Shell Polystyrene

Macromolecular Research, Vol. 19, No. 11, pp 1114-1120 (2011) DOI 10.1007/s13233-011-1110-7

www.springer.com/13233

One-Step Synthesis of Photoluminescent Core/Shell Polystyrene/Polythiophene Particles Yeon Jae Jung1, Seung Mo Lee1, Subramani Sankaraiah1, In Woo Cheong2, Sung Wook Choi*,3, and Jung Hyun Kim*,1 1

Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 120-749, Korea 2 Department of Applied Chemistry, Kyungpook National University, Daegu 702-701, Korea 3 Department of Biotechnology, Catholic University, Gyeonggi 420-743, Korea Received December 23, 2010; Revised May 23, 2011; Accepted June 8, 2011

Abstract: Photoluminescent polystyrene (PS)/polythiophene (PTh) particles with a core/shell structure were synthesized via a one-step process using radical polymerization for styrene and Fe3+-catalyzed oxidative polymerization for thiophene. Water-soluble potassium persulfate (KPS) and iron chloride (FeCl3) were used as intitiators for the polymerization of styrene and thiophene, respectively. Sodium dodecyl sulfate (SDS) served as a polymierization site in the form of a micelle as well as a collodial stabilizer. Analyzing the samples using field-emission scanning electron microscopy (FE-SEM) and Fourier transform infrared (FTIR) spectroscopy at different times revealed a plausible mechanism for the formation of the PS/PTh particles. In the mechanism of particle formation, the sulfate (OSO3- ) groups of SDS electrostatically induced Fe3+ ions to the perimeter of the micelle; thus, the polymerization of thiophene was carried out mainly at the perimeter of the SDS micelle, eventually forming small PTh aggregates within the SDS micelles. The styrene oligomers or monomers that shifted into the preformed PTh aggregates were polymerized in the core domain of the aggregates; thus, the particle size gradually increased until all the styrene monomers were consumed, resulting in core/shell PS/PTh particles. The core/shell structure of the PS/PTh particles was confirmed by observing their crumpled morphology after selective dissolution of the PS core using a solvent. The photoluminescence (PL) intensity of the particles was found to be higher than that of pure PTh particles, attributable to the core/shell structure. Keywords: core/shell structure, one-step synthesis, polythiophene, polystyrene, photoluminescent.

In general, it is difficult to polymerize unsubstituted thiophene in an aqueous medium due to the poor water solubility of thiophene, low oxidizing activity of a catalyst, and extremely low conversion.21 To polymerize thiophene in an aqueous medium and improve luminescence efficiency of PTh, we have focused on composite particles with a thin shell layer of PTh, expecting a decrease in luminescence quenching.22 In our earlier work, we had demonstrated the fabrication of monodisperse luminescent particles with a controlled shell layer of PTh by seeded emulsion and oxidative polymerization.22 However, the polymerization time was quite long due to the two-steps procedure: production of seed polystyrene (PS) particles and then polymerization of thiophene at their surface. Therefore, in this article, we demonstrated the fabrication of core/shell structured PS/ PTh particles via a one-step synthetic process based on a combined polymerization of two different methods: radical polymerization for styrene and oxidation polymerization for thiophene. In this approach, sodium dodecyl sulfate (SDS) served as a polymerization site in the form of micelle, as

Introduction Polythiophene (PTh), one of the most important classes of linear conjugated polymers, has been extensively studied as a light-emitting material because it emits a red color that is difficult to achieve in the other conjugated polymers.1 Besides light-emitting, its electroconductivity,2,3 thermochromism,4-6 solvatochromism,7,8 electrochromism,9,10 photoluminescence,11-13 and tunable electro-optical properties make PTh more attractive in many application fields.14 Recently, many researchers have strived to apply PTh to light-emitting diode and electroluminescent device.15 However, low luminescence efficiency of PTh is pointed out as a major problem for an industrial application.16 Although several groups had prepared composite particles from conjugated polymers to enhance luminescence efficiency of PTh,17-19 only few studies demonstrated organic core/shell luminescent particles.20 *Corresponding Authors. E-mails: [email protected] or [email protected] The Polymer Society of Korea

