High-Throughput Transformation of Colloidal Polymer Spheres to

Mar 27, 2012 - Discs Simply via Magnetic Stirring of Their Dispersions. Bing Liu. † .... molecules, pyrene (in chloroform), Nile red (in chloroform)...
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Article pubs.acs.org/Langmuir

High-Throughput Transformation of Colloidal Polymer Spheres to Discs Simply via Magnetic Stirring of Their Dispersions Bing Liu† and Dayang Wang*,†,‡ †

Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany Ian Wark Research Institute, University of South Australia, Adelaide SA 5095, Australia

‡

S Supporting Information *

ABSTRACT: In this article, we have successfully demonstrated the high-throughput production of colloidal discs via magnetic stirring of aqueous dispersions of monodisperse, sulfate-stabilized polystyrene (PS) spheres in the presence of a good organic solvent. The organic solvent could be water-miscible, such as tetrahydrofuran, or water-immiscible, such as chloroform. Water-immiscible organic solvents were mixed into aqueous dispersions of PS spheres in the presence of sodium dodecyl sulfate. The geometry of the resulting discs could be easily adjusted by the magnetic stirring time and speed, the stirring bar weight, and the amount of organic solvent. Our strategy is simple, scalable, and hardly dependent on the nature of the organic solvent and the PS sphere diameter; PS spheres with diameters ranging from 200 nm to 5 μm were deformed into discs with almost 100% yield. When organic solutions of fluorescent dyes and nanoparticles were used instead of pure organic solvents for PS sphere liquefaction, fluorescent discs were obtained, underlining the effective, efficient encapsulation of the fluorescent substance in the discs.

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INTRODUCTION Nonspherical particles have been of fundamental and technical interest for decades because the shape and interaction anisotropy can significantly affect the particle rheological behavior, phase behavior, packing structure, packing density, and so on.1−5 Recent research interest in molecular-mimetic particle self-assembly has fueled the development of various template-free or template-assisted strategies for producing particles with diverse anisotropic shapes to control the interaction directionality and thus the spatial configuration of neighboring particles.6−20 However, many currently available methods are limited by high cost, low yield, poor colloidal dispersibility and stability, process complexity, and large size and shape polydispersity. Thus, developing a reliable and highthroughput technique for producing uniform, nonspherical particles remains an imperative. Herein we present a simple and high-throughput way to produce colloidal polymer discs via simple magnetic stirring of aqueous dispersions of polystyrene (PS) spheres in the presence of a good organic solvent, either immiscible or miscible with water. Mechanical deformation of polymer spheres has been applied to produce polymer rods and ellipsoids.16−21 The deformation process, however, is rather complicated and cumbersome and involves at least four steps: (1) embedding polymer spheres in dry sacrificial films of, for instance, poly(vinyl alcohol) (PVA); (2) stretching the films at a temperature above the glasstransition temperature (Tg) of the polymer spheres or by using a given amount of an organic solvent that is good for polymer spheres but bad for PVA to liquefy the polymer spheres selectively; (3) cooling the films below the sphere Tg or extracting the organic solvent under stretching; and (4) © 2012 American Chemical Society

dissolving the PVA to release the polymer rods or ellipsoids. Compared with this process, our strategy allows the deformation of PS spheres directly in their aqueous dispersions under ambient conditions via magnetic stirring. To the best of our knowledge, this should be the simplest method for the direct transformation of polymer spheres to colloidal discs in water with almost 100% yield thus far. Furthermore, the currently available strategies have had little success in the production of colloidal discs.

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EXPERIMENTAL SECTION

Aqueous dispersions of monodisperse, negatively charged, sulfatestabilized PS spheres with diameters of 0.2, 0.75, 2.0, and 4.4 μm (10 w/v %) were purchased from Micropaticles GmbH (Berlin, Germany) and used without further treatment. Our experimental setup was a 2.5 mL bottle with a bottom outer diameter of 11 mm and an inner diameter of about 10 mm containing a magnetic stirring bar with a length of about 10 mm, a diameter of 6 mm, and a weight of 0.8 g (Figure S1a, Supporting Information). When a water-immiscible organic solvent such as chloroform was used for the liquefaction of PS spheres, a given amount of chloroform was first added to 500 μL of an aqueous solution of sodium dodecyl sulfate (SDS) (1 wt %) and sonicated for 1 min to form oil-in-water emulsions. Subsequently, 50 μL of an aqueous dispersion of PS spheres, corresponding to a PS sphere ensemble volume of 5 μL, was added to the resulting emulsions, followed by mixing via a rotary mixer for 24−48 h under ambient conditions. The amount of chloroform was varied from 15 to 30 and 100 μL, corresponding to ratios of the chloroform volume (VCF) to the PS sphere ensemble volume (VPS) of 3:1, 6:1, and 20:1, Received: January 4, 2012 Revised: March 20, 2012 Published: March 27, 2012 6436

