Crystal-Like Polymer Microdiscs - Macromolecules (ACS Publications)

Article pubs.acs.org/Macromolecules

Crystal-Like Polymer Microdiscs Shanqin Liu, Renhua Deng, Lei Shen,* Xiaolin Xie, and Jintao Zhu* Key Laboratory for Large-Format Battery Materials and System of the Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China

Downloaded by TEXAS A&M INTL UNIV on September 5, 2015 | http://pubs.acs.org Publication Date (Web): August 13, 2015 | doi: 10.1021/acs.macromol.5b00914

S Supporting Information *

ABSTRACT: Here we describe a facile yet effective route for the fabrication of crystal-like polymer microdiscs with a huge bump at the surrounding edge through hydrodynamic instabilities of emulsion droplets containing hydrophobic polymer and cosurfactant noctadecanol (OD). This strategy allows for the generation of polymer particles with tunable size and shape by tuning the cosurfactant concentration, emulsion droplet size, and/or solvent evaporation rate. The generation of polystyrene (PS) microdiscs is balanced by the interfacial instabilities of emulsion droplets, crystallization of OD, and capillary flow. Our approach can be extended to different hydrophobic polymers and allows for the functionalization of the discs with tunable chemical/physical properties by incorporating functional species. By introducing magnetic nanoparticles, we have been able to manipulate the spatial orientation of the magnetic microdiscs via an external magnetic field. We anticipate this simple and versatile route to be useful for the design and fabrication of well-defined microparticles for potential applications in the fields of targeting, separation, sensing, drug delivery, and formation of advanced materials.

1. INTRODUCTION In nature, numerous examples can be found in which physical features such as size, shape, and mechanical properties are shown to have a critical influence on biological functions.1−3 On the basis of this observation, researchers have tried to design and fabricate materials with controllable shapes and functionalities.4 Polymer particles with various sizes and shapes have been produced, including spherical particles,5 rod-shaped particles,6 ellipsoids,7 patchy particles,8 disc-like particles,2 and other complex structures.9 Particle shape has been proven to be a critical parameter that significantly affects particle functions, including targeting ability,4 cellular uptake,10 immune response,11 flow separation,12 rheological property,13 colloidal self-assembly,14 and so forth. Among these, discoidal structures with two-dimensional (2D) anisotropy demonstrate unique properties.15−17 For example, in drug delivery system, discshaped hydrophilic nanoparticles (NPs) were internalized more efficiently than nanorods by endothelial cells.10 Moreover, micrometer-scaled elliptical discs (1 × 3 μm) demonstrate better targeting efficiency for accumulation in tissues than spheres with various diameters (from 100 nm to 10 μm).18 Interestingly, nature has defined her own specific shape for function selectivity. For example, erythrocytes in vivo are able to avoid filtration in the spleen due to their discoidal shape and mechanical flexibility while platelets use a similar shape to assist their functions of adhesion and rolling on vascular endothelium.3 Therefore, it is desirable to generate disc-like particles not only for wide potential applications of drug delivery, targeting, sensing, and separation, but also for deeply understanding the shape-related phenomena, such as anisotropy colloidal-assembly, cell-particle interaction/recognition, vascular dynamics and transportation, and degradation profile. © 2015 American Chemical Society

Several methods have been reported for preparing disc-like polymer particles, including mechanical stretching,2 seed dispersion polymerization,5 block copolymer self-assembly and disassembly,19−21 crystallization-guided route,22 template and solvent annealing,23,24 jet and flash imprint lithography,10 microfluidic-assisted polymerization,25 among others.15,26 For example, Champion et al. used a stretching method to prepare anisotropic polymer particles with tunable shape based on the preformed polymer particles.2 Chen’s group prepared Janus polymer discs by using triblock copolymer under solvent annealing.24 Yet, the resulting discoidal structure was folded, and the size/structure of the nanodiscs was hard to control. Nowadays, it is still challenging to generate 2D polymer microdiscs with tunable size and structure in a straightforward and versatile means. Herein, we demonstrate a facile, yet effective approach to prepare crystal-like polymer microdiscs via interfacial instability of emulsion droplets containing polystyrene (PS) and cosurfactant n-octadecanol (OD). Upon solvent removal, the emulsion droplets shrunk and their interfacial areas increased spontaneously, i.e., the interfacial instabilities occurred. During this process, crystallization of OD on the surface of the droplets led to the formation of crystal-like discs. Using this approach, microdiscs with controllable size and shape can be generated by manipulating the OD concentration (COD), emulsion droplets size, and/or solvent evaporation rate. Our approach can be extended to different hydrophobic polymers and allows us to functionalize the microdiscs with tunable chemical/physical properties by incorporating functional species. Received: April 30, 2015 Revised: July 10, 2015 Published: August 13, 2015 5944

