Nitroxide-mediated polymerization of pentafluorostyrene initiated by PS–DEPN through the surface of APTMS modified fumed silica: towards functional nanohybrids

Abstract

Polypentafluorostyrene (PPFS) chains were anchored on the surface of silica nanoparticles (Aerosil A200 fumed silica) by nitroxide-mediated polymerization (NMP) with PS–DEPN as macroinitiator using a “grafting through” strategy with (acryloxypropyl)trimethoxysilane (APTMS)-modified silica. First, NMP of PFS was evaluated with and without silica in various solvents (namely N-methyl-2-pyrrolidone – MP, N,N-dimethylformamide – DMF, and toluene) that favor the dispersion of APTMS-silica in reaction medium. In both situations, polymerization presents all the features of a controlled process whatever the solvent is, with a marked impact of the solvent polarity on the kinetics. Moreover, NMP of PFS is faster in polar solvents when conducted in presence of APTMS-modified silica. By tuning polymerization time and/or solvent polarity, the weight ratio of organic matter to silica can be tuned from 5 to 32 wt%. The impact of such modification rate is demonstrated on the surface properties of SiO₂-PS-b-PPFS deposited on a silicon wafer: indeed, introduction of fluorinated segments in combination with dual roughness (due to inherent dimensions of fumed silica) lead to relevant hydrophobic surface properties with water contact angle as high as (132 ± 1.7)°.

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A Introduction

The grafting of polymers onto inorganic substrates has gathered attention due to their potential applications in the field of colloidal stabilization, surface property tailoring and microlithographic patterning.^1–7 Surface modification of inorganic particles is generally undertaken by physisorption or chemical bonding of chains. Typically, three main routes are usually reported to chemically attach a polymer chain to a surface: (i) the “grafting onto” method, where end functionalized polymers react with appropriate surface sites^8,9 (ii) the “grafting from”, where chains grow in situ from preformed surface-grafted initiators^10,11 and (iii) surface copolymerization through a covalently linked monomer (“grafting through”).¹² Silica particles are usually used as inorganic substrates to prepare inorganic–organic hybrids due to their mechanical resistance, high specific surface area and low cost.^13,14 In the literature, silica-based substrates coated with well-defined polymeric chains have been achieved by polymerization techniques such as controlled free radical polymerization (CRP) due to its simplicity and versatility permitting to build up highly dense polymer brushes from silicon wafer¹⁵ or silica gel.^16–19 In our previous studies,^20,21 well-defined polystyrene-coated silica nanoparticles were obtained using a “grafting from” procedure and the nitroxide mediated polymerization (NMP) technique. In addition, generation of low surface energy surfaces, solvent resistant coatings and ultra-low dielectric constant films can be obtained by the use of fluorine containing polymers.^22–24 In this context, polypentafluorostyrene (PPFS) and its corresponding copolymers are of particular interest.²⁵ Indeed, as a styrenic derivative the latters can be synthesized by various polymerization techniques, including CRP that makes the synthesis of functional sophisticated materials possible.^26–35 Recently, fabrication of superhydrophobic surfaces using raspberry like particles with immobilized PPFS was demonstrated using a multistep synthesis including an ATRP process.³⁶ In addition, the labile para-fluorine of the pentafluorophenyl group in PPFS-based materials, can undergo regioselective substitution by nucleophiles such as amines, thiols and alcohols.^37–39 Herein, we report for the first time the synthesis of hydrophobic PPFS-b-PS grafted silica particles by combining nitroxide mediated polymerization and the “grafting through” procedure. Indeed, the use of conventional free radical polymerization for the graft copolymerization of vinyl monomers with immobilized monomers does not permit to control the layer thickness that reaches a plateau after only a few hours polymerization. Moreover, due to the constant molecular weight of the grafted chains with the polymerization time, the graft density shows a similar behavior, as discussed by Ruhe.⁴⁰ This paper emphasizes both the effect of solvent on the PFS polymerization kinetics and the quantitative determination of the PS-b-PPFS grafting weight. Moreover, the corresponding surface properties are discussed in terms of water wettability.

