Janus and Strawberry-like Particles from Azo Molecular Glass and

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Janus and Strawberry-like Particles from Azo Molecular Glass and Polydimethylsiloxane Oligomer Chungen Hsu, Yi Du, and Xiaogong Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02815 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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Janus and Strawberry-like Particles from Azo Molecular Glass and Polydimethylsiloxane Oligomer Chungen Hsu, Yi Du, Xiaogong Wang∗ Department of Chemical Engineering, Laboratory of Advanced Materials (MOE), Tsinghua University, Beijing, P. R. China 100084

KEYWORDS Janus Particle, Strawberry-like Particle, Azo Molecular Glass, Polydimethylsiloxane, Photodeformable

∗

Corresponding author: Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT This study investigated Janus and strawberry-like particles composed of azo molecular glass and polydimethylsiloxane (PDMS) oligomer, focusing on controllable fabrication and formation mechanism of these unique structures and morphologies. Two materials, the azo molecular glass (IA-Chol) and PDMS oligomer (H2pdca-PDMS), were prepared for this purpose. The Janus and strawberry-like particles were obtained from the droplets of a dichloromethane (DCM) solution containing both IA-Chol and H2pdca-PDMS, dispersed in water and stabilized by poly(vinyl alcohol). Results show that the structured particles are formed through segregation between the two components induced by gradual evaporation of DCM from the droplets, which is controlled by adding ethylene glycol (EG) into the above dispersion. Without the addition of EG, Janus particles are formed through the full segregation of the two components in the droplets. On the other hand, with the existence of EG in the dispersion, strawberry-like particles instead of Janus particles are formed in the phase separation process. The diffusion of EG molecules from the dispersion medium into the droplets causes the PDMS phase deswelling in the interfacial area due to the poor solvent effect. Caused by the surface coagulation, the coalescence of the isolated IA-Chol domains is jammed in the shell region, which results in the formation of the strawberrylike particles. For the particles separated from the dispersion and dried, the PDMS oligomer phase of the Janus particles can adhere and spread on the substrate to form unique “particle-onpad” morphology due to its low surface energy and swelling ability, while the strawberry-like particles exist as “standstill” objects on the substrates. Upon irradiation with a linearly polarized laser beam at 488 nm, the azo molecular glass parts in the particles are significantly deformed along the light polarization direction, which show unique and distinct morphologies for these two types of the particles.


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1. Introduction In recent years, formation of complex structures by combining mutually incompatible components in a single particle has attracted a wide range of attention.1-10 Janus particle (JP) as a unique category of the nano/micro-objects was first introduced by de Gennes in 1992 to exhibit such distinguishing features and functions.11 JPs are characterized by their symmetry-breaking structures combining materials with different chemical or physical properties within a single particle.1,2 Owing to their great importance in both fundamental understanding and potential applications,5,6,8,9 JPs have been fabricated by series of innovative methods, such as selfassembly of block copolymers,12-15 subsequent surface modification,16,17 microphase separation in confined volume,18-21 seeded emulsion polymerization,22-25 electrohydrodynamic jetting and co-spinning,26-30 and microfluidics.31

Through the avenues of non-centrosymmetric

compartmentalization and distinct surface patching, different kinds of materials with diverse properties can be incorporated into one particle to realize different new functions. Rooted in the fascinating nature of the asymmetrical distribution of materials and symmetry-breaking structures, JPs have demonstrated remarkable application potential in areas such as particulate surfactants,32,33 electric/magnetic field-driving devices,34,35 and bio/life science.36,37 Besides JPs, other structured and functional particles, such as strawberry-like particles and shape-adaptable particles, have also been actively investigated in recent years.4,7,10,38 The way to control the structure formation is a critically important requirement for fabricating particles with complex architecture and new functions. Polymers and amorphous molecular materials containing azo chromophores (azo polymers and azo molecular glasses for short) have been intensively investigated for their interesting photoresponsive properties.39-41 Distinct from their high molecular weight counterparts, azo


