Butyl Acrylate

Jan 22, 2013 - Iodine and the two-monomer mixture were polymerized by sodium metabisulfite and potassium persulfate as a redox initiation system...
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Emulsion Copolymerization of Methyl Methacrylate/Butyl Acrylate/ Iodine System to Monosize Rubbery Nanoparticles Containing Iodine and Triiodide Mixture Hossein Riazi, Naser Mohammadi,* and Hadi Mohammadi Loghman Fundamental Research Group, Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, P.O. Box 15875-4413, Tehran, Iran ABSTRACT: Monosize poly(methyl methacrylate-co-butyl acrylate) rubbery nanoparticles, approximately 95 nm in diameter, containing iodine and triiodide with a molar ratio of 2.78 of the former to the latter, were synthesized by emulsion polymerization. Iodine and the two-monomer mixture were polymerized by sodium metabisulfite and potassium persulfate as a redox initiation system. Iodine evolution to triiodide during the copolymerization was traced by ultraviolet−visible spectroscopy. Iodine addition to the monomer mixture and its copolymerization not only reduced the copolymer average molecular weight by 30%, but also led to the formation of more MMA-enriched chains. The redox initiation system also changed 26% of initial iodine molecules into triiodide, while the rest were encapsulated as molecular iodine. During thermal annealing, the lower rate of triiodide decomposition into iodine and iodide in comparison with the iodine evaporation rate broadened both thermogravimetric and heat flow curves.

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INTRODUCTION Iodine-doped polymers are widely used as electrolytes in dyesensitized solar cells.1−3 In these systems, iodine molecules are converted into triiodides after reaction with iodides. In the presence of an electron-donating polymer such as poly(methyl methacrylate) (PMMA), triiodide and iodide form a redox system capable of transferring electrical charge.4,5 In spite of the intrinsic tendency of PMMA to electron donation, brittleness hinders its extensive applications as a solid polymer electrolyte.6,7 Moreover, depolymerization of PMMA at elevated temperatures via an unzipping mechanism is another important deficiency against its further development.8 MMA copolymerization with other electron-donating monomers such as n-butyl acrylate (BuA) has been practiced to simultaneously improve its mechanical and thermal properties. MMA copolymerizaion with BuA reduces the glass transition of PMMA-based electrolyte and facilitates transfer of the iodide/triiodide redox pair resulting in higher energy conversion.9 Dissolution of iodine and a polymer in their common solvent is the most usual approach to incorporate iodine into polymeric systems. Evaporation of the common solvent from cast polymer/iodine solution forms iodine-containing polymeric film.10−13 Miniemulsification of polymer/iodine solution in a nonsolvent for iodine, water for example, also leads to polymer hybrid particles containing iodine.14−17 Iodine vapor permeation through polymeric films can be nominated as another approach for preparing a solid electrolyte. By iodine vapor diffusion through a polymeric film, its molecules are precipitated.18−20 Nonetheless, the extensive application of organic solvents, a long period of solvent evaporation, and prolonged iodine vapor diffusion through polymer are the major drawbacks of the aforementioned methods, respectively. These economic and environmental problems have been the motives behind sustained scientific efforts for replacing solvent © 2013 American Chemical Society

and vapor based processes with more economical and environmentally friendly approaches. Polymerizations in aqueous mediaemulsion, miniemulsion, and suspensionwith lower expenses and environmental pollution appear as suitable methods for preparing solid polymer electrolytes. These techniques may lead to hybrid nanoparticles dispersed in an aqueous medium. By using such latexes, control of interfacial properties and morphological characteristics of the solid polymer electrolytes is more feasible.21,22 The mechanism of water-based polymerization in the presence of iodine molecules is, however, complex due to iodine chain transfer and retardation roles in the course of free radical polymerization.23 The chain transfer ability of iodine has been extensively used in a certain branch of controlled/living free radical polymerization named “reverse iodine transfer polymerization” (RITP).24 The most prominent advantage of this method is accurate control over the polymer molecular weight and its distribution. In spite of extensive advantages, the approach prevents iodine entrapment due to its covalent bond formation with the chain backbone.25 Deficiency of the C−I covalent bond in transferring electrical charge and the absence of iodine and triiodide as the redox pair in synthesized systems by RITP clarify its inappropriateness for production of solid polymer electrolytes. Modification of the initiation system of RITP, however, prevents C−I bond formation resulting in production of iodine-entrapped polymers.26 In this research, monosize poly(methyl methacrylate-co-butyl acrylate) nanoparticles, 95 nm in diameter, containing an iodine and triiodide mixture were prepared via emulsion copolymerization. Application of the redox initiation system led to the Received: Revised: Accepted: Published: 2449

