Zinc-Containing Block Copolymer as a Precursor for the - American

Article pubs.acs.org/Macromolecules

Zinc-Containing Block Copolymer as a Precursor for the in Situ Formation of Nano ZnO and PMMA/ZnO Nanocomposites Tomaž Kos,† Alojz Anžlovar,†,‡ David Pahovnik,† Ema Ž agar,†,‡ Zorica Crnjak Orel,†,‡ and Majda Ž igon*,†,‡ †

National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia Centre of Excellence PoliMaT, Tehnološki park 24, 1000 Ljubljana, Slovenia

‡

S Supporting Information *

ABSTRACT: We report on the synthesis of highly transparent and UV-absorbing PMMA/ZnO nanocomposites prepared by hydrolysis of a ZnO precursor, the A-b-(AB) diblock copolymer, poly(methyl methacrylate)-block-poly(methyl methacrylate-co-(zinc methacrylate acetate)), PMMA-b-P(MMA-co-ZnMAAc), synthesized by RAFT polymerization. The zinc content of the block copolymers was in the range from 3 to 13 wt %. The PMMA block provides inherent compatibility with the PMMA matrix, whereas the second block, P(MMA-co-ZnMAAc) with zinc ions, acts as a polymeric precursor for the formation of ZnO nanoparticles. The amphiphilic block copolymer self-organizes in THF and THF/H2O in ordered nanostructures, thereby influencing the nanoparticle formation during the hydrolysis of the precursor block copolymer with KOH in a solvent mixture THF/H2O. The ZnO nanoparticles were rod-shaped with lengths up to 80 nm and a diameter of 14 nm and were redispersible in THF. Dispersions in THF and thin films of PMMA/ZnO nanocomposite exhibit excellent transparency in the visible range and good absorption in the UV range below 400 nm. The block copolymer was characterized by SEC, NMR, DLS, and TGA, while PMMA/ZnO nanocomposites were characterized by IR, XRD, UV−vis, and STEM. dispersants,12,27 but their use for ZnO functionalization is seldom reported.28,29 Amphiphilic block copolymers have the additional advantage of self-organization, thereby forming confined spaces for NP synthesis.12,30 The self-assembly of polystyrene-block-poly(2-vinylpyridine) into micelles in solution was used as reaction vessels for the synthesis of ZnO NPs by Braun et al.28 and by El-Atwani et al.31 The majority of modifications/functionalizations of the NPs by polymers are performed on the already synthesized NPs.1 The polymer usually serves only as a template and is eliminated during the thermal synthesis of NPs at elevated temperatures (calcination).32,33 Reports of the in situ synthesis of ZnO NPs from polymeric precursors, prepared by free radical polymerization, are rare. Toprak et al.34 reported the formation of PMMA/ZnO nanocomposites by simultaneous in situ free radical polymerization of MMA and ZnO formation from zinc acetate dihydrate and monoethanol amine. In a study by Zhang et al.,35 random copolymers from MMA and asymmetric monomer zinc acetate methacrylate (ZnMAAc) were prepared by free radical polymerization. By thermal treatment of the precursor from 100 to 190 °C transparent composite thin films with ZnO NPs were synthesized. The authors do not provide information about the efficiency of UV absorption of the

1. INTRODUCTION Organic−inorganic nanocomposites are an emerging class of materials that hold significant promise due to their outstanding properties, which usually arise from a combined and/or synergistic effect of the properties of their organic and inorganic components.1 Many research groups have demonstrated the high potential of nano zinc oxide (ZnO) in various areas of applications such as electronics, optoelectronics, electrochemical and electromechanical devices,2 ultraviolet (UV) lasers,3 light-emitting diodes,4 field emission devices,5 high performance sensors,6 solar cells,7 and piezoelectric generators.8 PMMA/ZnO nanocomposite materials have gained a lot of attention due to their excellent optical and UV-absorbing properties.9−11 For applications of this material in field of optics, such as UV protecting films and plates, antireflective coatings, transparent barrier/protective layers, etc., it is crucial that nanoparticles (NPs) have a small size (up to 100 nm) and are homogeneously dispersed in the polymer matrix. Because NPs possess a high surface area to volume ratio, the aggregation/agglomeration of primary particles is unavoidable in many cases.9,12 It is possible to prevent aggregation by surface functionalization of the NPs.13,14 For this purpose small molecules like silanes,13−15 carboxylic or phosphoric acids,16−19 polyols,20 and polymers like PMAA,17,21 PVA,22 PVP,23 and PS,24 statistical copolymer P(MMA-coMAA),25 and P(S-co-MMA)26 are employed. It is also wellknown that amphiphilic block copolymers are very effective as © XXXX American Chemical Society

