Facile Fabrication of Polymer Nanocapsules with Cross-Linked

Article pubs.acs.org/Langmuir

Facile Fabrication of Polymer Nanocapsules with Cross-Linked Organic−Inorganic Hybrid Walls Tianyou Chen, Binyang Du,* and Zhiqiang Fan MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: A facile method was developed for the fabrication of polymer nanocapsules with organic−inorganic hybrid walls and controllable morphologies from a crosslinkable polymer, poly[3-(trimethoxysilyl)propyl methacrylate] (PTMSPMA). With the combination of emulsion, hydrolysis, and condensation reaction as well as the internal phase separation, cross-linked PTMSPMA nanocapsules with classic hollow structures, collapsed hollow structures with Kippah, and multi-fold morphologies could be successfully obtained by simply mixing the toluene solution of PTMSPMA with water under vigorous stirring for 48 h at different temperatures. The hydrolysis and condensation of methoxysilyl groups resulted in the phase separation of PTMSPMA inside the toluene droplets and the migration of PTMSPMA to the interface of toluene and water. The cross-linking reaction of methoxysilyl groups further fixed the interfacial phase of PTMSPMA, leading the formation of PTMSPMA nanocapsules with robust cross-linked organic−inorganic hybrid walls. Such nanocapsules with robust cross-linking structures may find potential applications for the encapsulations of many functional species.

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capsules with solid cores as templates.19,20 The solid cores needed to be removed after the formation of capsule layers in order to form the hollow structures. Lipid bilayers were employed as templates of nanocapsules, and the polymerization was performed within the hydrophobic interior of lipid bilayers to form cross-linked polymer nanocapsules.21−23 Recently, microemulsion polymerization was successfully applied for fabricating cross-linked polymeric nanocapsules.11,24 However, it was necessary to remove surfactants (such as OTAB, CTAB, and DTAB) for further applications. Hence, developing a facile and straightforward method for the fabrication of robust and cross-linked polymer capsules is still a challenging task. Herein, a facile method was developed for the fabrication of polymer nanocapsules with organic−inorganic hybrid walls and controllable morphologies from a cross-linkable polymer, poly[3-(trimethoxysilyl)propyl methacrylate] (PTMSPMA). PTMSPMA (Scheme 1) is a hydrophobic polymer and has been widely used for the preparation of organic−inorganic hybrid materials such as nanowires,25 nanotubes,26 capsules,13 and the like27−30 by utilizing the hydrolysis and condensation of methoxysilyl groups. In the present work, a portion of toluene solution of PTMSPMA was first mixed with deionized water under vigorous magnetic stirring to form an oil-in-water emulsion. PTMSPMA was thus confined inside the toluene

INTRODUCTION Polymer capsules with micro- or nanosizes possess large hollow spaces, which could be used for the encapsulation and release of various kinds of species. Polymer capsules have been widely studied for their potential applications in the fields of drug delivery,1 cancer therapy,2 protecting enzymes,3 and catalysis.4 In recent decades, many efforts have been devoted to the design and fabrication of polymer micro- or nanocapsules with controllable size, mechanical strength, permeability, and biocompatibility. Various types of synthetic methods have been developed for the preparation of polymer micro- or nanocapsules, including double emulsions,5−7 polymer precipitation by phase separation,8 layer-by-layer assembly,3,9,10 microemulsion polymerization,11 polymer growth by surface polymerization,12 vesicles assembled by amphiphilic copolymers,13,14 among others. However, most of those polymer micro- or nanocapsules were not fixed by chemical cross-linking and were easy to deform or decompose under environmental change. In order to preserve the structural integrity of polymer capsules under varying conditions, cross-linking was the key step to fabricate stable and robust capsules. Several successful cross-linking reactions, such as photodimerization,15,16 polymerization,17,18 and sol−gel reaction,13,14 were used for postcross-linking of polymer capsules. Although the structure of polymer capsules could be persisted with post-cross-linking, the preparation process became more complicated and timeconsuming. So far, a few methods were developed to prepare cross-linked polymer capsules during the formation of the © 2012 American Chemical Society

