Fabrication of ultraviolet-responsive microcapsules via Pickering emulsion polymerization using modified nano-silica/nano-titania as Pickering agents

Kunlin Chen and Shuxue Zhou*
Department of Materials Science and State Key Laboratory of Molecular Engineering of Polymers, Advanced Coating Research Center of Ministry of Education of China, Fudan University, Shanghai 200433, P.R. China. E-mail: zhoushuxue@fudan.edu.cn

Received 12th December 2014 , Accepted 22nd January 2015

First published on 22nd January 2015


Abstract

Fluoroalkyl silane (FAS)-loaded polystyrene microcapsules were prepared via Pickering emulsion polymerization using silica/titania nanoparticles as Pickering agents, wherein the nanoparticles were first modified with a Trixton X-100-tethered silane coupling agent. The microcapsules display rugged surfaces and have average sizes from 150 nm to 890 nm, depending on the nanoparticles-to-oil mass ratio. Under UV exposure, the microcapsules break up and release the encapsulated FAS, demonstrating their UV-responsive ability. The microcapsules were further embedded into waterborne coatings. Monitoring the evolution of the surface wettability of the coatings under UV-exposure indicates that the releasing rate of FAS could be tunable using the content of TiO2 nanoparticles in the microcapsules.


Introduction

Stimuli responsive capsules have received increasing interest in versatile applications such as self-healing materials,1–5 drug delivery,6–9 food and cosmetics.10,11 They can release the encapsulated substances upon external stimuli including pH, temperature, light, ultrasonic treatment, alternating magnetic field, electrical field, and applied mechanical force.12 Recently, triggered release of capsule contents using light has become appealing for a number of applications. Ultraviolet (UV)- and visible-sensitive capsules are used in cosmetics and agriculture13,14 while near-infrared (NIR)-sensitive capsules are of greater interest in biological systems.15,16

Two approaches have been reported to produce the light-triggered capsules: incorporation of either photo-responsive or photo-cleavable groups, such as azobenzenes, o-nitrobenzyl, thiols and 6-nitro-veratroyloxycarbonyl, in the shell and employment of metals and metal oxide nanoparticles (or nanorods) that are capable of absorbing NIR light or UV light.17,18 For example, Caruso et al. introduced gold nanoparticles to the shell via layer-by-layer (LbL) assembly. The capsules can release the encapsulated biomaterial on demand upon irradiation with short-pulsed laser light.19 Katagiri et al. reported UV-responsive polyelectrolyte microcapsules coated with SiO2/TiO2 via the LbL colloid-templating technique and sol–gel chemistry.20 Owing to the photocatalytic activity of the TiO2 component for decomposition of polyelectrolytes, the encapsulated low-molecular-weight dyes can be released from capsules under UV-irradiation. Nevertheless, incorporation of nanoparticles to the shell of capsules via LbL is too complicated.

Conventional miniemulsion polymerization is particularly attractive for fabrication of capsules.21–23 Besides surfactant-stabilized emulsion, Pickering emulsion is adopted in capsule preparation.24,25 The Pickering emulsion has the following advantages compared with the surfactant-stabilized emulsions: (i) excellent droplet stability due to the irreversible adsorption of modified nanoparticles at the oil/water interface,26–29 (ii) less foam and promising in practical applications,14 (iii) hierarchical structure, and (iv) combination of the properties of both organic and inorganic components.30–32 So far, stimuli responsive capsules via Pickering emulsions templates are seldom reported. Fielding et al. described pH-responsive fluorescent dyes-contained microcapsules via Pickering emulsions stabilized by core/shell polymer/silica nanocomposite particles.33 Duan et al. synthesized temperature-responsive poly-(N-isopropylacrylamide) (PNIPAM)/silica composite microcapsules via inverse Pickering suspension polymerization.34 Similarly, Zhang et al. also synthesized temperature-responsive PNIPAM/poly(methyl methacrylate)/silica hybrid capsules from inverse Pickering emulsion polymerization and the release rate of encapsulated drug could be raised by increasing the temperature.35 However, UV-responsive micro/nano-capsules synthesized by Pickering emulsion templates have not been reported.