DOI: 10.1021/acs.macromol.5b00914 Macromolecules 2015, 48, 5944−5950

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was blotted away using a strip of filter paper. The samples were allowed to dry in the atmosphere at room temperature for 1 day before observation. For the cross-section observation, the dried samples were embedded in epoxy resin (Electron Microscopy Sciences) and cured in an oven at 50 °C for 48 h. Thin cross sections with thickness of ∼70 nm were obtained by ultramicrotomy with an ultramicrotome (Leica, EM UC7). The thin cross-section was stained by RuO4 to enhance the contrast of the PS microdomain compared to the epoxy resin background before TEM investigation. Atomic Force Microscope (AFM). The thicknesses and surface morphology of polymer microdiscs were measured by AFM (Veeco dimension 3100) using tapping mode. X-ray Diffraction (XRD). The polymer microdiscs were identified by Powder XRD patterns using an X’Pert PRO (PANalytical B.V., Holland) diffractometer and 2θ varied over a range of 10−85°. The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. Differential Scanning Calorimeter (DSC). The phase transition behavior of OD and PS was measured with a DSC (Q2000, TA Instruments, U.S.A.). Ten milligram samples were first heated to 160 °C, maintained for ∼3 min, and then cooled to 10 °C at a rate of 10 °C/min. Transition temperatures were taken at the peak values from the cooling ramp.

2. EXPERIMENTAL SECTION Materials. Polystyrene (PS21k, we refer PS21k as PS with Mn = 21 kDa, Mw/Mn = 1.04; PS2.8k, Mw/Mn = 1.09; PS876k, Mw/Mn = 1.19; PS30k, Mw/Mn = 1.06), poly(methyl methacrylate) (PMMA24k, Mw/Mn = 1.25), PS-b-poly(methyl methacrylate) (PS25k-b-PMMA26k, Mw/Mn = 1.06), PS11.5k-SH (Mw/Mn = 1.08) were purchased from Polymer Source. Inc., Canada. Sodium dodecyl sulfate (SDS) (purity: 98%) was purchased from Sigma-Aldrich. n-Octadecanol (OD), n-dodecanol, ntetradecanol, n-hexadecanol, n-eicosanol, ethanol, and chloroform were all purchased from Shanghai Reagents Co., China. All of the materials were used as received without further purification. Sample Preparation. Monodisperse emulsion droplets with tunable size of ∼10−400 μm were generated through a microcapillary device.27 Typically, the dispersed organic phase contained 10 mg/mL polymers and OD with various concentration in chloroform, while 3 mg/mL SDS was added to the aqueous phase to stabilize the emulsion droplets against coalescence. The formed emulsion droplets were collected in a special homemade cell which had a flat bottom, and the rate of solvent evaporation was tuned by changing the height of water layer (h).27 Chloroform was allowed to evaporate at 30 °C. Depending on the height of water layer in the cell, evaporation process of the emulsion droplets will last tens of minutes to hours. The resulting suspension of polymer particles was placed in a dialysis tubing (DM27 EI9004, USA; cut off: 12 000− 14 000) to dialyze against deionized water for 5 days to remove SDS, while others were dialyzed against ethanol for 5 days to remove OD and residual SDS. Au NPs were prepared through a template-assisted procedure28 while Fe3O4 magnetic NPs were synthesized by using a reported approach.29 To prepare the microdiscs encapsulated with magnetic NPs, 2.0 wt % magnetic NPs compared to the polymers were added to the initial PS solution in chloroform. A similar procedure was carried out to produce the hybrid microdiscs, as described above. On the other hand, the formed PS microdiscs (with or without ethanol treatment) suspension was mixed with Au NPs dispersion in aqueous solution, followed by gently stirring to improve the incorporation of Au NPs on the surface of the microdiscs. Characterization. Optical Microscope (OM). An Olympus IX71 inverted optical microscope was used in bright-field or epifluorescence mode to monitor the evolution of shrinking emulsion droplet containing PS and OD as organic solvent removal. To prepare the samples for fluorescence microscopy investigation, 1 wt % Nile Red was dispersed in the initial PS solution in chloroform, followed by the emulsion generation and solvent removal to form the dye-labeled microdiscs. Scanning Electron Microscope (SEM). SEM was carried out on a Sirion 200 scanning electron microscope at an accelerating voltage of 10 kV. To prepare the samples for SEM, a drop of the dialyzed particles dispersion was dropped on a clean silicon wafer, and the water was allowed to evaporate. Then, the samples were coated with a thin film of gold. Transmission Electron Microscope (TEM). TEM investigation was performed on a FEI Tecnai G2 20 microscope operated at an acceleration voltage of 200 kV, equipped with a CCD camera (Gatan USC4000, Gatan). For TEM sample preparation, a drop of very dilute dispersion was placed onto TEM copper grid covered by a polymer supported film precoated with carbon thin film. After 10 min, excess solution

