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Silica/polystyrene and silica/polystyrene-b-polymethacryloxypropyltrimethoxysilane hybrid nanoparticles via surface-initiated ATRP and comparison of their wettabilities.

INTRODUCTION

The wettability of solid surfaces plays an important role in nature and many industrial applications, and is mainly governed by both their surface chemistry and roughness (1), (2). During the past few decades, superhydrophobic surfaces with water contact angles (WCAs) larger than 150[degrees] have exhibited numerous potential practical applications, such as self-cleaning surfaces, prevention of the adhesion of snow, and others (2), (3). In general, the WCA on a smooth surface of a hydrophobic material typically ranges from 100[degrees] to 120[degrees] (4) but can reach values higher than 150[degrees] with rough or hierarchical micro-/nanostructures (2), (5). Therefore, it is likely to modify the wettability of solid surfaces by strengthening the combination of the surface chemistry and roughness, especially the roughness to fabricate superhydrophobic surfaces.

On the other hand, polymer brushes grafted on solid surfaces can greatly increase the surface roughness to improve the wettability (6), (7). Among them, polymer brushes grafted inorganic nanoparticles, namely polymeric-inorganic hybrid nanoparticles, have attracted much attention due to their smartly designed surface roughness and other properties (8-10). Such polymeric-inorganic hybrid nanoparticles can be advantageously created by coupling inorganic nanoparticles with polymers of controlled architecture and precise properties (10). Extensive studies on polymer growths via controlled/living radical polymerization techniques including atom transfer radical polymerization (ATRP) have been performed on various inorganic substrates, in particular, silica ([SiO.sub.2]) (11-15). However, in the articles devoted to the grafting on [SiO.sub.2] nanoparticles, little attention was paid to [SiO.sub.2]/polystyrene(PS)-b-polymethacry-loxypropyltrimethoxysilane(PMPTS) hybrid nanoparticles via surface-initiated ATRP (SI-ATRP), especially characteristics of the PS-b-PMPTS block polymer and the morphology of the hybrid nanoparticles (it should be the crucial point which designs the roughness of micrometric particles). As far as we know, many articles reported substantial data on the grafting of PS chains from [SiO.sub.2] nanoparticles (16-19). Herein, we are interested in fabricating superhydrophobic [SiO.sub.2]/PS and [SiO.sub.2]/PS-b-PMPTS hybrid nanoparticles with rough surfaces and controlled PS and PMPTS chains grafted via SI-ATRP. Their wettabilities were measured and compared by WCA and surface roughness.

EXPERIMENTAL PART

Materials

[SiO.sub.2] (Aldrich), (3-aminopropyl)triethoxysilane (APTS, Alfa Aesar, 98%), 2-bromoisobutyrate bromide (BIBB, (ABCR), 98%), [gamma]-methacryloxypropyltrimethoxysilane (MPTS, ABCR, 98%), 1,1,4,7,7-pentamethyldiethylenetriamine (PMD ETA, Aldrich, 98%), copper (II) bromide (Cu (II) [Br.sub.2], Aldrich, 99%) toluene [99.5%, Sinopharm Chemical Reagent (SCRC)], methanol (SCRC, 99.5%), and p-toluene sulfonic acid (SCRC, 99%) were used as received directly. Styrene (St, SCRC, 98%) was purified by washing with 5% aqueous NaOH solution to remove inhibitor before polymerization. Copper (I) bromide (Cu (I) Br, Aldrich, 98%) was purified to remove Cu (II) by precipitation from the concentrated HBr acid by addition of water under nitrogen atmosphere (20). The [SiO.sub.2] used in this study has a specific surface area of 200 [m.sup.2] [g.sup.-1] [+ or -] 25 [m.sup.2] [g.sup.-1] and a mean particle diameter of 14 nm. The [SiO.sub.2] nanoparticles were surface functionalized with APTS and BIBB sequentially, as previously described by Zhang et al. (21).

