polarized at a 500:1 ratio. The microscope objective used for the Raman analysis was an Olympus Mplan 100 (numerical aperture 0.9). The absorption spectra of the pure block copolymer and the composites were recorded at 25 °C between 250 and 400 nm in a Shimadzu UV-160 spectrophotometer. The photo luminescence (PL) spectra of the solid samples were recorded at room temperature with a Perkin Elmer spectrofluorometer model L55, and the spectra were obtained under similar conditions, using an excitation wavelength of 320 nm. The XRD patterns were recorded by employing a Philips X0 PERT MPD diffractometer (Cu Ka radiation: k = 0.154056 nm at 40 kV and 30 mA) over the 2h range of 1.7–80° at a scan rate of 0.05°/min.
Please cite this article in press as: Pizarro GdC et al. Nanoreactors based on self-assembled amphiphilic diblock opolymers for the preparation of ZnO nanoparticles. Eur Polym J (2013), http://dx.doi.org/10.1016/j.eurpolymj.2013.08.008
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perature (TDT2) of approximately 379 °C. These results have been mentioned in a previous work by our research group [24]. 2.4. Formation of the ZnO nanoparticles The pure block copolymer was dissolved in THF at a concentration of 0.1%. Later, the metal ion ZnCl2 (20–50% in weight) was incorporated. After this, the solution was stirred for 48 h so that the system could achieve equilibrium and the Zn2+ associated with the functional groups could participate in the finished process. After this stage, the solution was left in another solution of NH4OH for 24 h to form Zn(OH)2. This step was followed by washing the solution in H2O to remove water salts and to decompose the unstable zinc hydroxides to ZnO. The ZnO remained insoluble in NH4OH, while it solubilized in the alkali bases NaOH and KOH. The conversion of Zn(OH)2 to ZnO–diblock copolymer nanoparticles was successfully achieved after drying by heating at 30 °C under vacuum for 48 h; see Scheme 2. The substitution of Cl by O was found to be a highly preferential process, whereby only one approach using a weak base (NH4OH) succeeded in effectively replacing Cl with O to result in spherical ZnO nanoparticles [15]. It is necessary to mention that the amount of ZnCl2 (20– 50% weight) did not represent 100% of the ZnO of the copolymer. However, the percentage composition of the nanoparticles were studied by SEM-EDS (five measurements for a sample), which showed values near the percentage of the precursor added. 3. Results and discussion 3.1. Characterization of the block copolymer composites The block copolymer composite structures were examined using FT-IR and Raman to verify the precursor association with the functional groups of the minority block. The FT-IR analysis was performed to examine bond stretching in the carboxyl groups, which would indicate that the metal cation in ZnCl2 is capable of interacting with the block copolymer and subsequently forming ZnO nanoparticles. The Raman spectra were obtained to characterize the functional group and the possible interaction with Zn2+ by determining certain characteristic peaks.
2.3. Block copolymer by ATRP 3.2. FTIR and Raman spectroscopic analysis The poly(S699-block-MMA232/MAA58) was prepared according to Scheme 1. The copolymer was dried until a constant weight was reached (yield 68.8%, Mn = 10.2 104 Daltons and Ð = Mw/Mn = 1.30). The FT-IR spectrum for the block copolymer shown in Fig. 1a exhibited the following absorption bands: at about 3424 cm1 AOH group vibration from ACOOH is observed; at 2921 cm1 [t(CH, CH2)]; at 1723 cm1 [t(AC@O)]; and 753 and 697 cm1 [t(aromatic ring of styrene)]. DSC showed a shift in the base-line; the transition corresponds to the glass transition temperature (Tg = 65 °C); TGA exhibited a two-step degradation with an extrapolated thermal decomposition tem-
The FT-IR spectra verified that the metal ion was associated with the minority block (MMA/MAA) of the block copolymer. The carbonyl band t(C@O group) at 1723 cm1 was replaced by two absorption bands at 1719 cm1 and 1631 cm1 with a lower intensity, which could be attributed to the stronger association of Zn2+ and the carboxyl groups (MAA) on the second block of the copolymer due to asymmetric and symmetric stretching (Scheme 2). Nevertheless, this peak also disappears and later appears as a new peak at 1600 cm1 due to the formation and interaction of the ZnO (20%) nanoparticle
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The study of the thermal stability was carried out with a Mettler Toledo STARe System with a thermogravimetric analyzer (TGA) at a heating rate of 10 °C/min and under nitrogen atmosphere (flow rate = 150 mL/min). The cyclic voltammogram (CV) of the block copolymer and their composites on the glassy carbon electrodes was obtained by an electrochemical analyzer (CH-Instruments, model CHI604C) using a platinum counter electrode and an Ag/AgCl reference electrode immersed in the electrolyte (tetrabutylammoniumperchlorate in DMF, 0.1 M) at a scan rate of 100 mV/s. In all of the experiments, pro analysis grade chemicals were used. Before each experiment, the working electrode (glassy carbon) was polished with alumina slurry (particle size 0.3 lm) on soft leather and subsequently washed with deionized water. Before the experiments, the solutions were purged with high-purity nitrogen at atmospheric pressure. The morphology of the block copolymer and composites was examined using conventional scanning electron microscopy (SEM) (JEOL, JSM 6380 LV) and transmission electron microscopy (TEM) (JEOL, JEM 1200 EX, operating at 120 kV, with a point resolution of approximately 4 Å). These measurements were performed on dispersed samples. TEM images were taken by placing a drop of the powder nanocomposite in THF onto a carbon-coated copper grid. For the preparation of the stable thin films, the copolymer solution was spin-coated (KW-4A, CheMat Technology, Inc., Northridge, United States). The surface characterization of the block copolymer and the composites (20%, 30% and 50% ZnO) was carried out by employing home-made atomic force microscopy (AFM) to determine the height profiles and their surface properties. This equipment was designed by Prof. Dr. Guido Tarrach at Pontificia Universidad Católica de Chile and was based on the hardware and software developed at the Guentherodt group at Basel University, Switzerland. This AFM could be operated in contact, non-contact and intermittent contact modes. The tips used for the measurements were pyramidal shaped with a height of 2 lm, a length of 450 lm, a width of 50 lm, a resonance frequency of 13 kHz and a force constant of 0.2 N/m. To obtain the highest lateral resolution, the image was taken with 256 256 pixels with a scan rate of one line per 2 s. The compounds (copolymer and composites) were dissolved in THF and were cast on glass slides using the spin coater with a rotation velocity of 500 rpm for approximately 30 s and 2000 rpm for 30 s.
