Evaporation-Induced Aggregation of Polystyrene-block-poly(acrylic

Lichao Gao, Linqi Shi, Yingli An, Wangqing Zhang, Xudong Shen, Shiyi Guo, and Binglin He ... Shan Qin , Hong Li , Wangzhang Yuan , Yongming Zhang...
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Evaporation-Induced Aggregation of Polystyrene-block-poly(acrylic acid) Micelles to Microcubic Particles Wangqing Zhang, Linqi Shi,* Yingli An, Xudong Shen, Yunyong Guo, Lichao Gao, Zhen Liu, and Binlin He State Key Laboratory of Functional Polymer for Adsorption and Separation, Institute of Polymer Chemistry, N&T Joint Academy, Nankai University, Tianjin 300071, China Received February 20, 2003. In Final Form: April 27, 2003 An evaporation-induced method for aggregation of polystyrene-block-poly(acrylic acid) (PS-b-PAA) micelles to unusual polymeric microcubic particles is reported. PS-b-PAA self-assembles into spherical nanomicelles in the block-selective solvent water. After addition of butanone, 1-propanol, acetic acid, N,N-dimethylformamide (DMF), or n-butylamine, evaporation of the solvent mixture induces aggregation of the nanomicelles into microcubic particles on the substrate a glass slide. The concentration of the additive, temperature, and substrate affect the morphology of the aggregation of the nanomicelles. Vaporizing the additive/water mixture at 120 °C results in the formation of highly symmetrical microcubic particles on a glass slide. The size of microcubic particles is in range from 200 × 200 × 200 nm3 to 600 × 600 × 600 nm3. Microcubic particles can also form on a silicon and gold wafer with a size similar to that of those formed on a glass slide. The morphology of the microcubic particles almost remains unchanged at temperatures lower than 150 °C, while it is destroyed above 200 °C.

1. Introduction Shape and size provide control over many physical and chemical properties of nanoscale and microscale materials.1,2 Essentially, there are two ways to approach particle shape control: growth-directed syntheses typical of precipitation processes and template-directed syntheses wherein the growth is directed by epitaxy via a pre-existing structure upon which nucleation and growth take place. However, the challenge of synthetically controlling particle shape has been met with limited success. Up to the present, little is known about bulk solution synthetic methods for nonspherical particles, although some methods do exist for making shape-controlled inorganic nanoparticles.3-11 To our best knowledge, little progress has also been made to form shape-controlled organic and polymeric microparticles. A very elegant approach was taken to synthesize rigid, nanometer size polystyrene rods using the method of template-directed syntheses. Dendrimers of polystyrene consisting of dendritic side chains and a polymer core were self-assembled into rodlike, rigid polystyrene particles by Stocker et al.12 Similarly to template-directed syntheses, * To whom correspondence should be addressed. E-mail: [email protected]. Telephone: 0086-22-23506103. (1) Alivisatos, A. P. Science 1996, 271, 933-937. (2) Lieber, C. M. Solid State Commun. 1998, 107, 607-610. (3) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924-1925. (4) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607-612. (5) Pinna, N.; Weiss, K.; Urban, J.; Pileni, M.-P. Adv. Mater. 2001, 13, 261-264. (6) Bradley, J. S.; Tesche, B.; Busser, W.; Maase, M.; Reetz, M. T. J. Am. Chem. Soc. 2000, 122, 4631-4636. (7) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schata, G. C.; Zheng, J. Science 2001, 294, 1901-1903. (8) Li, M.; Schnablegger, H.; Mann, S. Nature 1999, 402, 393-395. (9) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A. Nature 2000, 404, 59-61. (10) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115-2117. (11) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700-12706. (12) Stocker, W.; Schurmann, B.; Schluter A. Adv. Mater. 1998, 10, 793-797.