Synthesis of [SiO.sub.2]/PS and [SiO.sub.2]/PS-b-PMPTS Hybrid Nanoparticles

The surface grafting procedure is indicated briefly in Scheme 1. The SI-ATRP of St was carried out from the surface of the initiator-modified [SiO.sub.2] particles ([SiO.sub.2]/APTS-Br). In a typical grafting polymerization, [SiO.sub.2]/APTS-Br (0.20 g, 0.29 mmol), Cu (I) Br (42.8 mg, 0.29 mmol), some deactivator Cu (II) [Br.sub.2] (4.3 mg, 0.029 mmol), and dehydrous toluene (4.5 ml) were added to a dried round-bottom flask and sealed with a rubber septum. The flask was degassed and backfilled with nitrogen three times. Then, the monomer, St (3.3 ml, 29 mmol), was added to the suspension followed by three freeze-pump-thaw cycles to remove oxygen. Then, PMDETA (61 [micro]l, 0.29 mmol) was added to the flask. The molar ratio of each component of [CuBr]/[[CuBr.sub.2]][PMDETA]/[[SiO.sub.2]/APTS-Br]/[St] was 1:0.1:1:1:100. The polymerization was allowed to proceed at 110[degrees]C. To remove the untethered polymer chains, the final suspension was washed with tetrahydrofuran (THF) and centrifuged at 4000 r [min.sup.-1] (yield: 2.2 g, 70%).

[ILLUSTRATION OMITTED]

The SI-ATRP of MPTS was performed in a manner similar to that of the synthesis of [SiO.sub.2]/PS hybrid nanoparticles. The PS chain ends on the surface were reactivated to initiate PMPTS grafting. The molar ratio of each component of [CuBr]/[[CuBr.sub.2]]/[PMDETA]/[[SiO.sub.2]/PS]/[MPTS] was 1:0.1:1:1:100. The polymerization was allowed to proceed at 90[degrees]C (yield: 50%).

Cleavage of the Graft Polymer From the Silica Nanoparticles

A total of 500 mg of inorganic/organic silica particles was suspended in 100 ml of toluene in which 10 ml of methanol and 50 mg of p-toluene sulfonic acid were added. The mixture was heated to reflux overnight. After drying of the degrafted polymers, the molecular weights and molecular weight distribution were determined by gel permeation chromatography (GPC) measurements and compared with the untethered chains.

Preparations of the Films Based on [SiO.sub.2]/PS and [SiO.sub.2]/PS-b-PMPTS Nanoparticles

A suspension of 1% (wt/wt) [SiO.sub.2] nanoparticle in THF was slowly cast on cleaved glass slides. The solvent was evaporated at room temperature in a nearly saturated vapor of THF for over 3 days. The film was further dried under vacuum at room temperature for 6-8 h. To compare the surface morphology of a series of nanoparticles, all films were prepared according to this uniform procedure.

Measurements

Fourier transform infrared (FTIR) spectra were recorded from KBr pellets on a Nicolet Avatar 360 FTIR spectrophotometer. The cleaved polymers were measured by nuclear magnetic resonance ([.sup.1]H NMR) on a Bruker AV400 NMR spectrometer with deuterated chloroform as the solvent and tetramethylsilane as the internal reference. The molecular weight ([M.sub.n]) and molecular weight distribution ([M.sub.w]/[M.sub.n], polymer distribution index (PDI)) of the cleaved polymers and untethered polymers were measured by GPC. GPC was performed at 30[degrees]C using THF as the at the flow rate of 1 ml min-1 on a Waters instrument equipped with Waters Styragel columns (HT4, HT5E, and HT6) and a Waters 2414 refractive index detector. A series of multiple polymethyl methacrylate narrow standards supplied by Waters were used to generate a universal calibration curve. The amount of polymer grafted onto the [SiO.sub.2] surfaces was determined by thermogravimetric analysis (TGA) on a Netzsch TG 209 Fl Iris Thermal Analyzer. Samples were heated to 800[degrees]C from room temperature at the speed of 10[degrees]C min-1 under nitrogen. The morphologies of the pristine and grafted [SiO.sub.2] particles were observed using scanning electron microscope (SEM) on a Hitachi S-4800 electron microscope at accelerating voltage of 5 and 10 kV. Atomic force microscopy (AFM) observation was made on 5500ILM AFM (Agilent) in tapping mode to measure the film surface roughness. The WCAs of the [SiO.sub.2] hybrid nanoparticle films were recorded on a telescopic goniometer (SL200B, Solon Tech., Shanghai). For each angle reported, at least seven readings from different surface locations were averaged.

RESULTS AND DISCUSSION

As described earlier in this article, the [SiO.sub.2]/PS-b-PMPTS hybrid nanoparticles were prepared by the SI-ATRP of MPTS from the [SiO.sub.2]/PS hybrid nanoparticles. The wettability of [SiO.sub.2]/PS hybrid nanoparticles were also compared with that of the [SiO.sub.2]/PS-b-PMPTS hybrid nanoparticles. Herein, the characterization of their macro-molecular characteristics, surface morphologies, and wettabilities is described.