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Scheme 1. Structure of the poly(S699-block-MMA232/MAA58) block copolymer.
Fig. 1. FT-IR spectrum of (a) block copolymer; (b) block copolymer–Zn2+; and (c) block copolymer–ZnO 20%.
associated with the functional group, resulting in the formation of local chemical cross-links between chains; see Fig. 1a–c. In the Raman spectra, the band that appears at 1601 cm1 has a lower intensity in comparison with the IR spectra, and it is related to the C@C quadrant stretching of the aromatic ring as the pendent group of the copolymer. Furthermore, it may have little interaction with the CH in the plane of bending, and the band at 1454 cm1 could be attributed to the CH2 scissoring mode (semicircle stretching) and could possibly overlap with CH3 antisymmetric bending. Both signals are characteristics of the n-alkanes. In addition, the most intense band appears at 996 cm1 and one moderately strong band at 1081 cm1. These two peaks are related to ring bending and stretching vibration (‘‘breathing mode’’), along with an ‘‘in plane’’ ring CAH deformation (‘‘wagging mode’’), both of which are signs of benzenes; see Fig. 2. The carbonyl band near 1700 cm1 is strong in the IR (Fig. 1) and weaker in the Raman spectra (1716 cm1, Fig. 2). When the copolymer contains 20% ZnO nanoparticles in its structure, the signals above-mentioned are not observed. However, there appear three bands of medium– weak intensity in the Raman spectra that could be related to a vibration impurity mode at 2630 cm1, 2360 cm1 and 2119 cm1, which are conserved for composite 30% ZnO. Alternatively, the copolymer with 30% ZnO shows four bands characteristic of the copolymer, such as 1603 cm1
Scheme 2. The synthesis of ZnO using the nanoreactor based on the block copolymer.
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(C@C ‘‘quadrant stretching’’ of the benzene derivate), 1187 cm1 (bare ZnO [25]), 1083 cm1 and 996 cm1 (ring ‘‘breathing’’ mode). Lastly, when the copolymer has a large amount of ZnO nanoparticles (50%) in the system, the vibration mode is clearly affected. Thus, some bands appear that are not observed in the block copolymer, for example, in the 3053– 2893 cm1 range (antisymmetric CH3, anti symmetric CH2 and aromatic CH stretch). There is also an intensification of the Raman shifts in the bending modes. This behavior is reasonable because the ZnO nanoparticles into the copolymer increase the order and rigidity of the system.
Fig. 3. Optical properties: (a) UV–Vis absorption spectra of block copolymer and block copolymer composite (20% and 50% of ZnO) and (b) photoluminescence spectrum of block copolymer and block copolymer composite (50% and 20%) and commercial ZnO nanoparticles.
3.3. Optical properties The UV–Visible spectra of the block copolymer exhibited a maximum in the range 230–275 nm and its composites showed a high transparency that could be attributed to a lower fraction of the poly(MMA232/MAA58) segment due to the low percentage of the inter- or intra-molecular interaction of the acid functional group. All of the samples have absorptions below 350 nm, and the percentage of absorption increases in proportion to the amount of ZnO nanoparticles. Therefore, the resulting block copolymer composite can block UV rays, but is transparent to visible light (Fig. 3a). Additionally, the composites showed signals that not were observed for the copolymer pure, such as two bands at 378 nm and 423 nm for 20% and 50% ZnO, respectively. The emission spectra showed a blue shift attributed to a decreasing nanoparticle size. The photoluminescence spectrum of the pure block copolymer exhibited only a weak UV emission peak centered at 378 nm because the aromatic segments were unconjugated (Fig. 3b). Alternatively, the commercial ZnO showed three bands, one strong at 385 nm and two weak at 425 nm and 490 nm. The composite nanoparticles exhibited the same three bands but with a strong decrease in the first peak (385 nm). This behavior could be related to the interaction between the organic copolymer segment
and the semiconductor nanoparticles. According to these results, the incorporation of ZnO nanoparticles in the copolymer matrix is reflected in the system emission variation from monochromatic light to two-tone color lights, which altered the emission spectra.
3.4. XRD pattern The XRD patterns of the samples are shown in Fig. 4. These data indicate that all the peaks shown by the commercial nanoparticles are well indexed to the pure wurtzite crystalline phase of ZnO [26]. According to the pure ZnO nanoparticles, the average crystalline size was calculated as 35–50 nm using Sherre’s equation: D = 0.9 k/b cos h, where D is the crystallite size, k is a wavelength of the radiation, h is Bragg’s angle and b is the full width at half maximum. The block copolymer did not show a profile signal, most likely due to the system amorphicity (Fig. 4b). Alternatively, the formation of composite 50% ZnO showed multiple peaks that are similar at the Wurtzite crystalline phase of ZnO and one broad peak at 12.3° that was attributed to the interaction between the organic matrix and the nanoparticles (Fig. 4c).
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Fig. 2. Block copolymer Raman spectrum and block copolymer composite (10%, 20% and 50% of ZnO).
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Fig. 4. X-ray diffraction pattern of (a) ZnO; (b) block copolymer; and (c) block copolymer–ZnO 50%.
Fig. 6. Cyclic voltammetric response of a glassy carbon electrode cycled in a solution containing block copolymer and block copolymer–ZnO (20%, 30% and 50%) composites: (a) in the presence of oxygen; and (b) inert atmosphere. All the analyzes were conducted with a scan rate of 100 mV/s.
Fig. 5. TGA and DTG curves of block copolymer and block copolymer–ZnO 50% using a heating rate of 10 °C/min.
