Article pubs.acs.org/Langmuir
Emulsion Solvent Evaporation-Induced Self-Assembly of Block Copolymers Containing pH-Sensitive Block Yuqing Wu,† Ke Wang,† Haiying Tan,† Jiangping Xu,*,†,‡ and Jintao Zhu*,†,‡,§ †
Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering and ‡State Key Laboratory of Materials Processing and Mold Technology, Huazhong University of Science and Technology (HUST), Wuhan 430074, China § Shenzhen Research Institute of HUST, Shenzhen 518000, China S Supporting Information *
ABSTRACT: A simple yet efficient method is developed to manipulate the self-assembly of pH-sensitive block copolymers (BCPs) confined in emulsion droplets. Addition of acid induces significant variation in morphological transition (e.g., structure and surface composition changes) of the polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) assemblies, due to the hydrophobic−hydrophilic transition of the pH-sensitive P4VP block via protonation. In the case of pH > pKa(P4VP) (pKa (P4VP) = 4.8), the BCPs can self-assemble into pupa-like particles because of the nearly neutral wetting of PS and P4VP blocks at the oil/ water interface. As expected, onion-like particles obtained when pH is slightly lower than pKa(P4VP) (e.g., pH = 3.00), due to the interfacial affinity to the weakly hydrophilic P4VP block. Interestingly, when pH was further decreased to ∼2.5, interfacial instability of the emulsion droplets was observed, and each emulsion droplet generated nanoscale assemblies including vesicles, worm-like and/or spherical micelles rather than a nanostructured microparticle. Furthermore, homopolymer with different molecular weights and addition ratio are employed to adjust the interactions among copolymer blocks. By this means, particles with hierarchical structures can be obtained. Moreover, owing to the kinetically controlled processing, we found that temperature and stirring speed, which can significantly affect the kinetics of the evaporation of organic solvent and the formation of particles, played a key role in the morphology of the assemblies. We believe that manipulation of the property for the aqueous phase is a promising strategy to rationally design and fabricate polymeric assemblies with desirable shapes and internal structures.
1. INTRODUCTION Structured block copolymer (BCP) particles have received considerable attention on account that their chemical and physical properties (e.g., assembly structures, platelet adhesion, biodistribution, and bioresponse1−3) can be tuned by size, shape, internal structure, and composition. These advantages make them excellent candidates for electronics, photonics, biomedicine, separation, and other fields.4−9 Three-dimensional (3D) confined assembly of BCP is a powerful route to prepare polymeric particles with different morphologies.10−12 The emulsion solvent evaporation induced self-assembly of BCPs has been widely employed in the fabrication of BCP particles with various morphologies.13−16 Typically, the polymer is first dissolved in a water-immiscible organic solvent, which is then emulsified with surfactant aqueous solution to obtain emulsion droplets. After removal of the organic solvent by evaporation, nanostructured particles can be obtained. Several methods have been developed to manipulate the morphology of the © 2017 American Chemical Society
assemblies, including varying the strength of confinement,14,17,18 addition of additives like small molecule 3-npentadecyphenol (PDP),19,20 homopolymers,21 selective solvents,22−24 and inorganic ions.25−30 In this context, various particles with different internal structures, overall shapes and surface compositions, such as Janus particles,31 patchy particles,32,33 worm-like particles and onion-like particles34 have been prepared. When the emulsion solvent evaporation method was utilized for amphiphilic BCPs, interfacial instability of emulsion droplets was observed.35 During the evaporation process, organic solvent diffuses though oil/water interface and evaporates at water−air interface, resulting in the increase of the amphiphilic BCP concentration in the droplets, which will Received: July 5, 2017 Revised: August 12, 2017 Published: August 24, 2017 9889
DOI: 10.1021/acs.langmuir.7b02330 Langmuir 2017, 33, 9889−9896
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Figure 1. Evolution of the emulsion droplets during organic solvent evaporation: (a−h) Optical microscopy images showing the emulsion droplet during organic solvent evaporation at different time: (a) 1 min; (b) 1.5 min; (c) 3 min; (d) 4.5 min at pH 5.86; (e) 25 s; (f) 4 min; (g) 5 min; (h) 5.5 min at pH 2.47. The droplets are obtained by emulsification of PS9.8k-b-P4VP10k chloroform solution (10 mg/mL) in PVA (3 mg/mL) at different pH values. (i−l) Schematic illustration showing the evolution of the emulsion droplet during solvent evaporation process. The scale bar in panel a is also applied to panels b−h.
