One-step fabrication of 3-methacryloxypropyltrimethoxysilane modified silica and investigation of fluorinated polyacrylate/silica nanocomposite films

Abstract

Vinyl group containing silica (MSiO₂) nanoparticles were prepared by the co-condensation of tetraethoxysilane (TEOS) with 3-methacryloxypropyltrimethoxysilane (MPS) and applied in the miniemulsion polymerization of methyl methacrylate (MMA), butyl acrylate (BA) and 1H,1H,2H,2H-heptadecafluorodecyl methacrylate (FA) to prepare fluorinated polyacrylate/silica nanocomposite particles. The morphology and particle size of MSiO₂ was characterized by scanning electron microscopy (SEM). The nanocomposite films were investigated using atomic force microscopy (AFM), Fourier transform infrared (FT-IR) spectroscopy, water contact angle measurements, visible light spectroscopy, X-ray photoelectron spectroscopy (XPS) and thermogravimetry analyses (TGA). The effect of the amount of MSiO₂ on the surface properties of the nanocomposite films was studied. Results showed that the MSiO₂ nanoparticles presented a rough surface with a mean particle size of 200 nm. The nanocomposite film showed high transparency and the surface hydrophobicity of the fluorinated polyacrylate film was increased significantly by the incorporation of 0.04 g (0.46 wt% of monomers) of MSiO₂ nanoparticles. The presence of MSiO₂ contributed to the surface enrichment of the fluorinated components, and the nanocomposite film formation mechanism was proposed.

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1. Introduction

Polymer/inorganic nanocomposite particles have gained much attention from scientists owing to the remarkable properties that can be obtained by the combination and structuration of the organic and inorganic components inside the nanoparticles.^1–10 These nanocomposite materials are particularly promising in applications such as catalysis,^11,12 surface coatings^13–16 and biotechnologies.¹⁷ A number of approaches have been demonstrated for generating nanocomposite particles with different interesting morphologies, such as core–shell,^18,19 raspberry-like,^20–24 daisy-shaped,²⁵ currant bun-like²⁶ and snowman-like²⁷ structures. A lot of nanocomposites have been prepared in the last decade via encapsulation or grafting of polymers onto the surface of inorganic components, of which silica is the most widely studied due to its great potential in commercial applications.^28–31 The strategies for designing and fabricating polymer/silica nanocomposite particles have attracted wide interest because these composite particles exhibit enhanced mechanical and optical properties. Colloidal nanocomposite particles have various potential applications ranging from transparent, scratch-resistant coatings to durable exterior facade paints.^32–35

To improve the compatibility of silica with polymers, silica nanoparticles are necessarily pretreated with coupling agents,^36–39 oleic acid⁴⁰ or alcoholic components.^41,42 However, as far as we know, there is no report focused on the one-step fabrication of MPS modified large silica nanoparticles (MSiO₂). In addition, fluorinated polyacrylates have good adhesion to substrates, high transparency and amenable mechanical properties. They show many attractive characteristics including low surface energy, water and oil repellency, and high thermal, chemical, and weather resistance. Therefore, it is essential to spend effort on the preparation and investigation of the surface properties of fluorinated polyacrylate/silica nanocomposite films.

Herein, we describe the preparation of MPS modified silica nanoparticles by a one-step method of co-condensation of TEOS with MPS. Then, the fluorinated polyacrylate/silica (PFA/SiO₂) nanocomposite particles were prepared from a miniemulsion polymerization. The microstructure of the particles and the properties of the nanocomposite film were investigated, moreover, a film formation mechanism of the nanocomposite film was proposed.

2. Experimental

2.1 Materials

Tetraethoxysilane (TEOS), methyl methacrylate (MMA) and butyl acrylate (BA) were purchased from Xilong Chemical Co., Ltd (Guangzhou, China). MMA and BA were purified by passing through a neutral alumina column to eliminate inhibitors before use. The 1H,1H,2H,2H-heptadecafluorodecyl methacrylate (FA) monomer was prepared in our lab. 3-Methacryloxypropyltrimethoxysilane (MPS), bought from Nanjing Xiangqian Chemical Co., Ltd, was analytical grade and used as received. Ammonium persulfate (APS) was obtained from Tianjin Bodi Chemical Co., Ltd and recrystalized before use. Sodium bicarbonate (NaHCO₃), sodium dodecyl sulfate (SDS), hexdecane (HD) and ammonia (25 wt%) were all analytical grade and used without further purification. Deionized water was used for all polymerization and treatment processes.