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respectively. In the case of the use of a water-miscible organic solvent such as tetrahydrofuran (THF) for the liquefaction of PS spheres, a given amount of THF ranging from 0.01 to 0.5 mL was directly dropped into a 1.2 mL aqueous dispersion of PS spheres (1.67 w/v%), followed by incubation for 24−48 h similar to that during chloroform liquefaction. The dispersions of liquefied PS spheres were magnetically stirred at the room temperature. After 72 h, the resulting discs were washed via three repetitions of the centrifugation/decanting cycle of supernatants/redispersion in water in order to remove the residual organic solvents completely and finally were redispersed in water for characterization. Following the aforementioned protocol, solutions of organic dye molecules, pyrene (in chloroform), Nile red (in chloroform), coumarin 6 (in THF), and 5 nm octadecylamine-stabilized CdSe@ ZnS quantum dots (in toluene) were used to liquefy PS spheres to create fluorescent discs. Scanning electron microscopy (SEM) imaging was implemented using a field-emission scanning electron microscope (Gemini LEO 1550, operated at 3 kV). Confocal laser scanning microscopy (CLSM) images in fluorescence and transmission modes were obtained with a confocal microscope (Leica DM IRBE). Fluorescence spectra were measured on a Fluoromax-4 spectrophotometer.

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accordingly larger than that of the original spheres by 52% (Figure 2a). The D2/D1 ratio increases with the magnetic stirring time (Figure 2b). On the basis of low-magnification SEM imaging, we estimated the number percentage of deformed PS spheres to measure the deformation rate; note that any nonspherical particle was counted as a deformed sphere. The deformation rate rapidly increases to about 70% in the first 2 h of magnetic stirring and then slowly to 95% after another 22 h of stirring, whereas reaching a rate of 100% requires another 48 h of stirring (Figure 2b). The noticeable reduction in the D2/D1 ratio error bars with the stirring time, shown in Figure 2b, also highlights the improvement in the size and shape uniformity of the resulting discs. The magnetic stirring speed significantly affected the deformation of the PS spheres. The optimal stirring speed in our work was 1100 rpm, whereas a lower or higher speed caused a smaller D2/D1 ratio and/or poor uniformity of the shape and size of the resulting discs (Supporting Information, Figure S3). We also observed that an increase in the weight of the magnetic stirring bars used from 0.8 to 1.9 g increased the D2/D1 ratio from 1.46 ± 0.11 to 1.56 ± 0.13 (Supporting Information, Figure S4). These results lead to a plausible hypothesis that the deformation of PS spheres into discs in dispersion during magnetic stirring is a result of the grinding of the spheres by a magnetic stirring bar according to its dependence on the time and speed of magnetic stirring and the stirring bar weight. When PS spheres are located in between the magnetic stirring bar and the bottom of the glass vial during the magnetic stirring of their dispersions, they may be pressed and in turn deformed by the gravity of the stirring bar and the magnetic force between the stirring bar and the electromagnet underneath the glass vial. The rotation of the stirring bar causes not only the rapid transport of more spheres into the region between the stirring bar and the bottom of the glass vial but also an additional force grinding the spheres. Revealing a reasonable mechanism of deformation of PS spheres under magnetic stirring, however, requires the meticulous interrogation of local mechanic and hydrodynamic forces exerted on the spheres and their mechanical behavior alteration arising from the presence of chloroform, which is underway, especially with the help of theoretical modeling. The presence of chloroform in the aqueous dispersions of PS spheres was crucial to the sphere deformation. Regardless of the speed and time of the magnetic stirring and the weight of the stirring bars used, no PS sphere deformation was visible in the absence of chloroform under the same experimental conditions. The degree of deformation (D2/D1) of the PS spheres increased with VCF/VPS (Figure 3a; Supporting Information, Figure S5). We stained chloroform with pyrene prior to mixing into the aqueous dispersions of PS spheres. Thanks to its high sensitivity to the polarity of the pyrene microenvironment,23 the intensity ratio of the first to third bands of the pyrene monomer emission (I1/I3) was utilized to evaluate the volume fraction ( f CF) of chloroform in PS spheres according to the equation I1/I3 = f CF(I1/I3)CF + (1 − f CF)(I1/I3)PS, where (I1/ I3)CF and (I1/I3)PS are the I1/I3 ratios of pyrene in pure chloroform and PS, respectively. (I1/I3)CF was measured to be 1.23 at the pyrene concentration of 1 × 10−6 mol/L used. (I1/ I3)PS was assumed to be similar to the I1/I3 ratio of pyrene in pure styrene (1.02) because it was difficult to avoid the formation of pyrene excimers in solid PS. Figure 2b shows that mixing of the chloroform solution of pyrene into aqueous dispersions of PS spheres results in a significant decrease in the I1/I3 ratio in the first 24 h of incubation whereas the I1/I3 ratio