3.5. Thermal behavior The thermal behavior of the pure block copolymer and composites were examined using TGA under nitrogen at a heating rate of 10 °C/min. The TGA and DTG curves show that the block copolymer and its composites with ZnO have one main weight loss regions that are plotted in Fig. 5. The plot shows that the copolymer and composites degraded continuously at one stage process. The copolymer and composite were stable up to 389 °C and 386 °C, respectively. The presence of nanoparticles in the composites decreases slightly the degradation temperatures. Besides, the DTG curves showed the maximum decomposition of the material near at 431 and 427 °C for the same compounds. The presence of nanoparticles in the composites decreases slightly the degrada-
tion temperatures. At 161 °C and 166 °C, respectively, the weight loss is not significant, which was attributed to solvent loss or a small amount of monomer residue in the samples. The block copolymer had a weight loss of 75% at 500 °C, and for the composite, there was a loss of 65% at the same temperature. This effect may be due to the formation and rearrangement of the different interactions of the block copolymer with ZnO nanoparticles. The block copolymer presented a slightly higher thermal degradation temperature (TDT) than the composite, which can be principally attributed to the fact that the block copolymer composites presents in its structure a percentage of the ZnO nanoparticles, which can act like catalysts for the degradation reaction. 3.6. Electrochemical studies The electrochemical characterizations of the block copolymers and composites were carried out using CV to determine the redox potentials. The electrochemical behavior of the samples was tested under a potential range from 1.0 to +1.0 V and 1.5 to 1.5 V (at a scan rate of 100 mV/s). The CV characterization of the samples is shown in Fig. 6a and b. These voltammograms show that
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Fig. 8. SEM micrograph of (a) block copolymer–Zn2+ and (b–d) block copolymer–ZnO 50%.
the block copolymer does not exhibit an oxide-reduction process under the conditions of CV analysis. The range of electrochemical inertia of the copolymer can be extended to 1.0 V. This copolymer shows electrochemical stability, providing an ideal platform for fabricating novel functional nanostructured materials for potential applications in advanced technologies, such as nanomaterials, nanocomposites, and drug delivery and information storage. These voltammograms also show that the block copolymer composites exhibited an oxide-reduction process under the conditions of CV analysis (see Fig. 6a). The catalyst effect was observed at a low potential (electrocatalysis) (approximately from 0.7 to 0.5 V), and this reaction was catalyzed by the nanoparticle (ZnO) contained in the matrix. The observed oxide-reduction process corresponds to an oxygen reduction and peroxide oxidation in the presence of ZnO nanoparticles under the tested conditions. In inert atmosphere, block copolymer electrochemical inertia was observed due to the absence of oxygen in the medium. Furthermore, the block copolymer composites showed redox processes to negative potentials.
3.7. Self-assembled ZnO nanostructures The TEM images of the block copolymer composites in Fig. 7a–c give additional information regarding the size and size dispersion of the nanoparticles within the copolymer matrix forming spherical aggregates in the THF solution. The ZnO nanoparticles appear as dark-white spots, which are clearly observed in the matrix films, resulting in nanodomains of interconnected networks of spherical aggregates [13–15]. The average size range of the nanoparticles shows that the micelles tend to self-assemble into much larger spherical aggregates that predominate (Fig. 7a–c) due to the aggregation effect of the nanoparticles. Lastly, the size and stability of these spherical aggregates depends on the inorganic nanoparticle concentration [15]. 3.8. Morphological analysis A set of microphotographs obtained by the SEM (secondary electrons detector) show the morphology of the
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Fig. 7. TEM image of the block copolymer composites at different concentrations of nanoparticles: (a) 20% ZnO; (b) 30% ZnO; and (c) 50% ZnO.
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Fig. 9. AFM micrographics of the block copolymer and composites at different concentrations of ZnO (20%, 30%, and 50%).
poly(S699-block-MMA232/MAA58)-Zn2+ and the composite 50% ZnO (Fig. 8a–d). Fig. 8a shows an ordered structure, in which nano-islets of variable sizes coexist. However, the copolymer nanoparticle microphotographs (Fig. 8b–d) in an appropriate solvent (THF) showed larger spherical aggregates. These results indicate a good interaction between the copolymer block segments and the ZnO particles, producing nanoparticles of the copolymer block that contain occluded ZnO. These observations were confirmed using an FT-IR measurement and confirmed by Raman spectroscopy. In both cases, a highly ordered inorganic–organic nanoparticle was formed. The equilibrium structures were determined by the thermodynamics of the self-assembly process and the inter- and intra-aggregate forces. After comparing the size and distribution of the ZnO nanoparticles-polymer matrix from SEM and TEM images, it was observed that the self-assembled nanostructured films are suitable for the construction of devices based on inorganic nanoparticles and organic polymers. AFM images were obtained through the intermittent contact method (Fig. 9a–d). The micrographs show the morphological and topographical features of the pure copolymer block and the copolymer-ZnO (20%, 30% and 50%) nanoparticle, respectively. According to these results, the topographies of the pure block copolymer and the composite 30% and 50% ZnO thin
layer surfaces only show agglomerations of the cluster type. This behavior is completely random and does not depend on the amount of nanoparticles in the system. However, the composite 20% ZnO exhibited well-defined pores in the surface with an average height of 543 nm and an average pore diameter of 2.67 lm. This behavior is most likely due to the self-assembling property of the block copolymer with amphiphilic characteristics when interacting with the nanoparticles of ZnO. This result does not indicate that the surface of the pure copolymer and its composites of 30% and 50% ZnO do not have a porous surface, only that the probability of finding porosity in these composites is lower than in the 20% composite.