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well as a collodial stabilizer. To propose a mechanism of particle formation, the PS/PTh particles were analyzed by Fourier transform infrared (FTIR) spectroscopy, dynamic light scattering (DLS), and field-emission scanning electron microscopy (FE-SEM) at different times. The PS/PTh particles with a core/shell structure exhibited high conversion and photoluminescence (PL) efficiency in the emulsion state, compared to PTh particles.

Experimental Materials. Styrene monomer (Junsei Chemical, Japan) was purchased and purified using an inhibitor remover column (Aldrich, USA). The purified monomer was stored at -5 oC before use. Potassium persulfate (KPS, Junsei Chemical, Japan), sodium bicarbonate (NaHCO3, Aldrich, USA), thiophene (Aldrich, USA), iron chloride (FeCl3, Kanto Chemical, Japan), hydrogen peroxide (H2O2, Dong Yang Co, Korea), chloroform (CHCl3, Duksan Chemical, Korea), and sodium dodecyl sulfate (SDS, Samchun Chemical, Korea) were used without further purification. Double-distilled and deionized (DDI) water (Millipore Co.) was used in all the experiments. Preparation of PS/PTh Particles. Typically, DDI water (150 g) was added into a double-jacket reactor equipped with a mechanical stirrer, a condenser, and a thermometer. 0.012 g of SDS and 0.020 g of NaHCO3 were added to the reactor and stirred at 300 rpm and 25 oC. After adding 4 g of styrene and 4 g of thiophene monomers into the reactor, 0.020 g of KPS, 0.007 g of FeCl3, and 12 g of H2O2 were added to the reactor. The polymerization was carried out for 12 h at 80 oC under a nitrogen atmosphere. Preparation of Homo-Particles. Homo-particles (PS and PTh particles) were synthesized for comparison of the results with PS/PTh particles. For the preparation of PS particles, DDI water (75 g) was added into a double-jacket reactor. 0.006 g of SDS and 0.020 g of NaHCO3 were added to the reactor and stirred at 300 rpm and 25 oC. After adding 4 g of styrene monomer, 0.020 g of KPS was added to the reactor. The polymerization was carried out for 12 h at 80 oC under a nitrogen atmosphere. For the preparation of PTh particles, DDI water (75 g) and 0.006 g of SDS were added into the double-jacket reactor and stirred at 300 rpm and 25 oC. After adding 4 g of thiophene monomers into the reactor, 0.007 g of FeCl3 and 12 g of H2O2 were added. The polymerization was carried out for 12 h at 50 oC under a nitrogen atmosphere. Characterization. To evaluate the formation of the PS/ PTh particles, field-emission scanning electron microscopy (FE-SEM, JSM-6500F, JEOL), Fourier transform infrared spectroscopy (FTIR, a Tensor 27 FTIR spectrometer, Bruker), and dynamic light scattering analyzer (Zetasizer 3000HSA, MALVERN) were used. The core/shell morphology of the PS/PTh particles was observed using a transmission elecMacromol. Res., Vol. 19, No. 11, 2011

tron microscope (TEM, JEM 100CXII UHR, JEOL). The particles obtained at different time intervals during polymerization were characterized by above instruments. Thermal properties were analyzed by a thermo-gravimetric analyzer (TGA Q50, TA Instruments, USA). The samples were heated from 25 to 500 oC at a heating rate of 10 oC/min under a nitrogen atmosphere. The PL spectra of the prepared core/shell emulsions at the concentration of 0.1 wt% were obtained using a spectrofluorophotometer (RF-5301PC, Shimadzu). PL excitation was measured at 365 nm wavelength.

