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This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution
and sharing with colleagues.
Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information
regarding Elsevier’s archiving and manuscript policies areencouraged to visit:
Nanoreactors based on self-assembled amphiphilic diblockopolymers 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, Chileb Facultad de Física, Pontificia Universidad Católica de Chile, Vicuña Mackenna 4860, Santiago 7820436, Chilec Department of Nanobio Materials and Electronics, World-Class University (WCU), Gwangju Institute of Science and Technology (GIST), Gwangju 500-712,Republic of Koread Laboratory of Applied Macromolecular Chemistry, School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju,Republic of Korea
a r t i c l e i n f o
Article history:Received 22 April 2013Received in revised form 26 July 2013Accepted 4 August 2013Available online 19 August 2013
A self-assembled diblock copolymer containing styrene (S), methyl methacrylate and a cer-tain percentage of hydrophilic segment of poly(methacrylic acid) (i.e., poly(styrene)-block-poly(methyl methacrylate/methacrylic acid) was synthesized via the ATRP method in twosteps. This was followed by a partial hydrolysis of the methyl ester linkages of the methylmethacrylate block under acidic conditions. The resultant block copolymer had a narrowmolecular weight dispersity (Ð < 1.3) and was characterized using FT-IR and Raman spec-troscopy as well as size exclusion chromatography. The block copolymer was used as ananoreactor for inorganic nanoparticles (ZnO). The incorporation of a single source precur-sor, such as ZnCl2, into the self-assembled copolymer matrix and the conversion into ZnOnanostructures was carried out in the liquid phase using wet chemical processing tech-niques. We report the synthesis and characterization of nanocomposites with dual charac-teristics due to the functionalities incorporated into the matrix. The optical properties weredetermined by UV–Vis and fluorescence, the crystallinity was studied using X-ray diffrac-tion, and the thermal stability and studies of the cyclic voltammetry were obtained bythermogravimetric analyzes and potentiodynamic electrochemical measurements, respec-tively. The structural, topographical and morphological characterizations of the ZnO com-posite in relation to the precursor block copolymer were analyzed via scanning electronmicroscopy, transmission electron microscopy and atomic force microscopy.
According to the results, the block copolymer shows good transparency in the visibleregion (when containing 20, 30 and 50 wt.% nanoparticles) and was able to absorb UV irra-diation below 350 nm, indicating good UV-screening effects. The thermo gravimetric anal-ysis data showed that the block copolymer composite is more thermally stable comparedto the pure block copolymer. The self-assembled nanoscale morphology of the diblockcopolymer results in the formation of uniformly distributed spherical nano particles withinthe polymer matrix.
� 2013 Elsevier Ltd. All rights reserved.
1. Introduction
In recent decades, there has been increasing demand fornew materials in diverse areas in the technology industry.Organic materials for photovoltaic devices have beeninvestigated because of the increasing demand of energy
0014-3057/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.eurpolymj.2013.08.008
q This is an open-access article distributed under the terms of theCreative Commons Attribution-NonCommercial-No Derivative WorksLicense, which permits non-commercial use, distribution, and reproduc-tion in any medium, provided the original author and source are credited.⇑ Corresponding author.