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molecular glasses have been developed as a new type of photo/electroactive amorphous materials with a low molecular weight.40,42-45 Azo molecular glasses can exist in a stable amorphous state below the glass transition temperature (Tg). This property can be exploited to prepare transparent thin films for optical applications. Photoinduced mass transport along the light polarization direction is one of the most interesting functions of azo polymers and azo molecular glasses, which was first discovered in the case of surface-relief-grating (SRG) formation.46,47 In recent years, this effect has also been explored to induce the deformation of azo polymer microspheres,48 to create and modify surface patterns,49 among others.41 For photoinduced masstransport, azo molecular glasses usually show a much faster response compared with that of azo polymers owing to their low molecular weight, for which the entanglements of long polymer chains can be avoided.40,45 In a recent study, photoresponsive JPs have been prepared by using azo polymer and poly(methyl methacrylate) through the approach of microphase separation in confined volume.50

The symmetry-breaking deformation induced by light has been

demonstrated by irradiating JPs with the linearly polarized laser beam. However, even with this successful case, developing JPs to contain azo molecular glass is not a straightforward process. Preparing Janus particles through phase separation in dispersed droplets is a versatile and scalable approach.18-21 Typically, this method has been used to prepare Janus particles composed of two polymers as polymers have extremely strong segregation effects.1 Compared with the strong segregation effect between two polymers, a combination of low molecular weight molecule with oligomer or polymer usually shows a much weaker phase separation tendency.51 To fabricate JPs containing azo molecular glass, a material that can strongly segregate from it is highly desirable. Polydimethylsiloxane (PDMS) is the most common silicon-based organic polymer with unique properties, such as low glass transition temperature, low surface energy,


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optical transparency, and high flexibility.52 PDMS has been widely applied in membranes, microfluidic devices and other areas.53,54 PDMS-based components have also been utilized to prepare block copolymer,55 micro-droplets for encapsulation,56,57 and highly stretchable selfhealing elastomer.58

Compatibility of PDMS with different organic solvents has been

extensively investigated.54 Due to its non-polar nature, PDMS shows a strong tendency to segregate from polar substances, such as materials containing azo chromophores.

This

incompatibility can be used to develop particles composed of a PDMS-based component and azo molecular glass with phase-separated structures. Moreover, the nanoscale morphology of JPs was found to depend on a complex interplay of the compatibility between polymers and interfacial interaction between polymers and dispersion medium (water).18,20 Diminishing the differences between the interfacial tensions of the two components with the dispersion medium can reduce the chance to form core-shell structure.18 On the other hand, for a complex emulsion, diffusion of molecules of a separating agent from the continuous phase to dispersed phase can initiate phase separation of the dispersed droplets.59

Therefore, in addition to the special

molecular design, understanding of the mechanism and optimizing the procedure are necessary to fabricate JPs and other structured particles containing azo molecular glass through the microphase separation approach. However, to our knowledge, such controllable formation and related mechanisms have not been investigated for JPs and other structured particles containing azo molecular glass yet. In this study, we investigated the formation of the structured particles from an isosorbidebased azo molecular glass (IA-Chol) and a PDMS oligomer containing 2,6-pyridinedicarboxamide (H2pdca-PDMS) groups.

The solution of IA-Chol and H2pdca-PDMS in

dichloromethane (DCM) was prepared and dispersed in water, which was stabilized by


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poly(vinyl alcohol) (PVA) as the surfactant. Phase separation between the two components in the dispersed droplets was induced by gradual evaporation of DCM. Ethylene glycol (EG), known as a solvent having poor compatibility with PDMS, was added into the dispersion and its effect to control the particle structures was investigated. The assembling processes and particle structures were characterized by microscopic observations and energy dispersive spectrometer (EDS) analysis. The JPs and strawberry-like particles formed through above processes showed distinct morphologies and unique properties connected with the two components as well as the particle structures.

2. Experimental Section 2.1.

Materials.

Isosorbide,

4-nitrobenzoyl

chloride,

(N-ethyl-N-hydroxyethyl)aniline,

cholesteryl chloroformate were purchased from Alfa Aesar and used as received.

Bis(3-

aminopropyl)terminated poly(dimethylsiloxane) (H2N-PDMS-NH2, Mn = 5,000-7,000) and 2,6pyridinedicarbonyl dichloride were purchased from Sigma Aldrich. Concentrated sulphuric acid and glacial acetic acid were purchased from the commercial source and used for the azo-coupling reactions.

