Photoguided Shape Deformation of Azobenzene-Containing Polymer

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Photo-guided shape deformation of azobenzene-containing polymer microparticles Jingyi Li, Lingzhi Chen, Jiangping Xu, Ke Wang, Xiaofan Wang, Xiaohua He, Hai Dong, Shaoliang Lin, and Jintao Zhu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03610 • Publication Date (Web): 09 Nov 2015 Downloaded from http://pubs.acs.org on November 14, 2015

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Photo-guided Shape Deformation of Azobenzene-containing Polymer Microparticles Jingyi Li,1,# Lingzhi Chen,2,# Jiangping Xu,1 Ke Wang,1 Xiaofan Wang,2 Xiaohua He,3 Hai Dong,1,* Shaoliang Lin,2,* and Jintao Zhu 1,* 1

Key Laboratory for Large-Format Materials and Systems of the Ministry of Education, School

of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China 2

Shanghai Key Laboratory of Advanced Polymer Materials, School of Materials Science and

Engineering, East China University of Science and Technology, Shanghai 200237, China 3

School of Chemistry and Molecular Engineering, East China Normal University, Shanghai

200062, China #

These authors contributed equally to this work.

*To whom all correspondence should be addressed. E-mail: [email protected] (H. D.); [email protected] (S. L.); [email protected] (J. Z.)

ACS Paragon Plus Environment

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ABSTRACT: Here we present the generation of uniform microparticles with tunable diameters from azobenzene-based homopolymer by combining the microfluidics technique and emulsion-solvent evaporation route. In addition, the photo-induced deformation behavior of these microspheres, irradiated by a linearly polarized beam with different irradiation time and direction, are systemically studied. The deformation process through real time optical microscope observation can be investigated, benefiting from the uniform and microscaled size of the polymer particles. These results indicate that the deformation degree characterized by relative variation of the long axial for the particles can be controlled by the irradiation time. Moreover, elongated particles with tunable aspect ratio or tilted shape can be generated by manipulating the irradiation direction and/or time. Interestingly, the shape transformation kinetics displays a significant dependence on initial size of the polymer particle. In addition, the shape transformation of the polymer particle can lead to the variation of the orientation and distribution of the encapsulated anisotropic gold nanorods.

KEYWORDS: Microfluidics, Polymer particles, Azobenzene, Deformation, Emulsion droplets


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1. INTRODUCTION Polymer microparticles have found widespread applications in the fields of drug delivery, biodiagnostics, bio-imaging, electronics, optics, and advanced materials formation.1-6 Great efforts have been paid on the formation of polymer microparticles with various shapes, including spherical particles,7,8 rod-like particles,9 disc-like particles,10,11 elipsoids,12 Janus particles,13 patchy particles,14 and other complex structures.15,16 In particular, shape and morphology of the particles play essential role in determining their performance and functions, including rheological property,17 colloidal self-assembly,18 catalysis efficiency,19 cellular uptake,20 immune response,21 targeting ability,22 and others. For example, particle shape has been recognized as a new design parameter for drug delivery carriers. There is already evidence that the most basic function of particles, degradation to release therapeutic drug, will depend on particles shape.22 Recently, Mitragotri and coworkers incubated polystyrene (PS) microparticles with various shapes with rate alveolar macrophages, and the results indicated a crucial and interesting role of shape in phagocytosis. The local shape of the particles at the point where the cell attached, instead of the overall shape, directed whether or not a macrophage began internalization.23 Spherical particles were internalized from any point of attachment due to their symmetry. However, a macrophage attached to an ellipse at the pointed end internalized in a few minutes while a macrophage attached to a flat region of the same ellipse did not internalize the particle for over 12 h. Therefore, it is of great interest to design and formation of non-spherical polymer microparticles with well-controllable shape and functionality.24,25 Several synthesis strategies for generating non-spherical polymer particles have been developed, including mechanical stretching,1,10,12 microfluidics-assisted route,9,16,26 seed dispersion polymerization,11,27 lithography and photopolymerization,28,29 and modified emulsion-