B Experimental section

B.1 Materials

Aerosil*200 fumed silica (Aldrich) is characterized by an average diameter of 13 nm for the elementary particle (aggregates sizes ranging from 500 nm to 1 μm) and a specific surface area of 200 m² g⁻¹. Before use, it was dried at 150 °C under vacuum for 4 h. 2,3,4,5,6-Pentafluorostyrene (PFS, 99%, Aldrich) was distilled under vacuum and stored at −20 °C. Styrene (S, 99%, Aldrich) was distilled. DEPN (N-tert-butyl-N-(1-diethylphosphono-2,2-dimethyl)propyl nitroxide) was used as received from Polymer expert. Toluene (99.8%, Aldrich), N,N-dimethylformamide (DMF, 99%, Aldrich) and N-methyl pyrrolidone (MP, 99%, Aldrich) were distilled before used. (Acryloxypropyl)trimethoxysilane (APTMS) and all other chemical products (Aldrich) were used as received. The DEPN-based alkoxyamine (styryl-DEPN) was prepared using a procedure described in the literature.⁴¹

B.2 Analytical techniques

Fourier transform infrared (FT-IR). FT-IR spectra were recorded on a Bruker IFS 66/S spectrometer using pressed KBr pellet. The resolution of the measurement was 4 cm⁻¹ within a range of 500–4000 cm⁻¹ and an average of 32 scans.

Size exclusion chromatography (SEC). SEC characterizations were performed using a 2690 Waters Alliance System with CHCl₃ (HPLC grade) or THF (HPLC grade) as eluent at a flow rate of 1 mL min⁻¹. The chromatographic device was equipped with four Styragel columns HR 0.5, 2, 4 and 6 at 40 °C in series with a 2410 Waters refractive index detector and a 996 Waters photodiode array detector. A calibration curve was established with low dispersity index polystyrene standards.

Nuclear magnetic resonance spectroscopy (NMR). ¹H and ¹⁹F NMR spectra were recorded on a Bruker Advanced AM400 spectrometer at 400 MHz. Samples were dissolved in CDCl₃ and chemical shifts were determined in ppm with 64 scans for each kind of analysis.

Thermogravimetric analysis (TGA). TGA measurements were performed on a TA Q500 at a heating rate of 10 °C min⁻¹ between 50 and 750 °C under a nitrogen flow (40 mL min⁻¹).

Transmission electron microscope (TEM). TEM images were taken with a Philips CM120 transmission electron microscope operating at an accelerating voltage 80 kV. Crude products were deposited onto carbon layer covered cupper grid while pure products were dispersed into THF with a concentration of 1 mg mL⁻¹ before deposition.

Cryo-scanning electron microscope (cryo-SEM). Cryo-SEM images were taken by using a NEON 40 (Zeiss) scanning electron microscope.

Water contact angle analysis (WCA). Water contact angle (WCA) measurements were carried out using a Dataphysics Digidrop contact angle meter equipped with a CDD2/3 camera with the sessile drop method and by using Milli-Q quality water as probe liquid. The tabulated results are the average of at least five measurements on different parts of each sample.

Atomic force microscopy (AFM). AFM images were acquired in intermittent contact mode (tapping mode), on a wafer of 1 cm × 1 cm, in air at room temperature using an AFM Bruker Multimode 8 apparatus equipped with Nanoscope V controller. The scanning speed for image acquisition is 0.5 Hz. The used tips are TAP 150 with a radius of curvature of 8 nm. The surface roughness R_q was obtained by processing images using the Nanoscope Analysis software (version 1.5).

B.3 Preparation of macroinitiator (PS–DEPN)

Styrene (13.5 g; 0.13 mol), and styryl-DEPN (260 mg; 0.65 mmol) were introduced in a glass tube, degassed with freeze–pump–thaw cycles and heated at 110 °C for 40 min in an oil bath. The polymer was recovered by precipitation in excess methanol and dried in a high vacuum oven.