interfacial area) of the emulsion droplets during the evaporation process, and the following self-assembly of BCPs released from the droplets have not been investigated in previous reports. In this paper, we focus on the pH-induced interfacial instability phenomenon of the droplets containing pHresponsive hydrophobic PS-b-P4VP BCPs and the morphologies of the assemblies generated through this method. The protonation of P4VP permits hydrophobic−hydrophilic transition of this block, which plays a key role in determining the final morphology of the assemblies. It can even trigger the interfacial instability of the emulsion droplets. This interesting phenomenon is monitored through optical microscope by varying pH value in aqueous phase. Microparticles or nanoscale micelles with different morphologies can be obtained by just tuning the pH value. We also find that the structures of polymeric assemblies can be varied by the architectures of BCPs (molecular weight and the P4VP ratio), and also the fraction of homopolymer (varying molecular weight and ratio of homopolymer to BCP). Moreover, the temperature and stirring rate, which significantly affect the kinetics of evaporation, can obviously affect morphologies of the assemblies.
further decrease the interfacial tension. When the interfacial tension approaches to zero or even becomes transiently negative, the droplet no longer remains spherical, but instead undergoes an instability in which its interfacial area spontaneously increases or breaks into tiny droplets.35−37 Unlike traditional emulsion solvent evaporation-induced selfassembly, interfacial instability of emulsion droplets permits preparation of particles with unique microstructures on the surface. However, this method only works for amphiphilic polymer systems. Extending the scope of interfacial instability to hydrophobic polymers is one of the key issues. To achieve this point, addition of cosurfactants has been approved to be an effective method.38 Small amphiphilic molecular additives can also trigger the interfacial instability of emulsion droplets.39,40 Recently, a synergistic adsorption of polystyrene-block-poly(4vinylpyridine) (PS-b-P4VP) BCPs and sodium dodecyl sulfate (SDS) to the surface of the emulsion droplet induced interfacial instability at the particle surface, and PS-b-P4VP porous particles with tunable nanostructures and porosities are prepared by adjusting the concentration of SDS.40 Moreover, Wang et al. reported the morphologies transition induced by varying the amphiphilicity of BCP based on supramolecules41 or pH-sensitive BCP, 39 and multiple 3D macroporous architectures were generated. Guenoun and co-workers demonstrated that emulsions can be stabilized with a single BCP surfactant which was pH- and temperature-responsive.42,43 By varying the pH value and temperature, type of the emulsion was changed from oil-in-water to water-in-oil. More importantly, multiple emulsions could be obtained in this inversion process. Previous reports on self-assembly of BCPs by manipulating interfacial interaction involve the usage of different surfactant37,40 or supramolecular approach.41 However, as far as we know, the pH-induced interfacial instability (e.g., breakage of large droplet or spontaneous increase of
2. EXPERIMENTAL SECTION 2.1. Materials. Diblock copolymers PS9.8K-b-P4VP10K (the subscripts are the Mn of the blocks, Mw/Mn = 1.08), PS20K-b-P4VP17K (Mw/Mn = 1.08), PS17K-b-P4VP49K (Mw/Mn = 1.05), PS51K-b-P2VP18K (Mw/Mn = 1.15), and PS110 K-b-P4VP107 K (Mw/Mn = 1.15) were purchased from Polymer Source, Inc., Canada. Poly(vinyl alcohol) (PVA, average Mw: 13K−23K g/mol, 87−89% hydrolyzed) was purchased from Sigma-Aldrich. Sulfuric acid was purchased from Sinopharm Chemical Reagent. All of the materials were used as received without further purification. 9890