3. RESULTS AND DISCUSSION Formation of Polymer Microdiscs. Monodisperse emulsion droplets with tunable size of ∼10−400 μm were generated by a microcapillary device (Figure 1a and 2a).27 The

Figure 1. (a) Schematic illustration of the microcapillary device for the formation of monodisperse oil-in-water emulsion droplets; (b) Illustration showing the homemade cell for controlling the rate of solvent evaporation.

chloroform droplets containing PS and OD were stabilized by SDS. After collection of the formed emulsion droplets in a small cell, the organic solvent was allowed to evaporate (Figure 1b). Chloroform diffused through the aqueous phase and evaporated at the water/air interface. Thus, the rate of solvent evaporation was controlled by tailoring the height of the water layer (h) which regulated the diffusion distance of chloroform. Upon solvent removal, the emulsion droplets with an initial size of ∼20 μm shrunk and the surface area of the organic solvent/ water interface decreased (Figure 2b, c and Movie S1 in the Supporting Information (SI)), which concentrates the OD and SDS at the interface and thus triggers the formation of 5945


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Figure 2. (a) OM image of the monodisperse emulsion droplets formed from the capillary microfluidics technique; (b−g) OM images showing the morphology evolution of an emulsion droplet containing 10 mg/mL PS21k and 20 mg/mL OD at water layer height of h = 3 mm. SDS (3 mg/mL) was added to the aqueous phase to stabilize the emulsion droplets. The scale bar in (b) is applied to (c−g).

Figure 3. (a) Fluorescence microscopy image of the PS21k microdiscs, with the inset in the lower left the corresponding SEM image; (b) AFM topography image of the PS microdisc; (c) TEM image of crosssection of the PS disc. Inset in the upper right is the cartoon showing the structure of the disc; (d) XRD plots of the PS discs after water and ethanol dialysis. Insets are the TEM images and corresponding electron diffraction patterns before (a1, a2) and after (b1, b2) ethanol dialysis.