August 19, 2012 January 14, 2013 January 22, 2013 January 22, 2013 dx.doi.org/10.1021/ie303063b | Ind. Eng. Chem. Res. 2013, 52, 2449−2456

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of the copolymer was then calculated using appropriate Mark− Houwink−Sakurada constants.28 The same procedure was repeated for I2-MBC50 after removal of its iodine in a vacuum oven at 110 °C and a pressure of 200 mbar. Fourier Transform Infrared Spectroscopy (FTIR). Each cast and dried film of MBC50, I2-MBC50, and the reference blend was scanned 32 times by a Bomem instrument with a precision of 4 cm−1. Differential Scanning Calorimetry (DSC). After drying a part of each latex and extracting its ionic surfactant, heat flow curves of MBC50 and I2-MBC50 films were collected in a heating and immediate cooling cycle from −40 to 220 °C at 10 °C/min with a TA Instrument under nitrogen atmosphere. A film was also prepared by casting the reference blend solution and evaporating its solvent. A DSC thermogram of the reference blend film was recorded from 30 to 220 °C with the same temperature ramp. Thermogravimetric Analysis (TGA). The test was carried out under nitrogen atmosphere with a heating rate of 10 °C/ min using a Shimadzu TGA-50H thermal analyzer on MBC50, I2-MBC50, and reference blend films from 30 to 600 °C. Ultraviolet−Visible (UV−Vis) Spectroscopy. MBC50, I2MBC50, and reference blend solutions in chloroform were prepared. The spectrum of each solution was recorded by a Perkin-Elmer Lambda 25 spectrophotometer from 250 to 700 nm. In addition, I2-MBC50 films were annealed at 110 °C under a pressure of 200 mbar for 1.5 and 4 h. Subsequently, the annealed films were dissolved in chloroform and the UV−vis spectrum of each solution was recorded from 250 to 700 nm. The extinction coefficient of iodine at 510 nm was determined by UV−vis spectroscopy of its solutions in chloroform with known concentrations. The absorbance intensity of each solution at 510 nm was measured and plotted versus its concentration. Using a curve-fitting technique, the slope of the fitted line was determined and considered to be the extinction coefficient of iodine. Aqueous solutions of triiodide were also prepared by dissolution of iodine and sodium iodide in water.29 The aforementioned procedure was adopted to estimate the extinction coefficient of triiodide at 370 nm.

entrapment of nearly 74% of initial iodine molecules, while the rest were converted into triiodide during the course of the polymerization. Therefore, the copolymer and the final latex are unique in nanostructure, particle size, particle size distribution, and filled species. Accordingly, conventional approaches for making iodine-entrapped polymers can be replaced by the proposed method as systematic transfer of iodine to triiodide during the course of copolymerization is also feasible.