Herein, we describe a novel and effective method for the fabrication of UV-responsive organic/inorganic hybrid microcapsules based on Pickering emulsions templates. The strategy is shown in Scheme 1. For this purpose, the SiO2 and TiO2 nanoparticles, modified with Triton X-100-tethered silane coupling agent (T-IPTS), were used together to stabilize emulsions, and meanwhile, the TiO2 nanoparticles worked as the UV-responsive component because of its photocatalytic activity under UV light. The encapsulated materials-fluoroalkyl silane (FAS) can be released from the microcapsules through the photocatalytic degradation of polymer. Further, these microcapsules were embedded into waterborne coatings and the UV-induced release of FAS was demonstrated from the evolution of the surface wettability of coatings.


image file: c4ra16275g-s1.tif
Scheme 1 (a) Synthesis of Triton X-100-IPTS; (b) schematic illustration of the fabrication of UV-responsive microcapsules based on Pickering emulsion templates.

Experimental part

Materials

Styrene (St, ≥99%), ethyleneglycol dimethacrylate (EGDMA, ≥90%), 3-isocyana-topropyltriethoxysilane (IPTS, ≥95%), t-octylphenoxypolyethoxyethanol (Triton X-100, biochemistry grade), dibutyltin dilaurate (DBTDL, ≥95%), 2,2′-azobis(2-methylpropionitrile) (AIBN, ≥99%) were purchased from Aladdin Chemical Reagent Co. (China). Ammonia solution (NH3·H2O, 25 wt%), acetic acid (≥99.7%) were purchased from Sinopharm Chemical Reagent Co. (China). St and EGDMA were distilled under vacuum before use and the other reagents were used as received. Dodecafluoroheptyl-propyl-trimethoxysilane (FAS-12, C13H18F12O3Si) was purchased from Xeogia Fluorine-Silicon Chemical Co., Ltd. (China). Silica sol (30 wt% aqueous dispersion, pH: 6–8, diameter: 20 nm) were supplied by Zhangjiagang Churen New Material Technology Co., Ltd. (China). Nano-titania dispersion (ACTiV™ S5-300B) was supplied by Cristal. Silicone emulsion (BS-45, solid content: 50 wt%) were purchased from Wacker Chemicals (Germany). Deionized water was used throughout the experiments.

Synthesis of Triton X-100-IPTS (T-IPTS)

Briefly, 6.5 g of Triton X-100, 2.5 g of IPTS and 0.03 g DBTDL were added to a 50 mL three-neck flask and then stirred mechanically at 50 °C for 20 h under nitrogen atmosphere. The formation of T-IPTS can be confirmed by the disappearance of the two absorption peaks at 3400 and 2260 cm−1 ascribed to –OH and –NCO stretching modes and the occurrence of the new peak at 3300 cm−1 assigned to the N–H stretching vibrations in their FT-IR spectra.