Received: May 7, 2012 Revised: June 27, 2012 Published: July 3, 2012 11225

dx.doi.org/10.1021/la301872q | Langmuir 2012, 28, 11225−11231

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Scheme 1. Schematic of Free Radical Polymerization of TMSPMA and the Hydrolysis and Condensation Reactions of PTMSPMA

Instrumentation and Characterization. 1H NMR of the polymer PTMSPMA was performed on a 300 MHz Varian Mercury Plus NMR instrument with CDCl3 as solvent. Molecular weight and molecular weight distribution of the polymer PTMSPMA were determined by using a gel permeation chromatography (GPC, PLGPC 220, Polymer Laboratories Ltd.) with tetrahydrofuran as the eluent and monodisperse polystyrene as the calibration. Fourier transform infrared (FT-IR) spectra were recorded by a Vector 22 Bruker spectrometer in the transmission mode. Dynamic light scattering (DLS) measurements were performed on a 90 Plus particle size analyzer (Brookhaven Instrument Corporation) at a scattering angle θ of 90°. Each sample was measured for five runs with each run of 1 min. Multimodal size distribution (MSD) method was used to analyze the average size and size distribution of each sample. For the kinetic study of the formation process of PTMSPMA nanocapsules, 100 μL reaction solutions were taken out from the reaction mixture at a certain time and diluted with deionized water to 2 mL for DLS measurements. Transmission electron microscopy (TEM) measurements were performed on a JEOL JEM-1200 electron microscope operating at an acceleration voltage of 60 kV. The TEM samples were prepared by dip-coating. Carbon-coated copper grids were immersed in the corresponding solutions for a few seconds, and the solvent on the grids was then gently absorbed away by a filter paper. The grids were allowed to dry in air at room temperature for several hours before observation. Scanning electron microscopy (SEM) measurements were performed on a Hitachi S4800 electron microscope. A droplet of the sample solution was cast onto the aluminum foil at room temperature. After about 15 min, the excess solution was gently absorbed away by a filter paper. The aluminum foils were allowed to dry in air at room temperature. The samples were coated with platinum vapors before SEM observation.

droplets. Triethylamine (TEA) was then added into the emulsion to accelerate the hydrolysis and condensation of methoxysilyl groups. After 48 h stirring, the cross-linked PTMSPMA nanocapsules were successfully obtained. A possible mechanism was proposed. The effects of temperature and additives like surfactant and other silane agents on the outcome of the hydrolysis and condensation reaction of PTMSPMA were also investigated and discussed.