Received: May 17, 2013 Revised: July 24, 2013

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dx.doi.org/10.1021/ma4010296 | Macromolecules XXXX, XXX, XXX−XXX

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Table 1. Experimental Parameters for the Synthesis of the PMMA Macro-RAFT Agent by RAFT Polymerization (T = 60 °C) polymer A1 A2 A3 A1ext

mole ratio of MMA:AIBN:toluene:CPDB 1200 1200 1200 1800

1 1 1 1

400 400 400 600

polymerization time (h)

conversion (wt %)

Mn (SEC) (g/mol)

ĐM (SEC)

Mn (NMR)a (g/mol)

4 15 16 4

15 71 91 19

10 800 25 700 43 400 23 200

1.1 1.1 2.0 1.1

11 500 32 400 b 27 800

5 5 2 3c

a

Number-average molar mass Mn, determined by NMR, is calculated as a ratio of the integral of RAFT fragments at 7.7 ppm and that of the methoxy groups of PMMA at 3.6 ppm multiplied by the molar mass of MMA taking into account the ratio of protons of both groups. bNot evaluated due to the too low signal of the RAFT end group. cPolymer A1 was used as a macro-RAFT agent (see section 2.4 for details on the extension experiment).

Table 2. Experimental Parameters for the Synthesis of the PMMA-b-P(MMA-co-ZnMAAc) Block Copolymers in THF (T = 60 °C; t = 4 h, cpolymer = 37 mg/mL) block copolymer

PMMA block

n(PMMA) (mmol)

n(ZnMAAc) (mmol)

n(MMA) (mmol)

n(AIBN) (mmol)

Mn (SEC) (g/mol)

ĐM (SEC)

n(MMA):n(ZnMAAc) (NMR)

Zn2+ content (wt %) (NMR)

Zn2+ content (wt %) (TGA)

B1

A1

0.092

12.6

26.4

0.012

13

13

A1

0.092

3.1

6.6

0.012

1:0.14

7

8

B3

A3

0.023

2.8

0

0.152

1:0.11

6

5

B4

A2

0.039

1.6

0

0.012

1.15, 1.02 1.10, 1.09 1.09, 1.03 1.09, 1.02

1:0.33

B2

14 000,a 51 000 13 200, 42400a 43 600, 87400a 24 800, 57 400a

1:0.06

3

3

a

More intense peak in bimodal molar mass distribution of the copolymer.

2. EXPERIMENTAL SECTION

nanocomposite; only qualitative evidence about the absorption in the UV region below 400 nm is given. A few reports on the synthesis of polymer/ZnO nanocomposites from polymeric precursors by its chemical conversion were made by Mulligan36 and Ambrožič.37 Mulligan et al. synthesized amphiphilic block copolymers poly(norbornene)−block-poly(norbornene−dicarboxylic acid) by living polymerization. The subsequent binding of zinc ions and hydrolysis of the resulting ionomer in NH4OH at room temperature resulted in the formation of polymer/ZnO nanocomposite material. Ambrožič et al. employed zinc dimethacrylate as a starting monomer.37 By its free radical polymerization a highly cross-linked poly(zinc dimethacrylate) was obtained which served as a polymeric precursor for the synthesis of PMMA/ZnO nanocomposite by hydrolysis in a NaOH/1-butanol suspension at 80 °C. In either case no information about the optical properties of the resulting materials is given. The aim of this work was to prepare a homogeneous dispersion of nano zinc oxide in the PMMA matrix for optical applications using A-b-(AB) diblock copolymer poly(methyl methacrylate)-block-poly[(methyl methacrylate)-co-(zinc methacrylate acetate)], PMMA-b-P(MMA-co-ZnMAAc). We combined two features enabled by the use of amphiphilic block copolymers. First, we took advantage of the self-organization of block copolymers and their influence on the growth and stabilization of NPs by constraints in space. Second, ZnO NPs were synthesized in situ from the precursor block copolymer by KOH hydrolysis at room temperature. This way we synthesized PMMA/ZnO nanocomposites with homogeneous ZnO distribution in the polymer that is suitable for optical applications with high UV absorption and transparency in the visible range. Amphiphilic block copolymers PMMA-b-P(MMA-coZnMAAc) were prepared by reversible addition−fragmentation chain transfer (RAFT) polymerization since it allows for good control over the molar masses, dispersity, and end groups in the polymer and is suitable for the synthesis of block copolymers22 and for the preparation of the polymer/inorganic nanohybrids.1