Preparation of UV-responsive microcapsules

Firstly, SiO2 sols were added to 50 mL of water under agitation. Afterwards a definite amount of T-IPTS was dropped to the dispersion and the pH value of the dispersion was adjusted to 8 via dropping diluted ammonia solution at room temperature, and stirred mechanically for 24 h at 65 °C. Modified TiO2 nanoparticles were prepared with the same procedure. Then the modified SiO2 and TiO2 dispersion were mixed and the pH value of the mixture was adjusted to 7 with acetic acid solution. Afterwards, the mixture was charged into a 250 mL glass beaker that contained St, EGDMA, FAS and AIBN. The mixture was emulsified using an FLUKO homogenizer at 16[thin space (1/6-em)]000 rpm for 5 min. Afterthat, the resulting emulsion was added to 250 mL three-neck flask and deoxygenated by bubbling with N2 for 30 min. The prepolymer solution was then heated to 70 °C and stirred at that temperature for 24 h. The capsules were separated by centrifugation at 10[thin space (1/6-em)]000 rpm for 10 min and then rinsed with water three times. Subsequently, the capsules were dispersed in aqueous solution. Table 1 summarizes the formulations for the preparation of the UV-responsive microcapsules.
Table 1 The formulations for the preparation of the UV-responsive microcapsulesa
Run no Water phase Oil phase
T-IPTS (g) SiO2 sol (g) S5-300B (g) St (g) EGDMA (g) FAS (g) AIBN (g) mp/mob
a H2O 100 mL and pH = 7 are used in all formulations.b mp/mo: nanoparticles-to-oil mass ratio.
1 0 2.37 0.82 2 0.04 0.6 0.06 0.30
2 0.02 0.77 0.29 2 0.04 0.6 0.06 0.10
3 0.03 1.60 0.53 2 0.04 0.6 0.06 0.20
4 0.05 2.60 0.41 2 0.04 0.6 0.06 0.30
5 0.05 2.37 0.82 2 0.04 0.6 0.06 0.30
6 0.05 2.83 0 2 0.04 0.6 0.06 0.30
7 0.05 0 0 2 0.04 0.6 0.06


Preparation of coatings filled with UV-responsive microcapsules

In order to verify the UV-releasing properties, UV-responsive microcapsules were embedded into waterborne coatings. Typically, 3.5 g of BS-45 silicone emulsion and 7.5 g of the UV-responsive capsules aqueous dispersion (solid content: 20 wt%) were mixed at 500 rpm for 5 min. For the sake of comparison, a control coating was also fabricated with microcapsules (Run 6). The coatings were cast on aluminium plate and dried at 80 °C for 15 min. The content of microcapsules in the dried coatings was about 46 wt%.

Characterization

Fourier transform infrared spectroscopy (FT-IR) measurements were carried out with a Nicolet Nexus 470 spectrometer (ThermoFisher, USA), in the wavenumber range of 400–4000 cm−1, with a resolution of 0.5 cm−1 and an accumulation of 32 scans. The modified nanoparticles were sequently washed with water and ethanol and subsequently dried under vacuum at 60 °C for 10 h. The dried particles were blended with KBr to form sample plates.

The morphology of Pickering emulsion was characterized using HIROX digital microscope KH-7700 while the morphology of the microcapsules was observed using a scanning electron microscope (SEM Philips XL 30) at an accelerating voltage of 30 kV and a transmission electron microscope (TEM Hitachi H-800, Hitachi Corp, Japan) at an accelerating voltage of 10 kV. Elemental analysis was accomplished by energy dispersive spectroscope (EDS) attached to SEM. The size of microcapsules was determined by dynamic light scattering (DLS) method using Nano-ZS90 (Malvern).

Water contact angle (WCA) was determined with an OCA15 contact angle analyzer (Data-physics, Germany), using a 5 μL deionized water droplet. Average value from more than five parallel measurements on different sites of the same coatings was adopted. The three-phase contact angle (θ) of particles was measured with the same contact angle analyzer according to the following procedure: the dried nanoparticles were compressed into a circular disk and then placed at the bottom of an open, transparent quartz vessel. Afterwards, St and FAS (FAS/St = 0.3 wt/wt) was poured into the vessel. A 5 μL deionized water droplet was dropped on the particle flake and immediately photographed. The contact angles were then measured. Measurements were averaged from at least five droplets.36,37

UV irradiation was carried out using a 150 W high-pressure mercury lamp as the irradiation source (365 nm, 20 mW cm−2). The surface composition of coating was measured by X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI 5000C ECSA) using Al K radiation at a 90° take-off angle. All the binding energy values were calibrated using the reference peak of C1s at 284.6 eV.