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solvent evaporation route.30,31 For example, Kumacheva et al. and Doyle et al. formed liquid monomer droplets through microfluidic device, shaped the droplets in a microchannel and polymerized them to form solid particles of several non-spherical geometries.9,32 DeSimone et al. employed conventional soft lithographic molding methods to create isolated polyethylene glycol (PEG), poly(lactic-co-glycolic acid) (PLGA), polylactic acid (PLA) particles of various shapes by using a non-wetting perfluoropolyethers (PFPE) mold.28 Meanwhile, Champion group employed a stretching physical strategy to generate anisotropic polymer particles with tunable shapes based on the preformed polymer particles.1 Nowadays, it is still a great challenge to produce polymer microparticles with precisely controllable shape, aspect ratio and morphology in a facile and effective means for potential applications in the fields of drug delivery and targeting, bio-diagnostics, photo-storage, optical-switching, photonic crystals, and other photonic devices. On the other hand, polymers containing azobenzenes (azo polymers) have attracted considerable attention in recent years because of the photo-induced orientation properties; i.e., irradiation with linearly polarized light can induce orientation changes of the azo moieties, often used for light-controlled changes in the sample’s morphology. The light-induced trans-cis isomerization of the azo moieties makes azo polymers as optimal candidates for anisotropic particles fabrication through deformation. Upon light irradiation, azo-containing particles have the ability to transform the morphologies by orientating the azo moieties due to the Weigert effect.33-38 For example, Wang et al. studied the effects of functionality numbers, average diameters (~100-500 nm) and strong push–pull azo chromophores on the photo-induced deformation behavior of nanoscaled azo-colloids.39-43 The results indicated that polarized beam at normal incidence can stretch the azo polymer colloids from an amphiphilic azo polymers


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along the polarization direction of the beam. Some important factors that influence the colloidal deformation have been investigated in previous studies, which include composition of the colloids, amount of the azo chromophores in the colloids, different chain architectures, and diameter of the micelle-like aggregates.44 However, the current understanding of these work mentioned above are not clear because the characterization data are obtained from electron microscope (EM), light scattering, and UV-Vis absorption spectra which provide ex-situ results or indirect evidence. Thus, little is known for the deformation process and irritation angle effect for same polymer particle due to the lack of suitable technique for investigation of nanosized colloids. Recently, azo-containing block copolymer vesicles with micron size were formed and photo-induced circular process including fusion, damage and defect formation, disruption, disintegration and rearrangement in THF/water solution irradiated by UV light were observed.45 This direct optical microscope (OM) observations on the fusion and disruption provide useful information for understanding the fusion process of cells.46 Therefore, it will be highly desirable to gain insight in the deformation process of solid azo microparticles with uniform and tunable sizes through in-situ observation. Herein,

we

generate

poly[6-(4-methoxy-4’-oxy-azobenzene)

hexyl

methacrylate]

(PMMAzo) microparticles with uniform size, and systemically investigate their photo-induced shape transformation. The PMMAzo particles were synthesized by combining the microfluidics technique and emulsion-solvent evaporation strategy. We have examined the influence of polarization direction and irradiation time on PMMAzo particles morphologies. To the best of our knowledge, this is the first time to produce PMMAzo microparticles with novel morphology by combining variation of the polarization direction and irradiation time. The transformation of lance-shaped particles from lanky to extended spherical morphology was simply tuned by