4. Conclusions The insertion of ZnO nanoparticles into the block copolymer system was studied and confirmed through FT-IR and Raman spectroscopy, optical methods, X-ray diffraction, and thermal and morphological characterization techniques. The self-assembled nanoscale morphology of the composites results in the formation of uniformly distributed spherical nanoparticles within the polymer matrix, and the amount of ZnO nanoparticles cause changes in the copolymer behavior. The micelle-like supports for
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ZnO particles based on the self-assembled block copolymer (content from 20 to 50 wt.%), show good transparency in the visible region and excellent luminescent properties. Block copolymer composites with a high ZnO content are able to absorb UV irradiation below 350 nm, indicating that these composite films exhibit good UV screening effects. Raman spectroscopy showed clear vibrational bands of organic segments. Multiple peaks obtained by X-ray diffraction correspond to Wurtzite crystalline phase of ZnO and one broad peak was attributed to the interaction between the organic matrix and the nanoparticles. Thermogravimetric analyzes show that the ZnO nanoparticles were successfully formed into the polymer matrix and that this diblock copolymer composite has a good response to thermal variations; it is therefore stable against thermodynamic changes. The thermal stability of the pure block copolymer and composite-50% ZnO were found to be stable up to 386 °C. However, the composite exhibited a higher stability due to the presence of ZnO in the polymeric structure. The SEM images show that poly(St699-block-MMA211/ MAA53)-ZnO principally consists of auto-assembled spherical micellar aggregations, resulting in nanodomains of interconnected networks of spherical aggregates over the polymer surface and possibly into the polymer. Furthermore, for the sample corresponding to composite 20% of ZnO it was possible to determine very precisely the average height and diameter of the surface pore, 543 nm and 2.67 lm, respectively. Lastly, a certain degree of porosity that depends on the percentage of ZnO nanoparticles was found. The TEM studies indicate that the ZnO nanoparticles were uniformly dispersed on the polymer. The TEM images of the ZnO composite gave additional information regarding the size and size dispersion of the nanoparticles within the copolymer matrix formed. These results indicated good stability between the block copolymer containing ZnO.
Acknowledgments The authors acknowledge the financial assistance of this work by the Fondo Nacional de Investigación Científica y Tecnológica, FONDECYT Grants 1110836 and 11121281. The WCU Program funded by MEST (R31-10026). Thanks to Dra. Maria Jesus Aguirre (USACH) by Cyclic Voltammetry measurements.
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Macromolecular Nanotechnology
Nanoreactors based on self-assembled amphiphilic diblock opolymers for the preparation of ZnO nanoparticles q Guadalupe del C. Pizarro a,⇑, Oscar G. Marambio a, C.M. González Henríquez a, M. Sarabia Vallejos b, Kurt E. Geckeler c,d a
Departamento de Química, Universidad Tecnológica Metropolitana, J.P. Alessandri 1242, Santiago, Chile Facultad de Física, Pontificia Universidad Católica de Chile, Vicuña Mackenna 4860, Santiago 7820436, Chile Department of Nanobio Materials and Electronics, World-Class University (WCU), Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Republic of Korea d Laboratory of Applied Macromolecular Chemistry, School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 22 April 2013 Received in revised form 26 July 2013 Accepted 4 August 2013 Available online xxxx Keywords: Amphiphilic block copolymers ZnO Nanoparticles Nanoreactor Optical Thermal
a b s t r a c t A self-assembled diblock copolymer containing styrene (S), methyl methacrylate and a certain percentage of hydrophilic segment of poly(methacrylic acid) (i.e., poly(styrene)-blockpoly(methyl methacrylate/methacrylic acid) was synthesized via the ATRP method in two steps. This was followed by a partial hydrolysis of the methyl ester linkages of the methyl methacrylate block under acidic conditions. The resultant block copolymer had a narrow molecular weight dispersity (Ð < 1.3) and was characterized using FT-IR and Raman spectroscopy as well as size exclusion chromatography. The block copolymer was used as a nanoreactor for inorganic nanoparticles (ZnO). The incorporation of a single source precursor, such as ZnCl2, into the self-assembled copolymer matrix and the conversion into ZnO nanostructures was carried out in the liquid phase using wet chemical processing techniques. We report the synthesis and characterization of nanocomposites with dual characteristics due to the functionalities incorporated into the matrix. The optical properties were determined by UV–Vis and fluorescence, the crystallinity was studied using X-ray diffraction, and the thermal stability and studies of the cyclic voltammetry were obtained by thermogravimetric analyzes and potentiodynamic electrochemical measurements, respectively. The structural, topographical and morphological characterizations of the ZnO composite in relation to the precursor block copolymer were analyzed via scanning electron microscopy, transmission electron microscopy and atomic force microscopy. According to the results, the block copolymer shows good transparency in the visible region (when containing 20, 30 and 50 wt.% nanoparticles) and was able to absorb UV irradiation below 350 nm, indicating good UV-screening effects. The thermo gravimetric analysis data showed that the block copolymer composite is more thermally stable compared to the pure block copolymer. The self-assembled nanoscale morphology of the diblock copolymer results in the formation of uniformly distributed spherical nano particles within the polymer matrix. Ó 2013 The Authors. Published by Elsevier Ltd. All rights reserved.
1. Introduction q
This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. ⇑ Corresponding author. E-mail address: [email protected] (G. del C. Pizarro).