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EXPERIMENTAL SECTION

Chemicals and Materials. 3-(Trimethoxysilyl)propyl methacrylate (TMSPMA, 98%) and tetraethyl orthosilicate (TEOS, 98%) were purchased from Acros Organics and used as received. Anhydrous tetrahydrofuran (THF, Sinopharm Chemical Reagent Co. Ltd.) was dried by refluxing in the presence of a sodium flake and distilled prior to use. α,α′-Azodiisobutyronitrile (AIBN, Sinopharm Chemical Reagent Co. Ltd.) was recrystallized from methanol. Sodium dodecyl sulfate (SDS, 99%) and 3-aminopropyl triethoxysilane (APTES, 98%) were purchased from J&K Chemical Ltd. and used as received. Deionized water was used throughout the experiments. All other reagents were of analytical grade and used as received. Synthesis of Poly[3-(trimethoxysilyl)propyl methacrylate] (PTMSPMA). Poly[3-(trimethoxysilyl)propyl methacrylate] (PTMSPMA) (Scheme 1) was synthesized by free radical polymerization of TMSPMA in anhydrous THF with AIBN as the initiator.28 Typically, 50 mg of AIBN and 2 mL of TMSPMA were dissolved in 20 mL of anhydrous THF in a polymerization tube with magnetic stirring. Oxygen was eliminated by bubbling nitrogen through the solution for 1 h. The polymerization tube was then placed in an oil bath thermostated at 60 °C for 24 h. Afterward, PTMSPMA was collected by precipitation with n-hexane, dried in vacuum at 30 °C for 48 h, and then stored in a vacuum desiccator for further use. The obtained PTMSPMA was colorless grease. Preparation of Cross-Linked PTMSPMA Nanocapsules. In a typical preparation of cross-linked PTMSPMA nanocapsules, 10 mg of PTMSPMA was first completely dissolved in 100 μL of toluene. The PTMSPMA toluene solution was then added into 10 mL of deionized water in a dropwise manner in 1 min under vigorous magnetic stirring, forming an oil-in-water emulsion. Afterward, 10 μL of trimethylamine (TEA) was added into the emulsion to accelerate the hydrolysis and condensation of the methoxysilyl groups of PTMSPMA. The reaction was allowed to perform at the preset temperature (i.e., 50, 25, and 2 °C) under vigorous magnetic stirring (1200 rpm) for 48 h. After the completion of the reaction, the reaction mixture was purified by centrifugation and was washed with deionized water several times. Scheme 1 shows the hydrolysis and condensation reactions of PTMSPMA in the presence of the catalyst (TEA) in aqueous solution. The hydrolysis and condensation reaction of monomer TMSPMA were also carried out at 25 °C with the same procedure described above.

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RESULTS AND DISCUSSION Linear polymer poly[3-(trimethoxysilyl)propyl methacrylate] (PTMSPMA) was successfully obtained by free radical polymerization. The molecular weights and polydispersity index (PDI, Mw/Mn) of PTMSPMA were found to be 2.04 × 104 (Mw) and 2.4, respectively, as determined by GPC. The 1H NMR spectrum of PTMSPMA was coincided with its chemical structures (Figure S1a in the Supporting Information). The characteristic signals at 3.3−3.8 ppm were assigned to the [−Si(OCH3)3] groups of PTMSPMA. The FT-IR spectrum of PTMSPMA showed characteristic vibration peaks of the Si− O−CH3 group, that is, absorption peaks at 821 and 1087 cm−1 (Figure S1b). The cross-linked PTMSPMA nanocapsules were then fabricated by simply mixing the PTMSPMA toluene 11226


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solution with deionized water and further adding the catalyst TEA under vigorous magnetic stirring. Figure 1 shows the cross-linked PTMSPMA nanocapsules with classic hollow structures obtained at 50 °C. From the

Figure 3. Morphologies of the cross-linked PTMSPMA nanocapsules obtained at 25 °C. (A) TEM image and (B) SEM image.


TEM images, the average size and wall thickness of cross-linked PTMSPMA nanocapsules obtained at 50 °C were calculated to be ∼81 ± 11 nm and 20 ± 4 nm, respectively. Interestingly, fusion events of cross-linked PTMSPMA nanocapsules prepared at 50 °C were also observed, as shown in Figure 2, which might indicate that the rates of fusion of nanocapsules and the cross-linking reaction were at a similar time scale. Fusion events were frequently observed for polymeric vesicles.18,31 Five types of fusion events could be classified in Figure 2: A shows that two nanocapsules fully fused into one nanocapsule; B shows that two nanocapsules partly fused into an elliptical nanocapsule; C shows that two nanocapsules partly fused, and the wall just disappeared; D shows that two nanocapsules contacted and the wall thickness of the contact point was the same as the nanocapsules; and E presents that two nanocapsules just contacted with each. These fusion events suggested that the walls of PTMSPMA nanocapsules were fluidlike, which can flow and fuse each other at an early stage before they were completely fixed via the cross-linking reaction at the later stage. Large PTMSPMA nanocapsules, which were formed by the fusion of nanocapsules, are clearly observed in Figure 1A. The morphology of the PTMSPMA nanocapsules could be tuned by varying the reaction temperatures. At lower temperatures of 25 and 2 °C, PTMSPMA nanocapsules with Kippah32 and multi-fold morphologies were obtained, as shown in Figures 3 and 4. From the TEM images, the average sizes of the cross-linked PTMSPMA nanocapsules obtained at 25 and 2 °C were estimated to be 119 ± 36 nm and 194 ± 81 nm, respectively. The wall thicknesses of cross-linked PTMSPMA nanocapsules obtained at 25 and 2 °C were ∼9 ± 1 and 10 ± 2 nm, respectively. It is worth noting that only nanocapsules with Kippah morphologies, which were approximately double their actual wall thickness value, were included in the calculation of the wall thickness of cross-linked PTMSPMA nanocapsules