Styrene conversion = 15%, M_n = 4100 g mol⁻¹, Đ = 1.30.

B.4 Polymerization of PFS using PS–DEPN as macroinitiator

PFS was polymerized by NMP using PS–DEPN as macroinitiator in different solvents (toluene, DMF and MP). In a model reaction, PFS (5 mL, 36.22 mmol) was solubilized in 5 mL of solvent (toluene, DMF or MP) PS–DEPN (742 mg, 0.18 mmol) was added to get a mixture solution with [PFS]/[PS–DEPN] = 200. Then, the mixture was divided into 5 vials with the same volume (2 mL). The five flasks were degassed four times by freeze–pump–thaw cycles, flame-sealed under vacuum and then heated in a preheated oil bath at 110 °C for various reaction times (0.5 h, 1 h, 2 h, 4 h and 6 h). The polymerization was stopped by immersing the flask into liquid nitrogen before breaking it. An aliquot was withdrawn and further dedicated to conversion determination by ¹⁹F NMR. The reaction mixture was precipitated into cold methanol and filtered. Purification of the polymer was achieved by dissolving in THF and precipitated in cold methanol again. The pure polymers were collected and dried at 50 °C under vacuum for overnight.

B.5 Synthesis of APTMS-modified silica (SiO₂-APTMS)

A certain amount of APTMS, with a concentration of 4–40 μmol m⁻², was added to a toluene dispersion of SiO₂ (25 mg mL⁻¹). The reaction mixture was stirred for 30 min at room temperature and then at 110 °C for 24 h. The free non-grafted APTMS was eliminated by successive centrifugation/redispersion cycles in toluene. The APTMS-functionalized silica particles were collected and dried at 50 °C under vacuum overnight, then further characterized by FTIR and TGA. The latter technique was used to determine the APTMS grafting density according to the following eqn (1):where W_(SiO2-APTMS) and W_SiO2 are the weight loss (in %) of APTMS-modified silica and neat silica between 50 and 750 °C; M_APTMS (g mol⁻¹) is the molar mass of APTMS and S_spec is the specific surface area of silica; N_A is the Avogadro constant. The grafting yield, which corresponds to the fraction of APTMS effectively covalently attached to the silica surface, was determined using the following eqn (2):

APTMS grafting yield (%) = grafting density × 100/[APTMS] (2)

where [APTMS] (molecule nm⁻²) is the initial concentration of APTMS.

B.6 Polymerization of PFS in presence of APTMS modified silica and PS–DEPN by “grafting through” approach

For a typical reaction, a given amount of SiO₂-APTMS (0.3 g) with a grafting density of APTMS equal to 0.55 molecule per nm² was dispersed in 5 mL of solvent (toluene, DMF or MP). After sonication for 10 min and stirring for 30 min at room temperature, a solution of PFS (5 mL, 36.22 mmol) and PS–DEPN (742 mg, 0.18 mmol) ([PFS]/[PS–DEPN] = 200) was added. The reaction mixture was divided into five vials with the same volume (2 mL). The five flasks were degassed four times by freeze–pump–thaw cycles, flame-sealed under vacuum and then heated in a preheated oil bath at 110 °C for various reaction times (0.5 h, 1 h, 2 h, 4 h and 6 h). The polymerization was stopped by immersing the flask into liquid nitrogen before breaking it. An aliquot was withdrawn and further dedicated to conversion determination by ¹⁹F NMR. The reaction mixture (directly after polymerization and before purification) was dedicated to TEM and cryo-SEM analyses. Then free PS-b-PPFS chains possibly physisorbed to silica surface were removed by extensive centrifugation (11 000 rpm, 30 min)/redispersion cleaning cycles in THF (five times). After each cycle, the supernatant was collected, concentrated and precipitated in cold methanol to recover free polymer. Blank experiment involving neat silica was performed with the same procedure to confirm that ungrafted polymer chains were completely removed using such work up. After purification, the final PS–PPFS-grafted silica (SiO₂-PS-b-PPFS) particles and free polymers (PS-b-PPFS) were recovered separately and dried at 50 °C under vacuum overnight then were characterized by ¹H-NMR, ¹⁹F-NMR, FT-IR, TGA, and TEM. The PS-b-PPFS grafting weight was calculated by eqn (3),where W_{(SiO2-PS-b-PPFS)} is the weight loss (in %) of PS–PPFS grafted silica between 50 and 750 °C.