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Langmuir 2.2. Preparation of the BCP Assemblies. Emulsion-solvent evaporation method was applied to prepare the polymer particles.10,44 Typically, PS-b-P4VP was dissolved in chloroform at a concentration of 10 mg/mL. Subsequently, 0.1 mL of the solution was emulsified with 1.0 mL of PVA aqueous solution (3 mg/mL) with different pH values by membrane-extrusion emulsification or shaking vigorously. The pH value of PVA solution is adjusted by sulfuric acid. Then, the emulsion was collected in a 10 mL open vial to allow the slow evaporation of chloroform for 24 h at 30 °C. Then, the sample was washed with deionized water (DI water) to remove PVA and ions by repeated centrifugation three times (16 000 rpm for 15 min) and redispersed in neutral DI water under sonication for further characterizations. 2.3. In Situ Growth of Au NPs. The polymer particles were dispersed in an aqueous solution of HAuCl4·4H2O (0.25 mg/mL) to allow HAuCl4 to be preferentially absorbed to P4VP segment for ∼4 h under gently stirring, and then centrifuged at 14 000 rpm for 8 min to remove the unloaded HAuCl4·4H2O. Then, the particles were redispersed in DI water (1 mL) with concentration of ∼0.4 mg/mL; a freshly prepared NaBH4 solution cooled to 0 °C (0.5 mg/mL, 20 μL) was added dropwise under vigorous stirring. Afterward, the mixture was gently stirred for 4 h. Finally, the composite particles were separated by centrifugation at 14 000 rpm for 8 min and then redispersed in DI water. 2.4. Characterization. Internal structures of the polymer particles were investigated using FEI Tecnai G2 20 transmission electron microscope (TEM) operated at an accelerated voltage of 200 kV. Before TEM characterization, the samples were selectively stained with iodine vapor for 2 h at 30 °C (for P4VP block). Real-time evolution of the shrinking emulsion droplets was monitored by Olympus IX71 inverted optical microscope in bright-field optical mode. The measurement of pH value of aqueous phase was performed with a Mettler-Toledo pH meter FE20. To measure the water contact angle, BCP solutions were spin-coated on glass substrates and dried at 45 °C to form robust and uniform films. Then, a drop of water with varied pH values was dropped on the substrate. The water contact angles were measured and captured on an optical contact-angle measuring device (JC2000C1, Dataphysics Instruments Shanghai Zhongchen Digital Technic Apparatus co., ltd). In addition, the Fourier transform infrared spectroscopy (FTIR) was applied to characterize protonation interactions between the pyridine group and proton (Equinox 55, Bruker).
Figure 2. Plot of the interfacial tension of the emulsion droplets containing PS-b-P4VP with different molecular weight and block ratio at varied pH values. The interfacial tension for chloroform/PVA aqueous solution was measured through pendant drop tensiometry.
As the decrease of pH value in the aqueous phase, the interfacial tension decreased to ∼0.4 mN/m for pH value of 1.00 due to the adsorption of protonated BCPs at the chloroform/water interface. Presumably, interfacial tension would decrease further and approach zero when more protonated BCPs migrated to the chloroform/water interface during solvent evaporation, triggering the occurrence of interfacial instability of the droplets. Moreover, effects of BCP molar mass on the interfacial tension have been investigated. Owing to the lower mobility of BCP chains with higher molecular weight, the interfacial tension of emulsion droplets containing PS110k-b-P4VP107k is larger than that of PS9.8k-b-P4VP10k at the same pH value (see Figure 2). Furthermore, the interfacial tension of emulsion droplets containing PS51k-b-P4VP18k with shorter P4VP blocks is larger than that of PS17k-b-P4VP49k at the same pH value. As a result, the stability of the emulsion droplets of BCPs with different architectures can be controlled by adjusting the pH value of the aqueous phase. To further confirm the hydrophobic−hydrophilic transition of BCPs caused by protonation, the contact angle between water with different pH values and the BCP film was measured. The initial contact angle