Results and discussion

Pickering emulsions stabilized by modified silica/titania nanoparticles

Surface wettability and weak flocculation of nanoparticles are thought to be crucial in stabilizing oil/water Pickering emulsions.27,38,39 In order to improve the affinity of nanoparticles to the oil droplets, T-IPTS was used to modify the surface of SiO2 and TiO2 nanoparticles. Fig. 1 presents the FT-IR spectra of the modified SiO2 and TiO2 nanoparticles. The absorption bands at 2856 cm−1 and 2927 cm−1 due to the methyl groups were clearly observed, suggesting the successful modification of SiO2 and TiO2 with T-IPTS. The three-phase contact angle (θ) of the stabilizing particles at the water/oil interface was the most important parameter and when θ was around 90° the particles preferred to reside at the interface which was favourable for the stability of a Pickering emulsion.40,41 The T-IPTS modified nanoparticles/water/oil three-phase contact angle was therefore determined, as shown in Fig. 2. A contact angle of 87.8° was achieved for the modified nanoparticles. In contrast, a three-phase contact angle of 38.3° was shown for the unmodified nanoparticles. This fact demonstrates that T-IPTS modification can efficiently enhance the affinity of nanoparticles to oil droplets.
image file: c4ra16275g-f1.tif
Fig. 1 FT-IR spectra of modified SiO2 and TiO2 nanoparticles.

image file: c4ra16275g-f2.tif
Fig. 2 Photographs of a 5 μL water droplet and contact angle on the surface of a thin film of pure (a) and modified (b) nanoparticles (TiO2/SiO2: 1/5 wt/wt) at the oil/water interface, respectively.

To demonstrate the stabilizing effect of the modified nanoparticles, the storage stability of the emulsion within three days was examined and compared with those cases using unmodified nanoparticles, T-IPTS, or the mixture of T-IPTS/nanoparticles as the stabilizing agents. Fig. 3 shows the macroscopic view of the emulsions. For easy observation, 0.3 g of 0.1 wt% aqueous solution (pH 8) of phenol red was added to the water phase. It can be seen that the emulsions based on modified nanoparticles was uniform and had no obvious stratification phenomenon. However, complete phase separation took place for the emulsion stabilized by unmodified nanoparticles. This phenomenon was consistent with their three-phase contact angles. It suggests that T-IPTS modification is necessary for SiO2 and TiO2 nanoparticles acting as Pickering agents. Fig. 3 shows that T-IPTS has a stabilizing effect on the emulsion. Nevertheless, an oil layer was observed on the top of the emulsion. This indicates that T-IPTS is less efficient on stabilization of oil droplets because T-IPTS has low steric hindrance than T-IPTS modified nanoparticles. Similar phase separation was observed in the emulsion stabilized by the mixture of T-IPTS/unmodified nanoparticles. It implies that the unmodified nanoparticles are difficult to adhere to the oil droplets even though the interface of oil droplets was occupied with T-IPTS in advance.


image file: c4ra16275g-f3.tif
Fig. 3 Digital photographs of emulsions stabilized with (a) T-IPTS modified nanoparticles, i.e. Run 2, (b) unmodified nanoparticles, i.e. Run 1, (c) T-IPTS, i.e. Run 7, and (d) the mixture of T-IPTS and unmodified nanoparticles after three days' storage at room temperature.

Besides, the stabilizing effect of T-IPTS modified nanoparticles is revealed from the particle size of the emulsified droplets, as shown in Fig. 4. It is evident that the emulsions stabilized by modified nanoparticles have much smaller and more homogeneous sizes comparing with the droplet size of the emulsions stabilized by unmodified nanoparticles, pure T-IPTS, or their mixture. All these facts indicate that the T-IPTS modified nanoparticles work well as the Pickering agent.


image file: c4ra16275g-f4.tif
Fig. 4 Optical microscopic images of emulsions stabilized with (a) T-IPTS modified nanoparticles, i.e. Run 2, (b) unmodified nanoparticles, i.e. Run 1, (c) T-IPTS, i.e. Run 7, and (d) the mixture of T-IPTS and unmodified nanoparticles. Pictures were taken 10 minutes after preparation.