fabricated at 25 and 2 °C. Note that controlled experiments were carried out at 25 °C by replacing PTMSPMA with monomer TMSPMA. Only irregular solid particles (Figure S2) were obtained, indicating that the long chain structure of polymer PTMSPMA might also play an important role in the formation of hollow nanocapsules. By analyzing the TEM images, the cross-linked PTMSPMA nanocapsules obtained at 50 °C were with classic hollow structures, whereas the PTMSPMA nanocapsules with Kippah and multi-fold morphologies resulted from the collapse of classic hollow structures, as summarized in Figure 5. The

Figure 5. Summarized morphologies and structures of the cross-linked PTMSPMA nanocapsules obtained in the present work. The red portion in the cross section represents the boundary of the collapsed walls of the hollow nanocapsules.

Figure 2. Fusion events of cross-linked PTMSPMA nanocapsules with classic hollow structures obtained at 50 °C. 11227


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Figure 6. (A) Statistical apparent size distributions of cross-linked PTMSPMA nanocapsules prepared at 50, 25, and 2 °C, calculated from the TEM images. (B) Hydrodynamic diameter (measured at 25 °C) of cross-linked PTMSPMA nanocapsules prepared at 50, 25, and 2 °C, measured by DLS.

Scheme 2. Proposed Mechanism of the Formation of Cross-Linked PTMSPMA Nanocapsules via Three Stagesa

a

The interface energy controlled stage I; the viscosity controlled stage II; and the elasticity controlled stage III. Process a presents the initial hydrolysis and condensation of some methoxysilyl groups, resulting in the internal phase separation within the toluene droplets. Process b presents the migration of light cross-linked PTMSPMA phase to the interfacial region of toluene droplets and water, forming the interfacial PTMSPMA shells. Process c presents the further hydrolysis and condensation of residual methoxysilyl groups and the formation of robust cross-linked PTMSPMA shells.

detailed mathematical analyses of TEM images are shown in the Supporting Information. The TEM and SEM results also suggested that the nanocapsules with small sizes (∼100 nm) were prone to collapse and form the multi-fold morphologies. The statistical apparent size distributions of cross-linked PTMSPMA nanocapsules calculated from the TEM images are shown in Figure 6A. It can be clearly seen that the PTMSPMA nanocapsules prepared at low temperature had a larger average size and size distribution than those prepared at high temperatures, which were further confirmed by dynamic light scattering (DLS) measurements (Figure 6B). The hydrodynamic diameters of cross-linked PTMSPMA nanocapsules prepared at 50, 25, and 2 °C were 95 ± 10, 176 ± 30, and 228 ± 60 nm, respectively. Recently, Atkin et al. reported the preparation of polymer microcapsules by internal phase separation, which was induced by the evaporation of a low boiling point cosolvent, and the morphology was fixed by the removal of residual solvent.33 In the present work, the PTMSPMA nanocapsules with robust cross-linked organic−inorganic walls were successfully obtained by the combination of emulsion, hydrolysis, and condensation reaction as well as the internal phase separation. The hydrolysis reaction of the methoxysilyl group is exothermic, while the condensation reaction of hydroxysilyl is endothermic.34