between pure water and PS-b-P4VP film is ∼107.5° (Figure S1a−d in the Supporting Information (SI)). It remains constant for 10 s after dropping when the pH is 5.86 (Figure S1e). However, an obvious change of the contact angle can be observed at lower pH values with time extending. For instance, at the pH 3.00, the contact angle decreases to 77° after 10 s (Figure S1f). Further decreasing pH to 1.00, contact angle drops to 53° after 10 s (Figure S1h). Thus, P4VP blocks is believed to be protonated at lower pH value, resulting in the decrease of the water contact angle. Furthermore, FTIR analysis is carried out to verify the protonation of P4VP block (Figure S2). After the protonation of P4VP blocks, the original characteristic peaks associated with pyridine rings at 1592 and 1411 cm−1 shift to 1601 and 1416 cm−1, respectively. A new characteristic peak can be found at 1637 cm−1 due to the N−H bending vibration, which indicating the formation of ionized 4VP units. The above results confirmed the hydrophobic−hydrophilic transition of P4VP blocks when decreasing the pH value of the aqueous solution. Morphologies of the polymeric assemblies obtained from pH-sensitive hydrophobic PS9.8k-b-P4VP10k can be changed by varying pH value of the aqueous phase. As shown in Figure 3a,
3. RESULTS AND DISCUSSION 3.1. Effect of pH Value on the Interfacial Behavior of Emulsion Droplets. The interfacial behavior of emulsion droplets containing PS-b-P4VP can be affected by pH value of aqueous phase. When the pH > pKa(P4VP) (pKa (P4VP) = 4.8), as shown in Figure 1a−d at pH 5.86 (PVA solution 3 mg/mL), the interface of droplet is stable along with the evaporation of chloroform. However, when the pH is lower than pKa(P4VP) (e.g., pH = 2.47), as chloroform evaporates, the droplets shrink in volume and spontaneously become rough on surface (Figure 1e−f). Finally, the droplets breakup into tiny droplets (Figure 1g−h). The protons in the aqueous phase ionize P4VP blocks (Figure 1i−j), transforming them from hydrophobic to hydrophilic blocks. Thus, the amphiphilic ionized PS-b-P4VP will migrate to the chloroform/water interface to decrease the interfacial tension (Figure 1k). Meanwhile, the concentration of BCP in the emulsion droplets will increase as the evaporation of the chloroform. In this case, the interfacial instability of the droplet will be triggered to release the BCP chains to the aqueous phase, in which the BCPs assemble into micelles with different structures (Figure 1l). To further elucidate the role of pH value in the assembly of BCPs as described above, the interfacial tension of the organic/water was measured through pendant drop tensiometry as a function of pH value (Figure 2). 9891
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selectivity, and finally to nanoparticles with various morphologies by triggering the interfacial instability. 3.2. Effect of the Architectures of BCPs on the Structural Transition. The block ratio and molecular weight of the BCPs play essential roles in the structural transition induced by protonation. To systemically investigate these effects on the morphologies, four BCPs (Table 1) with different Table 1. Characteristics of the Diblock Copolymers Employed in This Study copolymers
Mn (kg/mol)
Mw/Mn
weight fraction w4VP
PS9.8K-b-P4VP10K PS20K-b-P4VP17K PS17K-b-P4VP49K PS51K-b-P4VP18K PS110K-b-P4VP107K
19.8 37 66 69 217
1.08 1.08 1.05 1.15 1.15
50.5% 45.9% 74.2% 26.1% 49.1%
molecular weights and block ratios were studied. As shown in Figure 4, symmetric BCP with larger molecular weight (PS110k-
Figure 3. TEM images of PS9.8k-b-P4VP10k particles obtained from emulsion-solvent evaporation route in PVA aqueous solution with varied pH values: (a) pH 5.86, (b) pH 3.00, (c) pH 2.47, and (d) pH 1.00. After staining with iodine vapor, the P4VP domains become black, while the PS domains look gray. Insets in the upper right are the cartoon showning BCP assemblies, where blue and yellow colors represent PS and P4VP block, respectively.
