Preparation of polystyrene spheres in different particle sizes and ...

SCIENCE CHINA Technological Sciences • RESEARCH PAPER •

November 2010 Vol.53 No.11: 3088–3093 doi: 10.1007/s11431-010-4110-5

Preparation of polystyrene spheres in different particle sizes and assembly of the PS colloidal crystals FANG JunFei, XUAN YiMin* & LI Qiang School of Power Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Received September 24, 2009; accepted October 29, 2009

Monodisperse polystyrene (PS) colloidal spheres were successfully prepared through emulsifier-free emulsion polymerization by controlling the polymerization reaction time, ionic strength of the system, concentration of the ionic copolymer (sodium p-styrenesulfonate) and other factors. The PS colloidal spheres were assembled into colloidal crystals whose structures were mainly face-centered cubic (fcc) close-packed. Then FDTD method was used to calculate the color-rendering characteristics of the colloidal crystals surface. The calculated results were consistent with the experimental results. polystyrene spheres, colloidal crystals, self-assembly, three-dimensional ordered, FDTD method Citation:

1

Fang J F, Xuan Y M, Li Q. Preparation of polystyrene spheres in different particle sizes and assembly of the PS colloidal crystals. Sci China Tech Sci, 2010, 53: 3088−3093, doi: 10.1007/s11431-010-4110-5

Introduction

Colloidal crystals are of the structures with a periodic arrangement which forms spontaneously when monodisperse colloidal spheres are in aqueous solution. The periodic arrangement is similar to that of atoms in a crystal [1–3]. Due to the size of the colloidal spheres is generally 0.1–1.0 μm, which is on sub-micron scale [1, 2], the colloidal crystals demonstrate an important application prospect for the preparation of photonic crystals in wave range of visible light and near-infrared light [3, 4]. Because the preparative process of colloidal crystals is relatively simple, using colloidal crystals as templates for fabricating ordered macroporous materials has become a very efficient method in recent years [5–8]. In addition, the colloidal crystals are widely used in sensors, filters, optical switches, and other optical devices due to their diffraction characteristics [4, 9–11].

*Corresponding author (email: [email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2010

At present, the familiar colloidal crystals are mainly SiO2 colloidal crystals and polymer colloidal crystals. The latter include polystyrene (PS) colloidal crystals and polymethyl methacrylate (PMMA) colloidal crystals. The emulsifierfree emulsion polymerization method is a new polymerization method developed from the classical emulsion polymerization and the colloidal spheres prepared by this method have the features of better monodispersity and cleaner surface [12, 13]. Therefore, this method has been widely used for preparing monodisperse polymer colloidal spheres. Gravitational sedimentation method, centrifugal sedimentation method and vertical deposition method are well-known three methods for assembling monodisperse colloidal spheres into three-dimensional ordered colloidal crystals [14]. Yu et al. [15] have successfully prepared the ordered SiO2 colloidal crystals by the electrophoresis-assisted deposition method. In this paper, monodisperse polystyrene (PS) colloidal spheres were prepared by emulsifier-free emulsion polymerization method. The polymerization reaction time, ionic strength of the system, concentration of the ionic copolymer, tech.scichina.com

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initiator concentration and other experimental conditions were altered to investigate their impacts on particle size of the polystyrene colloidal spheres. By using the centrifugal sedimentation method, the monodisperse polystyrene (PS) colloidal spheres of different particle sizes were assembled into three-dimensional ordered colloidal crystals.