RESULTS AND DISCUSSION

Monodisperse, negatively charged, sulfate-stabilized PS spheres with diameters ranging from 200 nm to 5 μm were used for proof-of-concept experiments. Inspired by emulsion polymerization,22 we mixed aqueous dispersions of PS spheres with given amounts of chloroform with the aid of SDS, followed by 24−48 h of incubation under ambient conditions. The amount of chloroform mixed into the aqueous dispersion of PS spheres was measured as the VCF/VPS ratio. Afterward, we conducted magnetic stirring of the resulting dispersions of PS spheres for 72 h, followed by centrifugation and redispersion in water (Supporting Information, Figure S1). SEM imaging demonstrates the complete transformation of PS spheres to discs (Figures 1 and S2). The degree of deformation of the PS spheres was measured as the ratio of the diameter of the resulting discs (D2) to that of the original spheres (D1). Independent of the diameters of original PS spheres, the D2/D1 ratio is 1.46 ± 0.11 and the surface area of the resulting discs is

Figure 1. SEM images of PS discs obtained via 72 h of magnetic stirring of the aqueous dispersion of PS spheres with diameters of (a) 4.4, (b) 2.0, (c) 0.75, and (d) 0.2 μm in the presence of chloroform. The stirring speed is 1100 rpm, the stirring bar weight is 0.8 g, and VCF/VPS = 3:1. 6437

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Figure 2. (a) Plot of the degree of deformation (D2/D1, ■) of PS spheres, obtained via 72 h of magnetic stirring of their aqueous dispersion in the presence of chloroform and the corresponding surface area increment (□) vs the original sphere diameter (D1). The surface area increment is calculated as [(D2/D1)2/2 + 2/3(D2/D1)] −1. (b) Plots of the deformation rate (□) and degree of deformation (D2/D1 ratio, ■) of 4.4 μm PS spheres obtained via magnetic stirring of their aqueous dispersion in the presence of chloroform vs the stirring time. The stirring speed is 1100 rpm, the stirring bar weight is 0.8 g, and VCF/VPS = 3:1.

Figure 3. (a) Plot of the degree of deformation (D2/D1, ■) of 4.4 μm PS spheres, obtained via 72 h of magnetic stirring of their aqueous dispersion in the presence of chloroform and the surface area ratio of the resulting discs with respect to the original spheres (□) vs VCF/VPS. (b) Plots of I1/I3 of pyrene and the volume fraction (f CF) of chloroform in 4.4 μm PS spheres vs the time over the course of sphere deformation. VCF/VPS is 3:1 (■), 6:1 (●), and 20:1 (▲). The stirring speed is 1100 rpm, and the stirring bar weight is 0.8 g. The I1/I3 ratios of pyrene at time zero are calculated by using VCF/VPS of the original dispersion of PS spheres. The time point for starting the magnetic stirring is highlighted by the dashed line.