Results and Discussion Polymerization Rates. Scheme I shows individual polymerization mechanisms for styrene and thiophene. Scheme I(A) shows the typical radical polymerization of styrene involving three major steps: initiation, propagation, and termination. Scheme 1(B) shows the oxidative polymerization of thiophene using FeCl3 and H2O2, involving four reaction steps: a radical cation, radical combination, deprotonation, and polymerization.22 First, thiophene monomer oxidized by Fe3+ ion converts into its radical cation (radical cation step). The second step involves the coupling reaction of two radical cations to produce a dihydro dimer dication (radical combination step), eventually forming a thiophene dimer after losing two protons (deprotonation step). The thiophene

Scheme I. Schematic diagrams for (A) radical polymerization of styrene and (B) oxidative polymerization of thiophene using FeCl3. 1115

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dimer turns into its radical cation form by Fe3+ ions and undergoes a further coupling reaction with a monomeric or oligomeric radical cation (polymerization step). During the oxidative polymerization, the Fe2+ ions formed by the oxidation reaction with thiophene monomers can be reoxidized to Fe3+ ions by H2O2. This recyclic process using Fe3+ ion can minimize the required amount of FeCl3 in the reaction and also increase the conversion of thiophene monomers to PTh.22 In our approach, styrene and thiophene were simultaneously polymerized in an aqueous medium in the presence of SDS surfactant by the different polymerization mechanisms, radical, and oxidation polymerizations, respectively. SDS served as Fe3+ inducer due to the electrostatic interaction between the sulfate (OSO3-) groups of SDS and Fe3+ ions of FeCl3 as well as a colloidal stabilizer.22 Figure 1 shows the conversion curves of PS, PTh, and PS/PTh particles over time. In a homo-monomer system, the polymerization of thiophene and styrene was completed in 3 and 4 h, respectively, indicating that the polymerization rate of thiophene was faster than that of styrene. In contrast, a mixture of thiophene and styrene was completely polymerized in 6 h. This retardation is due to the fact that oxygen formed from the direct reaction between H2O2 and Fe3+ ions makes free radicals (originated from KPS) unreactive.23,24 Therefore, it seems that PTh aggregates occupy the interior of the SDS micelle earlier than PS, eventually facilitating the entry of styrene oligomers into the micelle. FTIR, SEM, and TEM Analyses. Figure 2 shows the variation in FTIR spectra of the PS/PTh particles over time, where FTIR spectra of individual PTh and PS particles are also presented to compare the peak positions of the PS/PTh particles. The typical absorption band of PTh appeared at 1690 cm-1 due to the stretching vibration of the C=C bond.25

Figure 1. Conversion curves of styrene, thiophene, and a mixture of styrene and thiophene in various initiation systems. 1116

Figure 2. FTIR spectra of the PTh, PS, and PS/PTh particles at different times during polymerization.

As for PS particles, strong absorption peaks appeared at 1450, 1470, and 1590 cm-1, corresponding to the stretching vibration of the C=C bond on benzene ring. In addition, the two absorption bands appearing in the range of 600-800 cm-1 indicates that the benzene ring is singly substituted. As the reaction progressed, the intensity of the stretching vibration peak of the C=C bond on the benzene ring of styrene dramatically increased at 2 h. After the completion of the polymerization, the peak positions of the PS/PTh particles were observed to be exactly consistent with those of the PS and PTh homo-particles. To verify the mechanism of particle formation and individual domains of PS and PTh, the samples were collected from the reactor during polymerization at different times and analyzed by SEM before and after selective dissolution of PS using chloroform. Figure 3 shows SEM images of the PS/PTh particles at 30 min, 1, 2, 3, 5, and 6 h. Insets shows the SEM images taken after selective dissolution of PS of each sample. Small particles with less than 70 nm in size were observed at 30 min and there was no considerable change in the morphologies before and after the selective dissolution, suggesting that the initial particles were mainly composed of PTh due to the fast polymerization rate of thiophene. At 1 h, PS/PTh particles exhibited a spherical morphology with an increased size before the selective dissolution and a crumpled morphology after the selective dissolution, suggesting that styrene oligomers existed in the interior of the PS/PTh particles. Crumpled morphologies after the selective dissolution were obviously observed over 2 h, suggesting the core/shell morphology of the PS/PTh particles. Figures 3 and 4 confirmed that the particle size tended to gradually increase from 70 nm to around 1 µm Macromol. Res., Vol. 19, No. 11, 2011