2.1. Materials. Methyl methacrylate (MMA), methacrylic acid (MAA), azobis(isobutyronitrile) (AIBN), acetic acid, basic zinc carbonate (Zn5(CO3)2(OH)6), RAFT agent 2-cyano-2-propyl benzodithioate (CPDB), NaOH, KOH, THF, methanol, and toluene were of analytical grade, purchased from Sigma-Aldrich, and used as received without any further purification. MAA and MMA were distilled under reduced pressure; MMA was distilled after removal of the inhibitor with a 5% aqueous NaOH solution. AIBN was recrystallized from methanol. 2.2. Synthesis of Asymmetric Monomer Zinc Methacrylate Acetate (ZnMAAc). Basic zinc carbonate (11 g, 0.02 mol) was added in small portions to an aqueous solution (20 mL water) of acetic acid (6 g, 0.1 mol) and methacrylic acid (8.6 g, 0.1 mol). The progress of the reaction was clearly visible by the CO2 evolution. The resulting suspension was left mixing at room temperature for an additional 2 h. The insoluble precipitate was then filtered off and washed with 15 mL of water. Reactants and water were removed from the collected fractions by evaporation under reduced pressure at 50 °C. The resulting product mixture, a white crystalline solid of ZnMAAc and zinc diacetate, was purified by selective dissolution of ZnMAAc in THF (3 h at room temperature (RT), solvent:product mixture = 5:1 w/w). After solvent removal in a stream of argon, the product was collected as a white crystalline solid (8.6 g, 43%). 1H NMR (D2O, ppm): δ 1.94 (3H, s), 1.88 (3H, m), 5.38 (1H, m), 5.70 (1H, m). 2.3. Synthesis of PMMA Macro-RAFT Agent (A1−A3). The synthesis of the PMMA macro-RAFT agent was performed by RAFT polymerization. The general synthesis procedure is as follows: A round-bottom flask was charged with 28 g (280 mmol) of MMA, 10 mL of toluene, 40 mg (0.24 mmol) of AIBN, and 256 mg (1.16 mmol) of CPDB. The reaction mixture was degassed through three freeze− pump−thaw cycles. Polymerization was carried out by immersing the reaction flask into an oil bath at 60 °C for a certain period of time (Table 1). The polymer was isolated and purified by repeated precipitation from methanol. The polymer was dried at reduced pressure to a constant mass at room temperature. The monomer conversion was determined gravimetrically. 2.4. Extension of the PMMA Macro-RAFT Agent A1 (Synthesis of A1ext). 400 mg of macro-RAFT agent A1 and 2 mg (0.012 mmol) of AIBN were added to a solution of 2.2 g (22 mmol) of MMA in 700 mg of toluene. The reaction mixture was degassed through three freeze−pump−thaw cycles. The polymerization of the added B