2 Experiment 2.1

Experimental materials

Synthesis of polystyrene (PS) colloidal spheres

The polystyrene colloidal spheres were synthesized by emulsion polymerization method [8, 11, 12, 16, 17]. At first, a certain amount of potassium hydrogen carbonate (KHCO3) and sodium p-styrenesulfonate (C8H7SO3Na) were added into 200 mL deionized water and the solution was stirred until they were completely dissolved. Then the solution was transferred into a 500 mL four-mouth flask, and 26 mL styrene monomer was also added into the flask. It was stirred and reflowed under the protection of nitrogen. When the solution was heated to 72°C, 50 mL potassium persulfate (K2S2O8) solution (pre-heated to 72°C) which contained a certain amount of initiator was dropped into the solution in 30 min or so. After that the system was obturated and the solution was stirred for 28 h. Polystyrene colloidal spheres of different particle sizes were prepared by varying the experimental conditions. In order to analyze the effect of polymerization time on the particle size of the spheres, different samples were took out from the solution after it reacted for 2, 7, 11, 20 and 28 h, respectively. To investigate the effects of ionic strength and ionic copolymer concentration on particle size of the spheres, the following two experiments were designed (as shown in Tables 1 and 2), while the other experimental conditions were kept the same as before. Table 1 Effects of different amounts of potassium hydrogen carbonate (KHCO3) KHCO3 (g)

0.150

0.450

0.750

1.020

1.260

Table 2 Effects of different amounts of sodium p-styrenesulfonate (C8H7SO3Na) C8H7SO3Na (g)

0.0206

0.0310

2.3

0.0412

0.0652

0.0820

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Assembly of polystyrene (PS) colloidal crystals

The assembly of the colloidal crystals from polystyrene spheres liquor was conducted in a centrifuge. After about 24 h of centrifugation under 1000 r/min, the upper clear liquid was removed and the remanent liquor was naturally dried at room temperature. Then it was dried at 80°C in a vacuum oven for 1 h. Finally, the three-dimensional ordered colloidal crystals were prepared. 2.4

Styrene (C8H8, chemically pure) was alternately washed six times with 0.1 mol/L NaOH and deionized water in order to remove the polymerization inhibitor. Potassium persulfate (K2S2O8), sodium p-styrenesulfonate (C8H7SO3Na), potassium hydrogen carbonate (KHCO3) and sodium hydroxide (NaOH) were all analytically pure. 2.2

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Testing and characterization

The particle size of the polystyrene colloidal spheres was measured by JEM-2100 transmission electron microscope (TEM) (JEOL Corporation, Japan). The size distribution of the sample particles was determined with a Mastersizer Micro-p laser particle size analyzer (Malvern Instruments company, Britain). Infrared absorption spectra of the polystyrene were recorded by using a Verctor-22 Fourier transform infrared spectrometer (Bruker Company, Germany). The surface morphology of the colloidal crystals was characterized with an S-4800 field emission scanning electron microscope (SEM) (Japanese Hitachi High-Technologies Corporation) under an accelerating voltage of 15 kV, and gold spray processing was done to the sample surface before observation. Powder X-ray diffraction (XRD) patterns of the polystyrene were collected by using a D8 super speed diffractometer (Bruker-Axs company, Germany).

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Results and discussion

3.1 The effect of reaction conditions on particle size of polystyrene spheres 3.1.1 The effect of polymerization reaction time on particle size of spheres Figure 1 exhibits a set of TEM images of polystyrene colloidal spheres taken for different reaction times. Clearly, the particle size grows as the polymerization time increases. In the process of the styrene polymerization, a lot of polymer nuclei are rapidly produced at first and the system becomes unstable, so they gradually come together to form latex particles of larger size. As the reaction time increases, the monomer concentration in the system becomes smaller and smaller and the growth of the polymer sphere particle size becomes slow and finally stops [12, 17, 18]. 3.1.2 The effect of ionic strength on particle size of spheres The relationship between the particle sizes of polystyrene colloidal spheres and ionic strength of the system is illustrated in Figure 2. It is clear that the particle size increases with the increase of ionic strength. The reason may be that the stability of the emulsion in the reaction system is influenced by the ionic strength. When the static electricity force of the latex particles is greater than the coagulation force

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Figure 1 TEM images of polystyrene spheres at different reaction times. (a) 2 h; (b) 7 h; (c) 11 h; (d) 20 h; (e) 28 h.