In recent decades, there has been increasing demand for new materials in diverse areas in the technology industry. Organic materials for photovoltaic devices have been investigated because of the increasing demand of energy
0014-3057/$ - see front matter Ó 2013 The Authors. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2013.08.008
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around the world [1]. The confining of diblock copolymers in topographically or chemically patterned structures has frequently been employed to control the order and orientation of the nanostructures of the copolymer [2–4]. The synthesis of zinc oxide (ZnO) semiconductor nanoparticles within a microphase-separated diblock copolymer has also been reported [5–7]. Very small ZnO nanoparticles are well-known as multifunctional inorganic fillers that have unique properties, such as strong UV absorption in combination with good transparency in the visible range, a lowdielectric constant, and a large electromechanical coupling coefficient [8]. ZnO can be synthesized by various synthetic paths in various shapes and particle sizes [9–11]. Advanced applications require nanoparticles with a narrow particle size distribution and defined particle shape. ZnO is a material for semiconductor device applications because it has a direct and wide band gap of 3.37 eV, making it an excellent candidate for use in UV-light-emitting diodes (LEDs), lasers, and transparent transistors [12,13]. A large number of research in recent years has focused on the development of polymer-nanocomposite materials. Demir et al. [14] prepared ZnO/PMMA composite films with strong UV absorption and light transmittance in the visible range by the polymerization of particle dispersions in the monomer. Chae and Kim [15] successfully fabricated PS/ZnO nanocomposite films, which exhibited UV-absorbance without losing transparency at a low ZnO content by solution mixing. However, these films showed a transparency loss at a high concentration due to the aggregation of individual ZnO nanoparticles. ZnO has been incorporated into many other polymers, such as poly(hydroxyethylmethacrylate) [16], poly(amic acid) [17], polyimide [18], nylon [19], and PS-PMMA [20]. It is important to note that the ZnO is considered to be an interesting material for photovoltaic applications because of its unique combination of optical and semiconducting properties [21]. The goal of our work is to form self-assembled ZnO nanoparticles using the poly(S699-block-MMA232/MAA58) diblock copolymer matrix as a nanoreactor of this inorganic nanoparticles [22,23], studying the optical, thermal, topographical and morphological properties of the composites as well as the effect of the nanoparticles (20%, 30% and 50%) on the polymeric matrix. Additionally, with the goal of identifying semiconductor applications, potentiodynamic electrochemical measurements were conducted. The advantage in the use of this block-copolymer is focus in the easy synthetic route and low cost of it, beside we can obtain nanoparticles uniformly distributed in the polymeric matrix, also with a similar morphological shapes. These reasons are supported in the following comments, the diblock copolymer was synthesized via normal ATRP in two steps, followed by partial hydrolysis of the methyl ester linkages of the MMA block under acidic conditions and had a narrow molecular weight dispersity (Ð < 1.28). The amphiphilic diblock copolymer consisting of a majority polymer (styrene) and a minority polymer (methyl methacrylate/methacrylic acid) with a block repeat unit ratio of 699/264, this allows a spherical microphase separation and hence a spherical morphology for the metal oxide nanoparticles, achieved at room temperature in the liquid phase, using ZnCl2 as precursor dopant.
The phase separation occurs on the nanometer scale, as determined by the dimension of the blocks. If the polymer chains have narrow size distribution, the phase separation should produce ordered nanostructures. As result, the amphiphilic diblock copolymer, poly(styrene)-block-poly(methyl methacrylate/methacrylic acid), self-assembled materials, will produce spherical micelles in a continuous phase. The self-assembled nanoscale morphology of the composites results in the formation of uniformly distributed spherical nanoparticles within the polymer matrix, and the amount of ZnO nanoparticles cause changes in the copolymer behavior. This does not mean that the nanoparticles have similar size. 2. Experimental 2.1. Reagents Styrene and methyl methacrylate were purchased from Sigma–Aldrich Chemicals, Germany and Merck-Schuchardt OHG Chemicals, Germany, respectively. Both compounds were distilled under a reduced pressure before being utilized. Benzoyl peroxide (BPO), copper (I) bromide (CuBr), and N,N-bipyridine (Bpy) reagents were purchased from Sigma–Aldrich Chemicals, Merck-Schuchardt, Germany. 2.2. Measurements The number-average (Mn) and weight-average (Mw) for the molecular weight and the molecular weight distribution (dispersity, Ð = Mw/Mn) of the copolymer were determined using size exclusion chromatography (SEC) using a WATERS 600E instrument equipped with UV and RI detectors and using THF as the solvent (flow rate: 1.0 mL/min). The samples were measured at 30 °C with a concentration of 6 mg/mL, and the calibration was performed using polystyrene. FT-IR spectra were recorded on a Bruker Vector 22 spectrometer (Bruker Optics Inc., Ettlingen, Germany). The structural and vibrational properties of the pure block copolymer and the respective composites (20%, 30% and 50% of ZnO) were characterized by Raman spectroscopy with a LabRam 010 instrument from ISA equipped with a 5.5 mW, 632.8 nm He–Ne laser without a filter. The Raman microscope uses a back-scattering geometry, where the incident beam is linearly polarized at a 500:1 ratio. The microscope objective used for the Raman analysis was an Olympus Mplan 100 (numerical aperture 0.9). The absorption spectra of the pure block copolymer and the composites were recorded at 25 °C between 250 and 400 nm in a Shimadzu UV-160 spectrophotometer. The photo luminescence (PL) spectra of the solid samples were recorded at room temperature with a Perkin Elmer spectrofluorometer model L55, and the spectra were obtained under similar conditions, using an excitation wavelength of 320 nm. The XRD patterns were recorded by employing a Philips X0 PERT MPD diffractometer (Cu Ka radiation: k = 0.154056 nm at 40 kV and 30 mA) over the 2h range of 1.7–80° at a scan rate of 0.05°/min.
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perature (TDT2) of approximately 379 °C. These results have been mentioned in a previous work by our research group [24]. 2.4. Formation of the ZnO nanoparticles The pure block copolymer was dissolved in THF at a concentration of 0.1%. Later, the metal ion ZnCl2 (20–50% in weight) was incorporated. After this, the solution was stirred for 48 h so that the system could achieve equilibrium and the Zn2+ associated with the functional groups could participate in the finished process. After this stage, the solution was left in another solution of NH4OH for 24 h to form Zn(OH)2. This step was followed by washing the solution in H2O to remove water salts and to decompose the unstable zinc hydroxides to ZnO. The ZnO remained insoluble in NH4OH, while it solubilized in the alkali bases NaOH and KOH. The conversion of Zn(OH)2 to ZnO–diblock copolymer nanoparticles was successfully achieved after drying by heating at 30 °C under vacuum for 48 h; see Scheme 2. The substitution of Cl by O was found to be a highly preferential process, whereby only one approach using a weak base (NH4OH) succeeded in effectively replacing Cl with O to result in spherical ZnO nanoparticles [15]. It is necessary to mention that the amount of ZnCl2 (20– 50% weight) did not represent 100% of the ZnO of the copolymer. However, the percentage composition of the nanoparticles were studied by SEM-EDS (five measurements for a sample), which showed values near the percentage of the precursor added. 3. Results and discussion 3.1. Characterization of the block copolymer composites The block copolymer composite structures were examined using FT-IR and Raman to verify the precursor association with the functional groups of the minority block. The FT-IR analysis was performed to examine bond stretching in the carboxyl groups, which would indicate that the metal cation in ZnCl2 is capable of interacting with the block copolymer and subsequently forming ZnO nanoparticles. The Raman spectra were obtained to characterize the functional group and the possible interaction with Zn2+ by determining certain characteristic peaks.