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EXPERIMENTAL SECTION Materials and Copolymerization. Methyl methacrylate and butyl acrylate, the redox initiation system of potassium persulfate (K2S2O8) and sodium metabisulfite (Na2S2O5), ionic surfactant of sodium lauryl sulfate (SLS), and tetrahydrofuran were purchased from Merck and used without any further purification. Iodine and sodium iodide were received from Scharlau and Apply Chem., respectively. Double distilled water was supplied by Kimidaroo, while chloroform was purchased from Dr. Mojalali. Iodine-free poly(methyl methacrylate-co-butyl acrylate) with 50 wt % methyl methacrylate (MBC50) and its iodinecontaining version (I2-MBC50) were synthesized through emulsion copolymerization. For synthesizing MBC50, sodium metabisulfite was added to the polymerization medium first, while potassium persulfate was added afterward. A similar sequence was followed for synthesizing I2-MBC50 after iodine dissolution in the monomer mixture. Emulsion copolymerized iodine/monomer mixture by sodium metabisulfite was designated as I3−-MBC50. Polymerization of this sample was conducted via iodine dissolution in the monomer mixture and addition of sodium metabisulfite, respectively (Table 1). The Table 1. Recipes of Emulsion Copolymerizations ingredient (mol) 2

MMA (×10 ) BuA (×102) water K2S2O8 (×104) Na2S2O5 (×103) SLS (×104) iodine (×104)

MBC50

I2-MBC50

I3−-MBC50

5.5 4.3 4.94 4.44 1.26 1.32 −

5.5 4.3 4.94 4.44 1.26 1.32 6.54

5.5 4.3 4.94 − 1.26 1.32 6.54

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RESULTS The populations of particles in volume percent versus their size for MBC50 and I2-MBC50 latexes are shown in Figure 1. The average particle sizes of MBC50 and I2-MBC50 were estimated to be 110 and 95 nm, respectively, while the size distribution index of both latexes was equal to 1.09. The viscosity average

recipes were designed to obtain monosize nanoparticles. The ingredients of the recipes were added to 120 cm3 glass bottles with a filling factor of 0.83. After the bottles were sealed, each one was located in a tumbling reactor containing 40 °C water and turned over and over for 24 h at 40 rpm. The monomer conversion was found to be close to 100% using gravimetric analysis. Preparation of the Reference Blend. Iodine, 1.5 wt % with respect to the copolymer, was added to 2 wt % solution of MBC50 in chloroform. The reference film or MBC50/iodine blend was formed by casting the solution and evaporating its solvent at 60 °C and a pressure of 200 mbar. Nanoparticle Characterization. The size and size distribution of MBC50 latex particles were determined by dynamic light scattering (DLS) (Mastersizer 2000, Malvern). After part of the MBC50 latex was dried in an oven at 30 °C for 1 week, its ionic surfactant was extracted by repeated soaking in hot distilled water.27 Dried surfactant-free film was then dissolved in tetrahydrofuran, and the copolymer intrinsic viscosity was measured. The viscosity average molecular weight

Figure 1. Populations of particles of (a) MBC50 and (b) I2-MBC50 latexes in volume percent versus particle size. 2450

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molecular weight of MBC50 was 950 000 g/mol, which decreased to 640 000 g/mol by adding 1.5 wt % iodine to the monomer mixture before their copolymerization. The probability of covalent bond formation between iodine molecule and the copolymer backbone during polymerization was examined by FTIR (Figure 2). The peak of the MBC50

Figure 3. First heating cycles of DSC thermograms of MBC50 (○), the reference blend (□), and I2-MBC50 (△).

Figure 2. FTIR spectra of MBC50 (○), the reference blend (□), and I2-MBC50 (△).

