ARTICLE pubs.acs.org/Langmuir
Tailored Covalent Grafting of Hexafluoropropylene Oxide Oligomers onto Silica Nanoparticles: Toward Thermally Stable, Hydrophobic, and Oleophobic Nanocomposites Nelly Durand, David Mariot, Bruno Ameduri,* Bernard Boutevin, and Franc-ois Ganachaud* Institut Charles Gerhardt, Ingenierie et Architectures Macromoleculaires, UMR CNRS 5253, Ecole Nationale Superieure de Chimie de Montpellier, 8 Rue de l’Ecole Normale, 34296 Montpellier, France
bS Supporting Information ABSTRACT: The modification of silica nanoparticles with hexafluoropropylene oxide (HFPO) oligomers has been investigated. HFPO oligomers with two different average degrees of polymerization (DPn = 8 and 15) were first prepared by anionic ring-opening polymerization, deactivated by methanol, and in some cases postfunctionalized by aminopropyl(tri)ethoxysilane or allylamine. The “grafting onto” reactions of these oligomers were then carried out either on bare silica (reaction between a silanol surface and ethoxy-silanized HFPO) or on silica functionalized by amino groups (in an amidation reaction with methyl ester-ended HFPO) or mercapto groups (via the radical addition of allyl-functionalized HFPO). Hybrid nanoparticles thus obtained were characterized by solid-state 29Si NMR and FTIR spectroscopies as well as elemental and thermogravimetric analyses. The results assessed a significant yield of covalent grafting of HFPO oligomers when performing the hydrolysis-condensation of ethoxylated HFPO on the bare silica surface, compared to the other two methods that merely led to physically adsorbed HFPO chains. Chemically grafted nanohybrids showed a high thermal stability (up to 400 °C) as well as a very low surface tension (typically 5 mN/m) compared to physisorbed complexes.
’ INTRODUCTION The synthesis of core-shell nanohybrids (i.e., those composed of an inorganic core and a polymer shell) is of important scientific interest with a view toward constructing functional nanostructured materials with enhanced properties.1,2 Indeed, such core-shell structures can combine the unique electronic, optical, or magnetic properties of the filler with the relevant solubility, film formation, and processability of the polymer.3,4 In the specific case of nanocomposites, the content and dispersion of nanosized particles in a polymer matrix have a crucial impact on the final properties of the resulting materials. Achieving efficient dispersion generally requires the chemical or physical modification of the nanoparticle surface.1 Physical methods involve the physisorption of the polymer chains onto the nanoparticle surface through weak interactions such as van der Waals forces or hydrogen bonding.1 The chemical modification5,6 or “chemisorption” consists of the permanent covalent attachment of the macromolecular chains to the surfaces of nanoparticles. Three main approaches are classically used to generate chemical bonds between a polymer and a surface: (i) a “grafting-through”7 method involving the copolymerization between a growing polymer chain and a polymerizable group on a solid surface; (ii) a “grafting-from”8-18 technique through the initiation and propagation of polymer chains directly from the surface; and (iii) a “grafting-onto”2,19-29 pathway (i.e., covalent attachment of end-functionalized polymers onto the surface). r 2011 American Chemical Society