FTIR spectroscopy (Fig. 1) and 'H NMR spectrum (Fig. 2) are used to determine their chemical structures. In addition, the FTIR spectrum of [SiO.sub.2]/APTS-Br used as the macroinitiator of SI-ATRP was also measured and shown in Fig. 1. According to Fig. 1, one can see that all typical vibrations of the organic components are obvious in the above three typical samples corresponding to [SiO.sub.2]/APTS-Br, [SiO.sub.2]/PS, and [SiO.sub.2]/PS-b-PMPTS hybrid nanoparticles, respectively. From Fig. la, the absorption bands based on imide 0=C--NH linkage at 1645 [cm.sup.-1], Si--O--Si band at 1110 [cm.sup.-1], and Si--[CH.sub.3] band at 805 [cm.sup.-1] were observed, indicating that the [SiO.sub.2]/APTS-Br macroinitiator was synthesized. Compared with the absorption peaks shown in Fig. 1a, the sharp characteristic absorption peaks at 700 and 758 [cm.sup.-1]1 shown in Fig. 1b are due to the aromatic unit with five adjacent hydrogen out-of-plane deformations, which represents mono substitution (22). Medium to weak peaks of aromatic C--C bonds stretching at 1600 and 1490 [cm.sup.-1] were also detected (23). These observations reveal that the [SiO.sub.2]/PS nanoparticles were successfully synthesized. Compared with the absorption peaks shown in Fig. 1b, the adsorption band at 1727 [cm.sup.-1] shown in Fig. 1c can be attributed to stretching vibrations of the O--C=0 ester group of the PMPTS block, which proves the successful synthesis of [SiO.sub.2]/PS-b-PMPTS nanoparticles. Furthermore, we also measured the [.sup.1]H NMR spectra of both cleaved PS and PS-b-PMPTS polymers based on the handling of the samples described in the Experimental Part. As shown in Fig. 2a, the presence of characteristic signals of protons from aromatic group at [delta] = 6.33-6.74 ppm (3), and 6.95-7.21 ppm (4, 5) confirms that the [SiO.sub.2]/PS nanoparticles were successfully synthesized. In addition, as shown in Fig. 2b, the presence of characteristic signal of the methylene protons adjacent to ester groups in the PMPTS chain at [delta] = 4.19 ppm (8) verifies the successful synthesis of PS-b-PMPTS grafted [SiO.sub.2].

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Monomer conversion was measured by gravimetry through drying the sampled [SiO.sub.2]/PS or [SiO.sub.2]/PS-b-PMPTS suspension to constant weight in vacuum at 40[degrees]C (24). Figure 3a and b shows plots of [M.sub.n] as a function of conversion for the polymerizations of St and MPTS, respectively. Both plots show excellent linearity, indicative of living polymerization in a controlled manner. However, the experimental molecular weights were higher than the theoretical ones. This reveals that it is impossible that all the [SiO.sub.2]/APTS-Br macroinitiators effectively initiate monomer polymerization, namely, the initiation efficiency of [SiO.sub.2]/APTS-Br cannot reach 100%. GPC with THF as the eluent was used to study the PDI of the resulting polymers. GPC analysis in THF revealed a monomodal peak with a [M.sub.n] of 16,100 g [mol.sup.-1] and a PDI of 1.13 (Fig. 4a). The elution peak of PS is symmetric and exhibits no tailing at the lower molecular weight side. The typical GPC curve showed that the molecular weight ([M.sub.n]) of obtained block copolymer PS-b-PMPTS was 41,100 g [mol.sup.-1] and its PDI was 1.24 (Fig. 4b). According to the GPC results, the molecular weight of PMPTS moiety in PS-b-PMPTS was 25,000, which was obtained by subtracting the molecular weight (16,100) of PS from that (41,100) of PS-b-PMPTS. The elution peak of PS-b-PMPTS is slightly asymmetric and exhibits no apparent tailing at the lower molecular weight side. In general, the narrow molecular weight distributions also suggest that corresponding polymerizations proceed in controlled manners (24). These values are comparable to the untethered PS in this reaction, where [M.sub.n] = 19,100 g [mol.sup.-1] and PDI = 1.35 (Fig. 5a). Again, the result is comparable to untethered PMPTS, where [M.sub.n] = 27,900 g [mol.sup.-1] and PDI = 1.59 (Fig. 5b). GPC data of untethered polymer showed that the untethered polymers possess higher molecular weight and molecular weight distribution than that of the grafted polymers. This indicates that the untethered polymer was formed in an uncontrolled manner.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Although determination of the amount of untethered polymer will be discussed in detail in a future paper, preliminary work with as-prepared PS grafted silica ([M.sub.n]= 16,100 g [mol.sup.-1], PDI = 1.13, conversion 65%) and PS-b-PMPTS grafted silica ([M.sub.n] = 41,100 g [mol.sup.-1], PDI = 1.24, conversion 45%) showed that the fraction of untethered polymer estimated by TGA (Fig. 6) was only 9 and 11% after the particles were thoroughly washed with THF by three repeated cycles of dispersion centrifugation, respectively. We conclude that the immobilization of the initiator onto silica nanoparticlcs is an efficient method for preparing polymer brushes on nanoparticle surfaces. As the initiators were on the surface of silica particles, it can be concluded that the untethered polymer chains formed thermally. This thermal initiation can affect the polymerization. Especially in the final stage of the polymerization, more thermo-initiated untethered polymer chains come into being due to the absence of the free initiators. It is confirmed by the fact that the ratio of [M.sub.w]/[M.sub.n] increases during the polymerization (Fig. 3). Appropriate monomer concentration, initiator concentration, and lower polymerization temperature are advantageous to eliminate or minimize the generation of untethered polymer, which will be discussed in detail in future research.