remains fairly stable in the following 24 h. This I1/I3 ratio reduction suggests a polarity reduction in the pyrene microenvironment and thus the uptake of the pyrene chloroform solution into the PS spheres. The equilibrium f CF in PS spheres is calculated to be as large as about 36%, with little dependence on the VCF/VPS ratio of the dispersions. This large f CF suggests a significant uptake of chloroform in PS spheres and thus effective liquefaction of the PS spheres. Figure 3b also shows that magnetic stirring immediately triggers a reduction in the f CF from 36 to 24% and then slowly reduces the f CF with time. The f C reduction during magnetic stirring should be rationalized by the significant surface area increase during the deformation of PS spheres to discs as shown in Figures 2a and 3a, which drives the diffusion of chloroform from the deformed PS spheres back to the SDS micelle in the dispersion media. This f CF reduction during magnetic stirring should accordingly lead to the revitrifaction of the polymer chains in deformed PS spheres and thus the freezing of the shape of the deformed spheres. Figure 3b shows that although the VCF/VPS ratio of 3:1 or 6:1 leads to a reasonable large D2/D1 (1.46 ± 0.11 or 1.55 ± 0.11) and surface area increment (52 or 63%), f CF in the resulting discs is about 14% after 72 h of magnetic stirring. When the aqueous dispersions of the PS discs, obtained at VCF/VPS = 3:1, were heated to 95 °C (comparable to the Tg of PS) for 24 h, all

discs were transformed back to spheres (Supporting Information, Figure S6a), suggesting that the disklike shape is less thermodynamically favorable than the spherical shape. The PS spheres, recovered from heat treatment, were readily deformed into discs via magnetic stirring without the addition of chloroform to their aqueous dispersions; D2/D1 was 1.5 ± 0.11 and slightly larger than that derived from the original PS spheres (Supporting Information, Figure S6b). This further verifies the presence of a small amount of chloroform in the PS discs as well as the spheres recovered thereof. When the VCF/ VPS ratio used for the liquefaction of PS spheres increased to 20:1, the maximum D2/D1 (1.60 ± 0.12) and surface area increment (70%) were obtained after 72 h of magnetic stirring, and there was little chloroform left in the resulting discs (f CF = 0). Other good, water-immiscible or miscible, organic solvents such as toluene, THF, and dimethylformamide were successfully utilized to liquefy PS spheres. THF or other water-miscible organic solvents were directly mixed into the aqueous dispersions of PS spheres. Similar to our previous study,14 PS spheres remained intact over a large range of the volume fraction of THF (VTHF) in the dispersion (0.8−30%). The optimal VTHF for producing uniform disks was in the range of 4.8−9.1% (Supporting Information, Figure S7 and Table S1). Compared with chloroform, using THF for the liquefaction of 6438

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Langmuir PS spheres resulted in a faster deformation and a larger degree of deformation; D2/D1 was as large as 1.84 ± 0.15 after 96 h of stirring (Figure 4). The use of THF also effectively avoided

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CONCLUSIONS

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ASSOCIATED CONTENT

Article

We have successfully demonstrated the high-throughput production of colloidal discs via magnetic stirring of aqueous dispersions of PS spheres in the presence of a good organic solvent. The geometry of the resulting discs can be easily adjusted by the magnetic stirring time and speed, the stirring bar weight, and the amount of organic solvent. Our strategy is simple, scalable, and hardly dependent on the PS sphere diameter and the nature of the organic solvent. The use of organic solutions of functional substances for PS sphere liquefaction leads to colloidal discs with a variety of functions, thus significantly widening the range of the fundamental and technological applications of the discs. The present success leads to the implication of applying conventional macroscopic machining techniques to process the shape of nanoscopic particles in dispersions in situ, which is significant for research associated with anisotropic colloidal particles and their organization. Our preliminary results have demonstrated the formation of polymer rods with varied aspect ratios by using planar substrates to deform THF−liquefied PS spheres (Supporting Information, Figure S10). This has encouraged us to design magnetic stirring setups deliberately for elaborate control of the shape transformation of liquefied PS spheres, which is the focus of our ensuing research. In principle, the present approach should be generalized to deform other types of polymer spheres. We successfully applied the current protocol to deform PS-based composite particles such as carboxylated PS spheres into disk-shaped particles (Supporting Information, Figure 11). However, we were not able to produce uniform disk-shaped poly(methyl methacrylate) particles reproducibly. This should be due to the viscoelasticity difference between poly(methyl methacrylate) and PS. To extend the present approach to different polymer spheres, our current effort is devoted to the optimization of the amount and nature of organic solvents, the stirring bar weight, and the stirring speed to control the mechanical behavior of the polymer spheres deliberately during magnetic stirring.