when the pH > pKa(P4VP), pupa-like particles with alternatively stacking PS and P4VP domains are obtained at pH 5.86. The formation of this unique morphology is attributed to the neutral nature of the interface between polymer blocks and aqueous phase. The PVA chains not only stabilize the emulsion droplets but also creative a neutral interface for PS and P4VP.45,46 Interestingly, when pH is slightly lower than pKa(P4VP) (Figure 3b, pH = 3.00), the pupa-like particles transform to onion-like particles with P4VP at the outermost layer. In this case, partial of 4VP groups have been ionized, causing the transformation of the interfacial selectivity from neutral for both blocks to selective for P4VP block. However, no interfacial instability can be observed during the evaporation of chloroform, since partial protonation does not offer enough hydrophilicity to the BCPs. Further decrease of pH will increase the degree of protonation, resulting in the transformation of P4VP chains from hydrophobic to hydrophilic. This transformation of hydrophilicity triggers the interfacial instability. Due to the breakup of the droplets, the BCP chains self-assemble into various structures in the aqueous phase. For example, vesicles can be obtained at pH 2.47 (Figure 3c), while worm-like micelles can be observed at pH 1.00 (Figure 3d). As the increase of protonation degree, the repulsive interactions among P4VP corona enhance significantly, inducing the transition from vesicles to cylinders.47 It has been reported that spherical, worm-like, and vesicular or lamellar aggregates can be prepared from polystyrene-b-poly(acrylic acid) (PS-bPAA) by tuning the pH value or ion concentration.29 The transition of the aggregates can be ascribed to the changed repulsive interactions among the hydrophilic PAA coronas resulting from the protonation by adding acid or the charge screening by adding sodium chloride in selective solvent. As stated above, the interfacial stability and the morphology of assemblies can be simply tuned by adjusting the pH value. The introduction of acid induces the transition of pupa-like particles to onion-like microparticles by changing the interfacial
Figure 4. (a) Control of the patch size of PS110k-b-P4VP107k particles (L/R) by tuning pH value. The insets show the TEM images of PS110kb-P4VP107k particles obtained under different pH values: (i) pH 5.86; (ii) pH 5.00; (iii) pH 4.37. (b,c) TEM images of the PS110k-b-P4VP107k particles obtained under (b) pH 3.00 and (c) pH 1.00. The P4VP domains are selectively stained by iodine before TEM observation.
b-P4VP107k) presents diverse assembly behavior. When the pH is 5.86, patchy particles with bumpy surface are obtained (Figure 4a inset (i)). The possible mechanism for the formation of the patchy particles has been discussed in previous reports.48,49 The roughness of patchy particles is defined as the length (L) of protruding spherical P4VP domain to the radii (R) of patchy particle (L/R, Figure 4a lower-right inset). As shown in Figure 4a, the roughness decreases as the pH value decreases from 5.86 to 4.37. Presumably, protonation of P4VP blocks will decrease their solubility in chloroform, resulting in the formation of smaller P4VP patches.48 When the pH value equals to 3.00, microparticles with spherical P4VP domains are obtained (Figure 4b). Upon decreasing the pH value to 1.00, the initial droplet containing PS110k-b-P4VP107k does not break up completely, while similar processes of interfacial disturbance and tiny droplet ejection can be observed (Figure S3a−d). Presumably, the reason can be ascribed to the higher viscosity caused by larger molecular weight. Furthermore, capsules instead of patchy particles can be achieved due to the hydrophobic−hydrophilic transition of P4VP blocks (Figure 4c). 9892
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Langmuir Block ratio of the BCPs also plays an important role in the morphology of the assemblies. As shown in Figure 5,
Figure 6. Phase diagram summarizing the effect of pH and polymeric architectures on PS-b-P4VP aggregates obtained through an emulsionsolvent evaporation route. Figure 5. TEM images of PS-b-P4VP aggregates generated from emulsion-solvent evaporation route by adjusting the pH value in the aqueous phase: (a) PS51k-b-P4VP18k, pH = 5.86; (c) PS51k-b-P4VP18k, pH = 1.00; (b) PS17k-b-P4VP49k, pH = 5.86; (d) PS17k-b-P4VP49k, pH = 1.00.