2.2 Methods

The morphology of MSiO₂ was analyzed using a FESEM JSM-7500F instrument. The films formed on glass slides were sprayed with platinum (Pt) before observation. The morphology of the nanocomposite particles was characterized by transmission electron microscopy (TEM, JEM-2100, JEOL, Japan). A certain amount of latex was diluted with deionized water to a solid content of 1 wt%, then the diluted latex was dropped onto a carbon coated copper grid and negatively stained by a 2 wt% aqueous solution of phosphotungstic acid before TEM analysis. Fourier transform infrared (FT-IR) spectra were recorded on a NEXUS-470 FTIR analyzer (Nicolet, USA) from 4000 cm⁻¹ to 500 cm⁻¹ with a resolution of 8 cm⁻¹. ¹⁹F NMR spectra were recorded using an AVANCE III 400 MHz Bruker spectrometer with CDCl₃ as the solvent. Atomic force microscopy (Veeco DI, USA) was employed to investigate the film surface morphology in a tapping mode at ambient temperature by keeping a scan rate of 256 Hz and scan size of 5 μm × 5 μm. The transparency of the nanocomposite film was assessed by visible absorption spectrophotometry (UV-3600, SHIMADZU, Japan) from 400 nm to 800 nm, and the spectral slit width was set as 2 nm. The static contact angle of water on the latex films was measured using a sessile drop method with KRÜSS DSA20 (KRÜSS, Germany). 5 water contact angle values (5 μl per drop) on the film were averaged. X-ray photoelectron spectroscopy (XPS) studies were conducted on the dried nanocomposite film to get the chemical composition of the film surface in both survey and high-resolution mode on ESCALAB 250 systems equipped with an Al K_α X-ray source with a takeoff angle of 45°. Thermogravimetric analysis (TGA) was performed on a STA 3 Jupiter (Netzsch, Germany) from 50 °C to 500 °C at a heating rate of 10 °C min⁻¹ with argon protection.

2.3 One-step synthesis of MPS functionalized silica nanoparticles (MSiO₂)

A novel method was suggested for silica modification in the process of silica formation, and the reaction mechanism between MPS and the silanol groups is depicted in Scheme 1. It is known that TEOS will undergo hydrolysis and condensation reaction processes in alkaline conditions. As shown in Scheme 1, there are a lot of silanol groups after the hydrolysis of TEOS, and the presence of a large amount of silanol groups makes the grafting of MPS with silica more efficient. Therefore, by appropriately adjusting the addition time of MPS, the silanol groups can react with MPS easily and gently at room temperature. Based on this theory, the following reaction was designed and implemented.

Scheme 1 One-step approach to fabricate MSiO₂ nanoparticles.

Typically, 3.5 g of TEOS was mixed with 45 ml of ethanol, 10 ml of water and 2 ml of ammonia, and then stirred magnetically in a three-necked flask at room temperature. Subsequently, 0.3 g of MPS dissolved in 5 ml of ethanol was added dropwise into the flask. After completion of the addition, the reaction was continued for another 24 h at room temperature, and a white colored functionalized silica dispersion was obtained. The product was centrifuged and washed with ethanol three times, after which the silica was dried in an oven at 80 °C for 3 h, and then ground into powder before use.

2.4 Synthesis of the PFA/SiO₂ nanocomposite particles

The PFA/SiO₂ nanocomposite particles were prepared by miniemulsion polymerization. In a typical experiment, 0.04 g of MSiO₂ (PFA4, the amount of MSiO₂ was varied from 0 g to 0.15 g, and the nanocomposite was denoted as PFA0 to PFA15) was first dispersed in a mixture of monomers (4 g of MMA, 4 g of BA and 0.7 g of FA) and 0.4 g of HD with the aid of sonication for 20 min in an ice bath. Then, the dispersion was introduced into an aqueous solution of 0.08 g of SDS and 0.02 g of NaHCO₃. After the mixture was sonicated for 20 min, the resultant miniemulsion was transferred into a 250 ml three-necked round-bottomed flask equipped with a mechanical stirrer, reflux condenser and nitrogen inlet. The polymerization was started with 0.08 g of APS at 70 °C and the reaction was complete after 6 h under a protective stream of nitrogen.