spectrum at 622 cm−1 can be assigned to sulfur-containing initiator attachment to the copolymer backbone.30 The peak shifted to 604 cm−1 in the reference blend film, while its intensity diminished. A sharp intensity reduction of the 622 cm−1 peak was observed in the spectrum of I2-MBC50. This peak also overlapped with the 580 cm−1 peak, resulting in the appearance of a shoulder in the spectrum of I2-MBC50 near the aforementioned wavenumbers. The DSC thermogram of MBC50 in the temperature range 140−210 °C did not show an endothermic peak (Figure 3). An endothermic peak at 180−205 °C, however, appeared in the thermogram of the sample prepared by copolymerization of the monomer/iodine mixture, I2-MBC50. Finally, the reference blend film showed a minute endothermic peak at 185 °C corresponding to the iodine vaporization temperature (Figure 3). Weight loss curves versus temperature for MBC50, I2MBC50, and the reference blend are presented in Figure 4. The weight loss onsets of MBC50 and I2-MBC50 were detected at 160 and 70 °C, respectively. In fact, iodine addition to the monomer mixture and emulsion copolymerization of the system reduced the copolymer weight loss onset by 90 °C. Physical mixing of iodine with the copolymer (the reference blend), however, decreased its weight loss onset even further. The UV−vis spectrum of MBC50 solution in chloroform did not show an absorption peak at 510 nm (Figure 5). The reference blend solution, however, depicted a characteristic peak at this wavelength. I2-MBC50 solution in chloroform showed also an intense peak at 510 nm. The peak appearance at 510 nm for the reference blend and I2-MBC50 solutions and its absence for MBC50 were assigned to the presence and absence of iodine molecules, respectively.31 Two other peaks of I2MBC50 solution at 370 and 290 nm were assigned to

Figure 4. TGA spectra of MBC50 (○), the reference blend (□), and I2-MBC50 (△).

triiodide.31In fact, during the polymerization iodine-containing monomer droplets evolved into nanosize copolymer particles containing iodine and triiodide. The iodine-containing monomers copolymerized by sodium metabisulfite, I3−MBC50, showed only two peaks at 290 and 370 nm (Figure 5). The extinction coefficients of iodine at 510 nm and triiodide at 370 nm were calculated to be 914 and 2637 L/(cm·mol), respectively.

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DISCUSSION Monodispersed poly(methyl methacrylate-co-butyl acrylate) nanoparticles, 180 nm in diameter, with various comonomer compositions were synthesized using potassium persulfate as initiator and sodium dodecyl sulfate as surfactant.32 Nonetheless, emulsion polymerization of I2-MBC50 with that recipe and approach was unsuccessful. The failure was attributed to the neutralization of free radicals in the presence of iodine.24 A 2451

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S2 O82 − + I− → SO4•− + SO4 2 − +

1 I2 2

(4)

In reaction 4, S2O82− is reduced to SO42− while I− is oxidized to iodine. Standard electrode potentials for the former and the latter evolutions are 2.010 and −0.54 V, respectively.35 Thus, this reaction is favorable from an electrochemistry point of view and regeneration of iodine molecules is feasbile. In addition to eqs 1 and 4, reaction of sodium metabisulfite and potassium persulfate is also possible due to excess amount of sodium metabisulfite in the polymerization medium (eq 5). S2 O82 − + S2 O52 − → SO4•− + SO4 2 − + S2•O5−