[FIGURE 6 OMITTED]

Figure 7 shows the SEM micrographs of films of the [SiO.sub.2] particles before and after the SI-ATRP. The significant aggregation of the small spherical [SiO.sub.2] particles can be clearly observed from the SEM image as shown in Fig. 7a. Figure 7b shows that the [SiO.sub.2] particles grafted by PS via SI-ATRP of St are totally enveloped in a continuous PS phase. The [SiO.sub.2] nanoparticles are distributed fairly evenly in the polymer phase to form the [SiO.sub.2]/PS nanoparticles, which is important for future nanoparticle applications. Furthermore, the long polymer chains can twist among the particles to lead to further aggregation of the [SiO.sub.2] particles, which generally generates a primary roughness for water repellency. Furthermore, according to Fig. 7c, the aggregation phenomenon can still he observed. However, comparing Fig. 7c with Fig. 7b, it can found that an uniformer surface feature in the fine microstructure can be seen in the [SiO.sub.2]/PS-b-PMPTS nanoparticles due to the obvious microphase-separated morphology generated by the PS-b-PMPTS block polymers grafted from the [SiO.sub.2]/PS nanoparticles. In practice, it is well known that the block copolymer exhibits the microphase-separated structure with nanometer size because two different block sequences are chemically connected to each other, resulting in that each component cannot form large domains (25). In addition, the grafting of PMPTS on [SiO.sub.2]/PS nanoparticles creates larger interspaces between grafted [SiO.sub.2] aggregates, thus further roughening the [SiO.sub.2]-based films. Furthermore, comparing Fig. 7c with Fig. 7b, it is also found that the [SiO.sub.2]/PS-b-PMPTS nanoparticles grafted by PS-b-PMPTS block copolymers (sec Fig. 7c) present rougher surface and heavier density than that of the [SiO.sub.2]/PS nanoparticles (see Fig. 7b) grafted only by PS. The particle size of [SiO.sub.2]/PS and [SiO.sub.2]/PS-b-PMPTS estimated from SEM observation are approximately 45 and 90 nm, respectively. The particle size increase also indicates the grafting of PMPTS on to the surface of [SiO.sub.2]/PS.

[FIGURE 7 OMITTED]

To obtain the morphology and surface roughness of the [SiO.sub.2]/PS and [SiO.sub.2]/PS-b-PMPTS hybrid nanoparticles, the surfaces of the films were imaged using AFM. As seen in Fig. 8, the surface texture and the average size of the features were consistent with inorganic particles coated with polymers as observed in the SEM images (Fig. 7). Furthermore, the following data can be obtained using the software along with 5500ILM AFM. According to Fig. 8, the root mean square roughness ([R.sub.q]) of [SiO.sub.2]/PS nanoparticle film is 3.27 nm, while the average roughness ([R.sub.a]) of it is 2.41 nm over a scope of 1 [micro][m.sup.2]. After polymerization of MPTS on the surface of [SiO.sub.2]/PS, the [R.sub.q] of the [SiO.sub.2]/PS-b-PMPTS nanoparticles increases to 5.13 nm, while the [R.sub.a] of the [SiO.sub.2]/PS-b-PMPTS nanoparticles increases to 3.76 nm over a scope of 1 [micro][m.sup.2] The slight increase of the roughness might be caused by the micro-phase separation of [SiO.sub.2]/PS-b-PMPTS nanoparticles.

