Letter pubs.acs.org/macroletters
Azo Polymer Janus Particles and Their Photoinduced, SymmetryBreaking Deformation Xinran Zhou, Yi Du, and Xiaogong Wang* Department of Chemical Engineering, Laboratory of Advanced Materials (MOE), Tsinghua University, Beijing 100084, People’s Republic of China S Supporting Information *
ABSTRACT: We report the successful fabrication of photoresponsive Janus particles (JPs) composed of a methacrylatebased azo polymer (PAZO-ADMA) and poly(methyl methacrylate) (PMMA). The JPs are obtained through microphase separation in a confined volume of the dispersed droplets, which incorporates the azo polymer and PMMA into one single particle in a core-compartmentalized manner. It is observed that several unique types of symmetry-breaking deformations are induced upon irradiation with a linearly polarized laser beam at 488 nm. The JPs with such properties are valuable for fundamental understanding and smart photofabrication in micrometer scale.
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In this study, JPs composed of the azo polymer and PMMA in the core-compartmentalized manner are fabricated through microphase separation in confined volume of dispersed droplets. Figure 1a shows the optical microscope (OM) images
icro/nanoparticles possessing noncentrosymmetric architectures, Janus particles (JPs), have attracted great attention in recent years.1 Since brought in the spotlight by de Gennes in 1991,2 JPs have been intensively investigated for their great significance in both fundamental understanding and potential applications. Multifarious JPs have been fabricated by a variety of innovative methods, such as self-assembly of block copolymers,3 subsequent surface modification,4 microphase separation in confined volume,5 seeded emulsion polymerization,6 electrohydrodynamic jetting,7 and microfluidics,8 among others. The JPs have demonstrated several interesting properties with huge application potential in different areas, which include particulate surfactants,3a,9 electric/magnetic fielddriving devices,10 bio/life science,11 and so on. On the basis of the concepts of compartmentalization of JPs, different kinds of materials with alien properties can be incorporated into one single particle to achieve the asymmetry of the properties and realize different functionalities. Polymers containing azo chromophores, azo polymers for short, have demonstrated versatile photoresponsive properties in different aspects.12 The photoinduced deformation along light polarization direction has initially been revealed as the SRG formation.13 Such deformation has also been observed as an effect to cause the colloid shape deformation from microspheres to elongated particles such as ellipsoids.14 Corecompartmentalized JPs containing an azo polymer can be lightmanipulated to obtain JPs with unique shapes, which can hardly be obtained by other methods. Meanwhile, the basic understanding of the light-matter interaction obtained from this asymmetric photoresponsive system is of particular interest to the divisions of JP and azo polymer researches. However, to our knowledge, no such study has been documented in the literature yet. © XXXX American Chemical Society
Figure 1. Optical microscope images of azo polymer JPs with particle size about 10 μm before (a) and after (b) irradiation with a linearly polarized laser beam (200 mW/cm2, 488 nm) for 4 h; (c) the chemical structure of the azo polymer (PAZO-ADMA).
of the JPs,15 where the red part is the azo polymer phase and the transparent part is PMMA phase. Figure 1b shows the JPs after light irradiation with the horizontal polarization to demonstrate the photoinduced deformation behavior (Figure 1b). Several unique types of symmetry-breaking deformations are observed for these composition-asymmetrical particles as Received: December 20, 2015 Accepted: January 21, 2016
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DOI: 10.1021/acsmacrolett.5b00932 ACS Macro Lett. 2016, 5, 234−237
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ACS Macro Letters
and PAZO-AMDA, the particles obtained by the same procedure show more complicated phase-separated structures (Figures S1−S4 in the Supporting Information). The JPs with sizes in 1 μm scale were characterized by transmission electron microscopy (TEM).16 As shown in Figure 2a, even with the poor contrast between the polymers,
irradiated with the linearly polarized light, whose polarization direction has different intersection angles with the symmetric axis of the JPs. The preparation and characterization of the JPs are presented below in detail. A newly synthesized methacrylate-based azo polymers containing the side-chain adamantyl groups (PAZO-ADMA, Figure 1c) was adopted for the JP preparation. The synthesis and characterization of this polymer are given in the Supporting Information. JPs were obtained by evaporation of dichloromethane (DCM) from PAZO-ADMA/PMMA/DCM droplets dispersed in an aqueous solution. This method has been reported for preparing JPs of polystyrene/PMMA by evaporation of toluene from the droplets.5b,c However, due to the strong interaction between the azo chromophores, azo polymers can only be dissolved in strong polar organic solvents such as tetrahydrofuran (THF) and N,N-dimethylformamide (DMF).14 As these solvents are highly water-miscible, the solvent dispersion and evaporation route cannot be directly applied. Therefore, PAZO-ADMA with the high solubility in DCM was necessary to achieve the successful preparation of the JPs. The preparation of the JPs is illustrated in Scheme 1. PMMA and PAZO-AMDA were dissolved in DCM (1 mL) with the Scheme 1. Procedure to Form Azo Polymer JPs
Figure 2. (a) TEM images of the JPs with the size about 1 μm; (b) SEM images of the JPs and EDS spectrum for the two hemispheres.