In fact, nearly half of metabisulfite molecules react with whole iodine molecules according to eq 1. The rest of sodium metabisulfite molecules have the chance to react with persulfate molecules, leading to generation of some initiator species according to eq 5. The iodine to triiodide molar ratio in the synthesized polymer electrolyte depends on I− selectivity in reacting with either I2 or S2O82−. Therefore, electrolyte with a molar ratio of I2 to I3− equal to 2.78 can be formed via designing the molar ratio of 3 to 1 for sodium metabisulfite and potassium persulfate as the initiation system. Comparison of FTIR results of I2-MBC50, MBC50, and the reference blend confirmed the absence of I• as the polymerization initiator. In other words, there was no sign of C−I bond in the FTIR spectra of I2MBC50 and I3−-MBC50 (Figure 2). This bond usually appears as an absorption peak near 500 cm−1.30 A strong peak at 622 cm−1 in the FTIR spectrum of MBC50, however, was attributed to C−S covalent bond formation. The intensity and position of the 622 cm−1 peak for the reference blend decreased in comparison with MBC50. Formation of charge transfer complex between the free electrons of oxygen of the initiator and iodine seems to be reason for the observed phenomenon.36 The intensity of the 622 cm−1 peak in the FTIR spectrum of I2MBC50 declined more in comparison with the reference blend and changed into a shoulder with possible attribution to the simultaneous existence of iodine and triiodide.37 The shallow endothermic peak near 185 °C in the thermogram of the reference blend was assigned to iodine evaporation (Figure 3). Free iodine boils at 184.3 °C, while in the polymer matrix its evaporation temperature shifted toward higher values due to its physical interactions. Moreover, iodine molecules are partitioned between MMA-enriched or BuAenriched copolymers, a characteristic of emulsion copolymerized macromolecules which will be discussed later. Iodine entrapped in the BuA-enriched regions evaporates faster than iodine entrapped in MMA-enriched regions due to the lower glass transition temperature of the former. Therefore, the temperature range near 185 °C is defined as the iodine evaporation region. The thermophysical evolution of I2-MBC50 was very different from that of the reference blend. The endothermic peak of iodine evaporation started at 180 °C and continued up to 205 °C with much more heat absorption. Simultaneous existence of iodine and triiodide can be considered as the main reason for observing an intensive endothermic peak which displaced to higher temperatures in the I2-MBC50 thermogram. Besides the energy adsorption for iodine evaporation, a great amount of energy may be consumed by triiodide dissociation. In other words, adsorbed energy not only evaporated iodine, but also converted triiodide to iodine and iodide.38 Evaporation of iodine molecules, decomposition

Figure 5. UV−vis spectra of the reference blend (□), I2-MBC50 (△), MBC50 (○), and I3−-MBC50 (☆).

possible solution for relaxing the problem appeared to be replacing the initiation system by a higher decomposition rate one. Therefore, potassium persulfate was replaced with sodium metabisulfite as the initiator of emulsion copolymerization of MMA/BuA/iodine mixture. The role of iodine and sodium metabisulfite as a redox pair was reported before (eq 1).33 I 2 + S2 O52 − → I• + I− + S2•O5−

(1)

Similarities in the FTIR spectra of MBC50, I2-MBC50, and the reference blend in the region 450−550 cm−1 and the appearance of a 622 cm−1 peak in the spectra of all samples, with little difference in intensity and location, signified the absence and presence of C−I and C−S covalent bonds in the samples, respectively. This evidence led to the postulation that, in a competition between I• and S2•O5− for attacking the monomers, the latter wins (Figure 2). On the other hand, I• is recombined into iodine, while iodides react with the regenerated iodine molecules to form triiodide. The UV−vis spectrum of I3−-MBC50 displaying just two absorbance peaks at 290 and 370 nm not only confirmed formation of triiodide but also supported the proposed mechanism (Figure 5). In iodine-based polymer electrolytes, iodine and triiodide act as a redox pair to transfer electrical charge (eqs 2 and 3).5 I3− + 2e− → 3I−

(5)

(2)

3 I 2 + 3e− (3) 2 Considering the stoichiometry of reactions, the contribution of the iodine and triiodide pair with a molar ratio of 1.5 of the former to the latter corresponds to the maximum charge transfer. As the simultaneous presence of iodine and triiodide is required in a polymer electrolyte, the proposed mechanism presented by eq 1 is not a suitable approach to produce such systems. The problem is I3− enrichment and I2 depletion of the synthesized electrolyte. This drawback can be resolved by lowering the population of I− in the polymerization medium. This can be conducted via adding potassium persulfate to the polymerization medium. Potassium persulfate reacts with I− and results in reproduction of iodine molecules (eq 4).33,34 3I− →

2452

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of triiodide, and evaporation of regenerated iodine molecules vindicate absorption of more energy by I2-MBC50. The thermogravimetric analysis also confirmed the postulate of simultaneous iodine and triiodide formation during the copolymerization of the monomer mixture containing iodine (Figure 4). MBC50 showed avery small weight loss up to 240 °C due its moisture evaporation. On the other hand, both the reference blend and I2-MBC50 came out with more weight loss in the range 60−240 °C via both moisture and iodine evaporation (Figure 4). In spite of a minor difference between the final weight losses of the reference blend and I2-MBC50, their weight loss modes were quite different. The weight loss curve of the reference blend was narrower and appeared at lower temperatures (Figure 6). The weight loss broadness of