around the world [1]. The confining of diblock copolymersin topographically or chemically patterned structures hasfrequently been employed to control the order and orienta-tion of the nanostructures of the copolymer [2–4]. The syn-thesis of zinc oxide (ZnO) semiconductor nanoparticleswithin a microphase-separated diblock copolymer has alsobeen reported [5–7]. Very small ZnO nanoparticles arewell-known as multifunctional inorganic fillers that haveunique properties, such as strong UV absorption in combi-nation with good transparency in the visible range, a low-dielectric constant, and a large electromechanical couplingcoefficient [8]. ZnO can be synthesized by various syntheticpaths in various shapes and particle sizes [9–11]. Advancedapplications require nanoparticles with a narrow particlesize distribution and defined particle shape. ZnO is a mate-rial for semiconductor device applications because it has adirect and wide band gap of 3.37 eV, making it an excellentcandidate for use in UV-light-emitting diodes (LEDs), la-sers, and transparent transistors [12,13]. A large numberof research in recent years has focused on the developmentof polymer-nanocomposite materials. Demir et al. [14] pre-pared ZnO/PMMA composite films with strong UV absorp-tion and light transmittance in the visible range by thepolymerization of particle dispersions in the monomer.Chae and Kim [15] successfully fabricated PS/ZnO nano-composite films, which exhibited UV-absorbance withoutlosing transparency at a low ZnO content by solution mix-ing. However, these films showed a transparency loss at ahigh concentration due to the aggregation of individualZnO nanoparticles. ZnO has been incorporated into manyother polymers, such as poly(hydroxyethylmethacrylate)[16], poly(amic acid) [17], polyimide [18], nylon [19], andPS-PMMA [20]. It is important to note that the ZnO is con-sidered to be an interesting material for photovoltaic appli-cations because of its unique combination of optical andsemiconducting properties [21].
The goal of our work is to form self-assembled ZnOnanoparticles using the poly(S699-block-MMA232/MAA58)diblock copolymer matrix as a nanoreactor of this inor-ganic nanoparticles [22,23], studying the optical, thermal,topographical and morphological properties of the com-posites as well as the effect of the nanoparticles (20%,30% and 50%) on the polymeric matrix. Additionally, withthe goal of identifying semiconductor applications, poten-tiodynamic electrochemical measurements were con-ducted. The advantage in the use of this block-copolymeris focus in the easy synthetic route and low cost of it, be-side we can obtain nanoparticles uniformly distributed inthe polymeric matrix, also with a similar morphologicalshapes. These reasons are supported in the following com-ments, the diblock copolymer was synthesized via normalATRP in two steps, followed by partial hydrolysis of themethyl ester linkages of the MMA block under acidic con-ditions and had a narrow molecular weight dispersity(Ð < 1.28). The amphiphilic diblock copolymer consistingof a majority polymer (styrene) and a minority polymer(methyl methacrylate/methacrylic acid) with a block re-peat unit ratio of 699/264, this allows a spherical micro-phase separation and hence a spherical morphology forthe metal oxide nanoparticles, achieved at room tempera-ture in the liquid phase, using ZnCl2 as precursor dopant.
The phase separation occurs on the nanometer scale, asdetermined by the dimension of the blocks. If the polymerchains have narrow size distribution, the phase separationshould produce ordered nanostructures. As result, theamphiphilic diblock copolymer, poly(styrene)-block-poly(-methyl methacrylate/methacrylic acid), self-assembledmaterials, will produce spherical micelles in a continuousphase. The self-assembled nanoscale morphology of thecomposites results in the formation of uniformly distrib-uted spherical nanoparticles within the polymer matrix,and the amount of ZnO nanoparticles cause changes inthe copolymer behavior. This does not mean that the nano-particles have similar size.
2. Experimental
2.1. Reagents
Styrene and methyl methacrylate were purchased fromSigma–Aldrich Chemicals, Germany and Merck-SchuchardtOHG Chemicals, Germany, respectively. Both compoundswere distilled under a reduced pressure before being uti-lized. Benzoyl peroxide (BPO), copper (I) bromide (CuBr),and N,N-bipyridine (Bpy) reagents were purchased fromSigma–Aldrich Chemicals, Merck-Schuchardt, Germany.
2.2. Measurements
The number-average (Mn) and weight-average (Mw) forthe molecular weight and the molecular weight distribu-tion (dispersity, Ð = Mw/Mn) of the copolymer were deter-mined using size exclusion chromatography (SEC) using aWATERS 600E instrument equipped with UV and RI detec-tors and using THF as the solvent (flow rate: 1.0 mL/min).The samples were measured at 30 �C with a concentrationof 6 mg/mL, and the calibration was performed usingpolystyrene.