the snowman-like structure can be seen in the TEM images. The similar structure information can also be obtained from the images of the scanning electron microscopy (SEM; Figure 2b).17 The energy dispersive spectrometer (EDS) analysis indicates the different compositions in the two parts of the particles. From the results, it can be concluded that the hemispheres with the wrinkled surfaces are the PMMA phase owing to the lack of N element. The hemispheres with the smooth surfaces are the PAZO-ADMA phase. The atomic ratio of N and O, which is a little bit lower than 2:1, is close to the value estimated from the polymeric structure. Above results indicate that although the particles have the polydispersed size distribution, Janus structures are formed in the particles with different sizes. The photoinduced deformation was investigated by using a linearly polarized laser beam (488 nm) as the light source and JPs were exposed to the laser beam incident normal to the substrate.18 For the JPs with the sizes in 10 μm scale, the relatively high light intensity (200 mW/cm2) was adopted to induce the shape deformation. The morphologies of the JPs after continuous irradiation for 4 h are given in Figure 1b. The azo polymer parts of the JPs show the photoinduced elongation along the polarization direction of the laser beam, while the PMMA parts remain unchanged. The different morphologies shown in the figure can be explained by considering the polarization direction relative to the symmetric axis of JPs as shown in Scheme 2. When the polarization direction is fixed, three typical types of morphologies can be induced by the deformation along the polarization direction, which are named streamlined (I), mushroom-like (II), and snail-like (III) JPs, as discussed below. The deformation patterns can be more clearly seen for the JPs with the size in 1 μm scale. For the JPs, irradiation at the
concentration of 10.0 mg/mL for each of the polymers. The solution was added dropwise into the bottom of the poly(vinyl alcohol) (PVA) aqueous solution (2.5 wt %, 50 mL). During and after the adding process, moderate stirring was applied to promote the dispersion of the organic solution into microdroplets. When the solvent was volatilized slowly, phase separation occurred in the PAZO-ADMA/PMMA/DCM droplets to form the PAZO-ADMA and PMMA phases during the solvent evaporation. Spherical droplets were transformed to snowman-like shape during DCM evaporation. After forming Janus structures, the JPs were collected by sedimentation under gravity. Before further investigations, the JPs were centrifugalized to fractionate the particles according to their sizes and then washed to remove the residual PVA. Two types of JPs with the sizes in 10 and 1 μm scales were used to carry out the following studies. The JPs with the sizes in 10 μm scale could be readily observed with an optical microscope.15 Figure 1a shows the OM images of the JPs with the diameters about 10 μm. The JPs consist of two distinct parts whose volume ratio is about 1:1, which is very close to the original feed ratio. The red part is the PAZO-ADMA phase and the transparent part is PMMA phase. Due to high incompatibility between the two polymers, JPs with symmetric compartment volumes are obtained. It is also necessary to reduce the interfacial tension between the dispersed droplets and dispersion medium with a suitable surfactant for forming the snowman-like particles.5b,c Owing to the low volatilization rate of DCM, the phase separation is fully developed to give the smallest interface between the two polymer phases in each particle. For different ratios of PMMA 235
DOI: 10.1021/acsmacrolett.5b00932 ACS Macro Lett. 2016, 5, 234−237
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ACS Macro Letters Scheme 2. (a) Three Typical Types of the Photoinduced Deformations upon Irradiation with a Linearly Polarized Light at 488 nm; (b) the Definition of the Aspect Ratios for Photoinduced Deformation