Figure 2 Relation of sphere particle size and ionic strength.

between the particles, the system will become stable. However, the ionic strength of the system increases with the increasing of KHCO3 concentration and the electrostatic repulsion between the particles decreases. Therefore, the system becomes increasingly unstable, which makes the initial particles specially easy to gather, then the latex particles grow larger and form polymer spheres of larger particle size [12, 13, 17]. 3.1.3 The effect of ionic copolymer concentration on particle size of spheres Figure 3 shows the relationship between the particle size of the polystyrene colloidal spheres and the concentration of sodium p-styrenesulfonate. The results shown in Figure 3 illustrate that the higher the concentration of ionic copolymer, the smaller the particle size of polystyrene colloidal spheres. The sodium p-styrenesulfonate has a vinyl group, which makes it easily be triggered by initiator to generate oligomer free radicals. In addition, it has a hydrophilic

Figure 3 Relationship of sphere particle size and ionic copolymer concentration.

sulfonic group, which plays a role in keeping stability of the latex particles. Because of such structures, when the concentration of sodium p-styrenesulfonate increases, the concentrations of oligomer free radicals and sulfonic acid groups both increase, resulting in the formation of more active centers. Thus, the latex particles of smaller sizes increase, but the speed of reaction accelerates [12]. 3.2

Particle size distribution of polystyrene spheres

Figure 4 shows the particle size distribution diagram measured by laser particle size analyzer. The average size of the polystyrene colloidal spheres is about 240 nm. The measured result suggests that the size distribution of the colloidal polystyrene spheres is very narrow, indicating that the monodispersity of the polystyrene colloidal spheres is good and that it is quite easy for the colloidal spheres to be

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Figure 5 IR spectra of the polystyrene.

Figure 4 Size distribution of the polystyrene spheres.

assembled into three-dimensional ordered colloidal arrays [10, 15]. 3.3

Infrared analysis of the polystyrene (PS)

Figure 5 illustrates the infrared absorption spectra of the polystyrene spheres whose average particle size is about 560 nm. Obviously, there are several absorption peaks within the involved wavenumber range. There are absorption peaks at the wave numbers of 3060.8 and 3026.0 due to aromatic C-H stretching vibration absorption and there are three absorption peaks at the wave numbers of 1600.8, 1492.7, and 1452.2 due to aromatic C=C stretching vibration absorption. These absorption peaks indicate the existence of benzene rings. The absorption peaks at the wave numbers of 756.0 and 698.2 correspond to C-H out-of-plane bending vibration absorption and indicate that there is only one substituent in the benzene ring. Figure 5 also shows the

absorption peaks at the wave numbers of 2921.9 and 2848.6, corresponding to the existence of methylenes. These IR results have confirmed that the styrene reacts to produce polystyrene through polymerization reaction. In addition, the absorption peaks at the wave number of 3446.5 is for the stretching vibration absorption of O-H, which indicates the existence of hydroxyl. The hydroxyl may come from water or hydrolysis of strong acid weak alkali salt such as sodium p-styrenesulfonate, potassium hydrogen carbonate. 3.4 Assembly and morphology analysis of the polystyrene colloidal crystals Colloidal crystals were assembled by polystyrene colloidal spheres in several typical sizes in the experiment. The SEM images of the polystyrene (PS) colloidal crystals (particle sizes are about 200, 280, 450, 560 and 800 nm) are shown in Figure 6. Colloidal crystals, also known as colloidal arrays, are mainly arranged in the tetragonal or hexagonal

Figure 6 SEM images of the polystyrene colloidal crystals in different particle sizes. (a) 200 nm; (b) 280 nm; (c) 450 nm; (d) 560 nm; (e) 800 nm.

