Zhiqiang Feng, Zhipeng Wang, Changyou Gao, Jiacong Shen

Transcription

1 Materials Letters 61 (2007) Hollow microcapsules with a complex polyelectrolyte shell structure fabricated by polymerization of 4-vinylpyridine in the presence of poly(sodium 4-styrenesulfonate) and silica particles Zhiqiang Feng, Zhipeng Wang, Changyou Gao, Jiacong Shen Department of Polymer Science and Engineering, Zhejiang University, and Key Laboratory of Macromolecule Synthesis and Functionalization, Ministry of Education, Hangzhou , China Received 1 June 2006; accepted 28 September 2006 Available online 16 October 2006 Abstract Hollow microcapsules having shells of polyelectrolyte complexes were fabricated by polymerization of 4-vinylpyridine hydrochloride in a solution containing surface-modified silica particles and poly(sodium 4-styrenesulfonate), and followed by a core removal with HF. Here both the PSS molecules and the silica particles function as templates to guide the alignment of VPy monomers and support the deposition of the PSS/PVPy complexes, respectively. The continuous shells with a thickness of 5 15 nm on the silica particles are created by covalent linking of the PSS/PVPy complexes. The hollow and intact microcapsules in both wet and dry states were characterized and confirmed by transmission electron microscopy, scanning electron microscopy, and confocal laser scanning microscopy. The chemical compositions of the microcapsules were determined and confirmed by elemental analysis and FTIR spectroscopy Elsevier B.V. All rights reserved. Keywords: Microcapsule; Poly(4-vinylpyridine hydrochloride); Polymers; Thin films; Template polymerization; Composite particles 1. Introduction Polyelectrolyte microcapsules are of great interest because of their special structures, unique properties and potential applications. Besides the normal polyelectrolytes, various substances, such as nanoparticles, quantum dots, dye molecules, lipids and proteins, can be incorporated into the system, either on the capsule shells or in the capsules [1]. Due to the responsivity of the polyelectrolytes to environmental stimuli, the properties of the hollow capsules can be readily tuned by ph, temperature, ionic strength and solvent quality [2]. The variation of the capsule properties is of great importance for the loading and release of desired functional materials and bioactive species such as drugs, which is an indispensable step towards their applications. Many techniques, such as crosslinking of shells of liposomes [3] or micellar diblock or triblock copolymer systems [4], Corresponding author. Tel.: ; fax: address: cygao@mail.hz.zj.cn (C. Gao). emulsion polymerization [5], mini-emulsion polymerization [6], dispersion polymerization [7] and atomic transfer radial polymerization (ATRP) [8] with the assistance of preformed colloidal particles, have been developed to fabricate hollow microcapsules with variable features. Capsules fabricated by these methods usually have single component. By contrast, layer by layer (LBL) assembly on sacrificing cores is a feasible technique to produce microcapsules having multi-components [9]. It allows to precisely control the capsule structure (shape, size and wall thickness), and to freely choose the wall components with different combinations [1]. Template polymerization is another useful method to obtain multi-component materials [10]. In this technique, a polymer is used as the template to associate with the monomers by interactions of hydrogen bonding or/and electrostatic force, followed by polymerization of the monomers in which propagation of a radical occurs along the template polymer chain during most of its lifetime. Radical template polymerization can yield polymer polymer complexes held by cooperative interactions of complementary macromolecules. In these works, X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi: /j.matlet