Analytical grade dichloromethane (DCM), N,N’-dimethylformamide (DMF) and

ethylene glycol (EG) were purchased from commercial sources and used as received. Deionized water (resistivity > 18 MΩ cm) was obtained from a Milli-Q water puriﬁcation system. Poly(vinyl alcohol) (PVA 1788) was purchased from Shanghai Aladdin biochemical technology, which had the degree of hydrolysis of 87.0-89.0% (mol/mol). Other reagents were commercially available products and used as received without further purification. The azo compound (IAChol) was synthesized in this laboratory according to the previously reported method.45 The detail about the synthesis and characterization are given in the Supporting Information. The


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polydimethylsiloxane oligomer (H2pdca-PDMS) was synthesized according to the literature,58 and the synthesis and characterization details are also given in the Supporting Information. 2.2. Janus Particles from IA-Chol and H2pdca-PDMS. IA-Chol and H2pdca-PDMS were respectively dissolved in DCM to obtain two solutions with the concentrations of 10 mg/mL, respectively, which were then mixed with a volume ratio of 1:3 (IA-Chol solution/H2pdcaPDMS solution) to obtain a homogeneous solution used for the following preparation. This prepared solution (4 mL) was added dropwise into the bottom of the PVA aqueous solution (2.5 wt%, 50 mL). The moderate stirring was applied to promote the dispersion of the organic solution into micro-droplets during the adding process. Owing to the volatile nature of DCM (bp: 39.75 °C), the dispersion was kept in a refrigerator (5 °C) for 48 h to allow DCM to slowly evaporate. Owing to the high density of DCM, the droplets underwent slow sediment in the process. After the sedimentation under gravity, the supernatant about 2/3 of the volume was carefully decanted out and it was replaced with the same amount of deionized water stored in the refrigerator. The washing process was carried out regularly, every 3 h in the first day, every 6 h in the second day, every 12 h in the third day, and every 24 h in the following days. After each washing step, the beaker flask was kept in the refrigerator at 5 °C. During the process, Janus structure gradually formed in the droplets. Normally, the droplet sizes no longer reduced in 4-5 days, which evidenced the completion of the structure formation. After repeatedly washed with deionized water to remove residual PVA, the formed JPs could be dispersed in water with a gentle agitation. The dispersion was centrifugalized to fractionate the JPs according to their sizes. Typically, particles with the sizes in 5-15 µm scale were used to carry out the following studies.


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2.3. Strawberry-Like Particles from IA-Chol and H2pdca-PDMS. The solution of H2pdcaPDMS and IA-Chol in DCM used here is the same as mentioned above. The solution was added dropwise into the bottom of the aqueous solution (50 mL) of PVA (2.5 wt%) and EG (2.5 wt%), where the dispersion and washing procedure was the same as that described above. After repeatedly washed with deionized water to remove residual PVA and EG, the formed strawberrylike particles could be dispersed in water with a gentle agitation.

The dispersion was

centrifugalized to fractionate the particles according to their sizes. Typically, particles with the size in 5-15 µm scale were used to carry out the following studies. 2.4. Optical Setup for Photoinduced Deformation. A linearly polarized beam from a diodepumped frequency doubled solid state laser (488 nm) was expanded with a spatial filter (pinhole, 25 µm). Then, a convex lens was adopted to generate a parallel uniform beam with a diameter of 20 mm. A diaphragm with a diameter of 6 mm was used to select the central part of the beam. A half wavelength plate was adopted to obtain the linearly polarized light. After being spatially filtered, expanded and collimated, the laser beam was incident perpendicularly to the silicon wafer containing the particles. The intensity of laser beam was set to be 200 mW/cm2 and the light irradiation was performed for a required time period at room temperature under the ambient condition. 2.5. Instrument and Characterization. 1H NMR spectra were obtained on a JEOL JNMECA600 NMR spectrometer with tetramethylsilane (TMS) as the internal standard in a DMSOd6 solution at 30 °C. FT-IR spectra were collected on a Nicolet 560-IR spectrophotometer. The samples were mixed with KBr powder and then pressed into thin IR-transparent disks. Differential scanning calorimetry (DSC) experiments were performed using a TA Instrument (Model Q2000). The scanning temperature range was −90 °C to 100 °C for the H2pdca-PDMS