growth-directed formation of shape-controlled polymeric particles has also been scarcely studied, partly due to the structure of polymer chains. Self-assembly of block copolymers in bulk may form various ordered phases depending on the composition of the copolymer block.13 Utilizing the self-assembling properties of block copolymers, Liu’s group has successfully yielded nanofibers,14 nanochannels,15 and nanotubes.16 Self-assembly of block copolymers in a selective solvent is also an efficient way to construct some unique architectures. A binary system composed of an amphiphilic block copolymer and a block-selective solvent, such as block copolymer/oil or block copolymer/water, can self-assemble into nanomicelles with various morphologies.17-20 A ternary isothermal system, such as PEO-PPO-PEO/ water/oil, can self-assemble into several morphologies.21 However, the nanomicelles tend to form a disorderly aggregate when solvent is removed. Thus, controlling the aggregation of nanomicelles is necessary to form shapecontrolled polymeric nano- and microparticles. Polystyrene-block-poly(acrylic acid) (PS-b-PAA) is a typical amphiphilic block copolymer, whose self-assembly into core-shell nanomicelles in water has been studied extensively.19,20 The core-shell nanomicelle consists of a core composed of the insoluble block of PS and a shell of the soluble block PAA in water. It is easily understood that the shell of the soluble block PAA can be influenced by addition of ions19 and electric field. Our strategy is to (13) Bates, F. S.; Fredrickson, G. H. Annu. Rev. Phys. Chem. 1990, 41, 525-557. (14) Liu, G.; Qiao, L.; Guo, A. Macromolecules 1996, 29, 5508-5510. (15) Liu, G.; Ding, J.; Guo, A.; Herfort, M.; Bazett-Jones, D. Macromolecules 1997, 30, 1851-1853. (16) Yan, X.; Liu, F.; Li, Z.; Liu, G. Macromolecules 2001, 34, 91129116. (17) Chen, Z.-r.; Kornfield, J. A.; Smith, S. D.; Grothaus, J. T.; Satkowski, M. M. Science 1997, 277, 1248-1253. (18) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728-1731. (19) Zhang, L.; Eisenberg, A. Macromolecules 1996, 29, 8805-8815. (20) Zhang, L.; Eisenberg, A. Macromolecules 1999, 32, 2239-2249. (21) Alexandridis, P.; Olson U.; Lindman, B. Langmuir 1998, 14, 2627-2638.

10.1021/la034300g CCC: $25.00 © 2003 American Chemical Society Published on Web 06/18/2003

Aggregation PS-b-PAA Micelles to Microcubic Particles

achieve controlled aggregation of PS-b-PAA micelles into shape-controlled particles. In this article, we report the shape-controlled formation of PS-b-PAA microcubic particles by evaporation-induced aggregation of PS-b-PAA micelles in an additive/water solvent mixture. The mechanism of aggregation of micelles and the configuration of the microcubic particles will be studied further in our future research. Dynamic laser light scattering (DLLS) was used to measure the diameter of the micelles selfassembled by PS-b-PAA. Scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray fluorescence spectroscopy (XRF) were used to study the structure of the polymeric microcubic particles. 2. Experimental Section Polystyrene-block-polymethyl acrylate (PS115-b-PMA63) was synthesized by atom transfer radical polymerization (ATRP),22 and the polydispersity index (PDI) of PS115-b-PMA63 measured by gel permeation chromatography (GPC) was 1.28. PS115-bPMA63 (2 g) was hydrolyzed in 50 mL of 20 wt % NaOH aqueous solution at 90 °C for 72 h. The hydrolyzate product, PS115-bPAA63, was deposited by slowly dropping the hydrolyzate solution into 40 mL of 33 vol % hydrochloric acid. The precipitate was centrifuged and washed with 5 vol % dilute hydrochloric acid and deionized water six times, respectively. The product was then dried at 50 °C in a vacuum oven for 24 h. The block copolymer of PS115-b-PAA63 was first dissolved in N,N-dimethylformamide (DMF) to give a stock polymer solution of 2.0 mg/mL. The PS-b-PAA microcubic particles were prepared by three steps. First, 0.50 mL of the stock polymer solution was dropped into 4.5 mL of water to make a copolymer micelle solution. The micelle solution was then dialyzed against water for 4 days to remove DMF to produce a stock micelle solution with the polymer concentration of 0.20 mg/mL. A given amount of the mixture of additive such as butanone, 1-propanol, acetic acid, DMF, or n-butylamine and water was then added to the stock micelle solution. The mixed solution, with the polymer concentration 0.020 mg/mL, was shaken for 30 min. Finally, one drop (about 0.05 mL) of the mixed solution was dropped on the surface of the clean substrate glass slide and heated at 120 °C to vaporize the solvent quickly and then placed into a vacuum oven at 30 °C for 12 h. The diameter of the micelles in aqueous solution was measured by DLLS (BI-9000AT) at 25 °C. To prepare samples for DLLS measurement, the stock polymer solution and water or additives aqueous solution were clarified with a 0.2 µm Millipore filter to remove dust. X-ray fluorescence spectroscopy (XRF) (Rigaku 1800) and scanning electron microscopy (SEM) (Philips XL30) equipped with EDS (EDAX) were used to characterize the composition of the polymer and the polymeric microcubic particle. XRF analysis was performed at 50 kV and 50 mA under vacuum. SEM and AFM were used to study the structure of the polymeric microcubic particles. The SEM was performed on a HITACHI S3500N microscope at 20 or 25 kV. To prepare SEM samples for the study of the content and morphology of the aggregates, the samples for SEM observation were sprayed with a gold layer about 3 nm thick. AFM operated in the tapping mode was performed on a Digital Instrument NanoScope A. For studying the morphology of the microcubic particles at different temperatures, the microcubic particles on the glass slide were first heated in a furnace of thermogravimetric analysis (TGA) in air at different temperatures, then cooled to room temperature, and observed by SEM.