Increasing temperature could then effectively cause more cross-linking reactions of hydroxysilyl groups and increasing the rate of cross-linking reactions. Scheme 2 proposed the possible formation mechanism of PTMSPMA nanocapsules. In the first stage I, the oil-in-water emulsion was formed with toluene droplets containing cross-linkable polymer PTMSPMA. At this stage, the degree of cross-linking of PTMSPMA was light. The size distribution of toluene droplets was mainly determined by the interface energy. With addition of TEA, the acceleration of hydrolysis and condensation of methoxysilyl groups resulted in the internal phase separation of the PTMSPMA phase within the toluene droplets. In the second stage II, however, further hydrolysis and condensation of methoxysilyl groups led the cross-linked PTMSPMA phase migration to the interfaces of the toluene droplets and water, forming an interfacial cross-linked PTMSPMA layer. The size distribution of toluene droplets with interfacial cross-linked PTMSPMA layers was dominated by the viscosity of the crosslinked PTMSPMA phase. In the third stage III, the PTMSPMA completely located at the interfaces of toluene droplets and water. The completion of hydrolysis and condensation of residue methoxysilyl groups led to the formation of robust cross-linked PTMSPMA shells. The cross-linked PTMSPMA nanocapsules could be treated as elastic or rigid particles in this stage. 11228


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Figure 7. Evolutions of droplet (or particle) size and size distribution of the reaction mixtures at 50 °C as the function of reaction times measured by DLS. Note that the DLS measurements were performed at 50 °C. (A) Number average size and size distribution and (B) volume average size and size distribution.

Figure 8. Evolutions of droplet (or particle) size and size distribution of the reaction mixtures at 25 °C as the function of reaction times measured by DLS. Note that the DLS measurements were performed at 25 °C. (A) Number average size and size distribution and (B) volume average size and size distribution.

To test this hypothesis, DLS was used to measure the size distribution of toluene droplets or hollow structures of the reaction mixtures as a function of reaction times. The evolutions of droplet (or particle) size and size distribution of the reaction mixtures at 50 and 25 °C as the function of reaction times were followed by using DLS. Figures 7 and 8 clearly show that only particles with millimeter sizes were observed at the early stage, which could be attributed to the toluene droplets containing light cross-linked PTMSPMA. With the proceeding of reaction, the size distributions of particles were changed into nanometer scale until the completion of the reaction (48 h). The other possible reason for the size development could also be attributed to splitting larger droplets into many small ones with migration of hydrolyzed polymers. The proposed mechanism was basically verified by the DLS results. The transition points between the first stage and the second stage were 150 and 230 min for the cross-linked PTMSPMA nanocapsules prepared at 50 and 25 °C, respectively. These results also indicated that the average size and size distribution of cross-linked PTMSPMA nano-

capsules were determined in the second stage, that is, the viscosity controlled region. The above results and discussion suggested that the linear polymer PTMSPMA may behave like a surfactant after the hydrolysis of methoxysilyl groups. The hydrolyzed PTMSPMA with hydrophilic silanol groups (Si−OH) will migrate and accumulate at the interface of the toluene droplet and continuous water phase, forming the interfacial PTMSPMA shells. Therefore, one may speculate that any additive, which can alter the interfacial properties of PTMSPMA, toluene, and the water phases, will affect the final outcome of the condensation reaction. Indeed, if sodium dodecyl sulfate (SDS, 5 mg) was first dissolved in 10 mL of water, and then the toluene solution of PTMSPMA was added into the SDS aqueous solution (0.5 mg/mL) at 25 °C according to the same procedure described in Experimental Section for the fabrication of PTMSPMA nanocapsules, only cross-linked PTMSPMA solid nanoparticles were obtained after 48 h hydrolysis and condensation reaction, as shown in Figure 9. Furthermore, the surfaces of these PTMSPMA solid nanoparticles were rough 11229


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Figure 9. Morphologies of the solid cross-linked PTMSPMA nanoparticles obtained at 25 °C with the presence of sodium dodecyl sulfate (SDS). (A) TEM and (B) SEM images.