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oligomer at a heating and cooling speed of 10 °C/min. The UV-vis spectra of the samples were measured by using an Agilent 8453 UV-vis spectrophotometer. The molecular weights and molecular weight distributions were obtained by using a gel permeation chromatography (GPC) apparatus with THF as the eluent (1 mL/min). The instrument was equipped with a refractive index detector (Wyatt Optilab rEX) and fitted with a PL gel 5 µm mixed-D column. The measurements were carried out at 35 °C and the data were calibrated with linear polystyrene standards. A Nikon LV 100 POL microscope equipped with a Nikon DS-fi2 CCD camera was used to capture the optical images of the IA-Chol/H2pdca-PDMS droplets and particles. A scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS) from Zeiss corporation (Zeiss Merlin) was used to characterize the morphology and perform the elemental analysis. A high vacuum (4×10-6 mbar, approximately) condition was adopted, while the voltage and current were 1 kV and 100 pA, respectively.

3. Results and Discussion 3.1. Fabrication of Janus and Strawberry-Like Particles Figure 1 shows the fabrication procedure of the JPs and strawberry-like particles as well as chemical structure of the two materials used to fabricate the particles.

IA-Chol is an azo

compound containing a core of isosorbide moiety bearing two push-pull type azo chromophores and two cholesteryl groups on the periphery. H2pdca-PDMS is a PDMS oligomer containing 2,6-pyridinedicarboxamide (H2pdca) groups. Both materials were synthesized for this study according to the previous reports.45,58 The synthesis and characterization details are given in the Supporting Information (Figure S1-S9). Hydrophobic IA-Chol cannot dissolve in water, but can well dissolve in an organic solvent such as DCM. H2pdca-PDMS is composed of a hydrophobic poly(dimethylsiloxane) (PDMS) chain and hydrophilic 2,6-pyridinedicarboxamide (pdca)


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groups. H2pdca-PDMS is an oligomer with Mn = 17100, Mw =29700, and Mw/Mn =1.7, which was

obtained

by

the

condensation

reaction

between

bis(3-aminopropyl)-terminated

poly(dimethyl-siloxane) (H2N-PDMS-NH2) and 2,6-pyridinedicarbonyl dichloride.

H2pdca-

PDMS was originally designed by Li et al. to form the crosslinking complexes of 2,6pyridinedicarboxamide ligands and Fe(III) centers for the stretchable self-healing elastomer.58 In the current study, H2pdca-PDMS instead of conventional PDMS was used in order to enhance the cohesion between PDMS and IA-Chol parts in the particles. As shown in Figure 1, both Janus and strawberry-like particles were prepared through the phase separation in dispersed droplets in water. Under the typical conditions, the solution of IAChol and H2pdca-PDMS in DCM (4 mL) was added dropwise into an aqueous solution of PVA (2.5 wt%, 50 mL). In order to enhance the segregation between the two components, the dispersion was kept at 5 °C for several days and the water as the dispersion medium was regularly replaced with the neat water in the process. DCM was removed at the extremely slow rate through its evaporation and the water-exchange process. The segregation between the two components is enhanced by the gradual removal of DCM. JPs with the fully segregated parts in the droplets could be dispersed in water when PVA was gradually washed away in the waterexchange process. On the other hand, when a suitable amount of ethylene glycol (EG, 2.5 wt%) was added into the above aqueous dispersion, the strawberry-like particles instead JPs were formed from these two components in the process under the same conditions. The dispersions of JPs and strawberry-like particles in water were centrifugalized to fractionate the particles with proper sizes for further investigation. Figure 2(a) shows the typical optical microscopic (OM) images of the JPs dispersed in water, where the brownish red part is the azo molecular glass phase and the transparent part is H2pdca-


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PDMS phase. Similar to most azo dyes, IA-Chol shows bright colour from the push-pull type azo chromophores, which can be easily distinguished from the colourless H2pdca-PDMS part (see Figure S8 for their UV-vis spectra). Owing to the higher volume ratio of the H2pdca-PDMS phase and its slightly swelling with the high density DCM (1.325 g/mL at 25 °C), which will be discussed in Section 3.2 below, the JPs always float in water with the IA-Chol part up and the H2pdca-PDMS part down. This bottom-heavy buoying mode can be disrupted by little agitation, which can quickly return the original floating mode after stopping the disruption for a while. Figure 2(b) shows the OM images of the strawberry-like particles dispersed in water, where the brownish red patches are the azo molecular glass phase and the transparent matrix is the H2pdcaPDMS phase.