3. Results and Discussion 3.1. Formation of Microcubic Particles by Evaporation of the Water/Butanone Mixture. PS115-b-PAA63 block copolymer self-assembles into spherical nanomicelles in the block-selective solvent water. The diameter of the (22) Gao, L.; Shi, L.; Zhang, W.; et al. Chem. J. Chin. Univ. 2001, 10, 224-226.

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Figure 1. SEM image of the spheres formed on a glass slide by vaporizing a drop of the micelle aqueous solution at 120 °C.

micelles measured by DLLS is about 31 nm. Usually, nanomicelles tend to form a disorderly aggregate when solvent is removed. By drying a drop of the diluted aqueous solution of spherical micelles at 120 °C with great care, spherical nanoparticles are formed on a glass slide. Figure 1 shows the SEM image of the spherical nanoparticles with diameter about 30 nm on a glass slide following the conditions. Clearly, the nanospheres are formed by single micelles. After adding butanone into the micellar solution and then vaporizing the solvent mixture of butanone/water, microcubic particles formed on a glass slide. The SEM images in Figure 2 show the aggregation of PS-b-PAA nanomicelles into microcubic particles by evaporation of the butanone/water mixture at different concentrations of butanone. Figure 2A shows a typical SEM image observed from a sample prepared by evaporation of the water/butanone mixture, in which the butanone concentration is 1 vol %. We can clearly see coexistence of spherical nanoparticles with a size of about 30 nm and microcubic particles, as pointed out by arrow A in Figure 2A, with a size of about 200 × 200 × 200 nm3, indicating a starting of microcubic particles. When the concentration of butanone is increased to 3%, the number of nanospheres, as pointed out by arrow B in Figure 2B, decreases. The nanospheres totally disappear and only microcubic particles exist when the butanone concentration is further increased to 10%. The size of the microcubic particles in Figure 2C is in the range from 200 × 200 × 200 nm3 to 400 × 400 × 400 nm3, which is much larger than the nanospheres. The SEM image in Figure 2D shows the pile of the microcubic particles, which is just like an inorganic cubic crystal. The 3-dimensional structure of the microcubic particles was further confirmed by AFM, as shown in Figure 3. The average size of about 200 × 200 × 200 nm3 is almost equal to that found by SEM. Microcubic particles can also be shaped on substrates of silicon and gold wafers when evaporating the butanone/ water mixed solvent at temperatures lower than 100 °C. When samples are heated at 120 °C to vaporize the solvent mixture, no cubic particles can be observed by SEM. The SEM images in Figure 4 show morphologies of particles formed on a silicon (Figure 4A) and gold (Figure 4B) wafer by heating at 70 °C to volatilize the mixed solvent. We can see microcubic particles and unshaped particles coexisting on the silicon and gold substrate. The result suggests that the glass substrate is most suitable for forming the microcubic particles and that lower temperature favors formation of microcubic particles on a gold and silicon substrate. The substrate effect on formation of microcubic particles needs further study. We think the different substrate effect on formation of microcubic particles is possibly due to the interaction between the micelle solution and the substrate.

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Figure 2. SEM images of particles formed on a glass slide by evaporation of a drop of micelle mixed solution at 120 °C with different butanone concentrations [(A) 1%; (B) 3%; (C) 10%] and of the (D) 3-dimensional structure of the piled microcubic particles with 10% butanone.

Figure 3. AFM image of the microcubic particles on a glass slide.