FT-IR spectra were recorded on a Bruker Vector 22 spec-trometer (Bruker Optics Inc., Ettlingen, Germany). Thestructural and vibrational properties of the pure blockcopolymer and the respective composites (20%, 30% and50% of ZnO) were characterized by Raman spectroscopywith a LabRam 010 instrument from ISA equipped with a5.5 mW, 632.8 nm He–Ne laser without a filter. The Ramanmicroscope uses a back-scattering geometry, where theincident beam is linearly polarized at a 500:1 ratio. Themicroscope objective used for the Raman analysis was anOlympus Mplan 100� (numerical aperture 0.9).
The absorption spectra of the pure block copolymer andthe composites were recorded at 25 �C between 250 and400 nm in a Shimadzu UV-160 spectrophotometer. Thephoto luminescence (PL) spectra of the solid samples wererecorded at room temperature with a Perkin Elmer spec-trofluorometer model L55, and the spectra were obtainedunder similar conditions, using an excitation wavelengthof 320 nm.
The XRD patterns were recorded by employing a PhilipsX0 PERT MPD diffractometer (Cu Ka radiation:k = 0.154056 nm at 40 kV and 30 mA) over the 2h rangeof 1.7–80� at a scan rate of 0.05�/min.
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The study of the thermal stability was carried out with aMettler Toledo STARe System with a thermogravimetricanalyzer (TGA) at a heating rate of 10 �C/min and undernitrogen atmosphere (flow rate = 150 mL/min).
The cyclic voltammogram (CV) of the block copolymerand their composites on the glassy carbon electrodes wasobtained by an electrochemical analyzer (CH-Instruments,model CHI604C) using a platinum counter electrode and anAg/AgCl reference electrode immersed in the electrolyte(tetrabutylammoniumperchlorate in DMF, 0.1 M) at a scanrate of 100 mV/s. In all of the experiments, pro analysisgrade chemicals were used. Before each experiment, theworking electrode (glassy carbon) was polished with alu-mina slurry (particle size 0.3 lm) on soft leather and sub-sequently washed with deionized water. Before theexperiments, the solutions were purged with high-puritynitrogen at atmospheric pressure.
The morphology of the block copolymer and compositeswas examined using conventional scanning electronmicroscopy (SEM) (JEOL, JSM 6380 LV) and transmissionelectron microscopy (TEM) (JEOL, JEM 1200 EX, operatingat 120 kV, with a point resolution of approximately 4 Å).These measurements were performed on dispersed sam-ples. TEM images were taken by placing a drop of the pow-der nanocomposite in THF onto a carbon-coated coppergrid. For the preparation of the stable thin films, thecopolymer solution was spin-coated (KW-4A, CheMatTechnology, Inc., Northridge, United States). The surfacecharacterization of the block copolymer and the compos-ites (20%, 30% and 50% ZnO) was carried out by employinghome-made atomic force microscopy (AFM) to determinethe height profiles and their surface properties. This equip-ment was designed by Prof. Dr. Guido Tarrach at PontificiaUniversidad Católica de Chile and was based on the hard-ware and software developed at the Guentherodt groupat Basel University, Switzerland. This AFM could be oper-ated in contact, non-contact and intermittent contactmodes. The tips used for the measurements were pyrami-dal shaped with a height of 2 lm, a length of 450 lm, awidth of 50 lm, a resonance frequency of 13 kHz and aforce constant of 0.2 N/m. To obtain the highest lateral res-olution, the image was taken with 256 � 256 pixels with ascan rate of one line per 2 s. The compounds (copolymerand composites) were dissolved in THF and were cast onglass slides using the spin coater with a rotation velocityof 500 rpm for approximately 30 s and 2000 rpm for 30 s.
2.3. Block copolymer by ATRP
The poly(S699-block-MMA232/MAA58) was preparedaccording to Scheme 1. The copolymer was dried until aconstant weight was reached (yield 68.8%, Mn = 10.2� 104 Daltons and Ð = Mw/Mn = 1.30). The FT-IR spectrumfor the block copolymer shown in Fig. 1a exhibited the fol-lowing absorption bands: at about 3424 cm�1 AOH groupvibration from ACOOH is observed; at 2921 cm�1 [t(CH,CH2)]; at 1723 cm�1 [t(AC@O)]; and 753 and 697 cm�1
[t(aromatic ring of styrene)]. DSC showed a shift in thebase-line; the transition corresponds to the glass transitiontemperature (Tg = 65 �C); TGA exhibited a two-step degra-dation with an extrapolated thermal decomposition tem-
perature (TDT2) of approximately 379 �C. These resultshave been mentioned in a previous work by our researchgroup [24].