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changing the direction of polarizer and irradiation time. This finding not only provides a new design principle for obtaining light responsive particles containing azo polymers, but also extends new morphologies of the polymer particles besides the spindle-like structures. 2. EXPERIMENTAL SECTION 2.1 Materials. All reagents and solvents were purchased from Sinopharm Chemical Reagent. PS2k-SH (Mw/Mn =1.15) was purchased from Polymer Source Inc. Canada. All the chemicals were used without further purification unless where noted. 2.2 Synthesis of the Azo containing homopolymers. Homopolymer PMMAzo was synthesized through atom transfer radical polymerization (ATRP) in anisole.47 Typically, an oven-dried Schlenk tube was charged with MMAzo (1.3 g, 3.1 mmol), EtBriB (25.4 mg, 0.13 mmol), Me6TREN (38.0 mg, 0.26 mmol), CuCl (25.7 mg, 0.26 mmol), anisole (2.0 mL) and a magnetic bar. After degassing three freeze-pump-thaw cycles, the tube was sealed under vacuum and then placed in a thermostatic oil bath at 90 °C for 18 h. The reaction was stopped and quickly cooled down to room temperature with cold water. The mixture was further diluted with THF, removed copper salts through a plugged column of neutral aluminum oxide and precipitated into an excess amount of diethyl ether. The sample was then purified by reprecipitation for three times from CH2Cl2 to diethyl ether and dried in a vacuum oven overnight. Yield: 72 %, Mn (from GPC) =2.6×104, Mw/Mn=1.23. The chemical structure and UV-vis absorption spectrum of PMMAzo is shown in Figure S1 and S2 in the Supporting Information (SI). 2.3 Generation of the polymer particles. The microfluidic device consisted of a cylindrical collection tubes nested inside a square glass capillary. Round glass capillary tube (World Precision Instruments) was tapered to an orifice of ~ 40-80 µm using a micropipette puller (Narishige PC-10) and a microforge (Narishige MF-900).


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Monodispersed PMMAzo microparticles were prepared through microcapillary device. The continuous aqueous phase consisted of 5 mg/mL sodium dodecyl sulfate (SDS) while the dispersed organic phase was 10 mg/mL polymers in chloroform. The above two different fluids were pumped through the microfluidic device. Once the two fluids met at the orifice, uniform emulsion droplets would be generated under the strong shearing force of the continuous phase and the interfacial tension of oil/water. After generation, the emulsions were collected into a glass beaker. After evaporation of chloroform, the resulting suspension was dialyzed against deionized water for 3 days to remove SDS and residual chloroform. 2.4 Characterization. Real time structural evolution of shape change of the polymer particles by irradiation was monitored using Olympus IX71 inverted optical microscope (OM) in bright-field. The resulting particles morphology was observed by scanning electron microscope (SEM) which was carried out on a Sirion 200 SEM at an accelerating voltage of 10 kV. The extended structure and height variation of the polymer particles caused by irradiation were explored at room temperature with an atomic force microscope (AFM, Digital Instruments, Nanoscope IIIa) operated in tapping mode. The redistribution of inorganic NRs in the polymer particles were investigated using FEI TecnaiG2 20 transmission electron microscope (TEM) operated at an accelerated voltage of 200 kV. 2.5 Photo-induced shape deformation. For photo-induced shape deformation of the particles, samples were prepared by dropping the aqueous dispersion onto a glass slide and then dried in a 30 °C vacuum oven for 24 h. The samples were irradiated by a linearly polarized beam from a LED lamp (Uvata UP114) with a wavelength of 450 nm. After spatially filtered and collimated, the LED lamp had an intensity of 1400 mW/cm2. The linearly polarized light was incident perpendicularly to the substrate surfaces. The experiments were carried out at room temperature


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under an ambient condition. After the samples were irradiated for various time elapses, the OM observations were performed to probe the shape deformation. 2.6 Photo-guided inorganic NRs redistribution. Au NRs with high aspect ratio were prepared by a binary surfactant mixtures protocol while Au@PS2k NRs were obtained through the ligand exchange route as reported previously.48,49 Then, Au@PS2k NRs were dispersed with PMMAzo in chloroform and uniform emulsion droplets were formed through the microfluidics-assisted strategy, as described above. After evaporation of chloroform, the resulting suspension was dialyzed against deionized water for 3 days to remove SDS and residual chloroform. Subsequently, the particles were irradiated by a linearly polarized light at 450 nm, and distribution of the NRs within the particles was investigated through TEM. 3. RESULTS AND DISCUSSION 3.1 Preparation of Uniform PMMAzo Microparticles Colloid size is one of the most important factors that are commonly used to characterize colloid spheres and influence their rheological property, self-assembly, and biological functions (e.g., transport, adhesion, internalization, enhanced permeability and retention effect, and clearance from the body).50,51 Study of the size dependence of its photoinduced deformation behavior can lead to a better understanding of the nature of the photoinduced massive motion. Nowadays, the research on the size dependence of photo-induced deformation behavior is limited for the nanosized polymer colloid which can be observed by ex-situ characterization technique (e.g., electron microscope) instead of in-situ observation by optical microscope.52,53 In this report, monodispersed PMMAzo microparticles with tunable size of ~8-80 µm are generated through a microcapillary device.54,55 As shown in Figure 1, flow-focusing of the organic phase by the continuous aqueous fluid stretches the solvents out into a thread that subsequently breaks into