supersaturated state of the molecular packing at the interface. The interfacial area of droplets then increased spontaneously (Figure 2c,d), and similar phenomenon has been reported in our previous reports for the emulsion droplets containing amphiphilic block copolymers or a binary mixture of PS and nhexadecanol (HD) at lower concentration.27,30 The driving force for the interfacial instabilities was ascribed to the vanishing of the interfacial tension due to an interfacial excess of the amphiphiles.31,32 It has been demonstrated that the inset of long-chain alcohol cosurfactants into SDS monolayer at the organic solvent/water interface can markedly decrease the interfacial tension and enhance the surface curvature of shrunk droplets, initiating the instabilities of the droplets.27 Through this process, we have tuned the structural diversity of synthesized particles, such as spheres with well-controlled surface textures, dendritic or budding surfaces. In the present report, OD with a high COD was employed, and qualitatively different interfacial instabilities were observed. Due to the crystallization ability of OD,33 the wrinkled droplets further spread in plane and stretched into disc-like structures when COD at the interface was high enough for crystallization (Figure 2c−g). Evaporation led to supersaturation of the OD molecules on the interface and edge of the fluids, which would crystallize at the interface during the instabilities. Simultaneously, concentration of PS and thus the viscosity of the fluids within the droplets increased. The formed structures were then frozen due to the glassy nature of PS and crystallization of OD upon complete solvent removal. The whole process from occurrence of interfacial instabilities to fixation of the disc-like structures lasted for ∼2 min. Structural Characterization and Formation Mechanism of the Polymer Microdiscs. Figure 3 shows the characteristics of the PS microdiscs. Figure 3a,b show the typical fluorescence microscopy and AFM topography images of the hexagonal PS discs after removing the cosurfactant OD by ethanol dialysis. The polygon PS disc consists of an ultrathin

central area with ∼40 nm thickness surrounding by a 10 times thicker bump (also see Figure S1, S2 in the SI). This unique structure can be clearly confirmed through cross-sectional TEM image (Figure 3c). Through the analysis, we can deduce that structure of the disc presents a thin film surrounded by a symmetric thicker bump at the edge of the film, as illustrated in the inset of Figure 3c. Presumably, formation of this unique bump structure can be attributed to the coffee ring effect.34,35 During the solvent evaporation, fluids at the edge of the droplets expanded slowly although not completely pinned, and the capillary flow outward from the center of the droplet brought dispersed species to the edge as the evaporation proceeded. This point can be proved from the observation of the disc formation (Figure 2c−g), where the materials in the center of the droplets decreased (reflected from the weaker light intensity of the center) and the disc-like fluids expanded in plane. The process continuously occurred to supply the expansion, and the structures fixed when the viscosity was high enough and the OD crystallization formed. Besides triggering the interfacial instabilities, OD can also form crystallization outside hydrophobic PS domain, guiding the formation of the crystal-like PS discs. Presumably, OD molecules locate on the surface of the formed particles due to the amphiphilic character of the cosurfactant. During solvent evaporation, the OD and SDS molecules would concentrate at the oil/water interface. Because OD has longer hydrophobic tails than SDS, OD would pack more ordered than SDS,36 which initiates the crystallization of OD at the interface. The OD crystals have been demonstrated to grow along linear direction with monoclinic structure,37,38 which triggered the formation of PS domains within the OD backbones to produce microdiscs along the crystal direction of OD molecules. Thus, the microdiscs with polygonal shape were formed. To confirm this, we performed X-ray diffraction (XRD) investigation (Figure 3d). Clearly, before ethanol dialysis, the typical crystalline peaks at (−6 1 1) and (14 0 2) are attributed to the crystallization of OD33 (note: PS does not form crystalline 5946


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Figure 4. SEM images of PS particles formed from emulsion droplets containing 10 mg/mL PS21k and 20 mg/mL OD with D0 of: (a) 10, (b) 15, (c) 30, (d) 40, (e) 70, (f) 100, (g) 200, (h) 370, and (i) 440 μm. SDS (3 mg/mL) was added to the aqueous phase to stabilize the emulsions, and h = 3 mm.

Figure 5. (a) SEM images of PS particles with different morphologies formed from emulsion droplets containing 10 mg/mL PS21k and OD with various concentrations: (a) 0, (b) 0.5, (c) 1, (d) 5, (e) 10, and (f) 20 mg/mL. SDS (3 mg/mL) was added to the aqueous phase, and h = 3 mm.