Morphology of UV-responsive microcapsules

The morphologies of UV-responsive microcapsules are shown in Fig. 5. It can be seen that the capsules prepared at mp/mo ratios of 0.1, 0.2, and 0.3 have diameters approximately in the range of 100–200, 400–800, and 800–1500 nm, respectively. The rugged surface morphologies (the insets in Fig. 5a–c) clearly indicate that the capsules are covered with a dense layer of nanoparticles. These morphologies are also demonstrated in the TEM images (Fig. 5d–f). However, core/shell structure is not revealed from TEM images, suggesting that FAS-12 molecules are intended to be soluble in the polystyrene matrix rather than to form a distinctive oil core. EDS spectrum (Fig. 6) of the capsule (Run 3) shows that besides C atom O, Si and Ti atoms are existed. It further proves the coverage of the microcapsules with SiO2 and TiO2 nanoparticles.
image file: c4ra16275g-f5.tif
Fig. 5 SEM and TEM micrographs of UV-responsive microcapsules prepared at different mp/mo ratios. (a and d) Run 2, (b and e) Run 3, (c and f) Run 4.

image file: c4ra16275g-f6.tif
Fig. 6 EDS spectrum of UV-responsive microcapsules (Run 3).

The particle size and its distribution by DLS analysis are shown in Fig. 7. With the increasing of mp/mo ratio, the average diameter of the microcapsules decreased from 890 nm to a minimum of around 150 nm and meanwhile the size distribution became narrow. The results agree with the SEM images well. Therefore, the size of UV-responsive microcapsules is adjustable within a certain range by changing the mp/mo ratio.


image file: c4ra16275g-f7.tif
Fig. 7 Size distributions of UV-responsive microcapsules at different mp/mo ratios measured by DLS.

UV-induced release property of microcapsules in water

The UV-induced release of the encapsulated FAS-12 was first examined in water. The experiments were carried out as follows: the as-obtained microcapsules (Run 4 and 6) were separated by centrifugation at 10[thin space (1/6-em)]000 rpm for 10 min and then rinsed with water three times. Afterthat, the microcapsules were dispersed in aqueous solution and exposed to the UV-light for 5 h. The morphologies of the microcapsules after UV irradiation were observed, as illustrated in Fig. 8. It was found that the microcapsules (Run 4) completely broke up after UV irradiation. Interestingly, when the destroyed microcapsules were dried, the surface of dry microcapsules (Fig. 9b) displayed hydrophobic character and the WCA was as high as 132°. However, the surface of the control sample (Run 6) was hydrophilic (Fig. 9c). This suggested that FAS-12 had been released from Run 4 microcapsules. The dried microcapsules were washed using ethanol for three times and the extracted ethanol was characterized by the FT-IR. As shown in Fig. 9d, a peak at 1205 cm−1 assigned to C–F stretching vibration is revealed in the spectrum of Run 4 microcapsules. However, no peak is observed at this wavelength for the control Run 6 sample. The FT-IR results further confirmed the UV-induced release of FAS-12 from Run 4 microcapsules but not from Run 6 microcapsules. Since Run 4 microcapsules are different from Run 6 microcapsules only in the existence of TiO2 nanoparticles. The UV-induced release of Run 4 capsules should be attributed to the photocatalytic activity of the TiO2 component to decompose polystyrene (PS), causing the microcapsules to rupture.
image file: c4ra16275g-f8.tif
Fig. 8 (a) SEM and (b) TEM images of microcapsules (Run 4) after UV irradiation (20 mW cm−2, 300 min).

image file: c4ra16275g-f9.tif
Fig. 9 (a) Schematic representation of the release mechanism in UV-responsive microcapsules; (b) dry capsules after UV-irradiation for 5 h (left: Run 6; right: Run 4); (c) the water droplets on the surfaces of dry microcapsules (left: Run 6; right: Run 4); (d) FT-IR spectra of FAS-12 and the extracted ethanol of the microcapsules (Run 4 and 6).