By agitating the dispersion, it can be seen that the patches are uniformly

distributed on the particle surface. 3.2. Structure Characterization For the microscopic and EDS characterizations, the dispersions of the particles were dropped on clean silicon wafers and carefully dried under proper conditions. Figure 3(a) shows the SEM image of the JPs on the silicon wafer. The particles surrounded with the wrinkled coronae can be clearly seen form the image. Figure 3(b) shows the typical OM image of the dried JPs on the substrate, where the brownish red particles are IA-Chol parts surrounded with the H2pdca-PDMS coronae. Both SEM and OM images demonstrate that the IA-Chol phase consists in the upper part of the structure, while the H2pdca-PDMS phase forms the bottom of the particle that adheres on the silicon wafer. Figure 3(c) gives the enlarged SEM image of a typical JP, where the corona with the wrinkles can be more clearly observed. The EDS analysis was used to analyse the compositions of the structure, which was taken at the two positions marked in Figure 4(c). EDS spectra obtained from the top of the particle and its surrounding corona show the different


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compositions (Figure 3(d)). The existence of N element in the former clearly evidences that it is composed of the IA-Chol phase (from −N=N− group), which is fully consistent with the OM observation. EDS mapping was employed to further identify the compositions of the particle and corona surrounding it (Figure 4). From the EDS mapping, it can be concluded that the upper part of the particle is composed of IA-Chol with N element, and the wrinkled corona is the H2pdcaPDMS phase because of the lack of N element and the relatively high Si content. It is also indicated by EDS mapping that there is almost no Si element in the upper side of the JP, which confirms that H2pdca-PDMS is below the IA-Chol part and forms the pad. The formation of this unique “particle-on-pad” morphology can be attributed to the low surface energy of PDMS. The wrinkle formation is related to the swelling of H2pdca-PDMS with a small amount of DCM. DCM is known as a solvent that can highly swell PDMS and is difficult to be completely removed from PDMS.54 When the H2pdca-PDMS part of the JPs touches the surface, it spreads on the surface and the precursor film formed by the spreading is gradually dried in air. DCM evaporates faster on the side exposed to air, which shrinks much faster than the bottom of the film, where the evaporation is slow and shrinkage is restricted by the surface.

It is this

unbalanced shrinkage of the precursor film that results in the wrinkles. For comparison, the strawberry-like particles were deposited on the substrates, dried and characterized by the same procedure mentioned above. Figure 5(a) shows the OM image of the strawberry-like particle on the substrate. The brownish red patches are formed from the azo molecular glass and the transparent matrix is the H2pdca-PDMS phase. The strawberry-like structure is also confirmed by the SEM observation (Figure 5(b), 5(c)). The EDS spectra obtained from the two positions of the particle show the different compositions (Fig. 5(d)). The existence of N element in the patch indicates that it is the IA-Chol phase (from −N=N−), which