same wavelength was performed with the moderate intensity of 100 mW/cm2 for a shorter time period. Figure 3a shows the SEM image of the JPs upon the irradiation at 488 nm for 20 min, and Figure 3b−e show TEM images after the irradiation for 5, 10, 15, and 20 min. Similar to their larger-size counterparts, the JPs also demonstrate photoinduced deformation in these typical modes, as illustrated in Scheme 2, which is also caused by the different intersection angles (θ) between the polarization direction of the incident light and the particle symmetric axis. As shown in Figure 3, which can also be seen in Figure 1b, the JPs possess these typical symmetry-breaking modes caused by the deformation along the polarization direction. For the first type (streamlined JPs), the particle symmetric axis is parallel to the polarization direction (θ = 0), leading to subsequent deformation along the axis to form the water-drop-like structure. For the second type (mushroom-like JPs), the particle symmetric axis is perpendicular to the polarization direction (θ = π/2) and the azo polymer hemisphere is deformed vertically to the particle symmetric axis to give mushroom-like JPs. And, for the third type (snaillike JPs), as the angle θ can be an arbitrary value between 0 and π/2, the photoinduced deformation results in these unique shapes. As indicated by SEM, TEM, and OM observations, the JPs with the fully developed compartments of the PAZO-ADMA and PMMA phases can be photomanipulated to give particle with these symmetry-breaking shapes. Moreover, the deformation degree can be controlled by adjusting the irradiation time. For the deformation modes to produce type I and II JPs, the aspect ratios are defined according to the geometric dimensions given in Scheme 2. The averaged aspect ratios were estimated from TEM images. Five JPs with the similar sizes and shapes were selected to obtain the average value and deviation for each data point after irradiation for different time periods. As shown in Figure 4, the aspect ratios for both type I and type II JPs significantly increase in the first 20 min. For the case corresponding to type I morphology, the aspect ratio still gradually grows after 20 min irradiation and is saturated in about 50 min. On the other hand, for type II morphology, the
Figure 3. SEM (a) and TEM (b-e) images of the azo polymer JPs with particle size in 1 μm scale upon the light irradiation (100 mW/cm2, 488 nm). The irradiation time was 20, 5, 10, 15, and 20 min from (a) to (e), respectively. The double-sided arrows indicate the polarization direction of the laser beam.
aspect ratio levels off after 20 min, and the saturated aspect ratio has a significantly smaller value compared with that of type I JPs. This difference can be attributed to the restrictive effect of the PMMA part on the photoinduced deformation, where the interface restriction is more significant for the type II particles. It is interesting to observe that although a significant deformation is induced in the PAZO-ADMA parts for all the cases, they still tightly adhere to the hemispheres of PMMA phase, which offers the great potential for photofabrication to produce particles with a large deformation and diversified shapes. In summary, photoresponsive Janus particles composed of a methacrylate-based azo polymer (PAZO-ADMA) and PMMA were successfully developed in this study. The corecompartmentalized JPs were obtained through microphase separation in the dispersed droplets. Janus structure is fully 236
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ACS Macro Letters
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Figure 4. Aspect ratio variation of azo-containing JPs with particle size of about 1 μm under irradiation (488 nm, 100 mW/cm2) for different time periods.
developed in the confined volume through the phase separation due to high incompatibility of the two polymers and also the surfactant effect of PVA. Upon the irradiation with a linearly polarized laser beam at 488 nm, the JPs with the asymmetric photoresponsive properties show unique symmetry-breaking deformation behavior. Due to the orientation of the particle symmetric axis relative to the polarization direction, the streamlined, mushroom-like, and snail-like JPs among others can be readily fabricated.
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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/acsmacrolett.5b00932. Experimental details and supporting figures (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The financial support from the NSFC under Project 51233002 is gratefully acknowledged. REFERENCES
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