Besides UV-induced release of FAS-12, diffusion-induced release of FAS-12 from the dry capsules is thermodynamically possible. To inspect this release mechanism, the dry Run 4 microcapsules were heated at 100 °C for 2 h. The surface wettability of the microcapsules did not change, suggesting no leakage of FAS-12. However, if the heating temperature was raised to 140 °C, the microcapsules transformed from superhydrophilic character to hydrophobic character (WCA: 136°). It indicated that diffusion-induced release of FAS-12 was appreciable only at high temperature. Since the microcapsules are used at room temperature (far from 140 °C), the leakage of FAS-12 should be negligible, being favourable for their storage in dry powder state. The undetected diffusion-induced release of FAS-12 at low temperature should be due to the low diffusion coefficient of FAS-12 because of the dense structure of the crosslinked PS matrix. The other reason may lie in the low solubility of FAS-12 in the crosslinked polystyrene matrix, which was primarily demonstrated by immersing the crosslinked PS spheres into FAS-12.

Release properties of UV-responsive microcapsules in coatings

The as-obtained microcapsules (Run 4–6) were directly embedded into waterborne coatings to examine its' release property under UV-irradiation. Fig. 10a and b typically show the top view and the cross-section SEM images of the coating containing Run 4 microcapsules. It can be seen that UV-responsive microcapsules can be dispersed in coatings without aggregation. When the coatings were irradiated with UV light, the WCA of coatings containing Run 4 or Run 5 microcapsules gradually increased and finally converted into hydrophobicity (85° → 116°). In contrast, there were no changes of WCA for the control coatings with Run 6 microcapsules, reflecting virtually no FAS-12 release after irradiation (Fig. 11a). Moreover, the Run 5 microcapsules-embedded coating is faster to reach the plateau of WCA than the Run 4 microcapsules-embedded coating, suggesting that the releasing rate of FAS-12 is controllable through varying the TiO2 content in the microcapsules. Further, the atomic composition of surfaces as a function of UV irradiation time was monitored by XPS analysis, as displayed in Fig. 11b. The F atomic concentrations gradually increased as UV-irradiation went on, directly evidencing that FAS-12 slowly enriched at the surface of the coatings. In addition, the O/C atomic ratio considerably increased from 0.4 to 1.35 after 8 h of UV irradiation, being owed to the photo-catalyzed degradation of polystyrene in the presence of TiO2 nanoparticles. To further understand the releasing mechanism of FAS-12 in the coating, the coating containing Run 4 microcapsules was placed into a 100 °C oven for 24 h. The WCA of surface did not change within the thermal treatment period (Fig. 12b). Hence, it can be deduced that the variation of wettability of coatings should be attributed to the UV-induced rupture of the microcapsules and subsequent release of FAS-12, not to the diffusion of FAS-12 from the microcapsules.
image file: c4ra16275g-f10.tif
Fig. 10 SEM images of (a) the surface and (b) the cross section of the coating containing UV-responsive microcapsules (Run 4).

image file: c4ra16275g-f11.tif
Fig. 11 (a) Changes of WCA for the coatings containing UV-responsive microcapsules along with UV irradiation time; (b) evolution of surface atomic composition of the coating containing Run 4 microcapsules with UV irradiation time, determined by XPS analysis.

image file: c4ra16275g-f12.tif
Fig. 12 The WCA of surface of the coating contained microcapsules (Run 4) at 100 °C for 24 h.

Conclusions

UV-responsive FAS-loaded microcapsules have been prepared via O/W Pickering emulsion templates using T-IPTS modified SiO2 and TiO2 nanoparticles as Pickering agents. The size of the microcapsules can be adjusted by the nanoparticle-to-oil mass ratio. The encapsulated hydrophobic molecule, FAS-12, can be released under UV irradiation even the capsules are embedded into coatings. Moreover, the releasing rate is tunable, depending on the content of TiO2 nanoparticles. The release of FAS-12 is caused by the TiO2-catalyzed photodegradation of the microcapsules. These microcapsules are convinced to be useful to prolong the hydrophobic performance of the coatings especially those used outdoors, such as waterproof, anti-icing and anti-flashing coatings.

Acknowledgements

This work was supported by Specialized Research Fund for the Doctoral Program of Higher Education (20130071110002) and Nature Science Foundation of China (Grant no. 51133001).

Notes and references

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