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is also fully consistent with the OM observation. EDS mapping was employed to further identify the components of the two parts of the particle (Figure 6). The EDS mapping verifies that the patches are the IA-Chol phase with the N element and the body of the particle is made of H2pdca-PDMS because of the lack of the N element and the existence of the Si element. Distinct from the JPs, the strawberry-like particles can stand still on the substrates without obvious deformation and spreading, which is caused by the reasons discussed below. 3.3. Structure Formation Mechanism As discussed above, the Janus and strawberry particles were formed in the DCM droplets containing IA-Chol and H2pdca-PDMS dispersed in water, which was gradually induced by the solvent evaporation. In the process, IA-Chol first undergoes the binodal phase separation to form the concentrated and dilute phases, owing to its relatively poor solubility in DCM. Such understanding has been well recorded in the polymer physics literature.51 The development of the submicron-scale morphology depends on the incompatibility of the two components and the interfacial tensions of the components with water.18,20 PVA as the surfactant can reduce the differences between the interfacial tensions of both IA-Chol and H2pdca-PDMS with water, which restricts the formation of a core-shell structure. Owing to the low boiling point of DCM, the DCM evaporation was accomplished at the low temperature (5 °C) to enhance the complete segregation of the two components. In order to visibly monitor the process, after the droplets were dispersed in water at 5 °C for 48 h, the dispersion was carefully transferred from the container onto the glass slide for OM observation. Figure 7 shows the microscopic images of the droplets observed in a real-time manner. Although the DCM evaporation is greatly accelerated at the room temperature and the phase separation is not completed, the structure formation process can still be clearly seen. Since IA-Chol phase is brightly coloured, the phase separation


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was monitored by the optical microscope. As shown in Figure 7(a1)-(a7), in the first stage, the IA-Chol concentrated phase gradually separates from the H2pdca-PDMS rich phase in the single droplet. Then, in the second stage, the isolated IA-Chol domains gradually coalesce into a single domain. Phase segregation between both components is enforced by the evaporation of the DCM, JPs are finally formed to have the two fully segregated parts in the droplet (Figure 7(a8)). On the other hand, for the system contains EG, the coalescence of the isolated domains cannot be observed after the first stage segregation, i.e., the IA-Chol concentrated phase forms the patches on the droplet surface (Figure 7(b1)-(b7)). With the evaporation of the DCM, the strawberry-like particles are shaped and finally obtained (Figure 7(b8)). It indicates that although both processes start from the segregation occurring in the dispersed droplets stabilized by PVA, the morphology of the final particles depends on whether EG exists in the systems or not. The above observations can be rationalized by considering the compatibility of PDMS with the organic solvents and multiple diffusion processes. The compatibility of PDMS with the organic solvents has been thoroughly investigated by swelling experiments, which was correlated with Hildebrand solubility parameter (δ) through Hildebrand-Scatchard equation.54,60 According to Hildebrand-Scatchard equation, the compatibility between two components is maximal when their δ values are similar. As the δ value of DCM is 10.01 (cal/cm3)1/2, close to that of PDMS (7.3 (cal/cm3)1/2), it can swell PDMS very effectively. On the other hand, EG has the δ value of 14.6 (cal/cm3)1/2, which is known to be a solvent swelling PDMS the least.54 EG was chosen in the current study because of this property and its compatibility with DCM. The diffusion of EG molecules from water to the dispersed droplets will cause the deswelling of PDMS at the interfaces. Once they diffuse into a droplet form the dispersion medium, the solvent quality becomes poor for the PDMS chains, which causes their collapse and deswelling in the surface


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area. Due to the surface coagulation, the coalescence of the isolated IA-Chol domains is jammed in the shell region. For the system without EG, sufficient diffusion time and slow evaporation of DCM can cause the completed phase separation in the droplets. PVA plays an important role in reducing the surface free energy at the interface of the droplets with water and also allowing EG molecules to penetrate the interface. Due to incompatibility between the two components, JPs composed of IA-Chol and H2pdca-PDMS are obtained after the complete phase separation. On the other hand, for the strawberry-like particle formation, the diffusion of EG molecules from the water through the interface initiates coagulation at the surface of the droplets, which blocks the further coalescence of these small IA-Chol domains. Above results also show that after placed on the substrates and carefully dried, the JPs and strawberry-like particles show the different spreading behaviour and deformation degrees under gravity. To understand this difference, the particles of neat H2pdca-PDMS were prepared by the same procedure in the dispersions with or without the EG. Figure S10 (in the Supporting Information) shows the SEM images of the two types of dried particles on the silicon wafers. It can be seen that both types of the H2pdca-PDMS particles, obtained from the dispersions with or without EG, all show the spreading coronae around the particles. Nevertheless, with the EG participation, the H2pdca-PDMS particles show a smaller size corona around the particle due to the deswelling effect of the EG. It means that although EG has the effect to enhance the surface rigidity of H2pdca-PDMS, whose spreading on the surface cannot be fully impeded. It means that the IA-Chol patches on the particle surface also have the effect to impede the spreading of H2pdca-PDMS phase on the substrate surface. Because more likely the IA-Chol patches instead of H2pdca-PDMS matrix touch the surface directly, which blocks the routes for H2pdca-PDMS phase to spread, the strawberry-like structure can exhibit the “standstill” morphology.