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monomer proceeded at 60 °C for 4 h. The polymer was isolated by precipitation from methanol and dried under vacuum to a constant mass at RT. The monomer conversion was determined gravimetrically. 2.5. Synthesis of Zinc-Containing Block Copolymers (B1−B4). In a typical procedure, 0.5 g of the PMMA macro-RAFT agent (A1− A3) was dissolved in 15 g of THF; then the ZnMAAc monomer or a mixture of ZnMAAc and MMA and AIBN were added (Table 2). The reaction mixture was degassed through three freeze−pump−thaw cycles and polymerized at 60 °C for 4 h. The resulting A-b-(AB) diblock copolymer PMMA-b-P(MMA-coZnMAAc) was isolated by precipitation from methanol and dried under vacuum to a constant mass at RT. 2.6. Removal of the RAFT (Dithiobenzoate) End Group from Homopolymers and Zinc-Containing Block Copolymers. In a typical procedure, 0.5 g of polymer with RAFT end groups was dissolved in 15 g of THF, and 100 mg of AIBN was added. The reaction mixture was degassed through three freeze−pump−thaw cycles. End group removal was executed by heating the solution at 60 °C for 3 h. The polymer was isolated by precipitation into methanol and dried under vacuum to a constant mass at RT (white powder). Removal of the RAFT end group was confirmed by NMR and UV spectroscopy of the polymers. 2.7. Synthesis of ZnO Nanocomposite (ZnO/B2−ZnO/B4) by Hydrolysis of Zinc-Containing Block Copolymers. In a typical procedure, a solution of 150 mg of zinc containing block copolymer (B2 to B4) in 30 g of THF was prepared. 150 mg of H2O was then added and sonicated for 5 min. After the addition of 50 mg of KOH the reaction mixture was sonicated for an additional 5 min. After stirring for 40 h at RT, the solution was filtered through the PTFE filter (0.45 μm) and precipitated with MeOH. The nanocomposite was isolated by centrifugation at 8500 rpm for 30 min and purified by washing with methanol. Alternatively, excess KOH was removed from the nanocomposite powder by repeated extraction with water at RT. The yield was determined gravimetrically. The course of ZnO nanoparticle formation from the block copolymer precursor was determined by UV measurement by measuring the transmittance at 350 nm. 2.8. Synthesis of PMMA by Free Radical Polymerization (PMMAFRP). A solution of 40 mg (0.24 mmol) AIBN in 10 g (100 mmol) of MMA was transferred into a glass reactor (dimensions 10 × 10 × 0.15 cm) and polymerized for 18 h at 75 °C. Mn was 877 000 g/mol and molar-mass dispersity ĐM of 2.7 as determined by SEC. 2.9. Preparation of Thin Films from PMMA/(ZnO/B3 Nanocomposite). 50 mg of PMMAFRP solution in THF was mixed with 17 mg of ZnO/B3 nanocomposite in 3.5 g of THF. The dispersion was homogenized by ultrasound bath and then transferred to the glass plate creating a transparent polymer layer by slow evaporation of the THF at RT (film thickness 45 μm). 2.10. Characterization. Details are provided in the Supporting Information.

Scheme 1. Synthetic Approach to PMMA/ZnO Nanocompositesa

a

Reactions: (a) preparation of PMMA with RAFT (dithiobenzoate) end group, (b) preparation of PMMA-b-P(MMA-co-ZnMAAc) diblock copolymer as the polymeric precursor with RAFT end group, (c) removal of the RAFT end group from the diblock copolymer, and (d) in situ synthesis of ZnO nanoparticles by hydrolysis of the precursor block copolymer with KOH in a THF/H2O mixture.

Thus, we could synthesize PMMA/ZnO nanocomposites with homogeneous ZnO distribution in the polymer film, suitable for optical applications with high UV absorption and transparency in the visible range. 3.1. Synthesis of Asymmetric Monomer Zinc Methacrylate Acetate (ZnMAAc). The original synthesis procedure for ZnMAAc using ZnO as a starting material35,38 results in a mixture of asymmetric zinc methacrylate acetate and symmetric zinc diacetate (70:30) dissolved in water. Zinc dimethacrylate precipitates during the reaction due to its limited solubility and can be separated by filtration. In our work, we modified the original procedure38 by replacing ZnO with a basic zinc carbonate. The neutralization reaction is thus easy to follow due to the evolution of CO2 bubbles. The product of this reaction is a mixture of ZnMAAc and zinc diacetate Zn(Ac)2. Pure ZnMAAc was obtained by its selective dissolution in THF at RT. The effective separation of ZnMAAc and Zn(Ac)2 was confirmed by NMR (ratio of methyl methacrylate to acetate) and XRD analysis (Figure S1). 3.2. Synthesis of Block Copolymers. The RAFT polymerization method, one of the reversible-deactivation radical polymerizations39 (also called “controlled” or “living” radical polymerizations), was chosen for the preparation of block copolymers as it allows good control over the molar mass, molar mass dispersity, and polymer end groups and is suitable for the synthesis of block copolymers.1 2-Cyano-2propyl benzoditioate (CPDB) was selected as a RAFT agent, as it allows good control over methacrylate polymerization.40,41 In the first step, PMMA macro-RAFT agents A1−A3 were synthesized. As can be seen in Table 1, the molar mass of the PMMA macro-RAFT agent increases with conversion (from 10 800 g/mol after 4 h for A1 to 25 700 g/mol after 15 h for A2), whereas the dispersity remains at around 1.1, thereby confirming the controlled nature of the polymer growth. The