2 Z. Feng et al. / Materials Letters 61 (2007) template polymerization of sodium vinyl sulphonate in the presence of poly(allylamine) hydrochloride [10b] as well as the mechanism of template polymerization [10a] have been illustrated. Although the template polymerization has been developed for many years, it has not been extended to obtain hollow microcapsules assisted with sacrificing cores. Herein the template polymerization is employed to fabricate hollow microcapsules with a shell structure of polyelectrolyte complex, using poly(sodium 4-styrenesulfonate) and silica particles as the molecular template and sacrificing template respectively. The silica particles are modified by 3-(trimethoxysilyl) propyl methacrylate (MPS) firstly to introduce polymerizable C_C bonds. The protonized pyridine groups can associate with the negative sulfonate groups of the template PSS via electrostatic interactions. When the polymerization proceeds, the PSS/VPy complexes are transferred into the PSS/PVPy complexes. If the radicals in the complexes are not terminated, they are capable of copolymerization with the vinyl groups on the silica surfaces, resulting in immobilization of the complexes on the particle surfaces. Along with the on going polymerization, the particle surfaces are covered fully by the tiny complexes. After removal of the cores, hollow PSS/PVPy microcapsules can then be obtained. The resulting hollow capsules contain complexes which originated from water soluble polymers by one step, and have good dispersivity in water due to their hydrophilicity and charge repulsion. The present work bridges the template polymerization and the composite particles and hollow capsules, providing a new pathway to obtain multi-component and multifunctional microcapsules. 2. Experimental section 2.1. Materials Poly(sodium 4-styrenesulfonate) (PSS, M.W. CA. 70,000) and 4-vinylpyridine ( 96%, stabilized with 100 ppm hydroquinone) were purchased from Aldrich and Fluka, respectively, and were used as received. Potassium persulfate (K 2 S 2 O 8 ) was a commercial product and was used after recrystallization in water. 3-(Trimethoxysilyl)propyl methacrylate (MPS, N96%), toluene and ethanol (AR grade) were used as received. Tripledistilled water was used in all the experiments Fabrication of the microcapsules The silica microparticles with a diameter of 1 μm, prepared by a modified Stöber method [11], were dispersed in 40 ml anhydrous toluene, into which 2 ml MPS was added. The mixture was stirred overnight, centrifuged and washed sequentially with toluene and ethanol, each for 3 times. The obtained MPS grafted microparticles were dried under reduced pressure. In a typical procedure for core shell particle fabrication, 1 g of MPS grafted microparticles was dispersed in 25 ml of water Fig. 1. SEM image of (A) bare silica particles. (B) SEM and (C E) TEM images of silica particles covered by a polymerization product of PVPy in the presence of PSS. Inset of (C) is a higher magnification of the particle edge. Synthesis conditions: 0.03 g K 2 S 2 O 8, in (B, C) 0.3 ml VPy/1.6 g PSS, (D) 0.1 ml VPy/0.54 g PSS, (E) 0.03 ml VPy/0.16 g PSS and (F) 0.3 ml VPy/2.4 g PSS.

3 2562 Z. Feng et al. / Materials Letters 61 (2007) under ultrasonication, into which 1.6 g PSS, 0.3 ml 4- vinylpyridine (VPy, Fluka) and 0.03 g K 2 S 2 O 8 were added. The ph value of the system was adjusted to 4 by 1 M HCl solution to change the 4-vinylpyridine to hydrochloride salt. The mixture was stirred by a mechanic agitation machine with a speed of 400 rpm. The polymerization was conducted in N 2 atmosphere at 65 C for 2 h. The resulted core shell particles were collected by centrifugation at 4000 rpm and washed 3 times, and were then treated with 0.4 mol/l HF solution to etch off the silica template, obtaining the PSS/PVPy microcapsules. The experimental conditions such as initiator concentration, monomer concentration and monomer ratio were also varied to prove their influences on the quality of the resulting core shell particles Characterization The core shell microparticles and hollow microcapsules were characterized by transmission electron microscopy (TEM) (TECNAL-10, Philips and JEOL JEM-200CX, Japan) and scanning electron microscopy (SEM) (SIRION-100 microscope, RAITH). The fluorescent images were captured using a Bio-Rad Radiance 2100 confocal laser scanning microscope (CLSM). FTIR spectra were recorded on a VETOR 22 spectrophotometer. Elemental analyses were carried out on Flash EA 1112 instrument. The ζ-potentials of the composite particles were measured by a Zetasizer Nanoinstrument Nano Z equipment. The diameters of the free complexes formed in the polymerization were determined by Particle Size Analyzer (BI-90 Plus Brookhaven Instruments Co.). 