instability of droplets happens, inducing the transformation of microparticles to nanoparticles. Depending on the architectures of BCPs and pH value, vesicles, cylinders and spheres can be achieved. Thus, by tuning the pH value of aqueous phase, we can manipulate the behavior of the oil/water interface during solvent evaporation, offering a promising route to engineer the structures of polymeric assemblies. 3.3. Effect of Homopolymers on the Morphology of the BCP Assemblies. The addition of homopolymer also affects the interfacial phenomenon and the final morphology of the assemblies.21,51 The structure of the blend system dramatically depends on the molecular weight, mixing ratio, and interaction parameter among different components.44,51−54 Here, we focus on the structural transformation caused by the weight fraction (w) and molecular weight of homopolystyrene (hPS). Generally, hPS is immiscible with the PS brush of PS-bP4VP when the molecular weight of hPS is similar or higher than that of the PS block. This is the so-called dry-brush regime, where the mixture will segregate to form macrophase separation structure. While the molecular weight of hPS is smaller than that of the matrix, the blended polymers are miscible and a wet-brush regime is formed. Such relationship between homopolymer and BCP can be applied to control the internal structure of the BCP particles. As shown in Figure 7a−c for the blending of PS9.8k-b-P4VP10k and PS20k at pH 5.86, due to the dry-brush effect, hPS20k can not fully penetrate into the PS domain of PS9.8k-b-P4VP10k. In this case, macrophase separation occurs between homopolymer and BCP while microphase separation occurs within the BCP. As a result, lamella of PS blocks is significantly deformed when the whPS20k = 20 wt % (Figure 7a). Similar phenomenon has been observed in the previous report.21 Interestingly, when the whPS20k increases further to 80 wt %, the pupa-like particles transform to spherical particles with P4VP hoops structures (Figure 7c). Moreover, when pH = 3.00, partial hPS20k penetrate into the PS domain to induce the structural transformation of P4VP domain from lamellae (Figure 3b) to perforated lamellae (whPS20k = 20 wt %, Figure 7d), then to cylinder (whPS20k = 50 wt %, Figure 7e), and finally to particles without obvious internal phase separation (whPS20k = 80 wt %, Figure 7f). With further decrease of pH to 2.47, addition of hPS
asymmetric BCPs with similar molecular weight but different block ratios, PS51k-b-P4VP18k (weight fraction of P4VP, wP4VP = 0.26) and PS17k-b-P4VP49k (wP4VP = 0.74), show different assembly behaviors. When pH = 5.86, PS51k-b-P4VP18k forms microparticles with spherical P4VP domains under 3D confinement (Figure 5a),18 while PS17k-b-P4VP49k with longer P4VP block assembles into pupa-like particles with alternatively stacked PS layer (thickness: 17 nm) and P4VP layer (thickness: 38 nm) (Figure 5b).50 Interestingly, when the pH decreases to 1.00, interfacial instability of the droplet occurs. The evolution of emulsion droplets for PS17k-b-P4VP49k (see Movie S1 in the SI) and PS51k-b-P4VP18k (Figure S3e−h) are similar to that for PS10k-b-P4VP9.8k system. In this case, PS51k-b-P4VP18k selfassembles into vesicles (Figure 5c), while PS17k-b-P4VP49k forms a mixture of short worm-like and spherical micelles (Figure 5d). When the pH value equals to 1.00, the P4VP blocks are fully ionized, leading to the formation of hydrophilic P4VP coronas. In this case, the effective volume fraction of P4VP increases by both increasing its effective mass and also introducing anionic charge, which in turn increases the geometric packing parameter for the P4VP chains. Therefore, PS17k-b-P4VP49k, which have longer hydrophilic block and larger repulsion among P4VP chains than that of PS9.8k-bP4VP10k, assemblies into a mixture of short worm-like and spherical micelles. However, long worm-like micelles and vesicles can be obtained from PS9.8k-b-P4VP10k. Moreover, PS51k-b-P4VP18k with shorter hydrophilic block and smaller repulsion among P4VP chains forms vesicles. As summarized in Figure 6, when the pH value decreases from 5.86 to 3.00, the unique pupa-like structure obtained through 3D confined assembly gives rise to onion-like structure owing to the addition of acid. The interfaces of the droplets are stable, resulting in microparticles with tunable internal structure, overall shape and interfacial composition, which are governed by the pH value of aqueous phase and architectures of the BCPs. When the pH value is lower than 3.00, interfacial 9893