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uniformly-sized droplets just inside the orifice by varying the exit orifice size and/or flow rates of the oil and aqueous phases. SDS is added to the aqueous phase to stabilize the formed emulsion droplets. After generation, the emulsion droplets are then collected in a glass beaker to allow the slow evaporation of the chloroform in ambient environment, triggering the formation of the PMMAzo microparticles (Figure S3 in the SI). In order to purify the particles, the suspension is dialyzed against water for 3 days. Finally, the suspension is dropped on a glass slide and dried for 2 h at room temperature before the deformation investigation. Due to the uniform size of the PMMAzo microparticles, it is possible to observe the phenomenon and obtain statistic information of the deformation process for the PMMAzo particles through real time optical microscope. 3.2 Photo-induced Shape Deformation It is well known that azo moieties can undergo photo-induced orientation by polarized light, which can induce the shape deformation of azo-containing particles or mass transfer at the nanoor micro-scale.33 We have systematically investigated the photo-responsive deformation behavior of the PMMAzo microparticles with varied sizes. Figure 2 shows the real-time photo-responsive behavior and shape deformation of PMMAzo microparticles under optical microscope investigation after polarized light irradiation. We show that the particle size plays a crucial role in the deformation process of the PMMAzo particles. The particles with initial diameter of 9 µm are irradiated by 450 nm light for different time from 0 h to 17 h. As showed in Figure 2(a)-(i), significant elongation of the original spheres along the polarization direction of the polarizer with the increase of irradiation time is observed, leading to the formation of spindle-like particles. Notably, the particles tend to expand rather than elongate after irradiation for 7 h. The SEM and AFM images of the particles after irradiation for 17 h are shown in Figure 2 (j-l).


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Clearly, the height of the particles is ~2 µm, which is much less than the initial size of 9 µm. The result indicates that the irradiation process could be an elongation, expansion and finally flattening process. The spindle-like particles begin to crumple after the irradiation for 4 h (see arrows in Figure 2(e) and (f)), and the crinkle-like morphology disappears gradually as further irradiation. This phenomenon is observed and confirmed in all shape transformation experiments. As the innitial particle diameter is large enough, the shape deformation process occurrs only at the surface region within a thickness less than 1 µm,56 and the inner azo core remains unchanged. Along with the direction of particle elongation, the azo moieties on particle surface move to the two poles and the inner parts protrude outside during the irradiation. It should be noted that, due to the Weigert effect,57 the numbers of azo moieties with their transition moments normal to the light polarization direction gradually increases, resulting in the light-selective alignment, with transition moments of azo almost perpendicular to the polarization direction of the actinic light. Therefore, the azo moieties at the two poles are easy to reach the balance and will not deform with further irradiation, while the middle part of azo moieties keeps on deforming. Then, the continuous deformation part will extrude the balanced part, and presents as crumple because of the out-of-step deformation process between two poles and medium portion. Subsequently, the crumple part will deform under irradiation again until all moieties reach balance, and remain constant with further irradiation. 3.3 Shape deformation kinetics To further understand the deformation process, the relative variations of long axial of the elongated particles with different diameters are recorded as the increase of irradiation time (Figure 3 and Figure S4-S6 in the SI). As shown in Figure 3(a), the shape deformation rate changes as a function of the particle size. The particles with diameter of 9 µm show the most