Figure 7. Deconvolution of UV−vis spectrum of I2-MBC50 to four Gaussian functions whose peaks are at 510 (△), 370 (+), 290 (○), and 243 (□) nm, respectively.

coefficient of triiodide at 370 nm, and the characteristic peak area, the molar ratio of iodine to triiodide was quantified as 2.78. UV−vis spectra of annealed I2-MBC50 films for 1.5 and 4 h at 110 °C under a pressure of 200 mbar were used to estimate the rate of I3− decomposition and I2 evaporation (Figure 8).

Figure 6. First derivative of thermogravimetric analysis of MBC50 (○), I2-MBC50 (△), and the reference blend (□).

the reference blend was assigned to the matrix heterogeneity. This heterogeneity which is originated from the presence of MMA-enriched or BuA-enriched areas causes diversity in the intensity of iodine entrapment by the polymer matrix. The higher broadness of the I2-MBC50 weight loss curve, however, was attributed to the triiodide decomposition into iodine and iodide which was followed by the iodine and moisture evaporation (Figure 6). To quantify the extent of iodine to triiodide conversion in I2MBC50, its UV−vis spectrum was recorded (Figure 5). The UV−vis spectrum of MBC50 solution in chloroform did not show a noticeable peak in the scanned region. On the other hand, the reference blend spectrum depicted an intense peak at 510 nm due to its entrapped molecular iodine.31 This intense peak was also detected in the UV−vis spectrum of I2-MBC50. Two intensive peaks at 290 and 370 nm also appeared in the spectrum of I2-MBC50, implying the existence of triiodide.31 The molar ratio of iodine to triiodide was then calculated from deconvolution of the UV−vis spectrum of I2-MBC50 (Figure 7). The UV−vis spectrum of I2-MBC50 was fitted by four Gaussian curves. The peak area at 510 nm is directly proportional to the amount of iodine, while the peak area at 370 or 290 nm corresponds to the amount of triiodide. Therefore, by considering the Beer−Lambert law, the extinction coefficient of iodine at 510 nm, the extinction

Figure 8. UV−visible spectra of I2-MBC50 (○), I2-MBC50 annealed for 1.5 h (□) and I2-MBC50 annealed for 4 h (△) at 110 °C and pressure of 200 mbar. Inset: iodine evaporation (□) and triiodide decomposition (○) versus annealing time.

Annealing I2-MBC50 film for 1.5 h reduced its peak heights at 510, 370, and 290 nm. Increase of the annealing time to 4 h intensified the decline of the peak heights further. By deconvoluting spectra of the annealed films, the rates of triiodide decomposition and iodine evaporation were calculated and are depicted in the inset of Figure 8. One iodine molecule and one iodide ion are generated by decomposition of every triiodide ion. As the triiodide decomposition rate coincided 2453