2.4. Formation of the ZnO nanoparticles
The pure block copolymer was dissolved in THF at aconcentration of 0.1%. Later, the metal ion ZnCl2 (20–50%in weight) was incorporated. After this, the solution wasstirred for 48 h so that the system could achieve equilib-rium and the Zn2+ associated with the functional groupscould participate in the finished process. After this stage,the solution was left in another solution of NH4OH for24 h to form Zn(OH)2. This step was followed by washingthe solution in H2O to remove water salts and to decom-pose the unstable zinc hydroxides to ZnO. The ZnO re-mained insoluble in NH4OH, while it solubilized in thealkali bases NaOH and KOH. The conversion of Zn(OH)2
to ZnO–diblock copolymer nanoparticles was successfullyachieved after drying by heating at 30 �C under vacuumfor 48 h; see Scheme 2. The substitution of Cl by O wasfound to be a highly preferential process, whereby onlyone approach using a weak base (NH4OH) succeeded ineffectively replacing Cl with O to result in spherical ZnOnanoparticles [15].
It is necessary to mention that the amount of ZnCl2 (20–50% weight) did not represent 100% of the ZnO of thecopolymer. However, the percentage composition of thenanoparticles were studied by SEM-EDS (five measure-ments for a sample), which showed values near the per-centage of the precursor added.
3. Results and discussion
3.1. Characterization of the block copolymer composites
The block copolymer composite structures were exam-ined using FT-IR and Raman to verify the precursor associ-ation with the functional groups of the minority block. TheFT-IR analysis was performed to examine bond stretchingin the carboxyl groups, which would indicate that the me-tal cation in ZnCl2 is capable of interacting with the blockcopolymer and subsequently forming ZnO nanoparticles.The Raman spectra were obtained to characterize the func-tional group and the possible interaction with Zn2+ bydetermining certain characteristic peaks.
3.2. FTIR and Raman spectroscopic analysis
The FT-IR spectra verified that the metal ion was asso-ciated with the minority block (MMA/MAA) of the blockcopolymer. The carbonyl band t(C@O group) at1723 cm�1 was replaced by two absorption bands at1719 cm�1 and 1631 cm�1 with a lower intensity, whichcould be attributed to the stronger association of Zn2+
and the carboxyl groups (MAA) on the second block ofthe copolymer due to asymmetric and symmetric stretch-ing (Scheme 2). Nevertheless, this peak also disappearsand later appears as a new peak at 1600 cm�1 due to theformation and interaction of the ZnO (20%) nanoparticle
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associated with the functional group, resulting in the for-mation of local chemical cross-links between chains; seeFig. 1a–c.
In the Raman spectra, the band that appears at1601 cm�1 has a lower intensity in comparison with theIR spectra, and it is related to the C@C quadrant stretchingof the aromatic ring as the pendent group of the copoly-mer. Furthermore, it may have little interaction with theCH in the plane of bending, and the band at 1454 cm�1
could be attributed to the CH2 scissoring mode (semicirclestretching) and could possibly overlap with CH3 antisym-metric bending. Both signals are characteristics of the n-al-kanes. In addition, the most intense band appears at996 cm�1 and one moderately strong band at 1081 cm�1.These two peaks are related to ring bending and stretchingvibration (‘‘breathing mode’’), along with an ‘‘in plane’’ ringCAH deformation (‘‘wagging mode’’), both of which aresigns of benzenes; see Fig. 2. The carbonyl band near1700 cm�1 is strong in the IR (Fig. 1) and weaker in the Ra-man spectra (1716 cm�1, Fig. 2).