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structure. The porosity of colloidal crystals with the tetragonal structure is much greater, which may result in thermodynamic instability, so that the tetragonal structure can be easily transformed into the hexagonal arrangement that is a more stable structure. Therefore, the hexagonal arrangement always dominates in the assembly of colloidal crystals at room temperature [19]. As shown in Figure 6, each sphere is tightly surrounded by six spheres to form a hexagonal structure, although there may be some defects in the structure in one plane. The structure defects may be caused by the difference between particle sizes of the colloidal spheres, which can be minimized by strictly controlling the synthesis conditions to obtain spheres with uniform particle size. Colloidal crystals are usually face-centered cubic (fcc) close-packed and hexagonal close-packed (hcp). Theoretical analysis indicates that the face-centered cubic close-packed structure is more stable in thermodynamics. It is possible for one to make such structure dominant in colloidal crystals by carefully controlling experimental conditions [20] in order to obtain colloidal crystals with a high degree of regularity. Figure 6 (b) demonstrates an example that the prepared colloidal crystals are basically of face-centered cubic close-packed structure, in which the hexagonal structured surface is its (111) crystal plane. 3.5


Figure 7 XRD patterns of polystyrene spheres in different particle sizes. (a) 280 nm; (b) 450 nm; (c) 560 nm; (d) 800 nm.

XRD analysis of the polystyrene spheres

Figure 7 shows the XRD patterns of the polystyrene spheres with different particle sizes. Although the particle sizes of the polystyrene spheres are different, the observed phenomena reveal that a strong diffraction peak always emerges at the position that 2θ is equal to 20°, suggesting that the polystyrene spheres have certain crystallinity. 3.6 Reflection and transmission spectra analysis of the polystyrene colloidal crystals Obvious color change on the surface of the colloidal crystals can be observed when the observation angle varies, but there are some color differences as the particle sizes of the colloidal crystals become different. Such color display on the surface can be explained from the mechanism of physical optics. A calculation model was established on the basis of the microstructure of polystyrene colloidal crystals (as shown in Figure 8). The color characteristics of the colloidal crystals surface can be calculated by means of the finite difference time domain method (FDTD) [21]. The calculated reflection and transmission spectra are illustrated in Figure 9. Distinctly, the transmission peaks of the surface are very strong and the reason is that there are only three micro-ball layers being involved in the calculation model. Figure 9(a) shows that the strongest reflection peak appears in the 300–360 nm bandwidth, belonging to the UV region without any visible color. A strong reflection peak appearing within the red bandwidth of 700–800 nm

Figure 8 Calculation model.

indicates that the reflection of red light is relatively strong, which coincides very well with the observed phenomenon that the surface of the colloidal crystals appears to be red (as shown in Figure 10(a)). Figure 9(b) shows that the strong reflection peak appears within the 400–500 nm bandwidth, which belongs to the purple, indigo and green regions. Thus, it displays a complex color, which accords with the observed color on the surface of the colloidal crystals. Of course, the surface of colloidal crystals also reflects light within other bandwidth, so that one can observe that the sample shows different colors when the observational angle changes (Figure 10(b)).

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Conclusions

Monodisperse polystyrene colloidal spheres have been successfully prepared through emulsion polymerization by varying the polymerization reaction time, ionic strength of the system, concentration of the ionic copolymer and other factors. The experimental results have indicated that the spheres particle size increases with increasing either polymerization time or ionic strength of the system, but increase in concentration of the ionic copolymer leads to

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Figure 9 Reflection and transmission spectra according to calculation results. (a) Sphere particle size is 280 nm; (b) sphere particle size is 450 nm.

Figure 10

Colors of the sample surface.

decrease of particle size of the spheres. The sample tests by the laser particle size analyzer have shown that the monodispersity of the polystyrene colloidal spheres is good. The structures of the colloidal crystals are primarily face-centered cubic (fcc) close-packed. A calculating model which is based on the surface microstructure of the colloidal crystals was established. FDTD method was used to calculate the color characteristics of the colloidal crystals surface, which were well consistent with the observed phonomena. 1 2 3 4 5

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