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with the iodine evaporation rate during thermal annealing (Figure 8, inset), it can be concluded that the triiodide decomposition rate is much slower than the iodine evaporation rate. The UV−vis spectrum of I2-MBC50 justified the broadness of its DSC and TGA curves. In fact, the iodine and triiodide simultaneous existence and the slower decomposition rate of triiodide in comparison with the iodine evaporation rate, which were verified by UV−vis spectroscopy, led to the broadness enhancement in the DSC endothermic peak (Figure 3) and the TGA weight loss peak (Figure 6). Iodine dissolution in the monomer mixture and its emulsion copolymerization with sodium metabisulfite and potassium persulfate as the redox pair initiation system resulted in a copolymer with specific characteristics. The average particle size of the prepared nanoparticles was 95 nm, much lower than that of the one prepared by potassium persulfate as initiator, 180 nm.32 Nonetheless, the average particle sizes of MBC50 and I2MBC50 latexes differed by as much as 15 nm (Figure 1). The lower average particle size of I2-MBC50 can be assigned to the existing interaction among iodine and surfactant molecules. Probably, iodine molecules act as electron acceptors and interact with surfactant molecules as electron donors via charge transfer.39 This may increase the population of surfactant molecules near iodine-containing monomer droplets leading to average particle size reduction in I2-MBC50. Formation of a small amount of iodine radicals by the mechanism depicted in eq 1 and their interference with the copolymerization reaction caused the observed difference in the average molecular weight of copolymers. In other words, a fraction of these radicals act as chain terminators before their recombination into iodine, reducing the copolymer average molecular weight in I2-MBC50. Due to the low concentration of iodine radicals in the polymerization medium, their swift recombination, and facile dissociation of the C−I bond by light,24 no peak was detected in the infrared spectrum of I2-MBC50 near 500 cm−1 (Figure 2). The statistical nature of emulsion copolymerization results in chains with a range of different compositions.40 Consequently, this phenomenon caused the formation of either MMAenriched or BuA-enriched chains. Appearance of a broad Tg in the DSC thermograms of the copolymers in the temperature range −20 to 80 °C (not shown here) confirmed the formation of chains with a broad comonomer distribution. Comonomer content and its distribution along various chains were detected by the broadness of the glass transition and modeled by the first derivative of the copolymer heat flow.40−42 The heat flow first derivative is a superposition of the transitions of various chains with different compositions. The heat flow first-derivative peak of the glass transition of MBC50 appeared in the range −20 to 80 °C. On the other hand, it was narrowed by its shifting to the range of −10 to 80 °C for I2-MBC50. By analyzing the heat flow first-derivative curves of MBC50 and I2-MBC50, their comonomer distribution and normalized population were calculated and plotted versus the MMA weight fraction (Figure 9). Iodine addition to the monomer mixture and its emulsion copolymerization led to the formation of more MMA-enriched chains. The complex formation between iodine molecules and MMA or BuA monomers via charge transfer seems to be the key parameter governing the phenomenon.43 In spite of the electron-donating capability of both monomers, the complexation is more intense in the presence of butyl acrylate. In BuA, a butyl group with four carbon atoms attaches to the ester group,

Figure 9. Normalized comonomer distribution function of I2-MBC50 (△) and MBC50 (○) samples.

while in MMA a methyl group containing a single carbon atom connects to the ester group. As the butyl group has a higher tendency for electron donation in comparison with methyl,44 more intensified charge transfer complex is formed between iodine and butyl carrying monomer. Thus, during copolymerization, MMA moves much faster in the polymerization medium and its addition to the propagating macroradicals is easier. As a result, the population of MMA-enriched chains increased in I2-MBC50, while most of the BuA monomers contributed in a handful of but long BuAenriched chains (Figure 9).

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CONCLUSIONS Monosize poly(methyl methacrylate-co-butyl acrylate) nanoparticles containing Iodine and triiodide were synthesized via emulsion polymerization using a redox initiation system. The absence of C−I covalent bond in the FTIR spectra of the samples implied that most initial iodine molecules were encapsulated as I2 or contributed to I3− formation. Three peaks at 510, 370, and 290 nm in the UV−vis spectrum of I2MBC50 indicated the concurrent presence of iodine and triiodide in the synthesized copolymer. Deconvolution of the spectrum led to an iodine to triiodide molar ratio estimation of 2.78. The simultaneous presence of I2 and I3− was also supported by broad endothermic and broad weight loss peaks in DSC and TGA thermograms of I2-MBC50, respectively. The first derivative of the heat flow curve of I2-MBC50 confirmed that iodine addition to the monomer mixture and its copolymerization increased the population of more MMAenriched chains. Moreover, iodine addition to the monomer mixture not only reduced the copolymer molecular weight but also the decreased latex average particle size by 15 nm.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 982164542406. Fax: 982166468243. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 2454

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