OD, and final shape of the discs. Thus, microdiscs with various polygonal shapes were produced (Figure S5). Notably, neat OD in the droplet formed irregular structures instead of the crystal-like discs while neat PS formed smooth particles (Figure S6a). At low concentration of PS (i.e., < 20 mg/mL), particles with irregular structures, mixtures of the spherical particles and microdiscs were obtained (Figure S6b, 6c). Increasing the concentration of PS led to the formation of the microdiscs (Figure S6e). This indicated that the initial concentration of PS in the emulsion droplets played an important role in the formation of disc-like structures. Therefore, the formation of PS microdiscs reflects the interplay among the interfacial area expanding by interfacial instability, the capillary flow from the coffee ring effect, and the OD crystallization.

structures while OD does, confirmed by DSC and XRD measurement in Figure S3, S4). Yet, after ethanol treatment, a spectrum with broad distribution without crystalline peak was observed, and it was indicated that the OD molecules had been removed by ethanol, leaving behind the noncrystallized PS microdiscs with crystal-like shape. Therefore, we can conclude that OD molecules crystallized on the surface of the microdiscs, and PS domains located within the OD backbones. OD molecules can be removed by alcohol treatment, and the PS discs can keep their contours for future applications. It is noted that formation of the discs originates from the hydrodynamic instabilities of emulsion droplets, and slight variation of the local experimental parameters (e.g., evaporation rate) will affect the evolution of the emulsion droplets, the crystallization of 5947


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Macromolecules Experimental Parameters Affect the Formation of Polymer Microdiscs. Generally, polymer microdiscs can be successfully generated by simply adjusting the initial diameter of the emulsion droplets (D0), COD, and/or solvent evaporation rate. Uniform discs can be obtained when the initial D0 was in the range of ∼10 to 20 μm (Figure 4a,b). As the D0 increased from 30 to 70 μm, folded discs and large spread flower-like particles were generated (Figure 4c−e). When the D0 further increased from 100 to 440 μm, flower-like or irregular microparticles were observed (Figure 4f,g). Therefore, emulsion droplets with small size of ∼10−20 μm preferred to form the discs, and the reason can be ascribed to the lower driving force needed for spreading the small folded droplets to form discs. Also, as the evaporation continued, the OD at the interface concentrated faster and the crystallization of OD performed easier on the surface of small droplets, which would further benefit for microdiscs formation. In addition, the COD played a crucial role in the interfacial behaviors and the formation of the discs. When the COD was lower than 0.5 mg/ mL, polymer particles with a smooth or slightly wrinkled surface were obtained (Figure 5b). As the COD increased from 1.0 to 5.0 mg/mL, particles with enhanced wrinkles were obtained, similar to our previous report.27 However, as the COD further increased to ∼10−20 mg/mL, wrinkled droplets spread in plane and disc-like or even polygonal structures were observed (Figure 5e,f). Again, further increase of COD would not significantly affect the formation of the disc-like structures (Figure S7). In this case, the crystallization-guided formation of the polygonal structures played a dominant role, and thus, microdiscs instead of microparticles with textured surfaces were obtained. Furthermore, solvent evaporation rate, initial polymer concentration in the droplets, polymer molecular weight, and chain length of the cosurfactant are also important for the interfacial instabilities and disc formation (Figure S6, S8−S10). For example, when PS2.8k was employed, a similar phenomenon was observed and microdiscs formed initially. As time increased, most of the microdiscs dynamically retracted back and formed irregular or spherical particles due to the low viscosity of the polymer (Figure S10a). Yet, qualitatively different phenomenon was observed, and the interface of the droplets containing PS876k became unstable and rearranged into spherical or irregular structures due to high viscosity of the polymer (Figure S10c). Generality of This Technique. Our technique can be extended to other hydrophobic polymer systems, such as homopolymer PMMA, PS with a functional end group, and PSb-PMMA diblock copolymer. Both PMMA and PS-b-PMMA diblock copolymer can form microdiscs in a similar fashion (Figure 6), demonstrating the generality of our technique. TEM images with high magnification in Figure 6c,d show the lamellar structures on the surface of the PS-b-PMMA microdiscs due to the self-assembly of the copolymer in the discs. Due to the immiscibility between PS and PMMA block, PS-b-PMMA diblock copolymer with covalent linked together can self-assemble into PS and PMMA microphases. Thus, symmetric PS25k-b-PMMA26k can self-assemble into lamellar structure with domain size of ∼20 nm (Figure 6c and 6d). Moreover, PS11.5k-SH can also form microdiscs in a similar way. As shown in Figure 7, OD molecules still relocated on the surface of the discs without ethanol treatment, preventing the incorporation of Au NPs onto the surface of the discs. Therefore, no Au NPs can be observed in Figure 7b after