3. Results and discussion

3.1 SEM and FT-IR analyses of MSiO₂

The morphology and particle size of MSiO₂ are shown in Fig. 1a. It is seen that the particles present a sphere morphology with a rough surface, from which small granules can be observed at the surface of MSiO₂. As is well known, silica particles that are synthesized according to the Stöber procedure conventionally show a smooth surface because TEOS undergoes relatively complete hydrolysis and condensation reactions in ethanol/water solution. However, when MPS is added to the process of TEOS hydrolysis, MPS will also hydrolyze and further condense with the hydroxyl groups (–OH) of silanol (see Scheme 1), which results in silanol itself being unable to undergo complete condensation, therefore, silica particles showing a particular rough surface morphology were obtained. Fig. 1b presents the FT-IR spectra of pure silica and the MSiO₂ synthesized in the experiment. As shown in Fig. 1b, after modification with MPS, a peak for C=O appears in the MSiO₂ spectrum. The peak at 1100 cm⁻¹ is ascribed to the stretching vibration of Si–O–Si. The absorption at 950 cm⁻¹ is assigned to Si–OH. The stretching vibration of Si–O occurs at 802 cm⁻¹, and the peak at 470 cm⁻¹ belongs to flexural vibration of Si–O. After modification, absorption of the carboxyl group (C=O) at 1730 cm⁻¹ occurred, which indicates that MPS was grafted onto the surface of the silica nanoparticles. The absorption of –OH occurs in both silica and MSiO₂, indicating that there are still large numbers of –OH groups on the surface of MSiO₂, although the surfaces of the nanoparticles were grafted with MPS molecules.

Fig. 1 SEM image of MSiO₂ (a) and FT-IR spectra of pure silica and MSiO₂ (b).

3.2 ¹⁹F NMR and FT-IR analyses

Fig. 2 presents the ¹⁹F NMR spectrum of PFA0. As shown in Fig. 2, the chemical shift at −81 ppm is assigned to –CF₃, and the chemical shifts from −113 ppm to −127 ppm are ascribed to –CF₂. Therefore, it is confirmed that FA has copolymerized with the acrylic monomers and that the PFA copolymer was obtained as expected.

Fig. 2 ¹⁹F NMR spectra of FA and PFA0.

Fig. 3 presents the FT-IR spectra of the PFA/SiO₂ nanocomposite films. The absorption peaks at 2950 cm⁻¹ and 2870 cm⁻¹ are due to the stretching vibrations of C–H. It can be seen that after incorporation of MSiO₂ into fluorinated polyacrylate, the FT-IR spectrum of the film obviously changed. The peak at 3450 cm⁻¹ is assigned to the stretching vibration of –OH, and the absorption at 1629 cm⁻¹ belongs to the flexural vibration of –OH. It is seen that when MSiO₂ was added into the polymerization, absorptions for –OH appear in the FT-IR spectrum. The characteristic absorption that appears at 806 cm⁻¹ is due to the symmetric stretching vibration of Si–O. The characteristic peaks at 1240 cm⁻¹ and 1170 cm⁻¹ are the stretching vibration absorptions of C–F. Therefore, the results from the FT-IR spectra indicate that PFA/SiO₂ nanocomposite particles were obtained as designed.

Fig. 3 FT-IR spectra of the films: (a) PFA0, (b) PFA2 and (c) PFA8.

3.3 The surface properties of the PFA/SiO₂ nanocomposite films

3.3.1 The transparency of the PFA/SiO₂ nanocomposite films. Transparency is a significant feature of a copolymer film, therefore, it is crucial to investigate the transparency of the PFA/SiO₂ nanocomposite films. It was expected that as nanoparticles were added into the polymer matrix, the transparency of the film might be affected. Photographs of the hybrid films formed on glass slides and the visible light transmittance curves of the films containing various amounts of MSiO₂ are displayed in Fig. 4. It can be seen that as the amount of MSiO₂ increased, the transparency gradually decreased. However, although an increased amount of MSiO₂ was added, the hybrid film remained highly transparent, at more than 80% over the 400–800 nm range. This excellent optical transparency indicates that the silica particles are homogeneously dispersed at the nanoscale within each film. Moreover, the silica content of the films appear to strongly influence their transmission.

Fig. 4 The transparency of the hybrid films.

3.3.2 The water contact angle of the PFA/SiO₂ nanocomposite films. The surface hydrophobicity of films containing different amounts of MSiO₂ was measured by determining their water contact angles, and the results are shown in Fig. 5. It can be seen that after incorporation of MSiO₂ into fluorinated polyacrylate, the surface hydrophobicity of the films was increased significantly. As shown in Fig. 5, the PFA0 film showed a WCA of 94°, and when 0.04 g of MSiO₂ was applied in the polymerization to produce the PFA/SiO₂ nanocomposite particles, the surface hydrophobicity of the nanocomposite film increased dramatically from 94° to 110°. Following a further increase in the amount of MSiO₂ applied in the polymerization process, the film hydrophobicity decreased slightly, however, the surface hydrophobicity of the film still maintained a high level compared with that of PFA0. It is well known that the surface hydrophobicity of a solid surface depends on the chemical microstructure and composition. Therefore, the WCA data indicate that there was a change in the surface properties of the film after the incorporation of MSiO₂, which led to a change in the surface microstructure and the composition of the film. In order to investigate what changes have occurred at the composite film surface, AFM and XPS analyses were applied to study the film surface properties.