3. RESULTS AND DISCUSSION The aim of this work was to prepare a homogeneous dispersion of nano zinc oxide in a PMMA matrix for optical applications. PMMA-b-P(MMA-co-ZnMAAc) was prepared (Scheme 1) in order to synthesize ZnO nanoparticles in situ from the precursor A-b-(AB) diblock copolymer. PMMA was selected for the first block as it assures solubility in organic solvents and provides inherent compatibility with the PMMA matrix. Asymmetric zinc methacrylate acetate (ZnMAAc) was selected as a suitable monomer for the second block in order to insert Zn2+ directly into the polymer without the risk of chemical cross-linking, as in the case of zinc dimethacrylate. ZnO NPs were synthesized in situ by hydrolysis of the precursor block copolymer PMMA-b-P(MMA-co-ZnMAAc) at room temperature. After hydrolysis of the ZnMAAc segments, the resulting free carboxylate groups provide compatibility with the ZnO nanoparticles, covering the surface of the nanoparticle. C


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PMMA dead polymer chains that were created during the synthesis of the P(MMA-co-ZnMAAC) block. Since it is known that thiocarbonylthio compounds are prone to both acid and base-catalyzed hydrolysis,42 we attributed the loss of the end groups to the partial hydrolysis of the dithiobenzoate group by the basic acetate group from ZnMAAc monomer. This also means that RAFT polymerization of ZnMAAc monomer (under selected conditions) is partially impeded by hydrolysis, and consequently, the chain extension of the second block is only limited. The block copolymer B1 with 13 wt % Zn2+ was only poorly soluble even in polar solvents like DMAc. The reason for this could be the association of the polar groups in the block copolymer leading to a physical cross-linking, which becomes more pronounced with the length of the second block. The block copolymers with lower zinc content (below approximately 10 wt %) are soluble in solvents such as THF, DMAc, and DMSO. Therefore, we can estimate that the maximal acceptable loading of zinc in this system is below 10 wt %. For this reason copolymer B1 was not used for the preparation of ZnO nanocomposites. 3.3. Removal of the RAFT End Group. For transformation of PMMA-b-P(MMA-co-ZnMAAc) into transparent PMMA/ZnO nanocomposites, the chromophoric RAFT end group43−45 should be removed since it worsens transparency in the visible light range. We removed RAFT end groups by radical substitution with AIBN.43 The course of the reaction can be monitored qualitatively as the color of the polymer changes from pink to white. Removal of the RAFT end group is observed in NMR spectra as the signals of the RAFT end group in the 7.4−8.0 ppm region disappear. Quantitative evidence of end group removal is also provided by SEC measurements of the (co)polymers where the intensity of UV absorption at 310 nm provides evidence of 99% removal of the RAFT end groups. 3.4. Self-Organization of the PMMA-b-P(MMA-coZnMAAc) in THF. The basic hypothesis of this work is the self-organization of PMMA-b-P(MMA-co-ZnMAAc) amphiphilic block copolymer into micelles in a selective solvent, i.e., a good solvent for one block and a precipitant for the other.46−48 It can self-assemble into various nanostructures such as spherical, ellipsoid or elongated micelles, bilayers, etc., in solution31,49,50 or in film,12,30 thereby providing reaction vessels for ZnO NPs growth from the precursor block copolymer. Thus, it allows for homogeneous dispersion of ZnO NPs in the PMMA matrix due to the stabilization effect of the polar carboxylate and acetate groups originating from hydrolyzed and non-hydrolyzed ZnMAAc segments. In order to test this hypothesis, dynamic light scattering (DLS) was performed for a suspension of the precursor block copolymer in THF and a mixture of THF/1 wt % water. The block copolymer self-organized in THF into ordered nanostructures with an average size of 86 and 30 nm for B3 and B4, respectively. The addition of minute amounts (1 wt %) of water resulted in formation of larger aggregates with an average size of 241 and 128 nm for B3 and B4, respectively. Aggregates have a relatively narrow distribution with a dispersity factor σ below 0.2 (Figure S5). 3.5. Preparation of PMMA/ZnO Nanocomposites by Alkali Hydrolysis of PMMA-b-P(MMA-co-ZnMAAc). Synthesis of the ZnO nanoparticles was performed in a solution of the precursor block copolymer in mixed solvent THF/H2O (1 wt %). After the addition of KOH (0.2 wt %) to the precursor