The synthesis of nanocomposites that combine highly polar fillers (such as silica) with oleo- and hydrophobic coatings (typically fluorinated polymers) remains challenging. In particular, perfluoropolyethers (PFPEs) are chemically inert, low-Tg amorphous polymers that give, after filling and cross-linking, elastomers with outstanding thermal properties for high-tech industries.30 These nanocomposites may be used as scratchresistant coatings with very low (sometimes superhydrophobiclike) surface tensions to be used in a high-temperature range (typically above 300 °C). In most instances, fluorinated silica was obtained either by a sol-gel process using a perfluoroalkyl di- or trialkoxysilane31,32 or by the condensation of perfluoroalkyl alkoxy-33,34 (or chloro-35,36) silanes onto a silica surface. Recently, new nanohybrids covered with oligomers of hexafluoropropylene oxide (HFPO), a model PFPE,37,38 have been described. For instance, Erhardt and Nuzzo39 deposited a functional amphiphilic PFPE on a silica surface by spin casting and contact printing. The former method produced a thin film, whereas the latter method led to complex surface structures composed of beaded domains and depleted regions that result from dewetting processes. Spontaneous dewetting was used to generate self-organizing PFPE bead patterns by microcontact Received: December 8, 2010 Revised: January 26, 2011 Published: March 10, 2011 4057
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Scheme 1. Synthesis of Hexafluoropropylene Oxide (HFPO) Oligomers and Their Functionalization by Condensation with an Alcohol and a Primary Amine50
printing. Also, Gosalawit et al. modified different nanoparticles such as silica40 and montmorillonite41,42 with HFPO oligomers. The grafting reaction consisted of reacting COF-terminated PFPE oligomers, freshly prepared by ring-opening polymerization (ROP) of HFPO, with amino-functionalized particles using a water-conjugating agent (namely, 1-(3-dimethylaminopropyl)3-ethylcarbodiimide hydrochloride).40 The originality of the present study lies in the different chemistries tested to graft hexafluoropropylene oxide oligomers onto silica nanoparticle surfaces and the variety of characterization techniques carried out to discriminate between physi- and chemisorption. The final aim of this work is to choose, from these, the best method to obtain a true and long-lasting chemical modification, which to our knowledge is an issue that was not specifically addressed in the papers mentioned above.
’ EXPERIMENTAL SECTION Materials. Unless otherwise stated, all products were purchased from Sigma-Aldrich. Hydrophilic fumed silica, a gift from the Bluestar Silicones Company (AEROSIL 150, Evonik AG, data on the specific surface area according to the manufacturer was confirmed by BET analysis), is composed of primary particles with an average diameter of 14 nm. It was used after a specific drying treatment (2 h at 200 °C under primary vacuum). Hexafluoropropylene oxide (HFPO, 97% purity), 3-aminopropyltriethoxysilane (APTES, 99.9% purity), and 3-aminopropyldimethylethoxysilane (APDMES, 99.9% purity) were purchased from ABCR. Tetraethyleneglycol dimethyl ether or tetraglyme (99% purity) and acetonitrile (99% purity) were stirred over CaH2 under argon for 48 h and then vacuum distilled (∼89 °C/0.02 mmHg). KF (99% purity) was dried at 90 °C under vacuum (0.02-0.03 mmHg) for at least 12 h. Methanol (99.9% purity) and allylamine (98% purity) were used as received. The ethanol solution used for modifications contained 90 vol % absolute ethanol (99.5% purity) and 10 vol % distilled water. 1,1,1,3,3-Pentafluorobutane or Solkane fmc 665 (C4F5H5, 97% purity, kindly delivered for free by Solvay) was distilled prior use. 3-Mercaptopropyltriethoxysilane (MPTES) (94% purity) was delivered by Alfa Aesar and used without any purification. Tertbutyl peroxypivalate (TBPPi) dissolved in isodecane (Trigonox 25-C75, purity 75%) was a gift from Akzo Nobel (Chalons sur Marne, France). 1,4Bis(trifluoromethyl)benzene or hexafluoroxylene (HFX, purity 98%) was purchased from Apollo Scientific. Methods. The weight percentages of X (where X represents carbon or fluorine atoms) were assessed by elemental analysis on different silica samples at the CNRS-Service Central d’Analyse (Solaize, France).