B.7 Deposition of the hybrid modified silica onto silicon wafer

Silicon wafers were purchased from Sil'tronix. They were cut into appropriate dimensions (typically 1 cm × 1 cm) before use, and activated by ozonolysis for 30 min prior to any deposition. Free PS-b​-PPFS and hybrid SiO₂-PS-b-PPFS particles were dispersed into THF with a concentration of 30 mg mL⁻¹, and then 0.1 mL of such solutions was spin-coated onto the cleaned silicon wafer with a speed of 2000 rpm for 30 s using a Polo Spin 150i/200i spin-coater type from SPS-Europe. Then THF was evaporated at room temperature for two days before any analysis.

C Results and discussion

In our synthetic strategy (Scheme 1), three steps were used to anchor PS-b-PPFS copolymer chains to the surface of silica particles. First, a macroalkoxyamine based on DEPN and PS was prepared by nitroxide mediated polymerization, and APTMS-grafted silica particles were synthesized separately according to literature²¹ in order to introduce at the surface of the silica particles C=C double bonds that are reactive in radical (co)polymerization. Indeed, the thermal cleavage of PS–DEPN produces PS-based macroradicals that can react with both pentafluorostyrene and the vinyl group in grafted APTMS molecules leading to chemically bonded block copolymer chains at the silica surface (Scheme 1).

Scheme 1 Reaction scheme for nitroxide mediated polymerization of pentafluorostyrene (PFS) initiated by PS–DEPN through the surface of APTMS functionalized fumed silica.

Then, PS-b-PPFS chains with controlled molecular weights and narrow polydispersities were attached through the APTMS functionalized nanoparticles surface using a known amount of PS–DEPN macroinitiator (Scheme 1). The chain length of the grafted and the free PPFS-based copolymer is controlled by the pentafluorostyrene to PS–DEPN molar ratio, polymerization time and solvent that is used. It should be also mentioned that modified nanoparticles are multifunctional sites that may participate in crosslinking reactions.

C.1 Kinetic of the nitroxide mediated polymerization of PFS in presence of PS–DEPN and without silica

We have focused our study on the influence of the solvent on the kinetic of the nitroxide mediated polymerization of PFS employing a polystyrene macroinitiator (PS–DEPN). Indeed, it is well known that the homolysis of the C–ON bond in alkoxyamines at the initiation stage (k_d) plays a key role in the success of nitroxide polymerization.⁴² Moreover, it is of crucial importance to find the most suitable solvent for the forthcoming grafting reaction in presence of APTMS-grafted silica so the solvent effect was investigated in three solvents including toluene, DMF and MP. The extension of PS–DEPN homopolymer (M_n = 4100 g mol⁻¹, Đ = 1.3; see Experimental part) with PFS in solution was carried out at 110 °C for different polymerization times and the monomer-to-macroinitiator ratio (200/1) was chosen such that the number-average molecular weight M_n at complete conversion of monomer would equal 40 000 g mol⁻¹. The structural characterization of the copolymers was achieved using both ¹H NMR and ¹⁹F NMR spectroscopy. For example, Fig. S1† illustrates the ¹⁹F NMR spectrum of the crude PS-b-PPFS solution in MP after 4 hours. The conversion of PFS monomer in the withdrawn samples was calculated from the ¹⁹F NMR spectrum (Fig. S1†) by use of the normalized area for the aromatic fluorines (para position at 156.5 ppm; I_P) for PFS as compared to the area of the aromatic fluorines (para position at 154.5 ppm; I_P′) for PPFS according to the following equation: conversion = I_P′/(I_P′ + I_P).