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3.4. Photoinduced Deformation Behaviour The dispersions prepared by the above method were dropped on the silicon wafers and carefully dried to obtain the samples in the solid state. The photoinduced deformation was induced by irradiating the particles with a linearly polarized laser beam (488 nm, 200 mW/cm2), which was parallel with the optical bench surface and incident perpendicular to the substrate surface. Figure 8(a) and 8(b) give the top-view SEM images of typical JPs after the light irradiation for 2 and 3 h. For the JPs with “particle-on-pad” morphology, the IA-Chol part is significantly elongated along the light polarization direction, and the deformation degree increases with increasing irradiation time. Figure 8(c)-8(f) show EDS mapping images of the deformed JPs after the irradiation for 2 h, it further confirms that the IA-Chol part of the particle shows the photoinduced elongation along the polarization direction of the laser beam, whereas the H2pdca-PDMS part remains unchanged.

Such photoinduced deformation behaviour is

similar to those observed on the azo polymer microspheres and JPs.48,50 On the other hand, for the strawberry-like particles, only the IA-Chol patches are elongated along the light polarization direction as shown by the SEM images (Figure 9(a)-9(c)).

EDS mapping images of the

strawberry-like particle after the light irradiation for 2 h are given in Figure 9(d)-9(h). It further confirms that only the patches composed of the IA-Chol are deformed by the light irradiation, where the elongation is parallel to the light polarization direction. Figure 10 gives the side-view SEM images of the deformed particles with the view angle of 55°. For the JPs (Figure 10(a) and 10(b)), it can also be seen that the IA-Chol part is deformed along the light polarization direction. By comparing the images after the irradiation for 0.5 and 2 h, it can be concluded that the deformation degree increases with the irradiation time. Figure 10(c) shows the strawberry-like particle before the light irradiation as a control, and 10(d) and


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10(e) give the side-view SEM images of the particles after the light irradiation for 2 h, viewed from the two directions relative to the polarization. It can be seen that the IA-Chol patches are elongated along the light polarization direction in the illuminated area. To our knowledge, the strawberry-like particles with photodeformable function has not been reported in literature yet. Such unique substances, both Janus and strawberry-like particles, are valuable to be further explored to understand the nature of the photoinduced mass transport behaviour in the future.

4. Conclusions We present the one-step method to controllably fabricate Janus and strawberry-like particles composed of azo molecular glass and polydimethylsiloxane (PDMS) oligomer, which provides a versatile route and new platform for the development of structured and shape-adaptable particles. Photoresponsive particles composed of isosorbide-based azo molecular glass (IA-Chol) and PDMS oligomer (H2pdca-PDMS) were successfully developed by the method.

The phase

separation between IA-Chol and H2pdca-PDMS was induced by the slow evaporation of DCM from the dispersed droplets that were stabilized by PVA in water. Without or with the ethylene glycol (EG) existence in the dispersion, JPs or strawberry-like particles are formed after the segregation developed in the confined volume. EG plays the key role of deswelling by diffusion through the interface between the dispersed phase and the dispersion medium. The dried JPs and strawberry-like particles exhibit “particle-on-pad” and “standstill” morphologies on the substrates. Upon the irradiation with a linearly polarized laser beam at 488 nm, the IA-Chol parts of the particles in the solid state show the deformation along the light polarization direction. Owing to the different structures of JPs and strawberry-like particles, the photoinduced deformation results in the two types of unique morphologies after the irradiation. To our knowledge, the JPs and strawberry-like particles containing azo molecular glass as one


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component has not been reported in the literature yet.

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Especially, the photodeformable

strawberry-like particles belong to a new type of the colloidal particles. The study also proves that the PDMS oligomer is the valuable component to fabricate structured and functional particles. These results can shed the new light on the understanding of the structure formation process in the confined volume for fabricating functional particles with controlled morphology.