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significant deformation rate under the same irradiation window. While particles with diameter of 58 µm show the lowest deformation degree (Figure S5). Though particles larger than 25 µm deform much slower than smaller ones because the larger size of the particles needs more external energy for deformation, it can be seen that all the particles present an initial linear deformation process followed by a slow elongation. The morphology of PMMAzo particles remains constant eventually. Clearly, the transition points of the deformation kinetics decrease with the increase of the particle sizes, and the deformation rate decrease after reaching a plateau (Figure 3b and Figure S7). Presumably, the reason can be ascribed to the flat of the PMMAzo particles without further elongation. Notably, for the particles with size of 25 µm, there is a significant decrease of the relative variation of the long axial for the particles after reaching the transition point, followed by the increase of the relative variation in the long axial. This can be ascribed to the readily flat of the particles in the short axial, driving the retraction of the particles in the long axial. This is confirmed from the curve of axial ratio of the polymer particles as a function of irradiation time (Figure S7). For the initial growth stage of the first 500 min as shown in Figure 3(b), a fitting model function can be applied for the deformation rate. As the particle size is smaller than 38 µm, the deformation rate exhibits a nearly linear process. Yet, the deformation process becomes much slower when particles are larger than 38 µm. Figure S4 displays the full deformation process of PMMAzo particles with diameter of 38 µm. Notably, for particle diameter of 58 µm, only the two poles deform while the medium parts remain constant (Figure S6). Since more than one whisker is found in each pole, the particles are too large to deform in a relative short time. Given more irradiation time, we can anticipate that the medium parts of the particles will transfer from center to the two poles, and the particles thus become


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elongated. From these results, we can rationally design and fabricate azo-containing polymer particles with desirable aspect ratio and shape by carefully tailoring the irradiation time. 3.4 Morphology Transformation by Varying the Polarization Direction To manipulate the shape of the PMMAzo particles, we have employed the polarization direction as an experimental parameter to reshape the particles. Interestingly, lanky lance-shaped particles are prepared by changing the polarization direction at 30° after irradiation for another 2 h after shape deformation. Figure 4a records the deformation processes of lanky lance-shaped particles. The two poles of the spindle-like particles begin to elongate along the new polarization direction after a short time. As the number of azo moieties increases in the two poles, the particles change from spindle-like to lance-shaped morphology. The swollen lance-shaped particles are formed by changing the polarization direction from 45° to 60° (Figure 4b and 4c). By manipulation of the irradiation time and the polarization direction, the flat rod-like, rice-like (Figure 4c) and extended spherical particles (Figure 4d) can be obtained. This result consolidates our technique as a facile and powerful means to tune the shape and structure of the azo polymer particles through varying the irradiation time and the polarization direction. 3.5 PMMAzo/Au NRs Hybrid Microparticles This deformation behavior can also be employed to align the redistribution and orientation of anisotropic nanoparticles (NPs) encapsulated in the azo polymer particles. As a proof of concept, gold nanorods (NRs) are encapsulated into the PMMAzo particles to fabricate functional hybrid materials. The orientation of Au NRs is tuned through the deformation process of microparticles under irradiation. Typically, we introduce PS2k-tethered gold NRs (Au@PS2k NRs) with high aspect ratio (length: 80 nm; diameter: 16 nm) as model anisotropic NPs into the PMMAzo microparticles. Figure 5 demonstrates the preliminary result for the dispersion and orientation of