lower ratio of CPDB to AIBN in A3 allows higher molar masses of the resulting macro-RAFT agents, but at the cost of higher molar-mass dispersity ĐM. The PMMA dithiobenzoate end groups of CPDB fragments were confirmed by NMR spectroscopy (Figure S2), also showing a good correlation between the number-average molar masses (Mn) and those determined by SEC. From the literature it is known that in statistical copolymerizations of MMA and ZnMAAc a substantial amount of gel can be formed through physical cross-linking.38 Because of that, the second block of PMMA-b-P(MMA-co-ZnMAAc) with ZnMAAc units should be much shorter than the PMMA block. In order to evenly disperse polar functional groups and thus provide better solubility of the copolymer, MMA was also used as a comonomer in the second block (B1, B2). THF was selected as a solvent for the polymerization reaction as it is a good solvent for both monomers and the polymer up to approximately 10 wt % Zn2+. Solution concentration during the synthesis of the P(MMA-co-ZnMAAc) block was kept relatively low in order to minimize the extent of gel formation. The length of the second block was adjusted through the molar ratio of ZnMAAc and MMA monomers as well as the initiator content, while the temperature and time of polymerization were kept constant. Table 2 summarizes the synthetic parameters for the preparation of block copolymers together with the molar masses of the second block and the zinc content in the block copolymers. Zinc content in block copolymers was determined as a ZnO residue at 600 °C from the TGA measurements, being in the range between 3 and 13 wt %. These values are in good correlation with the zinc content calculated from the ratio of MMA to ZnMAAc units in block copolymers by NMR spectroscopy (Table 2, Figure 1, and Figure S3).

Figure 1. 1H NMR spectrum of the copolymer B1, in solvent DMSOd6 with added TFA.

The SEC chromatograms of PMMA-b-P(MMA-coZnMAAc) block copolymers exhibit a bimodal distribution (Figure S4). The peaks at higher eluation volumes are attributed to the PMMA homopolymers and those at lower elution volumes to the block copolymers. The extension experiment (sample A1ext, Table 1 and Figure S4) shows that the PMMA macro-RAFT agent is capable of further growth. The UV absorption of the polymer (at higher elution volume peak) above 300 nm is smaller than that of the peak of PMMA macro-RAFT agent, indicating a loss of dithiobenzoate end groups. It can be concluded that this peak belongs to the D


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formed PMMA/ZnO nanocomposites in the visible light range and so it was not considered for further studies. After hydrolysis of B3 and B4, the obtained PMMA/ZnO nanocomposites were redispersed in THF. Their UV−vis spectra (Figure 2b) were evaluated according to Demir et al.51 We estimated the transmittance at 550 nm (as a reference value for absorption in visible range) and transmittance at 350 nm (as a reference value for absorption in ultraviolet range). From the absorption at 350 nm and the Beer−Lambert law (eq 2a in Supporting Information), it is possible to calculate the concentration of the ZnO in PMMA/ZnO nanocomposites in THF, where the molar absorption coefficient ε of 971 L mol−1 cm−1 is calculated from literature data for nano ZnO of comparable size.51 By comparing the calculated values of ZnO with zinc content in the precursor polymer (Table 2), a conversion of the Zn2+ salt to ZnO nanoparticles was calculated to be in the range of 80% (Table 3). This value should be regarded as an estimation since there is a very pronounced blue shift of the absorption edge of ZnO as the particle size decreases, which could lead to an underestimation of the conversion efficiency.52 From the data in Table 3 and the UV−vis spectra in Figure 2b, we can conclude that nanocomposites exhibit high transparency in the visible region and excellent absorption in the UV region of light. 3.6. Analysis of the PMMA/ZnO Nanocomposites. We characterized the synthesized PMMA/ZnO nanocomposites by XRD, STEM, and IR spectroscopy. Figure 3 shows the XRD

solution at room temperature the ZnMAAc segments hydrolyzed into zinc oxide. The yield of the reaction after isolation and purification was 84% and 58% for ZnO/B3 and ZnO/B4, respectively. Formation of ZnO NPs was monitored by UV absorption at 350 nm. As can be seen from the inset in Figure 2b, the biggest change in the transmittance occurs in the first 30

Figure 2. (a) UV−vis transmission spectra of thin films on a glass substrate prepared from PMMAFRP (solid line) and nanocomposite ZnO/B3 (broken line, c(ZnO) = 1.5 wt %). (b) UV−vis transmission spectra of the PMMA/ZnO nanocomposite ZnO/B4 (solid line, c(ZnO/B4)a = 1 wt %) and ZnO/B3 (broken line, c(ZnO/B3) = 0.67 wt %) in THF. Inset shows transmittance at 350 nm during the hydrolysis of precursor polymer B2 with KOH in THF/H2O.