Fig. 5 The WCA data for the PFA/SiO₂ nanocomposite films.

3.3.3 The surface microstructure of the PFA/SiO₂ nanocomposite films. Fig. 6 shows the AFM height and SEM images of the films. It can be seen that as MSiO₂ is added to the polymerization, the hybrid film displays a rougher surface than that of PFA0. As shown in Fig. 6c, the R_q of the film is 3.68 nm, and needle-like peaks can be seen on the film surface. It can be seen that more needle-like peaks occurred on the film surface as a greater amount of MSiO₂ was added into the miniemulsion, however, the R_q of the films changed little when the amount of MSiO₂ was more than 0.04 g. Meanwhile, when the hybrid film contains more than 0.08 g of MSiO₂, domains of aggregates can be seen (as seen from Fig. 6e and f), which might be caused by the aggregation of MSiO₂ particles near the film surface. In addition, as shown in Fig. 6a1, c1 and f1, there is no special morphology at the film surface. This result indicates that although MSiO₂ was added into the film, there was almost no variation in the hybrid film surfaces. We can see that there is no obvious contrast at the film surface and only a smooth film is observed in the SEM images. This is reasonable because the presence of a small number of MSiO₂ nanoparticles cannot change the surface morphology that can be distinguished by SEM. In order to find out the chemical composition changes in the films after adding MSiO₂, XPS was employed to characterize the surface chemical compositions of the films.

Fig. 6 AFM height images of the films: (a) PFA0, (b) PFA2, (c) PFA4, (d) PFA6, (e) PFA8 and (f) PFA10; (a1), (c1) and (f1) are the corresponding SEM images of (a), (c) and (f), respectively.

3.3.4 The surface composition of the PFA/SiO₂ nanocomposite films. XPS is a highly surface-specific technique with a typical sampling depth of 2–10 nm.⁴³ As shown in Fig. 7a and d, a signal for Si2p was detected at the PFA8 nanocomposite film surface, while a Si2p signal from the PFA4 film surface was not obvious. The Si2p signal acts as a unique elemental marker for silica nanoparticles and provides further evidence that this component is present either at or very near to the surface of the nanocomposite films. Therefore, it can be concluded that more silica nanoparticles aggregate at the PFA8 film surface than at that of the PFA4. It can be seen from Table 1 that the Si2p content at the PFA8 film surface was 1.97 atom% while this value was only 0.22 atom% for the PFA4 film surface. Fig. 7b shows the C1s scan spectrum of the PFA4 film, and the characteristic peaks of CF₃ and CF₂ were observed. Fig. 7c presents the F1s scan spectra of the PFA4 and PFA8 films. It is seen that the F1s intensity of PFA4 is much stronger than that of PFA8, which suggests that the fluorine content at the PFA4 film surface is larger than that at the PFA8 film surface, and Table 1 shows that the fluorine content at the PFA4 and PFA8 film surfaces is 22.71 atom% and 14.01 atom%, respectively.

Fig. 7 XPS survey of the PFA4 and PFA8 films (a). Spectra for the PFA4 nanocomposite films: C1s scan (b), F1s scan (c), Si2p scan (d).

Table 1 Surface composition of the PFA/SiO₂ nanocomposite films

	PFA0	PFA4	PFA8
C1s	58.66	60.06	60.17
O1s	26.85	17.01	20.06
F1s	7.36	22.72	14.01
Si2p	0	0.22	1.97
N1s	2.79
Na1s			1.81
S2p	4.34		1.98

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It can be seen that after the incorporation of MSiO₂ into the PFA copolymer, the nanocomposite film surface is more fluorine enriched, and depending on what amount of MSiO₂ is used in the miniemulsion polymerization, the silica content at the film surface is varied. When the MSiO₂ content was increased from 0 g to 0.04 g, the surface hydrophobicity of the film increased, and with a further increase in the amount of MSiO₂ in the polymerization, the film surface became more hydrophilic (see Fig. 5). According to XPS, this phenomenon can be explained as follows: a certain amount of MSiO₂ (e.g. 0.04 g) makes the film more fluorine enriched and favors the film surface being more hydrophobic. As the amount of MSiO₂ becomes more than 0.08 g, some aggregates of silica occur at the film surface, which causes a decrease in the film hydrophobicity. However, the nanocomposite films containing silica are more hydrophobic than PFA0 (no silica), which is due to the increased fluorinated segments and rough surface that were obtained simultaneously in the nanocomposite films.