2.3. Block copolymer by ATRP 3.2. FTIR and Raman spectroscopic analysis The poly(S699-block-MMA232/MAA58) was prepared according to Scheme 1. The copolymer was dried until a constant weight was reached (yield 68.8%, Mn = 10.2 104 Daltons and Ð = Mw/Mn = 1.30). The FT-IR spectrum for the block copolymer shown in Fig. 1a exhibited the following absorption bands: at about 3424 cm1 AOH group vibration from ACOOH is observed; at 2921 cm1 [t(CH, CH2)]; at 1723 cm1 [t(AC@O)]; and 753 and 697 cm1 [t(aromatic ring of styrene)]. DSC showed a shift in the base-line; the transition corresponds to the glass transition temperature (Tg = 65 °C); TGA exhibited a two-step degradation with an extrapolated thermal decomposition tem-
The FT-IR spectra verified that the metal ion was associated with the minority block (MMA/MAA) of the block copolymer. The carbonyl band t(C@O group) at 1723 cm1 was replaced by two absorption bands at 1719 cm1 and 1631 cm1 with a lower intensity, which could be attributed to the stronger association of Zn2+ and the carboxyl groups (MAA) on the second block of the copolymer due to asymmetric and symmetric stretching (Scheme 2). Nevertheless, this peak also disappears and later appears as a new peak at 1600 cm1 due to the formation and interaction of the ZnO (20%) nanoparticle
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The study of the thermal stability was carried out with a Mettler Toledo STARe System with a thermogravimetric analyzer (TGA) at a heating rate of 10 °C/min and under nitrogen atmosphere (flow rate = 150 mL/min). The cyclic voltammogram (CV) of the block copolymer and their composites on the glassy carbon electrodes was obtained by an electrochemical analyzer (CH-Instruments, model CHI604C) using a platinum counter electrode and an Ag/AgCl reference electrode immersed in the electrolyte (tetrabutylammoniumperchlorate in DMF, 0.1 M) at a scan rate of 100 mV/s. In all of the experiments, pro analysis grade chemicals were used. Before each experiment, the working electrode (glassy carbon) was polished with alumina slurry (particle size 0.3 lm) on soft leather and subsequently washed with deionized water. Before the experiments, the solutions were purged with high-purity nitrogen at atmospheric pressure. The morphology of the block copolymer and composites was examined using conventional scanning electron microscopy (SEM) (JEOL, JSM 6380 LV) and transmission electron microscopy (TEM) (JEOL, JEM 1200 EX, operating at 120 kV, with a point resolution of approximately 4 Å). These measurements were performed on dispersed samples. TEM images were taken by placing a drop of the powder nanocomposite in THF onto a carbon-coated copper grid. For the preparation of the stable thin films, the copolymer solution was spin-coated (KW-4A, CheMat Technology, Inc., Northridge, United States). The surface characterization of the block copolymer and the composites (20%, 30% and 50% ZnO) was carried out by employing home-made atomic force microscopy (AFM) to determine the height profiles and their surface properties. This equipment was designed by Prof. Dr. Guido Tarrach at Pontificia Universidad Católica de Chile and was based on the hardware and software developed at the Guentherodt group at Basel University, Switzerland. This AFM could be operated in contact, non-contact and intermittent contact modes. The tips used for the measurements were pyramidal shaped with a height of 2 lm, a length of 450 lm, a width of 50 lm, a resonance frequency of 13 kHz and a force constant of 0.2 N/m. To obtain the highest lateral resolution, the image was taken with 256 256 pixels with a scan rate of one line per 2 s. The compounds (copolymer and composites) were dissolved in THF and were cast on glass slides using the spin coater with a rotation velocity of 500 rpm for approximately 30 s and 2000 rpm for 30 s.
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Scheme 1. Structure of the poly(S699-block-MMA232/MAA58) block copolymer.
Fig. 1. FT-IR spectrum of (a) block copolymer; (b) block copolymer–Zn2+; and (c) block copolymer–ZnO 20%.
associated with the functional group, resulting in the formation of local chemical cross-links between chains; see Fig. 1a–c. In the Raman spectra, the band that appears at 1601 cm1 has a lower intensity in comparison with the IR spectra, and it is related to the C@C quadrant stretching of the aromatic ring as the pendent group of the copolymer. Furthermore, it may have little interaction with the CH in the plane of bending, and the band at 1454 cm1 could be attributed to the CH2 scissoring mode (semicircle stretching) and could possibly overlap with CH3 antisymmetric bending. Both signals are characteristics of the n-alkanes. In addition, the most intense band appears at 996 cm1 and one moderately strong band at 1081 cm1. These two peaks are related to ring bending and stretching vibration (‘‘breathing mode’’), along with an ‘‘in plane’’ ring CAH deformation (‘‘wagging mode’’), both of which are signs of benzenes; see Fig. 2. The carbonyl band near 1700 cm1 is strong in the IR (Fig. 1) and weaker in the Raman spectra (1716 cm1, Fig. 2). When the copolymer contains 20% ZnO nanoparticles in its structure, the signals above-mentioned are not observed. However, there appear three bands of medium– weak intensity in the Raman spectra that could be related to a vibration impurity mode at 2630 cm1, 2360 cm1 and 2119 cm1, which are conserved for composite 30% ZnO. Alternatively, the copolymer with 30% ZnO shows four bands characteristic of the copolymer, such as 1603 cm1
Scheme 2. The synthesis of ZnO using the nanoreactor based on the block copolymer.