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ASSOCIATED CONTENT Supporting Information It includes more characterization results of the materials and the scanning electron microscope (SEM) images. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by NSFC under Project 51233002.

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Captions of Figures Figure 1. (a) Schematic illustration of the formation of the Janus particles and strawberry-like particles by the phase separation in the dispersed droplets with or without the ethylene glycol participation. (b) Chemical structure of H2pdca-PDMS and IA-Chol. Figure 2. Optical microscopic images of the particles in water dispersion, (a) IA-Chol/H2pdcaPDMS Janus particles, (b) IA-Chol/H2pdca-PDMS strawberry-like particles. Figure 3. Typical microscopic images and EDS spectra of IA-Chol/H2pdca-PDMS Janus particles: (a) SEM image (inset: a magnified SEM image of one particle), (b) Optical microscopic image, (c) magnified SEM image (marking the areas of the EDS analysis with the crosses), (d) EDS spectra. Figure 4. (a) EDS mapping of a typical IA-Chol/H2pdca-PDMS Janus particle, (b) C element (light purple), (c) O element (indigo), (d) N element (green), and (e) Si element (pink). Figure 5. Typical microscopic images and EDS spectra of IA-Chol/H2pdca-PDMS strawberrylike particles: (a) Optical microscopic image with the SEM image as the inset, (b) SEM image, (c) magnified SEM image (marking the areas of the EDS analysis with the crosses), (d) EDS spectra. Figure 6. (a) EDS mapping of a typical IA-Chol/H2pdca-PDMS strawberry-like particle, (b) C element (light purple), (c) O element (indigo), (d) N element (green), and (e) Si element (pink). Figure 7. Real-time optical microscopic images, (a1-a8) the Janus particle formation observed at different time, a1: 1 s, a2: 11 s, a3: 22 s, a4: 48 s, a5: 67 s, a6: 92 s, a7: 103 s, a8: the steady state; (b1-b8) the strawberry-like particle formation observed at different time. b1: 1 s, b2: 12 s, b3: 13 s, b4: 18 s, b5: 23 s, b6: 37 s, b7: 49 s, b8: the steady state.


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Figure 8. (a) Top-view SEM image of a typical Janus particle irradiated with the p-polarized light for 2 h, (b) Top-view SEM image of a typical Janus particle irradiated with the ppolarized light for 3 h, (c) EDS mapping for C element (light purple); (d) EDS mapping for O element (indigo); (e) EDS mapping for N element (green), and (f) EDS mapping for Si element (pink). The light intensity was 200 mW/cm2 and the doublesided arrows indicate the polarization direction of the laser beam. Figure 9. (a-c) Top-view SEM images of the strawberry-like particles irradiated with the ppolarized light for 2 h, (d) EDS mapping of a typical strawberry-like particle irradiated with p-polarized light for 2 h, (e) C element (light purple); (f) O element (indigo), (g) N element (green), and (h) Si element (pink). The light intensity was 200 mW/cm2 and the double-sided arrows indicate the polarization direction of the laser beam. Figure 10. Side-view SEM images (view angle = 55°), (a) Janus particles irradiated with the ppolarized light for 30 min, (b) Janus particle irradiated with the p-polarized light for 2 h, (c) strawberry-like particle before the light irradiation, (d, e) strawberry-like particle irradiated with the p-polarized light for 2 h, viewed from two directions relative to the light polarization. The light intensity was 200 mW/cm2 and the double-sided arrows indicate the polarization direction of the laser beam.


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Figure 1. (Hsu et al.)


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For Table of Contents Use Only

Janus and Strawberry-like Particles from Azo Molecular Glass and Polydimethylsiloxane Oligomer Chongen Hsu, Yi Du, Xiaogong Wang* Department of Chemical Engineering, Laboratory of Advanced Materials (MOE) Tsinghua University, Beijing, 100084, P. R. China

Janus and strawberry-like particles are fabricated through microphase-separation in the dispersed droplets. Without or with the ethylene glycol (EG) in the dispersion, Janus or strawberry-like particles are formed after the segregation developed in the confined volume. The particles show unique morphologies and deformations upon the polarized light irradiation.


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Janus and Strawberry-like Particles from Azo Molecular Glass and

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