Figure 6. (a, b) SEM images and (c, d) bright-field TEM images of microdiscs formed from emulsion droplets containing 10 mg/mL OD and 10 mg/mL (a) PMMA24k; (b-d) PS25k-b-PMMA26k with water layer height h = 4.5 mm. (c) and (d) represent the high-magnification TEM images of the edge and inner thin layer of the microdiscs. The microdiscs were dialyzed with ethanol to remove the OD and stained by RuO4 to enhance the contrast of PS microdomains compared to that of PMMA.

Figure 7. (a) Schematic illustration showing the structure of Au NPs incorporated PS hybrid microdiscs; (b, c) Bright-field TEM images of PS11.5k-SH microdiscs. The microdiscs were dialyzed with (b) water and (c) ethanol, respectively, and then interacted with Au NPs (size: 6.5 nm) aqueous suspension.

treatment of the PS11.5k-SH discs with Au NPs suspension. Yet, with the ethanol treatment, SH groups were exposed outside the discs, triggering the interaction between Au NPs and SH groups on the surface of the microdiscs (Figure 7c). Therefore, this result indirectly confirmed that OD molecules located on the surface of the microdiscs. Through this strategy, functional NPs can be fixed on the surface of the microdisc from thiol5948


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terminated PS after removal of OD (Figure 7), potentially useful in the area of sensing and catalysis. By incorporating active materials (e.g., magnetic Fe3O4 NPs) into the microdiscs, it would be possible to produce multifunctional colloids with unique shapes for various new applications. Introduction of magnetic NPs to the initial organic phase will lead to the formation of composited discs. We have been able to manipulate the spatial orientation of the magnetic microdiscs by using an external magnetic field. Figure 8 displays a series of


AUTHOR INFORMATION

Corresponding Authors

*(L. S.) E-mail: [email protected]. *(J. Z.) E-mail: [email protected]. Fax: +86-27-87543632. Tel: +86-27-87793240. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is funded by National Basic Research Program of China (973 Program, 2012CB821500) and National Natural Science Foundation of China (51473059). We also thank the HUST Analytical and Testing Center for allowing us to use its facilities.

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REFERENCES

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Figure 8. OM images of a magnetic PS microdisc dispersed in aqueous media. Orientation of the same microdisc can be controlled by using an external magnetic field. The scale bar in (a) is applied to (b−f).

OM images of a magnetic disc with diameter of ∼15 μm suspended in an aqueous media, clearly demonstrating that the disc can be rotated by externally controlling the orientation of the permanent magnetic.

4. CONCLUSION In summary, we have developed a facile and effective approach to fabricate polymer microdiscs with unique 2D structure by using the interfacial instability of shrinking emulsion droplets containing hydrophobic polymer. Emulsion droplets stabilized by SDS as surfactant and OD as cosurfactant were prepared through a microfluidics technique. With the assistance of OD crystallization at the solvent/water interface, the droplets transformed into microdiscs with a huge bump at the surrounding edge during solvent evaporation. It is demonstrated that the interfacial area expanding in plane via interfacial instabilities, capillary flow from the coffee ring effect, and OD crystallization surrounding the droplets together determine the formation process of the disc. This method can be extended to other hydrophobic homopolymers and block copolymers to generate functional 2D structures. These polymer microdiscs with unique shape and functionalities will find potential applications in the areas of targeting, separation, sensing, catalysis, drug delivery, and formation of advanced materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00914. Additional EM images and plots for the disc-like particles, Figures S1−S9 (PDF) Movie showing the formation process of PS microdiscs (AVI) 5949


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