[FIGURE 8 OMITTED]

The wettability of the modified surface and the impact of nanometer-scale roughness on WCAs have also been studied by CA and AFM measurement. As shown in Fig. 7a inset, the WCA of the pure [SiO.sub.2] nanoparticles is about 28[degrees]. After modified by PS and PS-b-PMPTS sequentially, the WCAs dramatically increase to 153[degrees] (Fig. 7b inset) and 143[degrees] (Fig. 7c inset), respectively. The static WCA measurements show that the [SiO.sub.2]/PS nanoparticles are superhydrophobic (153[degrees]), and the possible reason is that the grafting of PS generates a uniform hydrophobic surface with PS film outside.

At the same time, PS has good hydrophobicity and is often used as a typical hydrophobic polymer for fabricating superhydrophobic surfaces (26). Besides surface chemistry, the surface roughness at the micro-/nanometer scale is another important factor affecting the wetting behavior of solid-state materials. As mentioned before, the surface root mean square (RMS) roughness of the [SiO.sub.2]/PS and [SiO.sub.2]/PS-b-PMPTS nanoparticles is 3.27 and 5.13 nm over a scope of 1 [micro][m.sup.2]. respectively. Comparison of Fig. 6a and b indicates that the static WCA of the [SiO.sub.2]/PS-b-PMPTS nanoparticles (143[degrees]) is smaller than that of the [SiO.sub.2]/PS nanoparticles (153[degrees]). However, the surface roughness of the former is slightly rougher than that of the latter, which seems to be unreasonable. Our opinion is, as the wettability is determined by surface chemistry and surface geometrical microstructure (1), (2), this phenomenon can be attributed to the fact that the PMPTS block is not so hydrophobic than that of the PS block, although the surface of the [SiO.sub.2]/PS-b-PMPTS nanoparticles is slightly rougher than that of [SiO.sub.2]/PS. Therefore, the combination and competition of surface chemistry and roughness of a solid material can finally determine its wettability. Although the hydrophobicity seems to drop with the PMPTS chains, the advantages of coatings obtained from the end product with silicone-containing polymer are the consequence of their ability to crosslink in situ after treatment of substrate. Due to the fact that the crosslinking reactions through several reactive groups are possible, the design of coating system tailored for various substrates can be proposed.

CONCLUSIONS

We have demonstrated the SI-ATRP from [SiO.sub.2] nanoparticles can be used to fabricate superhydrophobic [SiO.sub.2]/PS and [SiO.sub.2]/PS-b-PMPTS hybrid nanoparticle films in a controlled manner. Corresponding polymer growths via SI-ATRP are well controlled as demonstrated by the macromolecular characteristics of the grafted chains. The measured wetting data show that the static WCA of [SiO.sub.2]/PS-b-PMPTS hybrid nanoparticles is smaller than that of [SiO.sub.2]/PS hybrid nanoparticles, and the surface of [SiO.sub.2]/PS-b-PMPTS hybrid nanoparticles is yet slightly rougher than that of [SiO.sub.2]/PS hybrid nanoparticles, which shows that the material wettability can be mainly affected by the combination and competition of surface chemistry and roughness.

ACKNOWLEDGMENTS

The authors acknowledge the State Key Laboratory of Physical Chemistry of Solid Surfaces at Xiamen University for providing AFM facilities and assistance.

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Hai-Jiang Yu, Zheng-Hong Luo

Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People's Republic of China

Correspondence to: Z.-H. Luo; e-mail: luozh@xmu.edu.cn

Contract grant sponsor: Nation Defense Key Laboratory of Ocean Corrosion and Anti-Corrosion of China; contract grant number: 51449020205QT8703; contract grant sponsor: Fujian Province Science and Technology Office of China; contract grant number: 2005H040.

Published online in Wiley Online Library (wileyonlinelibrary.com).

[C]2010 Society of Plastics Engineers

DOI 10.1002/pen.21809
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