In order to find the reason that causes the above results, the morphology of the nanocomposite particles was characterized and the result is shown in Fig. 8.

Fig. 8 The TEM image (a) and film-forming mechanism (b) of the PFA/SiO₂ nanocomposite particles. The inset in (a) is the TEM image of MSiO₂.

As shown in Fig. 8a, the PFA polymeric particles show a white color in the TEM image because the nanocomposite particles were previously negatively stained by phosphotungstic acid, and all the polymeric particles exhibit a uniform sphere morphology. Meanwhile, it is observed that the MSiO₂ particles showing high contrast (dark color) in the TEM image were encapsulated in the PFA polymeric particles. Due to the small amount of MSiO₂ (0.46 wt% of the total monomers) in the miniemulsion, only a limited amount of MSiO₂ can be found in the TEM image. However, this particular morphology of the nanocomposite particles is believed to affect film formation and the resulting film surface properties significantly. As a result, the probable film forming mechanism for the PFA/SiO₂ nanocomposite particles is depicted in Fig. 8b. It is believed that the fluorinated polyacrylate particles are grafted onto silica due to the fact that vinyl groups are enriched at the MSiO₂ surface (see Scheme 1), and every MSiO₂ particle has several polymeric particles on its surface. When the hybrid latex is cast onto a glass substrate, the polymeric particles gradually form a polymer layer at the surface of MSiO₂, and as the hybrid film is formed at last, more fluorinated segments will exist at the film surface than at that of the pure fluorinated polyacrylate. Therefore, this particular morphology of hybrid particles is useful for the surface enrichment of the fluorinated components at the surface after film formation.

3.3.5 The thermal resistance of the PFA/SiO₂ nanocomposite films. The Thermal resistance performance of the PFA/SiO₂ nanocomposite films is shown in Table 2 and Fig. 9. T₁₀ is taken as a criterion for evaluating the thermal stability of the films. The TGA curves of the PFA/SiO₂ nanocomposite films show that the thermal stability is affected by the addition of MSiO₂ nanoparticles. As shown in Table 2, the T₁₀ values of the nanocomposite films increased with the amount of MSiO₂. It can be seen that the T₁₀ value of PFA0 containing no MSiO₂ is 357.04 °C, and as the amount of MSiO₂ was increased to 0.15 g, T₁₀ increased to 361.31 °C. It can be seen that the thermal stability of the nanocomposite film was obviously improved, and a similar result has been obtained in previous work.⁴⁴ The increase in the thermal resistance of the PFA/SiO₂ nanocomposite films was due to the barrier effect of the nanoparticles in the film, which improves the fire-retardant behavior of the nanocomposite films.³¹

Table 2 The T₁₀ values of the PFA/SiO₂ nanocomposite films (°C)

	MSiO2 content (g)
	0	0.02	0.08	0.15
T10	357.04	357.04	359.63	361.31

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Fig. 9 The TGA curves of the PFA/SiO₂ nanocomposite films.

4. Conclusion

A novel process for the synthesis of MPS-functionalized large silica nanoparticles was described. Copolymerizing acrylic monomers of MMA and BA with 1H,1H,2H,2H-heptadecafluorodecyl methacrylate (FA) in the presence of MPS-functionalized silica leads to the formation of film-forming colloidal nanocomposite particles. Drying the aqueous nanocomposite dispersions results in highly transparent films, as judged by visible transmittance spectroscopy. The films showed improved surface hydrophobicity after the incorporation of a small amount of silica into the fluorinated polyacrylate, which is caused by the surface enrichment of the fluorinated components and increased surface roughness. This illustrates the utility of the in situ copolymerization approach for the synthesis of colloidal nanocomposite particles with excellent film-forming properties, good hydrophobicity and high transparency. Introducing only a small amount of silica into the fluorinated polyacrylate can promote the film hydrophobicity; this approach can also decrease the cost of the fluorinated component, while showing preferable surface properties, which will find potential applications in many fields.

Acknowledgements

We are thankful for financial support from the National Natural Science Foundation of China (Grant No. 51173006).

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