min, while after that a steady decrease with time is evident. We presume that at the beginning of the reaction nuclei are formed, which then grow gradually over time. Precursor block copolymers B3 and B4 with 5 and 3 wt % Zn2+, respectively, were used for the preparation of the PMMA/ZnO nanocomposites and their characterization according to Demir et al.51 (Table 3). Preliminary results on the precursor copolymer B2 showed inferior properties of the

Figure 3. XRD pattern of ZnO/PMMA nanocomposite ZnO/B3. Red lines represent the XRD reference spectrum of ZnO (Zincite, JCPDS 00-036-1451). The peak at 44° (assigned by asterisk) originates from the sample holder.

diffractogram of the purified sample ZnO/B3. Typical lines arise from ZnO NPs (at 31.8°, 34.4°, 36.3°, 47.5°, and 56.6°), showing good correlation with the ZnO reference XRD spectrum (Zincite, JCPDS 00-036-1451, wurzite crystal structure type). In addition, there is a broad peak with a maximum at around 15°, characteristic of amorphous polymers and originating from the copolymer PMMA-b-P(MMA-coZnMAAc).25 In comparison to the reference spectrum of zincite, we can see an increase in the intensity of the (002) reflection peak at 34.4° which is narrower than other peaks. We attribute this characteristic to a nonsymmetric rod-shaped particle form. The average size of the crystallite particles was 20 nm, as evaluated by Scherrer’s formula.51 The STEM micrographies of ZnO nanocomposites are presented in Figure 4 and support the XRD results. We can see a fine dispersion of ZnO nanorods in the PMMA matrix that have lengths of approximately 80 nm and widths of around 14

Table 3. UV Properties of the Selected PMMA/ZnO Nanocomposite Dispersions in THF nanocomposite concentration in THF (wt %) transmittance at 550 nm (%) transmittance at 350 nm (%) conversion of Zn2+ to ZnO (%) calculated ZnO content (wt %)

ZnO/B3 (5 wt % Zn2+)

ZnO/B4 (3 wt % Zn2+)

0.67

1.00

94

94

0.04

0.18

78

83

0.03

0.03

E


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where a band of the CO group of PMAA shifted from 1575 to 1550 cm−1. Tang et al. assigned the shift of the CO band toward the lower wavenumber to the strong interaction of the polymer and the nano ZnO at the interface.21,25 3.7. Thin Films. In order to demonstrate the practical use of the synthesized PMMA/ZnO nanocomposites in commercial PMMA materials, synthesized by radical polymerization, the UV-absorbing thin films were prepared on a glass substrate from ZnO/B3 and PMMAFRP (Mn = 877 000 g/mol). Thin film can be prepared directly from the nanocomposite dispersion after the hydrolysis or from the isolated/purified nanocomposite dispersed in THF (Scheme 2). The nanoScheme 2. Application of a Mixture of PMMA and PMMA/ ZnO Nanocomposite in THF onto a Glass Substrate Results in a Transparent and UV Absorbing Thin Film

Figure 4. Dark field STEM micrograph of the PMMA/ZnO nanocomposite ZnO/B3.