In order to examine the controlled radical polymerization process, kinetics, molecular weight and dispersity were analyzed as a function of time. As it can be seen in Fig. 1, a linear ln([M₀]/[M]) vs. time plot was obtained for all the samples showing that radical concentration remains unchanged during the polymerization and therefore polymerization proceeds in a controlled manner.

Fig. 1 Kinetics of nitroxide mediated polymerization of PFS at 110 °C; [PFS] = 3.6 mol L⁻¹; [PS–DEPN] = 0.018 mol L⁻¹ in different solvents.

Assuming pseudo-first order kinetics for the steady-state stage of polymerization, the reaction rate equation can be stated as follows: ln([M₀]/[M]) = k_appt^2/3 where, k_app is the slope of such linear variation and is known as apparent rate constant of polymerization. According to the results, k_app varied from 4.86 × 10⁻⁵ s^−2/3 (MP as solvent) to 0.29 × 10⁻⁵ s^−2/3 (toluene as solvent). One can observe that the PFS polymerization rate is higher when using MP as solvent. As discussed by Marque⁴³ and Billon,⁴⁴ the main factor that determines the decomposition rate of nitroxide adducts is a stabilization of radicals so one can suggest that the reason of the increase in k_app value in polar solvents is a stabilization of the liberated nitroxide due to its solvation by the polar molecules.

Moreover, SEC traces of PS-b-PPFS copolymers showed the increasing molecular weights of copolymers with increasing polymerization time and peaks with monomodal distribution, indicating the successful initiation of nitroxide mediated copolymerization by PS–DEPN, whatever the solvent is (Fig. 2, MP as solvent).

Fig. 2 SEC traces of PS-b-PPFS copolymers versus polymerization time (MP as solvent).

For all the solvent (MP, DMF and toluene), a linear correlation between number average molar mass M_n and conversion was obtained with polydispersity lower than 1.3. As an illustration, Fig. 3 presents the results obtained for MP used as the solvent.

Fig. 3 Linear dependence of M_n versus conversion and dispersity evolution during the nitroxide mediated polymerization of PFS at 110 °C in MP.

One can observe that the molar masses determined by ¹H NMR are close to the theoretical ones. Considering Fig. 1–3, we can conclude that the nitroxide mediated polymerization of PFS exhibits all the criteria of a controlled radical polymerization.

C.2 Synthesis of the PS-b-PPFS-grafted silica

First, similarly to our previous results in this field,²¹ preliminary experiments were dedicated to the determination of the optimal conditions for grafting APTMS at the surface of silica. The corresponding APTMS grafting densities were determined by using thermogravimetric analysis and eqn (1) (see Experimental part). Indeed, it is well known that heating functionalized silica particles in an inert atmosphere removes the organic moieties and restores the silica structure. Fig. S2† reports on the effect of the APTMS concentration on the grafting density. We can observe that the grafting density (determined by TGA) increases with increasing the APTMS content and reaches a plateau at high concentrations: a maximum grafting density of 0.55 molecule per nm² is achieved for a APTMS content of 18 μmol m⁻² and more. Then, the copolymerization of PFS with vinyl-grafted silica (4.3 wt%) in presence of PS–DEPN with a [PFS] to [PS–DEPN] ratio of 200/1 was investigated according to a “grafting through” approach (Scheme 1). It results in the formation of polymer chains attached to the surface of silica and free chains dispersed in the solution. The polymerization of PFS was performed according to the same experimental procedure as in the absence of silica and similar conclusions were drawn: a linear ln([M₀]/[M]) vs. time plot was obtained for all the samples (Fig. S3†) and MP permitted to obtain higher conversion than toluene and DMF in presence of a silica content of 4.3 wt% (Table 1). Moreover, it should be noted that addition of silica nanoparticles (especially in DMF and MP) favorably affects the kinetics of PFS polymerization in polar solvents with apparent rate constants of polymerization increasing from 4.86 × 10⁻⁵ s^−2/3 to 8.41 × 10⁻⁵ s^−2/3 for MP as solvent (Fig. S3†). As discussed by Salami-Kalajahi et al.,⁴⁵ this phenomenon could be attributed to the partially polarizing effect of the silica nanoparticles on the reaction medium and thereby its acceleration effect on the polymerization rate. Herein, residual pendant hydroxyl groups on the surface of silica particles could possibly cause adsorption of DEPN radicals thus prolonging the lifespan of the polymeric radicals and finally leading to a higher conversion.