3. Results and discussion The silica particles before and after polymerization were characterized with SEM firstly (Fig. 1A). The PSS/PVPy covered particles have very rough surfaces, compared with the smooth bare silica particles (Fig. 1B). Many tiny particles with a diameter of 50 nm are observed on the polymer-covered particles. A TEM image shows a consistent morphology of the polymer-covered particles (Fig. 1C), Fig. 3. FTIR spectrum of the hollow microcapsules, demonstrating their chemical compositions. where tiny particles extruding of the silica particle surfaces can be clearly visualized. The film around the silica particle is not homogeneous, but it forms a continuous and closed layer, whose thickness is estimated as 10 nm from the magnified image (Fig. 1C, inset). We further determined the particle size in the solution, finding a numberaverage diameter of 55.4 nm, which is consistent with the SEM and TEM observations. Hence, it is likely that the rough morphology of the core shell particles is formed by deposition of the aggregated polyelectrolytes onto the silica particles during the polymerization. The synthesis conditions such as initiator concentration, VPy concentration and VPy/PSS ratio were further varied to testify their influence on the quality of the resulting core shell particles. Keeping all other conditions constant, no significant influence of initiator amount from 0.01 g to 0.03 g was found. Keeping the molar ratio of VPy/PSS constant, 0.3 ml, 0.1 ml and 0.03 ml VPy resulted in a shell thickness of 10 nm (Fig. 1C), 7 nm (Fig. 1D) and 5 nm (Fig. 1E), respectively. Accompanying with this thickness reduction, the particle surface became more inhomogeneous. Keeping all other factors constant but increasing the amount of PSS from 1.6 g to 2.4 g, the shell thickness increased from 10 nm to 15 nm, and the shell became more homogeneous (Fig. 1F). By contrast, decreasing the PSS amount to 0.8 g, a mixture of composite silica particles and free complexes was formed Fig. 2. (A) SEM, (B) TEM and (C) CLSM images of the resulting hollow microcapsules at dry (A, B) and wet (C) states. Insets of (A) and (B) are higher magnification of an intact microcapsule. In (C) rhodamine 6G was added to visualize the capsule walls by fluorescent emission. Capsules are derived from the core shell particles synthesized at conditions corresponding to Fig. 1C.

4 Z. Feng et al. / Materials Letters 61 (2007) (image not shown), which could not be separated. This phenomenon should be caused by the insufficient stabilization of PSS to the newly polymerized PVPy, leading to formation of big complexes. In summary, the particle morphology and shell thickness can be well mediated by the experimental conditions such as monomer concentration and monomer/dispersant ratio. After removal of the silica particles by HF etching, intact hollow capsules were obtained, which show a collapsed morphology in the dry state as representatively shown in Fig. 2A. The hollow nature is further demonstrated by TEM observation (Fig. 2B), in which core residues in the microcapsules are hardly found. It is worth mentioning that the folds and creases are not very apparently observed as those in the traditional LBL polyelectrolyte microcapsules. The microstructures, namely irregular polyelectrolyte complexes and regular LBL assemblies for the present and the LBL microcapsules respectively, should be responsible for their corresponding morphologies. Moreover, the very rough surface morphology of the present microcapsules may also weaken the contrast between the folds and their neighboring regions. Confocal laser scanning microscopy observed a very good dispersion of the microcapsules in water (Fig. 3C), demonstrating no aggregation or bridging between the capsules. Moreover, all the capsules keep their spherical shape with no microscopically detectable breakages on the capsule walls. The good dispersivity is mainly attributed to the strong surface charge, since the ζ-potential of the composite particles was determined as 55.1 mv. The strong negative ζ-potential also implies that the particle surfaces were mainly covered by PSS. Actually, the good dispersivity of the core shell particles was evidenced by dynamic light scatting (DLS), which detected an average diameter of 816 nm with a very narrow distribution. No aggregate particles were found between 1 and 5 μm by DLS. The mechanism of shell formation is similar to that of the core