Nitrogen isotherms were achieved using a Micrometrics ASAP2020 instrument. BET (Brunauer-Emmett-Teller) theory43 was used to assess the physical adsorption of gas molecules (nitrogen) onto silica and a modified silica surface to calculate the specific surface area of these materials. These methods allowed the calculation of the tethering density (ηg) in μmol/m2 from the Berendsen44 equation ηg ¼
106 %X ½100 MX NX - %XðMsilane - 1Þ SBET
ð1Þ
where %X represents the weight percent of X, MX represents the molar mass of the X atom (g/mol), NX represents the number of X atoms in the grafted silane molecule, Msilane represents the molar mass of the silane, and SBET represents the specific surface area of the bare silica (SBET = 150 m2/g). Fourrier transform infrared attenuated total reflection (FTIR-ATR) spectra were recorded with a Perkin-Elmer Spectrum 100 in the wavenumber range of 650-4000 cm-1 with an accuracy of (2 cm-1. Samples were deposited on a diamond/ZnSe crystal and pressed with a dynamometric key. 19 F and 1H liquid NMR (400 MHz) spectra of bulk oligo(hexafluoropropylene oxide) fluids were recorded on a Bruker AC400 instrument at 25 °C using a capillary filled with acetone-d6 as an external locking reference. The experimental conditions were as follows: flip angle, 30°; acquisition time, 0.7 s; pulse delay, 2 s; number of scans, 68 and 32; and pulse width, 5 μs. 29 Si solid-state NMR was used to analyze the surface grafting qualitatively. Spectra were recorded using cross-polarization magic angle spinning (CP-MAS) on a Varian-ASX400 instrument (400 MHz) at 25 °C with a 3.2-mm-diameter rotor. The experimental conditions for recording 29Si CP/MAS NMR spectra were as follows: flip angle, 90°; recycle delay, 5 s; and spinning at 6 kHz. Thermogravimetric measurements were performed on a TA Instrument Q50 apparatus. Samples were heated under a mixed atmosphere of nitrogen and oxygen (60 and 40 mL/min) with a ramp temperature of 20 °C/min from room temperature to 800 °C. Transmission electron microscopy (TEM) measurements were carried out on a JEOL 1200 EXII apparatus operating at 100 kV. Samples were deposited directly onto copper grids and observed at a magnification of 100 000. The sessile drop method was used for static contact angle measurements at ambient temperature with an automatic video contact angle (CA) testing apparatus (Contact Angle System OCA, Data Physics). The probe liquids were water (θH2O) and diiodomethane (θCH2I2). The average CA value was determined on five different drops of 0.8 μL 4058
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Scheme 2. Silica Surface Activation with 3-Amino or 3-Mercaptopropyltriethoxysilane
those above, a round-bottomed flask was fed with 3.00 g (2.24 mmol) of 1340 g/mol oligo(HFPO) and 0.14 g (2.46 mmol) of allylamine. A viscous orange liquid (1.83 g, 90% yield) was obtained and labeled HFPO8-allyl. Silica Activation. Dried silica (1.00 g, referred to as SiO2 in this article) was homogeneously dispersed into a 90% ethanol solution (100 mL) under vigorous stirring for 15 min under argon. (3-Aminopropyl)triethoxysilane (APTES, 0.37 g, 1.7 mmol) or (3-mercaptopropyl)triethoxysilane (MPTES, 0.40 g, 1.7 mmol) was added to a SiO2ethanol suspension. The mixture was refluxed at 80 °C for 24 h, filtered, and washed several times with distilled water to obtain a yellow powder (SiO2-NH2, Scheme 2) or a white powder (SiO2-SH, Scheme 2).
Tethering of HFPO onto Functional Silica Nanoparticles.