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with the evaporation of chloroform, interfacial instability occurred at one hemisphere of the droplet to trigger the release of BCPs, while the other part of the droplet remained stable and finally retracted to a solid microparticle (Figure S5a− d). As shown in Figure S4d−e, besides the microparticles, there are many worm-like and spherical micelles, which confirm the existence of interfacial instability during the evaporation. Presumably, the decrease of the relative content of BCP will result in the increase of the degree of protonation, as the initial proton concentration is the same as that of the system without hPS. However, further increasing the whP180k to 80 wt %, particles without obviously internal structure can be observed. In this case, there are no enough amphiphilc BCP chains to reduce the interfacial tension. Thus, the interfacial instability of the hemisphere can not be observed. When pH = 1.00, similar phenomena can be observed as the increase of w (Figure S6a−c for PS2.8k, Figure S6d−f for PS180k). As stated above, we can conclude that the addition of hPS not only changes the relative ratio of two blocks of PS-b-P4VP but also the interfacial interaction. 3.4. Effect of Temperature and Stirring on the Structural Transition. To understand the kinetic controlled assembly processing, we investigated the effects of stirring (500 rpm) and temperature (80 °C) on the structural transition. The emulsions were left in an open vial, and stirred gently or heated. As shown in Figure 8 for PS9.8-b-P4VP10k, onion-like structure
Figure 7. TEM images of PS9.8k-b-P4VP10k and hPS20k blend aggregates at different pH and whPS20k: (a) w = 20 wt %, (b) w = 50 wt %, (c) w = 80 wt % at pH 5.86; (d) w = 20 wt %, (e) w = 50 wt %, (f) w = 80 wt % at pH 3.00; (g) w = 20 wt %, (h) w = 50 wt %, (i) w = 80 wt % at pH 2.47; (j) w = 20 wt %, (k) w = 50 wt %, (l) polymeric particles load with Au nanoparticles and w = 80 wt % at pH 1.00.
induces the transformation of mixture of vesicles and onion structures to a mixture of vesicles and microparticles, and even to particles without obvious internal structures (Figure 7g−i). Upon decreasing the pH to 1.00, the hPS20k serving as a nonvolatile, selective “solvent” for the PS block tunes the relative ratio of two blocks of PS9.8k-b-P4VP10k to vary the interfacial curvature, resulting in the formation of spherical micelles (Figure 7j). More interestingly, the interfacial behavior can be tuned by the whPS20k. For instant, further increasing whPS20k to 50 wt %, the blend polymer chains form a mixture of nanoparticles and microparticles (Figure 7k). Furthermore, as shown in Figure 7l, upon increasing whPS20k to 80 wt %, microparticles without obvious internal structures becomes the dominating assemblies. In fact, the edge of microparticles is consisted of BCPs. This can be verified by loading gold nanoparticles (AuNPs) through the in situ reduction method based on the selective interaction between P4VP and HAuCl4.55 As shown in Figure 7l, the AuNPs are mainly located at the periphery of the microparticles, as the P4VP is protonated and becomes hydrophilic. As a result, the interfacial behavior can be well controlled by the addition of hPS20k. When the pH is lower than pKa, the invasion of hPS20k can obviously enlarge the PS domain. When the pH is larger than pKa, the addition of hPS20k can facilitate the transition from nanoparticles to microparticles. Molecular weight of hPS also plays an important role in the morphologies of polymeric assemblies. Here, the hPS2.8k with much lower molecular weight is employed to shape the morphology. When pH = 3.00, as the whPS2.8k increases from 20 to 80 wt % (Figure S4a−c), the shape of P4VP domains transform from lamellae (Figure 3b) to cylinders (Figure S4a) then to spheres (Figure S4b), and finally to particles without obviously internal phase structure (Figure S4c). In this case, the hPS2.8k can penetrate into the PS domains of the BCPs (wetbrush), significantly enlarging the volume fraction of PS domains. However, hPS180k with huge molecular weight can not penetrate into the PS domains of the BCP; the macrophase separation induces the formation of Janus particles (Figure S4e). More interestingly, the addition of hPS180k can trigger particular interfacial instability of the polymeric droplets. As
Figure 8. TEM images of PS9.8k-b-P4VP10k aggregates generated from emulsion-solvent evaporation route: (a−c) pH = 3.00: (a) without stirring, 30 °C, (b) stirring, 30 °C, (c) without stirring, 80 °C; (d−f) pH = 2.47: (d) without stirring, 30 °C, (e) stirring, 30 °C, (f) without stirring, 80 °C.