Table 1 Effect of solvent on PFS conversion with and without APTMS-grafted silica

Time (h)	Without silica			With 4.3 wt% silica-APTMS
	PFS conversion (%)			PFS conversion (%)
	Toluene	DMF	MP	Toluene	DMF	MP
0.5	2	5	17	1.7	17	17
1	5	16	20	2.2	19	31
2	8	22	39	4.3	34	48
4	10	25	55	7.0	47	74
6	16	40	65	12.2	62	76

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After quantitative removal of the free block copolymer chains by extensive washings, analysis of the recovered silica powder by FTIR gave clear evidence of PPFS grafting (Fig. 4).

Fig. 4 FT-IR spectra of (a) SiO₂, (b) SiO₂-APTMS, (c) SiO₂-PS-b-PPFS and (d) free PS-b-PPFS copolymer.

Compared with silica, PS-b-PPFS-grafted silica exhibits a FTIR peak at 1740 cm⁻¹ indicative of a carbonyl group from APTMS and a double band in the range of 1450–1550 cm⁻¹ that can be ascribed to both the hydrogenated and the fluorinated aromatic ring vibrations.³⁷ Indeed, it can be observed in Fig. S4† that the band located at 1490 cm⁻¹ can be attributed to the PS- and PPFS-based aromatic vibration while the 1525 cm⁻¹ one corresponds to the PPFS-based aromatic vibration only. Further insight into the grafting reaction was obtained by determining the amount of polymer chains grafted on silica particles. Fig. 5d presents a typical thermogram for free PS-b-PPFS copolymer exhibiting a decomposition temperature close to 450 °C. It should be also noted that the decomposition temperature of PS-b-PPFS copolymer slightly increases after grafting onto the silica particles, as expected (Fig. 5c). TGA results show that free PS-b-PPFS copolymer can be completely decomposed at a temperature of 550 °C; therefore, the weight amount of PS-b-PPFS copolymer that is covalently attached to the silica surface is estimated by the weight loss of PS-b-PPFS-grafted silica sample between 200 °C and 550 °C after subtracting the contribution of APTMS groups (Fig. 5b). From Fig. 5, the PS-b-PPFS weight grafting is around 32% when polymerization proceeded in MP for 6 hours.

Fig. 5 TGA of (a) SiO₂, (b) SiO₂-APTMS, (c) SiO₂-PS-b-PPFS and (d) free PS₃₆-b-PPFS₁₄₂.

Fig. 6 reports on the effect of the PFS polymerization time on the polymer grafting weight on silica as a function of the solvent.

Fig. 6 Amount of polymer grafting weight as a function of PFS polymerization time ([PFS]/[PS–DEPN] = 200/1, 4.3 wt% of SiO₂-APTMS particles, 110 °C). The line is a guide for the eye.

It is well known that the use of a “grafting onto” approach in order to functionalize a substrate by a reactive end terminated polymer leads to a decrease of the molar coverage by increasing the polymer molecular weight due to the corresponding reduction in the entropy effect. Herein, by using the “grafting through” approach, it is observed the reversed situation so the PS-b-PPFS grafting weight increases with increasing PFS polymerization time (i.e. the PS-b-PPFS molar mass) whatever the solvent is. This behaviour is probably due to the chemical affinity of APTMS-grafted silica particles for the PFS polymerization medium by increasing the chain length of the grafted PPFS block. However, the use of toluene as solvent does not permit to obtain high polymer weight content even after 6 h regarding the low PFS conversion when studying the effect of solvent on PFS conversion (Table 1). Interestingly, by using MP as solvent, the PPFS content on the silica surface varies from 5 wt% to 32 wt% and levels off after a polymerization time of 4 h (Fig. 6).