Figure 11. Morphologies of the cross-linked PTMSPMA sheets obtained at 25 °C with the presence of 3-aminopropyl triethoxysilane (APTES). (A) TEM and (B) SEM images.

and exhibited certain nanostructures. This observation was understandable. It is well-known that SDS is an anionic surfactant, which is usually used as the emulsifier for the preparation of polymer nanoparticles by emulsion polymerization. SDS will also tend to locate at the interface of the toluene droplet and water phase in order to decrease the interfacial energy of the system in the present work. With the presence of SDS, most PTMSPMA will locate inside the SDS micelles formed after mixing with deionized water, leading to the formation of solid PTMSPMA nanoparticles after the hydrolysis and condensation reaction of methoxysilyl groups. The interfacial SDS may interfere with the cross-linking reaction of PTMSPMA, resulting in rough surfaces of PTMSPMA nanoparticles. Note that SDS did not take part in the cross-linking reaction of PTMSPMA. Interestingly, if tetraethyl orthosilicate (TEOS, 50 μL) was chosen to first mix with the PTMSPMA toluene solution before adding into the SDS aqueous solution (10 mL, 0.5 mg/mL), solid nanoparticles with smooth surfaces were obtained at 25 °C, as shown in Figure 10. TEOS has four ethyloxysilyl groups and is widely

linked PTMSPMA.33 With the pure PTMSPMA, cross-linked PTMSPMA nanocapsules could be obtained.

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CONCLUSIONS The PTMSPMA nanocapsules with robust cross-linked organic−inorganic walls were successfully obtained by simply mixing the toluene solution of PTMSPMA with water under vigorous stirring. The hydrolysis and condensation of methoxysilyl groups of PTMSPMA at the interface of toluene droplets and continuous water phase led to the formation of cross-linked hollow nanocapsules. The morphologies of the cross-linked PTMSPMA nanocapsules could be controlled by varying the reaction temperatures. Such nanocapsules with robust cross-linking structures may find potential applications for the encapsulations of catalysts, dyes, and other functional species.

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ASSOCIATED CONTENT

S Supporting Information *

1 H NMR and FT-IR spectra of polymer PTMSPMA, TEM image of monomer TMSPMA after hydrolysis, and the analysis of different TEM morphologies of polymer nanocapsules. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

Figure 10. Morphologies of the solid cross-linked PTMSPMA nanoparticles obtained at 25 °C with the presence of sodium dodecyl sulfate (SDS) and tetraethyl orthosilicate (TEOS). (A) TEM and (B) SEM images.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (Nos. 20874087 and 21074114) for financial support.

used to prepare solid SiO2 nanoparticles. Therefore, TEOS will take part in the hydrolysis and condensation reaction of PTMSPMA, which might lead to the formation of smooth cross-linked PTMSPMA/SiO2 nanoparticles. Furthermore, if 3aminopropyl triethoxysilane (APTES, 5 μL) was used as the additive to first mix with the PTMSPMA toluene solution before adding into the deionized water, flat and large sized cross-linked PTMSPMA/APTES sheets were obtained, as shown in Figure 11. APTES could react with PTMSPMA and made the cross-linked PTMSPMA phase more hydrophilic. However, the mechanism for the formation of flat polymer sheets was still unclear. Nevertheless, these experimental results strongly indicated that the interfacial properties of the crosslinked PTMSPMA phase, toluene phase, and water phase determined the final structures and morphologies of the cross-

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Facile Fabrication of Polymer Nanocapsules with Cross-Linked

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