When the copolymer contains 20% ZnO nanoparticles inits structure, the signals above-mentioned are not ob-served. However, there appear three bands of medium–weak intensity in the Raman spectra that could be relatedto a vibration impurity mode at 2630 cm�1, 2360 cm�1 and2119 cm�1, which are conserved for composite 30% ZnO.Alternatively, the copolymer with 30% ZnO shows fourbands characteristic of the copolymer, such as 1603 cm�1
Scheme 1. Structure of the poly(S699-block-MMA232/MAA58) block copolymer.
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 cm�1 (bare ZnO [25]), 1083 cm�1 and 996 cm�1 (ring‘‘breathing’’ mode).
Lastly, when the copolymer has a large amount of ZnOnanoparticles (50%) in the system, the vibration mode isclearly affected. Thus, some bands appear that are not ob-served in the block copolymer, for example, in the 3053–2893 cm�1 range (antisymmetric CH3, anti symmetricCH2 and aromatic CH stretch). There is also an intensifica-tion of the Raman shifts in the bending modes. This behav-ior is reasonable because the ZnO nanoparticles into thecopolymer increase the order and rigidity of the system.
3.3. Optical properties
The UV–Visible spectra of the block copolymer exhib-ited a maximum in the range 230–275 nm and its compos-ites showed a high transparency that could be attributed toa lower fraction of the poly(MMA232/MAA58) segment dueto the low percentage of the inter- or intra-molecularinteraction of the acid functional group. All of the sampleshave absorptions below 350 nm, and the percentage ofabsorption increases in proportion to the amount of ZnOnanoparticles. Therefore, the resulting block copolymercomposite can block UV rays, but is transparent to visiblelight (Fig. 3a).
Additionally, the composites showed signals that notwere observed for the copolymer pure, such as two bandsat 378 nm and 423 nm for 20% and 50% ZnO, respectively.The emission spectra showed a blue shift attributed to adecreasing nanoparticle size.
The photoluminescence spectrum of the pure blockcopolymer exhibited only a weak UV emission peak cen-tered at 378 nm because the aromatic segments wereunconjugated (Fig. 3b). Alternatively, the commercial ZnOshowed three bands, one strong at 385 nm and two weakat 425 nm and 490 nm. The composite nanoparticlesexhibited the same three bands but with a strong decreasein the first peak (385 nm). This behavior could be related tothe interaction between the organic copolymer segment
and the semiconductor nanoparticles. According to theseresults, the incorporation of ZnO nanoparticles in thecopolymer matrix is reflected in the system emission vari-ation 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 com-mercial nanoparticles are well indexed to the pure wurtz-ite crystalline phase of ZnO [26]. According to the pure ZnOnanoparticles, the average crystalline size was calculatedas 35–50 nm using Sherre’s equation: D = 0.9 k/b cosh,where D is the crystallite size, k is a wavelength of the radi-ation, h is Bragg’s angle and b is the full width at halfmaximum.
The block copolymer did not show a profile signal, mostlikely due to the system amorphicity (Fig. 4b). Alterna-tively, the formation of composite 50% ZnO showed multi-ple peaks that are similar at the Wurtzite crystalline phaseof ZnO and one broad peak at 12.3� that was attributed tothe interaction between the organic matrix and the nano-particles (Fig. 4c).
Fig. 2. Block copolymer Raman spectrum and block copolymer composite(10%, 20% and 50% of ZnO).
Fig. 3. Optical properties: (a) UV–Vis absorption spectra of blockcopolymer and block copolymer composite (20% and 50% of ZnO) and(b) photoluminescence spectrum of block copolymer and block copoly-mer composite (50% and 20%) and commercial ZnO nanoparticles.
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3.5. Thermal behavior
The thermal behavior of the pure block copolymer andcomposites were examined using TGA under nitrogen ata heating rate of 10 �C/min.
The TGA and DTG curves show that the block copolymerand its composites with ZnO have one main weight loss re-gions that are plotted in Fig. 5. The plot shows that thecopolymer and composites degraded continuously at onestage process. The copolymer and composite were stableup to 389 �C and 386 �C, respectively. The presence ofnanoparticles in the composites decreases slightly the deg-radation temperatures. Besides, the DTG curves showedthe maximum decomposition of the material near at 431and 427 �C for the same compounds. The presence of nano-particles in the composites decreases slightly the degrada-
tion temperatures. At 161 �C and 166 �C, respectively, theweight loss is not significant, which was attributed to sol-vent loss or a small amount of monomer residue in thesamples. 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 for-mation and rearrangement of the different interactions ofthe block copolymer with ZnO nanoparticles.