The characteristics of free PS-b-PPFS block copolymers synthesized in presence of APTMS-grafted silica are listed in Table S1.† As shown in the latter Table S1,† the well-defined PS-b-PPFS copolymers with different content of PPFS segments were successfully obtained whatever the solvent is.

Insight into morphology was provided by both cryo-SEM and TEM coupled with EDX analysis. First, the evolution of the morphologies occurring with time during the polymerization of PFS was analysed. Cryo-fracture SEM image of the crude product prepared in MP at low conversion (17%, Table 1) shows the presence of PS-b-PPFS based nodules surrounded by silica nanoparticles suggesting a phase separation (Fig. 7a). By increasing the PFS conversion to 31%, one can observe that the nodules have disappeared and the silica particles are homogeneously dispersed within the organic matrix due to favoured chemical affinity of the PS-b-PPFS grafted silica for the organic matrix, as discussed earlier (Fig. 7b).

Fig. 7 Cryo-fracture SEM images (a and b) and TEM images (c and d) of SiO₂-PS-b-PPFS crude product in MP (samples (a and c) – conversion = 17%, samples (b and d) – conversion = 31%, see Table 1).

Moreover, TEM images of PS-b-PPFS grafted silica crude product in MP suggest that silica agglomerates are partly destroyed upon polymerization (Fig. 7c and d). Indeed, Fig. 7c shows micrometer-sized domains of stringy shaped aggregated particles for a PFS conversion of 17% while the silica particles appear into domains with a size close to 300 nm for a PFS conversion of 31% (Fig. 7d). In addition, the TEM image of pure PS-b-PPFS grafted silica shows that majority of particles are spherical shaped with diameter about 10 nm and that they are surrounded at the surface by a fuzzy polymer layer (Fig. 8).

Fig. 8 TEM image of pure SiO₂-PS-b-PPFS nanoparticles (t = 6 h; MP) cast from THF suspension (c = 1 mg mL⁻¹).

The EDX spectra of PS-b-PPFS-grafted silica nanoparticles evidences that the particles observed by TEM (dark regions in Fig. 8) are homogeneously composed of carbon, of oxygen, silicon and fluorine elements, arguing that silica particles are modified with the aforementioned block copolymer (Fig. S5†).

Silica particles modified with fluorinated polymers such as PPFS chains constitute relevant candidates to impart hydrophobic surface properties to a given substrate. Thus, dispersions of SiO₂-APTMS and SiO₂-PS-b-PPFS in THF were spin-coated onto activated silicon wafers and the surfaces of the resulting thin films were analyzed by atomic force microscopy (AFM) and through water contact angle (WCA) measurements (Fig. 9).

Fig. 9 AFM topographic images of deposition resulting from (a) SiO₂-APTMS (b) SiO₂-PS-b-PPFS (grafting weight = 32%, Fig. 6) with the corresponding height profile and wettability properties.

First, compared to neat silicon wafer (R_q value of 0.25 nm, AFM image in Fig. S6†), AFM images of both SiO₂-APTMS and SiO₂-PS-b-PPFS enable to ascertain the presence of modified silica at the surface of the silicon wafer (Fig. 9). Indeed, the height profiles of AFM images reveal a dual surface structuration with two main range sizes in lateral dimension (from few tens to few hundreds nm) as the signature of fumed silica particles which can be considered as elementary particles self-assembled in aggregates. This topology corroborates the two-level morphology of SiO₂-PS-b-PPFS evidenced by TEM analysis (Fig. 8). The roughness (R_q) of SiO₂-APTMS is close to 23 nm while the one of SiO₂-PS-b-PPFS increases from 32.5 nm to 55.8 nm when the polymer grafting weight ranges from 5 wt% to 32 wt% (Fig. 6).