To our knowledge, only inorganic compounds and a few organic compounds can form a highly symmetrical crystal. It seems incredible that an amorphous polymer, such as PS-b-PAA, can self-assemble into a highly symmetrical structure such as a cube. To confirm the composition of the microcubic particles, XRF was used to analyze the powder of PS-b-PAA. We did not detect any signals corresponding to inorganic compounds, indicating our purification of the powder of the copolymer of PS-b-PAA eliminated any inorganic impurity. EDS was also used to analyze the composition of a microcubic particle. Considering the various inorganic elements in the glass slide, we chose the microcubic particles formed on a silicon wafer as the sample for EDS analysis. Figure 5 shows the EDS spectrum of a microcubic particle formed on a silicon wafer. The result shows there is no signal corresponding to an inorganic element other than Au from the gold-spray and Si from the silicon substrate. Therefore, we believe that the microcubic structure is formed by the PS-b-PAA micelles. As discussed above, the butanone additive plays a key role in the formation of the microcubic particles. However,

the addition of butanone does not affect the size of the micelles in solution. The data listed in Table 1 show the diameter of the micelles self-assembled by PS-b-PAA in the block-selective solvent the water/butanone mixture at different concentrations of butanone. Clearly, the copolymer self-assembles into spherical nanomicelles with a diameter of about 31 nm in the butanone/water mixture over the butanone concentration range covered in this work. The diameter of the micelle is almost a constant of about 31 nm, since the nanomicelles are kinetically frozen in the water-rich solution.23 Eisenberg et al. prepared a needlelike solid at the glassair interface by drying an aqueous solution of spherical micelles self-assembled by PS180-b-PAA28.18 They proposed that the repeat unit assembling the needlelike solid was a large micelle ball. Sascha General et al. studied the formation of crystalline-like structures and nanoparticles formed by the complexes of poly(ethylene oxide)-b-poly(ethylene imine) and dodecanoic acid.24 Here, we report the orderly aggregation of the nanomicelles forming the microcubic particles. Clearly, the formation of all the crystalline-like structures is owed to the kinetics of the solvent casting or the additives such as dodecanoic acid and butanone, although the mechanism needs further study. We think the additive of butanone possibly plays two roles in the aggregation. First, the addition of butanone into the micelle solution forms a ternary system of copolymer/water/additive. With the evaporation of part of the additive and water, a suitable ratio of copolymer/ water/additive is achieved and induces the nanomicellar cubic phase to form.16 Next, the additive may have a suitable affinity for the exterior of the micellar cubic phase because of hydrogen bonding, which facilitates the aggregating of the cubic micellar phase into a microcubic particle. Besides, the formation of the microcubic particles is partly due to the mode of the evaporation of the solvent mixture of the additive and water, which will be discussed subsequently. (23) Zhang, L.; Shen, H.; Eisenberg, A. Macromolecules 1997, 30, 1001-1011. (24) General, S.; Thunemann, A. F. Macromolecules 2001, 34, 69786984.

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Figure 4. SEM images of particles on a silicon (A) and gold wafer (B) by evaporation of butanone/water mixed solvent at 70 °C.

Figure 5. EDS spectrum of a microcubic particle formed on a silicon wafer by evaporation of butanone/water mixed solvent at 70 °C. Table 1. Diameters of the Nanomicelles in a Butanone/ Water Mixture Measured by DLLS with Different Concentrations of Butanone at 25 °C butanone conc (by vol) 0% 1% 2% 4% 6% 8% 10% diameter/nm 31.2 31.2 31.4 31.2 31.3 31.5 31.5

3.2. Formation of Microcubic Particles on a Glass Slide by Evaporation of Other Additive/Water Mixed Solvents. Figure 6 shows the SEM images of microcubic particles formed on a glass slide by evaporation of DMF/ water, 1-propanol/water, acetic acid/water, or n-butylamine/water mixed solvent, respectively, at 120 °C. The size of the microcubic particles obtained by evaporation of the n-butylamine/water mixture as shown in Figure 6D is similar to that obtained by evaporation of the butanone/water mixture. The size of the microcubic particles obtained by evaporation of DMF/water and acetic acid/water mixtures is slightly larger, ranging from 300 × 300 × 300 nm3 to 600 × 600 × 600 nm3 (see Figure 6A and C). Of all the microcubic particles we have obtained, those formed by evaporation of a 1-propanol/water mixture are the most nonuniform, ranging from 200 × 200 × 200 nm3 to 600 × 600 × 600 nm3, as shown in Figure 6B. Whereas the microcubic particles can be formed by evaporation of water mixtures with the five additives mentioned above, we find the coexistence of microcubic particles and some irregular shaped areas in the sample prepared by evaporation of the DMF/water mixture, as shown in Figure 6A. This possibly indicates the dissolution of microcubic particles by DMF because DMF is a solvent for both blocks of the block copolymer, while the other four additives are block-selective solvents for the copolymer. Of all the additives, butanone and n-butylamine are more volatile than water; thus, their concentration seems to diminish and water will remain at the last stage of the evaporation of the micellar solution. Why the volatile