The block copolymer presented a slightly higher ther-mal degradation temperature (TDT) than the composite,which can be principally attributed to the fact that theblock copolymer composites presents in its structure a per-centage of the ZnO nanoparticles, which can act like cata-lysts for the degradation reaction.
3.6. Electrochemical studies
The electrochemical characterizations of the blockcopolymers and composites were carried out using CV todetermine the redox potentials. The electrochemicalbehavior of the samples was tested under a potential rangefrom �1.0 to +1.0 V and �1.5 to 1.5 V (at a scan rate of100 mV/s). The CV characterization of the samples isshown in Fig. 6a and b. These voltammograms show that
Fig. 4. X-ray diffraction pattern of (a) ZnO; (b) block copolymer; and (c)block copolymer–ZnO 50%.
Fig. 5. TGA and DTG curves of block copolymer and block copolymer–ZnO50% using a heating rate of 10 �C/min.
Fig. 6. Cyclic voltammetric response of a glassy carbon electrode cycled ina solution containing block copolymer and block copolymer–ZnO (20%,30% and 50%) composites: (a) in the presence of oxygen; and (b) inertatmosphere. All the analyzes were conducted with a scan rate of 100 mV/s.
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the block copolymer does not exhibit an oxide-reductionprocess under the conditions of CV analysis. The range ofelectrochemical inertia of the copolymer can be extendedto �1.0 V. This copolymer shows electrochemical stability,providing an ideal platform for fabricating novel functionalnanostructured materials for potential applications in ad-vanced technologies, such as nanomaterials, nanocompos-ites, and drug delivery and information storage.
These voltammograms also show that the block copoly-mer composites exhibited an oxide-reduction process un-der the conditions of CV analysis (see Fig. 6a). Thecatalyst effect was observed at a low potential (electroca-talysis) (approximately from �0.7 to �0.5 V), and this reac-tion was catalyzed by the nanoparticle (ZnO) contained inthe matrix. The observed oxide-reduction process corre-sponds to an oxygen reduction and peroxide oxidation inthe presence of ZnO nanoparticles under the tested condi-tions. In inert atmosphere, block copolymer electrochemi-cal inertia was observed due to the absence of oxygen inthe medium. Furthermore, the block copolymer compos-ites showed redox processes to negative potentials.
3.7. Self-assembled ZnO nanostructures
The TEM images of the block copolymer composites inFig. 7a–c give additional information regarding the sizeand size dispersion of the nanoparticles within the copoly-mer matrix forming spherical aggregates in the THF solu-tion. The ZnO nanoparticles appear as dark-white spots,which are clearly observed in the matrix films, resultingin nanodomains of interconnected networks of sphericalaggregates [13–15]. The average size range of the nanopar-ticles shows that the micelles tend to self-assemble intomuch larger spherical aggregates that predominate(Fig. 7a–c) due to the aggregation effect of the nanoparti-cles. Lastly, the size and stability of these spherical aggre-gates depends on the inorganic nanoparticleconcentration [15].
3.8. Morphological analysis
A set of microphotographs obtained by the SEM (sec-ondary electrons detector) show the morphology of the
Fig. 7. TEM image of the block copolymer composites at different concentrations of nanoparticles: (a) 20% ZnO; (b) 30% ZnO; and (c) 50% ZnO.
Fig. 8. SEM micrograph of (a) block copolymer–Zn2+ and (b–d) block copolymer–ZnO 50%.
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poly(S699-block-MMA232/MAA58)-Zn2+ and the composite50% 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 sphericalaggregates.