can be obtained when pH = 3.00 (Figure 3b and Figure 8a). However, when stirring is engaged during the evaporation, interfacial instability happens, and worm-like micelles can be observed (Figure 8b). Moreover, similar assembly behavior can be achieved by raising temperature to 80 °C during solvent evaporation (Figure 8c). Obviously, stirring and heating during solvent evaporation will trigger the interfacial disturbance as well as increase the evaporation rate, and therefore enhance mass transfer between oil and aqueous phase.56 Interestingly, when the pH value decreases to 2.47, a mixture of onion-like structure and vesicles (Figure 3c and Figure 8d) can be achieved without stirring or heating. However, the BCPs selfassemble into worm-like micelles under stirring (Figure 8e) or hearting at 80 °C (Figure 8f). Notably, as can be seen in Figure 8f, lamellar assemblies with cylindrical protrusions at the edges 9894
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Langmuir (e.g., octopus-like structures) can be obtained, which represents the intermediate states between worm-like micelles and vesicles, providing some insight into the evolution process of micellar structure.56 We can conclude that the heating and stirring during evaporation will accelerate the mass transfer rate at the oil/water interface, enhancing the protonation of P4VP blocks. As a result, interfacial instability and morphological transition of BCP assemblies can be observed in this case. Therefore, through the kinetic manipulation of BCP selfassembly in droplets, we are able to fabricate nanoscale aggregates with various structures.
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ACKNOWLEDGMENTS
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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.langmuir.7b02330. Additional water contact angle and FTIR measurement showing the protonation of P4VP blocks (Figure S1− S2), optical microscopy images showing the interfacial instabilities of emulsion droplets during solvent removal, and TEM images of the BCP assemblies (Figure S3− S4,S6) (PDF) Real time video showing the interfacial instabilities of emulsion droplets containing PS17K-b-P4VP49K at pH 1.00 (AVI)
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We gratefully acknowledge funding for this work provided by the National Natural Science Foundation of China (51473059 and 51525302), Shenzhen Science and Technology Project (JCYJ20150630155150194), and Open project of State Key Lab of Material-Oriented Chemical Engineering (KL16-04). We thank the HUST Analytical and Testing Center for allowing us to use its facilities.
CONCLUSIONS In summary, we have demonstrated a pH-induced interfacial stable-instable transition phenomenon of the droplets containing pH-responsive hydrophobic PS-b-P4VP BCPs. Through this approach, BCP assemblies with various morphologies can be prepared. The addition of acid into the aqueous phase will make P4VP blocks ionized, triggering the transformation of P4VP from hydrophobic to hydrophilic nature. The pH value can be harnessed to manipulate the hydrophilicity of the PS-bP4VP BCPs and thus the internal structures, surface composites, and overall shapes of the polymeric assemblies. Upon reducing pH value to 2.47, the degree of the protonation of the P4VP increases to trigger interfacial instability of the emulsion droplets, which drives microspheres to transform into nanoparticles. In this case, depending on the architectures of BCPs and the pH value, vesicles, cylinders, and spheres can be obtained. Moreover, the addition of homopolymers also affects the interfacial phenomenon during the evaporation and plays significant roles in the final morphologies of the assemblies. Interestingly, by adjusting temperature and stirring speed, we can kinetically control the evaporation process and thus the final morphology of the assemblies. The combination of these strategies offers us a versatile platform to design and fabricate polymeric assemblies with various structures.
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Article
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (J. X.). *E-mail:
[email protected] (J. Z.). ORCID
Jintao Zhu: 0000-0002-8230-3923 Notes
The authors declare no competing financial interest. 9895
DOI: 10.1021/acs.langmuir.7b02330 Langmuir 2017, 33, 9889−9896
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