Moreover, when water is deposited onto APTMS grafted silica and PS-b-PPFS grafted silica, SiO₂-APTMS exhibits a WCA of 27.8° while SiO₂-PS-b-PPFS displays a WCA in the range 85.2–132.1° depending on the polymer grafting weight (Table 2). It is admitted that water contact angles of hydrophobic surfaces are higher than 90° while hydrophilic surfaces have smaller water contact angles (0–30°).⁴⁶ Herein, the presence of polar acrylate groups in SiO₂-APTMS mainly explains its low WCA. Hydrophobicity for SiO₂-PS-b-PPFS samples may be attributed to the nano-scale silica phase well mixed and strongly adhered to the PS-b-PPFS chains. Indeed, both the presence of low surface energy fluorinated groups and the corresponding undulating topology for SiO₂-PS-b-PPFS provide hydrophobic features to the surface, as shown in Fig. 9b. It should also be noted that the WCA determined on a PS-b-PPFS film without silica equals (100.2 ± 1.0)° so the latter result confirms that the large WCA increase observed for SiO₂-PS-b-PPFS containing film is due to both the surface chemistry emanating from the hydrophobic character of long PPFS chains (M_n = 31 600 g mol⁻¹) and the peculiar roughness stemming from the inherent dimensions of fumed silica particles.

Table 2 Water contact angles for APTMS grafted silica and PS-b-PPFS grafted silica based samples

Polymer grafting weight (%)	Water contact angle (°)	Rq (nm)
a SiO2-APTMS.
0^a	27.8 ± 1.5	23.2
5	85.2 ± 2.0	32.5
18	126.5 ± 1.1	42.4
32	132.1 ± 1.7	55.8

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D Conclusions

Well-defined polystyrene-block-polypentafluorostyrene-coated silica nanoparticles were prepared by nitroxide-mediated polymerization using a “grafting through” approach. The composition structure, chain length, and the polymer grafting weight were precisely controlled and the growth process exhibited all criteria of controlled radical polymerization. It was shown that addition of silica nanoparticles affects the kinetics of PFS polymerization favorably in polar solvents with apparent rate constants of polymerization increasing by a factor two which may be due to adsorption of DEPN radicals. Moreover, contrary to the use of a “grafting onto” approach, it was observed that the PS-b-PPFS grafting weight increases with increasing PFS conversion probably due to the much favorable chemical affinity of APTMS-grafted silica particles for the PFS polymerization medium by increasing the chain length of the grafted PPFS block. Cryo-fracture SEM and TEM observations confirmed that the dispersibility of silica in MP/PFS solution was improved when the PFS conversion was higher than 17% and the silica particles appear into domains with a size varying from 1 μm to 300 nm by increasing the PFS conversion from 17% to 31%. Finally, it was shown that the hydrophobicity of PS-b-PPFS grafted silica hybrid films can be tailored by the PS-b-PPFS grafting weight.

Future work will examine the influence of the regioselective substitution of para-fluorine atoms of the pentafluorophenyl groups on the interactions between both the grafted polymer chains and silica particles. It is expected that the corresponding results will open the route to the elaboration of nanocomposite with potential applications in materials science.

Acknowledgements

We are very grateful to Pierre Alcouffe (IMP, UMR 5223, Villeurbanne) for his help in the SEM and TEM observations and all the staff of the Technological Centre of Microstructures of the University of Lyon 1 for their technical help. We thank Sébastien Pruvost (IMP, UMR 5223, Villeurbanne) for his helpful suggestion in the AFM analysis. We thank the NMR Polymer Centre and the “Plateforme de Caractérisation par Chromatographie Liquide” of Institut de Chimie de Lyon (FR 3023) for access to the NMR and SEC facilities, respectively. Q. Y. acknowledges the Chinese Scholarship Council (CSC) for a Ph.D. grant.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra08973a

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