shell particles made through dispersion polymerization [12]. In order to effectively capture the living complexes, silica particles should be modified with polymerizable C_C groups. We used here MPS, which can anchor its methacryl end group onto the silica surface by condensation of its methoxyl groups with the silanol groups of the silica. The mechanism of this process and characterization of MPSmodified silica particles have been documented previously [13]. The modification procedure endows the particle with hydrophobicity and reactivity. The hydrophobic surface facilitates the adsorption of PSS onto the particle surface too. Besides binding with the VPy monomers, PSS also plays a role of stabilization of the silica particles via charge repulsion. When the polymerization proceeds, the propagating poly(4- vinylpyridine) hydrochloride chain will separate from the continuous solution phase, together with the PSS. The PSS/PVPy complexes still can copolymerize with the methacryl groups on the particle surface thus can be immobilized, while the other part of the left living complexes will further propagate or be terminated to form free complexes. The amount of PSS here is stoichiometrically larger than the polymerized PVPy, thus the excess PSS can stabilize the formed PSS/PVPy complexes too. Here, the C_C double bonds on silica particles are of course indispensable, through which the covalent bonding between the particle surfaces and the PAA/PVP complexes can be achieved. These two components within the microcapsules are identified by the FTIR spectrum (Fig. 3). The absorbance peaks at 1635 cm 1 and 1618 cm 1 are assigned to the stretching vibrations of protonized pyridine ring, confirming the existence of poly(4-vinylpyridine) hydrochloride. The peaks at 1122 cm 1 and 1033 cm 1 are assigned to the asymmetric and symmetric stretching vibration of SO 3 1 respectively, confirming the existence of PSS. Elemental analysis measured 54.76% (wt) carbon and 4.97% (wt) nitrogen, from which the weight percentages of PVPy and PSS are calculated as 37.6% (wt) and 47.6% (wt), respectively (small counter ions are not considered), corresponding to a monomer units ratio (VPy/SS) of This result reveals that the PVPy is the major component of the microcapsules, although excess PSS was used. The phenomenon may be explained by the fact that a part of the PVPy is covalently linked to the particle surfaces, making the PVPy hardly rinsed off. By contrast, PSS has no covalent binding with the particles, resulting in partial loss during the extensive washing procedure. 4. Conclusion In a solution containing the MPS-modified silica particles and PSS molecules, polymerization of VPy results in continuous shell layers on the silica particle surfaces. Here both the PSS molecules and the silica particles function as the templates to guide the alignment of VPy monomers and support the deposition of the PSS/PVPy complexes, respectively. After sacrificing the cores, hollow microcapsules are obtained. Their hollow nature is confirmed by TEM, SEM and CLSM. The shell thickness can be tuned by experimental conditions such as the monomer concentration and the ratio of monomer to template polymer. FTIR spectroscopy and elemental analysis confirm that the capsule walls are indeed composed by 37.6% (wt) PVPy and 47.6% (wt) PSS. This method can broaden the capsule fabrication, and enrich the polyelectrolyte capsule family, which is a kind of important micro/nano-structures with versatile properties and extensive applications. Acknowledgements We thank Q. H. Zhao and F. Wang for their valuable discussions and help. This study is financially supported by the Natural Science Foundation of China ( ), the National Science Fund for Distinguished Young Scholars of China ( ) and the Open Project of Key Laboratory for Supramolecular Structure and Materials (KLSSM ). References [1] C.S. Peyratout, L. Dähne, Angew. Chem., Int. Ed. 43 (2004) [2] (a) A.A. Antipov, G.B. Sukhorukov, S. Leporatti, I.L. Radtchenko, E. Donath, H. Möhwald, Colloids Surf., A Physicochem. Eng. Asp. 198 (2002) 535; (b) G. Ibarz, L. Dähne, E. Donath, H. Möhwald, Adv. Mater. 13 (2001) 1324; (c) S. Leporatti, C. Gao, A. Voigt, E. Donath, H. Möhwald, Eur. Phys. J., E Soft Matter 5 (2001) 13; (d) Y. Lvov, A.A. Antipov, A. Mamedov, H. Möhwald, G.B. 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