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(C@C ‘‘quadrant stretching’’ of the benzene derivate), 1187 cm1 (bare ZnO [25]), 1083 cm1 and 996 cm1 (ring ‘‘breathing’’ mode). Lastly, when the copolymer has a large amount of ZnO nanoparticles (50%) in the system, the vibration mode is clearly affected. Thus, some bands appear that are not observed in the block copolymer, for example, in the 3053– 2893 cm1 range (antisymmetric CH3, anti symmetric CH2 and aromatic CH stretch). There is also an intensification of the Raman shifts in the bending modes. This behavior is reasonable because the ZnO nanoparticles into the copolymer increase the order and rigidity of the system.
Fig. 3. Optical properties: (a) UV–Vis absorption spectra of block copolymer and block copolymer composite (20% and 50% of ZnO) and (b) photoluminescence spectrum of block copolymer and block copolymer composite (50% and 20%) and commercial ZnO nanoparticles.
3.3. Optical properties The UV–Visible spectra of the block copolymer exhibited a maximum in the range 230–275 nm and its composites showed a high transparency that could be attributed to a lower fraction of the poly(MMA232/MAA58) segment due to the low percentage of the inter- or intra-molecular interaction of the acid functional group. All of the samples have absorptions below 350 nm, and the percentage of absorption increases in proportion to the amount of ZnO nanoparticles. Therefore, the resulting block copolymer composite can block UV rays, but is transparent to visible light (Fig. 3a). Additionally, the composites showed signals that not were observed for the copolymer pure, such as two bands at 378 nm and 423 nm for 20% and 50% ZnO, respectively. The emission spectra showed a blue shift attributed to a decreasing nanoparticle size. The photoluminescence spectrum of the pure block copolymer exhibited only a weak UV emission peak centered at 378 nm because the aromatic segments were unconjugated (Fig. 3b). Alternatively, the commercial ZnO showed three bands, one strong at 385 nm and two weak at 425 nm and 490 nm. The composite nanoparticles exhibited the same three bands but with a strong decrease in the first peak (385 nm). This behavior could be related to the interaction between the organic copolymer segment
and the semiconductor nanoparticles. According to these results, the incorporation of ZnO nanoparticles in the copolymer matrix is reflected in the system emission variation from monochromatic light to two-tone color lights, which altered the emission spectra.
3.4. XRD pattern The XRD patterns of the samples are shown in Fig. 4. These data indicate that all the peaks shown by the commercial nanoparticles are well indexed to the pure wurtzite crystalline phase of ZnO [26]. According to the pure ZnO nanoparticles, the average crystalline size was calculated as 35–50 nm using Sherre’s equation: D = 0.9 k/b cos h, where D is the crystallite size, k is a wavelength of the radiation, h is Bragg’s angle and b is the full width at half maximum. The block copolymer did not show a profile signal, most likely due to the system amorphicity (Fig. 4b). Alternatively, the formation of composite 50% ZnO showed multiple peaks that are similar at the Wurtzite crystalline phase of ZnO and one broad peak at 12.3° that was attributed to the interaction between the organic matrix and the nanoparticles (Fig. 4c).
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Fig. 2. Block copolymer Raman spectrum and block copolymer composite (10%, 20% and 50% of ZnO).
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Fig. 4. X-ray diffraction pattern of (a) ZnO; (b) block copolymer; and (c) block copolymer–ZnO 50%.
Fig. 6. Cyclic voltammetric response of a glassy carbon electrode cycled in a solution containing block copolymer and block copolymer–ZnO (20%, 30% and 50%) composites: (a) in the presence of oxygen; and (b) inert atmosphere. All the analyzes were conducted with a scan rate of 100 mV/s.
Fig. 5. TGA and DTG curves of block copolymer and block copolymer–ZnO 50% using a heating rate of 10 °C/min.
3.5. Thermal behavior The thermal behavior of the pure block copolymer and composites were examined using TGA under nitrogen at a heating rate of 10 °C/min. The TGA and DTG curves show that the block copolymer and its composites with ZnO have one main weight loss regions that are plotted in Fig. 5. The plot shows that the copolymer and composites degraded continuously at one stage process. The copolymer and composite were stable up to 389 °C and 386 °C, respectively. The presence of nanoparticles in the composites decreases slightly the degradation temperatures. Besides, the DTG curves showed the maximum decomposition of the material near at 431 and 427 °C for the same compounds. The presence of nanoparticles in the composites decreases slightly the degrada-
tion temperatures. At 161 °C and 166 °C, respectively, the weight loss is not significant, which was attributed to solvent loss or a small amount of monomer residue in the samples. The block copolymer had a weight loss of 75% at 500 °C, and for the composite, there was a loss of 65% at the same temperature. This effect may be due to the formation and rearrangement of the different interactions of the block copolymer with ZnO nanoparticles. The block copolymer presented a slightly higher thermal degradation temperature (TDT) than the composite, which can be principally attributed to the fact that the block copolymer composites presents in its structure a percentage of the ZnO nanoparticles, which can act like catalysts for the degradation reaction. 3.6. Electrochemical studies The electrochemical characterizations of the block copolymers and composites were carried out using CV to determine the redox potentials. The electrochemical behavior of the samples was tested under a potential range from 1.0 to +1.0 V and 1.5 to 1.5 V (at a scan rate of 100 mV/s). The CV characterization of the samples is shown in Fig. 6a and b. These voltammograms show that
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Fig. 8. SEM micrograph of (a) block copolymer–Zn2+ and (b–d) block copolymer–ZnO 50%.
the block copolymer does not exhibit an oxide-reduction process under the conditions of CV analysis. The range of electrochemical inertia of the copolymer can be extended to 1.0 V. This copolymer shows electrochemical stability, providing an ideal platform for fabricating novel functional nanostructured materials for potential applications in advanced technologies, such as nanomaterials, nanocomposites, and drug delivery and information storage. These voltammograms also show that the block copolymer composites exhibited an oxide-reduction process under the conditions of CV analysis (see Fig. 6a). The catalyst effect was observed at a low potential (electrocatalysis) (approximately from 0.7 to 0.5 V), and this reaction was catalyzed by the nanoparticle (ZnO) contained in the matrix. The observed oxide-reduction process corresponds to an oxygen reduction and peroxide oxidation in the presence of ZnO nanoparticles under the tested conditions. In inert atmosphere, block copolymer electrochemical inertia was observed due to the absence of oxygen in the medium. Furthermore, the block copolymer composites showed redox processes to negative potentials.