additives can influence the aggregation of the micelles during evaporation is possibly due to two reasons. The first is attributed to the interaction between the additive and the nanomicelles. For n-butylamine, the result is mainly from the valence bonding between the amino group of n-butylamine and the carboxyl of the PAA block. For butanone, we think it is possibly involved in the association between butanone and the nanomicelle. The interaction affects the Brownian motion of the molecules of the additives, which further influences the evaporation of the additives. The second reason possibly owes to the mode of the evaporation. To vaporize the additive and water synchronously, a small quantity of the micellar solution of about 0.05 mL as discussed above was outspread on the surface of the substrate and then heated at 120 °C, which was much higher than the boiling point of both the additive and water. This fast evaporation provided the opportunity for the additive to influence the aggregation of the micelles. As discussed above, the cubic structure can also form on a silicon and gold wafer at the lower temperature of 70 °C, but the morphology is much more defective than those formed at 120 °C. This is partly due to the substrate effect and the anisochronous volatilization of butanone and water at lower temperature. 3.3. Thermal Stability. Figure 7 shows the SEM images of the microcubic particles on a glass slide after heating in a TGA furnace at different temperatures for 2 h and then cooling at room temperature. The SEM image in Figure 7A shows the morphology of microcubic particles before heating, which is similar to that shown in Figure 2C. The morphology of the microcubic particles remained almost unchanged after heating at 150 °C for 2 h (Figure 7B). When heated at 200 °C for 2 h, part of the microcubic particles were deformed, as shown in Figure 7C. When heated at 280 °C for 2 h, almost all of the microcubic particles were destroyed and a little larger unshaped

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Figure 6. SEM images of microcubic particles formed on a glass slide by evaporation of a drop of micelle solution at 120 °C with different concentrations of additives: (A) 5 vol % DMF; (B) 3 vol % 1-propanol; (C) 10 vol % acetic acid; (D) 5 vol % n-butylamine.

Figure 7. Morphology transition of the microcubic particles at room temperature (A) and heated at 150 °C for 2 h (B), at 200 °C for 2 h (C), at 280 °C for 2 h (D), and at 400 °C for 2 h (E).

island structure formed (Figure 7D). When heated at 400 °C for 2 h, part of the unshaped island structure disappeared (Figure 7E). When further heated at 600 °C for 2 h, almost nothing could be observed by SEM on the glass slide. The reason that the microcubic particles remained

stable at 150 °C for 2 h is possibly due to the ordered structure of the microcubic structure. The formation of the larger unshaped island structure is possibly due to the fusion of the 3-dimensional microcubic structure (see in Figure 7C and D). At temperatures above 200 °C, the

Aggregation PS-b-PAA Micelles to Microcubic Particles

PS block becomes able to flow; thus, the microcubic structure is destroyed and spread on the substrate as shown in Figure 7C and D. Besides, the morphology transition of microcubic particles, such as the deformation of some of the microcubic particles at 200 °C as shown in Figure 7C and the transformation from the 3-dimensional structure to the 2-dimensional island structure as shown in Figure 7D, confirms further that the microcubic particle is really polymeric but not inorganic, because the melting point of inorganic salts such as NaCl and CaCO3 is higher than 800 °C. 4. Conclusion PS-b-PAA self-assembles into spherical nanomicelles in the block-selective solvent water. After addition of butanone, 1-propanol, acetic acid, DMF, or n-butylamine, evaporation of the solvent mixture induces aggregation of the nanomicelles into microcubic particles on a glass slide. The size of microcubic particles is from 200 × 200 × 200 nm3 to 600 × 600 × 600 nm3. The concentration of the additives affects the formation of the microcubic particles. When the concentration of butanone is 10%, all nanomicelles are organized to form microcubic particles.

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For additives of DMF, 1-propanol, acetic acid, and nbutylamine, the suitable concentration is 5%, 3%, 10%, and 5%, respectively. The formation of the microcubic particles is possibly due to the effect of the additives and the mode of the evaporation of the mixture solvent of additive/water. Microcubic particles can also form on a silicon and gold wafer at 70 °C, but the formation of microcubic particles is not as perfect as that on a glass slide. The morphology of the microcubic particles remains almost unchanged below 150 °C. When heated at 200 °C, some of the microcubic particles are deformed. When the temperature is raised to 280 °C, almost all of the microparticles are destroyed and a little larger unshaped island structure forms. At 600 °C, almost nothing can be observed by SEM on the glass slide. Acknowledgment. The work is supported by the National Natural Science Foundation of China (No. 50273015) and Chinese Education Ministry Foundation for Nankai University and Tianjin University Joint Academy. LA034300G