deposited on the same sample and with both solvents. The surface tensions (γs) of nanoparticles were calculated with the method of Kaelble45,46 and Owens and Wendt,47 who used an extension of Fowkes’ equation48 γs ¼ γds þ γps
ð2Þ
qffiffiffiffiffi 1 þ cos θCH2 I2 pffiffiffiffiffi γL γds ¼ 2
ð3Þ
qffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffi γ ð1 þ cos θ Þ 2 γds γdL H O LV 2 p pffiffiffiffiffi γs ¼ 2 γPL
ð4Þ
where γds , γps , γdL and γPL stand for the dispersive and polar components of the surface tension of the surface and of the liquid, respectively, and γL stands for the liquid-vapor surface tension of the liquid. Here, we used 21.8 and 51.0 mN/m for the dispersive and polar components of water, respectively, and 50.8 and 0 mN/m for the dispersive and polar components of diiodomethane, respectively. Synthesis of Hexafluoropropylene Oxide Oligomers. The experimental conditions used to synthesize HFPO oligomers were similar to those recently optimized by Kostjuk et al.49 As a typical example, a Hastelloy reactor was charged with 0.41 g (7.1 mmol) of KF, 2.9 mL (13.2 mmol) of tetraglyme, and 10 mL of 1,1,1,3,3-pentafluorobutane (C4F5H5). Then, 45 g (0.27 mol) of hexafluoropropylene oxide was slowly transferred into the reactor. Polymerization started when the temperature reached 0 °C and was stopped after 2 h by adding 15 mL of methanol. After esterification, the polymer was washed three times with distilled water to remove tetraglyme, methanol, and catalyst. The resulting colorless liquid polymer (40.5 g, hereafter called HFPOCOOMe, Scheme 1) was obtained by removing the solvent with a rotary evaporator (10-20 mmHg/40-50 °C). The final results are as follows for HFPO15-COOMe HFPO conversion = 89%; Mn (as assessed by NMR) = 1900 g/mol. Functionalization of HFPO Oligomers. In the first route (Scheme 1), 3.00 g (2.24 mmol) of 1340 g/mol HFPO8-COOMe and 0.54 g (2.46 mmol) of APTES (or 0.40 g (2.46 mmol) of APDMES) were inserted under argon in a round-bottomed flask fitted with a condenser. The reaction was stirred and held at 70 °C for 4 h. Methanol formed during the reaction was removed under vacuum and the oligomer was kept under argon at 3 °C. A yellow viscous liquid (3.47 g, 98% yield) was obtained and named HFPO8SiOEt3 (or HFPO8-Si(CH3)2OEt). Under conditions similar to
Oligo(HFPO) (2.6 g, 1.7 mmol) functionalized with amidopropyltriethoxysilane groups (HFPO8-SiOEt3) was added to the SiO2ethanol suspension (vide supra). The mixture was refluxed at 80 °C for 24 h, filtered, and washed several times with distilled water to obtain a white powder (SiO2-SiOx-HFPO8, Scheme 3). Such a recipe was also applied to the grafting of simple HFPO8-COOMe as a blank experiment and to the monoethoxysilane oligomer (HFPO8-Si(CH3)2OEt), in order to interpret the grafting reaction better (vide infra). In another procedure, the SiO2-NH2 (1.00 g) compound was stirred with 2.24 g (1.7 mmol) of oligo(HFPO) functionalized with methyl ester (HFPO8-COOMe) in 100 mL of 90% ethanol solution (HFPO8COOMe/aminosilica molar ratio of 1:1). The mixture was vigorously stirred at room temperature for 24 h, filtered, and washed with distilled water to obtain a yellow wax (SiO2-NH-HFPO8, Scheme 3). In a third procedure, the SiO2-SH (1.00 g) compound was stirred with 2.24 g (1.7 mmol) of oligo(HFPO) functionalized with an allylamine (HFPO8-allyl) and 0.47 g (2.03 mmol) of tert-butyl peroxypivalate (TBPPi) in 100 mL of acetonitrile in a 1:1 molar ratio of HFPO8-allyl/mercaptosilane. The mixture was vigorously stirred at 74 °C for 48 h, filtered, and washed with acetonitrile to obtain a yellow wax (SiO2-S-HFPO8, Scheme 3). These modified silica particles were characterized once, systematically thoroughly washed with HFX on average two times, and characterized again to determine the content of physisorbed molecules that were eliminated (main text).