Figure 4. Variation in size of the PS/PTh particles over time during polymerization.

Figure 3. SEM images of the PS/PTh particles at different times during polymerization. The insets are SEM images of the PS/PTh particles after selective dissolution of the PS core using chloroform, where scale bars are 500 nm.

over time. Although one of the advantages of emulsion polymerization process is uniformity in particle size, the resulting PS/PTh particles showed a broad size distribution. Basically, this can be due to the complex mechanism of particle formation involving two different polymerization procedures at the same time. In addition, the difference in the number of PTh forming aggregates in each micelle at an initial stage might cause the uneven induction of styrene oligomers and monomers, leading to the increase in the polydispersity of the particles over time. TEM images of the PS/PTh particles taken at 3 and 6 h during polymerization are shown in Figure 5. Although core/shell morphology was not clearly observed in large PS/PTh particles at 6 h due to the limitation in transmission of TEM imaging, the significant difference in contrast between core and shell domains was observed at the PS/ PTh particles at 3 h, which could be another evidence for Macromol. Res., Vol. 19, No. 11, 2011

Figure 5. TEM images of the PS/PTh particles at (A) 3 and (B) 6 h during polymerization.

the core/shell structure. These results confirmed the individual core and shell domains occupied by PS and PTh, respectively. Figure 6 shows the thermal degradation profiles of PS, PTh, and PS/PTh particles. The initial decomposition of PTh and PS homo-particles began approximately 208 and 395 oC, respectively. In contrast, PS/PTh particles decomposed at two different temperatures at around ~185 and 390 oC, showing a combined tendency of both PTh and PS. The thermal degradation for PTh was observed to be initiated approximately at around 100 oC, which is presumably due to the presence of PTh with a relatively low molecular weight. It was also found that the composition of PS/PTh particles matched to 1:1 weight ratio of introduced styrene and thiophene monomer because the TGA profile of the PS/ PTh particles was presented in the middle of between PTh and PS profiles over 430 oC. In addition, we evaluated thermal degradation behavior of PTh particles and crumpled PTh shell that was obtained after the selective dissolution of the PS/PTh particles. In addition, we evaluated thermal deg1117

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Figure 6. Thermal degradation profiles of the PS, PTh, and PS/ PTh particles.

Figure 7. Thermal degradation profiles of the PTh particles and crumpled PTh shell that remained after selective dissolution of PS from the PS/PTh particles.

radation behavior of PTh particles and crumpled PTh shell remained after the selective dissolution of the PS/PTh particles. As shown in Figure 7, there was no difference in the profiles between the samples, suggesting the PTh shell in the PS/PTh particles had similar properties to the PTh homo-polymer. Mechanism of PS/PTh Particle Formation. A schematic diagram depicting a mechanism of core-shell PS/PTh particle formation is shown in Scheme II. At an initial stage (Scheme II(A)), thiophene monomers are rapidly polymerized by Fe3+ ions mainly at the perimeter of the SDS micelle, occupying the interior of the micelles in the form of PTh aggregates. With a slight time interval, styrene monomers 1118

Scheme II. A schematic diagram depicting the formation of PS/ PTh particles in an aqueous phase.