These results indicate a good interaction between thecopolymer block segments and the ZnO particles, produc-ing nanoparticles of the copolymer block that contain oc-cluded ZnO. These observations were confirmed using anFT-IR measurement and confirmed by Raman spectros-copy. In both cases, a highly ordered inorganic–organicnanoparticle was formed. The equilibrium structures weredetermined by the thermodynamics of the self-assemblyprocess and the inter- and intra-aggregate forces. Aftercomparing the size and distribution of the ZnO nanoparti-cles-polymer matrix from SEM and TEM images, it was ob-served that the self-assembled nanostructured films aresuitable for the construction of devices based on inorganicnanoparticles and organic polymers.
AFM images were obtained through the intermittentcontact method (Fig. 9a–d). The micrographs show themorphological and topographical features of the purecopolymer block and the copolymer-ZnO (20%, 30% and50%) nanoparticle, respectively.
According to these results, the topographies of the pureblock copolymer and the composite 30% and 50% ZnO thin
layer surfaces only show agglomerations of the clustertype. This behavior is completely random and does not de-pend on the amount of nanoparticles in the system.
However, the composite 20% ZnO exhibited well-de-fined pores in the surface with an average height of543 nm and an average pore diameter of 2.67 lm. Thisbehavior is most likely due to the self-assembling propertyof the block copolymer with amphiphilic characteristicswhen interacting with the nanoparticles of ZnO.
This result does not indicate that the surface of the purecopolymer and its composites of 30% and 50% ZnO do nothave a porous surface, only that the probability of findingporosity in these composites is lower than in the 20%composite.
4. Conclusions
The insertion of ZnO nanoparticles into the blockcopolymer system was studied and confirmed throughFT-IR and Raman spectroscopy, optical methods, X-ray dif-fraction, and thermal and morphological characterizationtechniques. The self-assembled nanoscale morphology ofthe composites results in the formation of uniformly dis-tributed spherical nanoparticles within the polymer ma-trix, and the amount of ZnO nanoparticles cause changesin the copolymer behavior. The micelle-like supports for
Fig. 9. AFM micrographics of the block copolymer and composites at different concentrations of ZnO (20%, 30%, and 50%).
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ZnO particles based on the self-assembled block copolymer(content from 20 to 50 wt.%), show good transparency inthe visible region and excellent luminescent properties.Block copolymer composites with a high ZnO content areable to absorb UV irradiation below 350 nm, indicatingthat these composite films exhibit good UV screening ef-fects. Raman spectroscopy showed clear vibrational bandsof organic segments. Multiple peaks obtained by X-ray dif-fraction correspond to Wurtzite crystalline phase of ZnOand one broad peak was attributed to the interaction be-tween the organic matrix and the nanoparticles.
Thermogravimetric analyzes show that the ZnO nano-particles were successfully formed into the polymer matrixand that this diblock copolymer composite has a good re-sponse to thermal variations; it is therefore stable againstthermodynamic changes. The thermal stability of the pureblock copolymer and composite-50% ZnO were found to bestable up to 386 �C. However, the composite exhibited ahigher stability due to the presence of ZnO in the poly-meric structure.
The SEM images show that poly(St699-block-MMA211/MAA53)-ZnO principally consists of auto-assembled spher-ical micellar aggregations, resulting in nanodomains ofinterconnected networks of spherical aggregates over thepolymer surface and possibly into the polymer. Further-more, for the sample corresponding to composite 20% ofZnO it was possible to determine very precisely the aver-age height and diameter of the surface pore, 543 nm and2.67 lm, respectively. Lastly, a certain degree of porositythat depends on the percentage of ZnO nanoparticles wasfound.
The TEM studies indicate that the ZnO nanoparticleswere uniformly dispersed on the polymer. The TEM imagesof the ZnO composite gave additional information regard-ing the size and size dispersion of the nanoparticles withinthe copolymer matrix formed. These results indicated goodstability between the block copolymer containing ZnO.
Acknowledgments
The authors acknowledge the financial assistance of thiswork by the Fondo Nacional de Investigación Científica yTecnológica, FONDECYT Grants 1110836 and 11121281.The WCU Program funded by MEST (R31-10026). Thanksto Dra. Maria Jesus Aguirre (USACH) by Cyclic Voltamme-try measurements.
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