Figure 4. Plots of the deformation rate (□) and degree of deformation (D2/D1, ■) of 4.4 μm PS spheres obtained via magnetic stirring of their aqueous dispersion in the presence of THF vs the stirring time. The stirring speed is 1100 rpm, the weight of the stirring bar is 0.8 g, and VTHF is 9%.

SDS contamination on the sphere surfaces, so we could fix the THF-liquefied PS spheres on positively charged substrates to measure their mechanical properties. The Young’s modulus of THF-liquefied spheres was as low as 1.2 MPa, underlining a glassy-state to rubbery-state transition in the PS spheres (Supporting Information, Figure S8). Note that the Young’s modulus of polystyrene is 3−3.5 GPa.24 Intriguingly, when VTHF was below 2.5%, a large number of PS spheres were deformed to cylinders, in coexistence with irregular discs (Supporting Information, Figure S9). Figure 5 shows that when the organic solutions of organic dyes or fluorescent inorganic nanoparticles were used instead of pure organic solvents for the liquefaction of PS spheres, highly fluorescent PS discs were obtained after magnetic stirring. This demonstrates not only the direct formation of colloidal discs in dispersions but also the effective and efficient encapsulation of various functional lipophilic molecules and/or nanoparticles into the resulting PS discs, thus diversifying the disk functionality.

S Supporting Information *

Photographs of a magnetic stirring setup for the transformation of PS spheres in water. SEM images of PS discs obtained at different magnetic stirring speeds using different magnetic stirring bars. AFM measurement of the Young’s modulus of

Figure 5. CLSM fluorescence images of PS discs obtained via magnetic stirring of (a) aqueous dispersions of 4.4 μm PS spheres in the presence of a chloroform solution of Nile red, (b) a toluene dispersion of octadecylamine-stabilized, 5 nm CdSe@ZnS quantum dots, and (c) a THF solution of coumarin 6. The stirring speed is 1100 rpm, and the weight of the stirring bar is 0.8 g. The scale bar is 5 μm. 6439

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(16) Keville, K. M.; Franses, E. I.; Caruthers, J. M. Preparation and characterization of monodisperse polymer microspheroids. J. Colloid Interface Sci. 1991, 144, 103−126. (17) Ho, C. C.; Keller, A.; Odell, J. A.; Ottewill, R. H. Preparation of monodisperse ellipsoidal polystyrene particles. Colloid Polym. Sci. 1993, 271, 469−479. (18) Jiang, P.; Bertone, J. F.; Colvin, V. L. A lost-wax approach to monodisperse colloids and their crystals. Science 2001, 291, 453−457. (19) Lu, Y.; Yin, Y.; Xia, Y. Preparation and characterization of micrometer-sized “egg shells”. Adv. Mater. 2001, 13, 271−274. (20) Champion, J. A.; Katare, Y. K.; Mitragotri, S. Making polymeric micro- and nanoparticles of complex shapes. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 11901−11904. (21) Hu, Y.; Ge, J.; Zhang, T.; Ying, Y. A blown film process to diskshaped polymer ellipsoids. Adv. Mater. 2008, 20, 4599−4602. (22) Gilbert, R. G. Emulsion Polymerization: A Mechanistic Approach; Academic Press: London, 1996. (23) Kalyamanasundaram, K.; Thomas, J. K. Environmental effects on vibronic band intensities in pyrene monomer fluorescence and their application in studies of micellar systems. J. Am. Chem. Soc. 1977, 99, 2039−2044. (24) Mott, P. H.; Dorgan, J. R.; Roland, C. M. The bulk modulus and Poisson’s ratio of “incompressible” materials. J. Sound Vib. 2008, 312, 572−575.

THF−liquefied PS spheres. SEM images of PS rods obtained via the deformation of THF−liquefied PS spheres in water. Morphology summary of deformed PS spheres liquefied with different amounts of THF. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+61) 8-830-23683. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Max Planck Society is acknowledged for financial support. We thank Prof. H. Möhwald for research support and valuable discussions and Dr. P. Fernandes for assistance with the assessment of the mechanical properties of polystyrene spheres. D.W. is grateful to the Australian Research Council for financial support (DP 110104179).

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