are polymerized by KPS via free radical polymerization in the aqueous medium, forming oligomers. As the chain length of the styrene oligomers increases to z-mer (•MzSO4-), they enter into the interior of the SDS micelle preoccupied by PTh aggregates (Scheme II(B)). Styrene and thiophene monomers diffused from the monomer mixture droplets into the SDS micelle are continuously polymerized and the particle size of PS/PTh particles gradually increases until all styrene monomers are consumed (Scheme II(C) and (D)). A large amount of thiophene is polymerized mainly at the perimeter of the particle aggregates surrounded by SDS due to the ionic interaction of Fe3+ with sulfate (SO4-) of SDS. In addition, PTh can be considered as a relatively hydrophilic polymer due to the charge-polarized sulfur molecule.22 From these two reasons, PTh mainly exist at a shell layer of the resulting particles, eventually forming PS as a core and PTh as a shell. In our previous work, PS/PTh particles were synthesized by emulsifier-free emulsion polymerization and Fe3+-catalyzed oxidative polymerization.21 The difference between previous and current works is the employment of a surfactant, greatly affecting on the particle formation mechanism. Specifically, the composition of initially-formed particles should be quite different. In the present approach, the initial particles consisted of mainly thiophene oligomers within the SDS micelle because SDS provides a polymerization site at the very early stage. However, in our previous work, thiophene oligomers rapidly polymerized by Fe3+ were believed to form a shell layer at the surface of the droplets of the monomer-swollen PTh aggregates, which eventually served as a nano-reactor for the further polymerization of both the styrene and thiophene. PL Properties. To evaluate the PL properties of the core/ Macromol. Res., Vol. 19, No. 11, 2011


results provide a facile route to fabricate luminescent particles in a one-step process.

Figure 8. Photographs of the PS/PTh particles in emulsion state under (A) white and (B) UV lights under 365 nm of wavelength and (C) their PL spectrum of the PTh and PS/PTh particles in emulsion state at the excitation wavelength of 365 nm.

Acknowledgements. This work was supported by the National Research Foundation (NRF) grant funded by the Korea government (MEST) through the Active Polymer Center for Pattern Integration (No. R11-2007-050-000000), a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. (K0006005), the Nano R&D program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (2009-0083233), the Pioneer Research Center Program though the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (No. 2010-0019308), Mid-career Researcher Program through NRF grant funded by the MEST (No. 2007-0052622), and the Research Fund, 2010 of The Catholic University of Korea.

References shell PS/PTh particles, photographs of the PS/PTh particles in emulsion state were taken under white and UV lights (Figure 8(A) and (B)). The PS/PTh particles emitted orangered light only under UV light at the wavelength of 365 nm. In addition, we analyzed PL spectrum for two types of PTh and PS/PTh particles in emulsion state (solid content = 0.1 wt%) at the excitation wavelength of 365 nm to compare PL intensity. As shown in Figure 8(C), the PL intensity of the PS/PTh nanoparticles was higher by 6.6 times than that of the PTh particles at the emission wavelength of 544 nm. This result suggests that there was no difference in the PL properties between the PTh and PS/PTh particles except the emission intensity. The rationale for the increase in PL property of the PS/PTh particles is as follows: the PTh particles typically showed a self-absorption, eventually leading to a reduction in the PL intensity due to their thick morphology.21 In contrast, the PL intensity of the PS/PTh particle was rarely affected by self-absorption due to the presence of thin shell layer of PTh.26

Conclusions Photoluminescent PS/PTh particles with a core/shell structure were successfully prepared via a one-step synthetic process using free-radical polymerization for styrene and Fe3+-catalyzed oxidative polymerization for thiophene in an aqueous medium. It was found that thiophene polymerized mainly at the perimeter of the SDS micelles at a higher polymerization rate than styrene. TEM and SEM images confirmed the individual core and shell domains of the PS/ PTh particles. In addition, the PS/PTh particles showed excellent PL property due to the core/shell structure. Our Macromol. Res., Vol. 19, No. 11, 2011

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