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Au@PS2k NRs before and after shape deformation process of the PMMAzo particles which is observed by TEM. Clearly, at the initial state, Au@PS2k NRs are randomly distributed in the PMMAzo particles. After deformation of PMMAzo particles, the Au@PS2k NRs tend to separate and rearrange along the deformation direction within the particles after irradiation for 5 h (Figure 5b-d). Therefore, this observation proves the concept that deformation of the polymer particles can direct the alignment of encapsulated anisotropic NPs, potentially useful for the shape and internal structures re-arrangement of the hybrid polymer particles. 4. CONCLUSION In summary, PMMAzo microparticles with uniform and tunable sizes are fabricated by combining the microfluidic technique and emulsion solvent evaporation route, and their shape deformation processes under linearly polarized irradiation are systematically investigated. The microparticle size plays a crucial rule in the deformation behavior of the azo polymer particles, which presents a linear growth step followed by a slow elongation process to reach a constant eventually. We show that the rate of the shape deformation decreases with the increase of initial particle diameter. When the particles are smaller than 38 µm, the deformation rate presents as a linear decrease process. Increase of the initial size of the particles will slow down the deformation process. Interestingly, different morphologies of particles, including lance-shaped, flat rod-like, rice-like, and extended spherical structures are obtained by tuning the irradiation time and/or the polarization direction. Furthermore, the reorientation of inorganic NPs can be realized by incorporating Au NRs within the PMMAzo microparticles under irradiation. These observations not only deepen the understanding of the nature of interaction between light and azo polymers at the micrometer scale, but also provide guidance for the rational design and fabrication of photo-responsive anisotropic materials with desirable structures.


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ASSOCIATED CONTENT Supporting Information Chemical structure and Uv-vis spectrum of the PMMAzo, and additional OM images showing the shape deformation processes of azo polymer particles with initial diameter of 18 µm, 38 µm, and 58 µm. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (H. D.); [email protected] (S. L.); [email protected] (J. Z.) Notes The authors declare no competing financial interest. Acknowledgements We gratefully acknowledge funding for this work provided by National Basic Research Program of China (973 program, 2012CB932500), National Natural Science Foundation of China (51103050, 51473059 and 51573046). S. L. also appreciated the support from Projects of Shanghai Municipality (14SG29) and Fundamental Research Funds for the Central Universities (NCET-12-0857 and WD1213002). We also thank HUST Analytical and Testing Center for allowing us to use its facilities.


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Figures:

Figure 1. Schematic illustration showing the preparation process of uniform PMMAzo microparticles through microfluidics-assisted strategy. The outer aqueous phase and internal oil phase consisted of a SDS aqueous solution and PMMAzo chloroform solution, respectively. The PMMAzo in chloroform was sheared into droplets at the tip of the orifice by the outerflow. The collected emulsion drops were exposed under the polarized light, and the nonspherical particles with different axial aspects were obtained by controlled irradiation time.


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Figure 2. Optical microscope (OM) images of the PMMAzo microparticles with initial diameter of 9 µm after irradiation for: (a) 0 h, (b) 0.5 h, (c) 1 h, (d) 2 h, (e) 4 h, (f) 5 h, (g) 7 h, (h) 11 h, and (i) 17 h. (j) Scanning electron microscope (SEM) and (k) Atomic force microscope (AFM) images of PMMAzo particles after irradiated 17 h. (l) Height image of particle marked in (k). Scale bar in (a) can be applied in (b-i).


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Figure 3. (a) The relative variation of the long axial of the PMMAzo particles as a function of the irradiation time. lini and l represents the length of the long axial of the PMMAzo particles before and after irradiation, respectively. (b) Relationship between the deformation rate and the initial particle sizes. We define the deformation rate for the first stage of the deformation process (~ 500 min) as the relative variation of the long axial of the particles per min, e.g., slope of the curve in Figure 3a at the first stage.


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Figure 4. OM images of PMMAzo particles before and after irradiation for 2 h. The particles were irradiated for 0.5 h, 1.0 h, 1.5 h, and 2.0 h after changing (rotating) polarization angle 30°, 45°, 60°, and 90°, respectively. The solid arrows show the polarization direction while the dashed arrows represent the original polarization direction. Scale bar in (a) can be applied in all the other images.


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Figure 5. Representative TEM images and inset schematic illustration of Au@PS2k NRs dispersed in PMMAzo particles before (a) and after (b-d) irradiation for 5.0 h.


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For the table of contents use only: Title: Photo-guided Shape Deformation of Azobenzene-containing Polymer Microparticles Authors: Jingyi Li, Lingzhi Chen, Jiangping Xu, Ke Wang, Xiaofan Wang, Xiaohua He, Hai Dong,* Shaoliang Lin,* and Jintao Zhu*

TOC graph:


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Photoguided Shape Deformation of Azobenzene-Containing Polymer

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