3.7. Self-assembled ZnO nanostructures The TEM images of the block copolymer composites in Fig. 7a–c give additional information regarding the size and size dispersion of the nanoparticles within the copolymer matrix forming spherical aggregates in the THF solution. The ZnO nanoparticles appear as dark-white spots, which are clearly observed in the matrix films, resulting in nanodomains of interconnected networks of spherical aggregates [13–15]. The average size range of the nanoparticles shows that the micelles tend to self-assemble into much larger spherical aggregates that predominate (Fig. 7a–c) due to the aggregation effect of the nanoparticles. Lastly, the size and stability of these spherical aggregates depends on the inorganic nanoparticle concentration [15]. 3.8. Morphological analysis A set of microphotographs obtained by the SEM (secondary electrons detector) show the morphology of the
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Fig. 7. TEM image of the block copolymer composites at different concentrations of nanoparticles: (a) 20% ZnO; (b) 30% ZnO; and (c) 50% ZnO.
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Fig. 9. AFM micrographics of the block copolymer and composites at different concentrations of ZnO (20%, 30%, and 50%).
poly(S699-block-MMA232/MAA58)-Zn2+ and the composite 50% ZnO (Fig. 8a–d). Fig. 8a shows an ordered structure, in which nano-islets of variable sizes coexist. However, the copolymer nanoparticle microphotographs (Fig. 8b–d) in an appropriate solvent (THF) showed larger spherical aggregates. These results indicate a good interaction between the copolymer block segments and the ZnO particles, producing nanoparticles of the copolymer block that contain occluded ZnO. These observations were confirmed using an FT-IR measurement and confirmed by Raman spectroscopy. In both cases, a highly ordered inorganic–organic nanoparticle was formed. The equilibrium structures were determined by the thermodynamics of the self-assembly process and the inter- and intra-aggregate forces. After comparing the size and distribution of the ZnO nanoparticles-polymer matrix from SEM and TEM images, it was observed that the self-assembled nanostructured films are suitable for the construction of devices based on inorganic nanoparticles and organic polymers. AFM images were obtained through the intermittent contact method (Fig. 9a–d). The micrographs show the morphological and topographical features of the pure copolymer block and the copolymer-ZnO (20%, 30% and 50%) nanoparticle, respectively. According to these results, the topographies of the pure block copolymer and the composite 30% and 50% ZnO thin
layer surfaces only show agglomerations of the cluster type. This behavior is completely random and does not depend on the amount of nanoparticles in the system. However, the composite 20% ZnO exhibited well-defined pores in the surface with an average height of 543 nm and an average pore diameter of 2.67 lm. This behavior is most likely due to the self-assembling property of the block copolymer with amphiphilic characteristics when interacting with the nanoparticles of ZnO. This result does not indicate that the surface of the pure copolymer and its composites of 30% and 50% ZnO do not have a porous surface, only that the probability of finding porosity in these composites is lower than in the 20% composite.
4. Conclusions The insertion of ZnO nanoparticles into the block copolymer system was studied and confirmed through FT-IR and Raman spectroscopy, optical methods, X-ray diffraction, and thermal and morphological characterization techniques. The self-assembled nanoscale morphology of the composites results in the formation of uniformly distributed spherical nanoparticles within the polymer matrix, and the amount of ZnO nanoparticles cause changes in the copolymer behavior. The micelle-like supports for
Please cite this article in press as: Pizarro GdC et al. Nanoreactors based on self-assembled amphiphilic diblock opolymers for the preparation of ZnO nanoparticles. Eur Polym J (2013), http://dx.doi.org/10.1016/j.eurpolymj.2013.08.008
ZnO particles based on the self-assembled block copolymer (content from 20 to 50 wt.%), show good transparency in the visible region and excellent luminescent properties. Block copolymer composites with a high ZnO content are able to absorb UV irradiation below 350 nm, indicating that these composite films exhibit good UV screening effects. Raman spectroscopy showed clear vibrational bands of organic segments. Multiple peaks obtained by X-ray diffraction correspond to Wurtzite crystalline phase of ZnO and one broad peak was attributed to the interaction between the organic matrix and the nanoparticles. Thermogravimetric analyzes show that the ZnO nanoparticles were successfully formed into the polymer matrix and that this diblock copolymer composite has a good response to thermal variations; it is therefore stable against thermodynamic changes. The thermal stability of the pure block copolymer and composite-50% ZnO were found to be stable up to 386 °C. However, the composite exhibited a higher stability due to the presence of ZnO in the polymeric structure. The SEM images show that poly(St699-block-MMA211/ MAA53)-ZnO principally consists of auto-assembled spherical micellar aggregations, resulting in nanodomains of interconnected networks of spherical aggregates over the polymer surface and possibly into the polymer. Furthermore, for the sample corresponding to composite 20% of ZnO it was possible to determine very precisely the average height and diameter of the surface pore, 543 nm and 2.67 lm, respectively. Lastly, a certain degree of porosity that depends on the percentage of ZnO nanoparticles was found. The TEM studies indicate that the ZnO nanoparticles were uniformly dispersed on the polymer. The TEM images of the ZnO composite gave additional information regarding the size and size dispersion of the nanoparticles within the copolymer matrix formed. These results indicated good stability between the block copolymer containing ZnO.
Acknowledgments The authors acknowledge the financial assistance of this work by the Fondo Nacional de Investigación Científica y Tecnológica, FONDECYT Grants 1110836 and 11121281. The WCU Program funded by MEST (R31-10026). Thanks to Dra. Maria Jesus Aguirre (USACH) by Cyclic Voltammetry measurements.
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