nm for ZnO/B3. In the case of ZnO/B4 (in Figure S6), nanostructures are smaller (average lengths below 20 nm). This difference in size between both ZnO NPs is in correlation with the DLS measurements, which in the case of ZnO/B3 in THF also shows bigger nanostructures than for ZnO/B4. This result can be seen as direct evidence of the correlation between the size of the ZnO NPs and Zn2+ content as well as the length of the second block of the precursor PMMA-b-P(MMA-coZnMAAc). Förster and Antonietti rationalize several examples of ionic block copolymers with cylindrical or worm-like morphologies in suitable solvents in terms of the geometric packing parameter which is increased by solvent ionic strength and an increase in the core-block length.50,53 Since STEM micrographs of block copolymers B3 and B4 prepared from THF and THF/H2O reveal only a spherical morphology of the micelles (with some aggregates having irregular shapes, see Figure S7), we presume that the change in shape from spherical into rod-like micelles occurs upon addition of KOH which increases the ionic strength of solvent. The formed ZnO nanorods thus reflect the shape of the micelles, which is supported by several literature reports28,31 claiming that nanoparticles retain the shape of the micelles (nanoreactors) after synthesis. The nature of the interaction between the block copolymer and the surface of the ZnO NPs was explored using FTIR spectroscopy (Figure S8). The IR spectrum of the asymmetric monomer ZnMAAc (Figure S8a) is dominated by two strong and broad absorption bands at 1561 and 1424 cm−1. Zhang et al.35 assigned these bands to absorption of unidentate coordination modes of the acetate group with zinc. The band at 1655 cm−1 can be attributed to the stretching of the CC double bond.35 In the IR spectrum of the block copolymer (Figure S8c) the PMMA bands prevail. The absorption band at 1730 cm−1 is characteristic of the CO stretching vibrations of PMMA,25 and the bands at 1484, 1449, 1437, and 1388 cm−1 correspond to the polymer backbone (Figure S8b). A distinct (although) broad band at 1586 cm−1 of the asymmetric stretching vibrations of the CO group can be assigned to the interaction of Zn2+ with COO− groups in the block copolymer B4. After hydrolysis, the IR spectrum of the resulting PMMA/ ZnO nanocomposite ZnO/B4 (Figure S8d) shows a nearly identical spectrum to the block copolymer (Figure S8c), except for characteristic shift of the CO band to a lower value (1560 cm−1). The same trend was observed by Tang et al.21 when modifying nano ZnO with poly(methacrylic acid), PMAA,

composite dispersion in THF was mixed with a PMMAFRP solution in THF to obtain the targeted ZnO content. The mixture was then applied onto a glass substrate. Transparent thin films of around 40 μm were formed by slow evaporation of the solvent at room temperature (Scheme 2). The UV−vis transmission spectra of thin films of the PMMA homopolymer and ZnO/B3 nanocomposite on glass substrate are shown in Figure 2a. The UV−vis spectrum of the PMMA exhibits transmittance in the visible range higher than 90%, while in UV region transmittance decreases with decreasing wavelength. The UV−vis spectrum of the ZnO/B3 nanocomposite shows transmittance in the visible range higher than 85% and substantially lower values below 390 nm due to the UV absorption of ZnO NPs, which makes it a good candidate for further use in optical applications as a UV absorber.

4. CONCLUSION The aim of this work was to synthesize the A-b-(AB) diblock copolymer PMMA-b-P(MMA-co-ZnMAAc) by RAFT polymerization as a precursor for the preparation of highly transparent UV-absorbing thin films of PMMA/ZnO nanocomposites. The PMMA block provides compatibility with the PMMA matrix and organic solvents whereas the P(MMA-co-ZnMAAc) block serves as a polymeric precursor for the in situ ZnO NPs formation. By RAFT polymerization it was possible to prepare block copolymers with a Zn2+ content ranging between 3 and 13 wt %. As shown by DLS, the block copolymer self-organized in THF into ordered nanostructures with an average size of 30− 90 nm, depending on the Zn2+ content, while in the mixed solvent THF/1 wt % H2O the size increased to 130−240 nm, with narrow dispersities in both solvents. Nano ZnO embedded in the PMMA matrix was prepared in situ by hydrolysis of precursor diblock copolymer with KOH in THF/H2O (1 wt %) at room temperature. ZnO formation proceeded fast in the first 30 min and then steadily decreased as shown by UV−vis spectroscopy. XRD confirmed the presence of a ZnO crystalline phase. STEM microscopy revealed rodshaped ZnO NPs of relatively uniform size of up to 80 nm length and 20 nm width. The sizes of the nanorods were smaller for the amphiphilic block copolymer with a lower Zn2+ F


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content. This finding is consistent with the smaller sizes of ordered structures for block copolymers with lower Zn2+ content as determined by DLS, leading to the conclusion that ZnO NPs size is in correlation with the Zn2+ content of the block copolymer. The ZnO NPs are homogeneously dispersed in PMMA matrix due to the stabilization effect of carboxylate groups originating from hydrolyzed ZnMAAc units as shown by IR spectroscopy.

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ASSOCIATED CONTENT

S Supporting Information *

A detailed description of the characterization techniques used (section 2.10) and Figures S1−S8. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the Slovenian Research Agency for their financial support (program P2-0145) and the PoliMaT Centre of Excellence for providing TGA measurements. The authors thank Dajana Japić for the UV measurements and dr. Helena Gradišar for the DLS measurements.

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REFERENCES

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Zinc-Containing Block Copolymer as a Precursor for the - American

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