’ RESULTS The different strategies used to tailor the grafting of hexafluoropropylene oxide oligomers onto a silica surface are presented in Schemes 1-3. Three pathways were tested here: the first one is a hydrolysis condensation of hexafluoropropylene oxide (HFPO) oligomers primarily functionalized with a triethoxysilane (HFPO8-SiOEt3) or a monoethoxysilane (HFPO8-Si(CH3)2OEt) on a bare silica surface. Second, a grafting-onto reaction was tested via a condensation between an oligo(HFPO) functionalized with a methyl ester (HFPO8COOMe) and the amino-functionalized silica. The last grafting reaction consisted of a radical addition between a silica activated with a mercapto group and an oligo(HFPO) bearing an allyl end group (HFPO8-allyl). Synthesis and Modification of Hexafluoropropylene Oxide Oligomers. PFPE oligomers were prepared by the anionic
ring-opening polymerization of HFPO with KF/TG in pentafluorobutane following a recent procedure described elsewhere49 (Experimental Section). All oligomers were obtained in almost quantitative yield (above 85% monomer conversion) and were systematically characterized by 1H (e.g., Figure 1a) and 19F NMR (e.g., Figure S1) spectroscopy. Their number-average degrees of polymerization (DPn) were assessed according to eq S1 given in the Supporting Information. Polymerization carried out at 20 °C 4059
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Figure 1. 1H NMR spectra of hexafluoropropylene oxide oligomers terminated with (a) a methyl ester (HFPO8-COOMe), (b) a triethoxysilane (HFPO8-Si(OEt)3), and (c) an allyl end group (HFPO8-allyl). Spectra recorded with two deuterated acetone probes at 400 MHz and 298 K.
Scheme 3. Expected Grafting Reactions of Hexafluoropropylene Oxide Oligomers on Bare Silica and Modified Silicas
generated oligomers of about 8 monomer units (denoted as HFPO8-F), whereas polymerization performed at 0 °C depressed transfer reactions and led to 15 unit oligomers (denoted as HFPO15-F) (F stands for the different functional groups). These HFPO oligomers were then modified by ester aminolysis with an allylamine or 3-amino(propyl) triethoxysilane (APTES)50 at 70 °C (Scheme 1). Figure 1b displays the
1
H NMR spectrum of an HFPO oligomer modified with APTES. Signals centered at 8.7 ppm (a0 ) are ascribed to the -NHamide groups. The resonances at 4.1 (b0 ), 3.5 (c0 ), 2.2 (d0 ), 1.6 (e0 ), and 1.0 ppm (f0 ) are assigned to Si-O-CH2-, NHCH2-, -CH2-CH2-CH2, -O-CH2-CH3, and -CH2CH2-Si, respectively. The rather broad signals, compared to those in the 1H NMR spectrum of monoethoxysilane (Figure S2
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Figure 2. TGA thermograms of a series of hexafluoropropylene oxide (HFPO) oligomers of different chain lengths (-, HFPO8-COOMe; - 3 -, HFPO15-COOMe) modified with (3-aminopropyl)triethoxysilane ( 3 3 3 , HFPO8-Si(OEt)3; - 3 3 -, HFPO15-Si(OEt)3) and allylamine (---, HFPO8-allyl).
in Supporting Information), indicate that some condensation reactions may already have taken place but still a sol product was delivered (vide infra). The 1H NMR spectrum of HFPO8-allyl oligomers (Figure 1c) shows signals centered at 7.5 ppm (a00 ) ascribed to the -NH- amide groups. The resonances at 3.0 (b00 ), 4.8 (c00 ), and 4.1 ppm (d00 ) are assigned to NH-CH2-, -CH2-CHdCH2, and -CH2-CHdCH2, respectively. Thermogravimetric analyses of the different oligomers were then carried out in air. PFPEs are known for their outstanding thermally stability:49 for instance, HFPO oligomers bearing an ester end group and a degree of polymerization of 8 were heated in a reactor at 250 °C (temperature of the oil bath) without showing any degradation by 19F NMR spectroscopy. Thermograms presented here thus display only the volatilization of oligomers at low temperature, swept along by the air flux. Both chain lengths (8 or 15 units) and chain ends (methyl ester, allyl, and SiOEt3) influenced the oligomers’ volatilization (Figure 2). Increasing the chain length of oligo(HFPO)s terminated with an ester group shifted the thermograms toward higher temperature. The onset temperatures, given by the maximum of the derivatives of weight loss versus temperature curves (Table 1 and Figure S3), were 190 and 270 °C for HFPO8-COOMe and HFPO15-COOMe, respectively. The temperatures at 95% weight loss were 210 and 290 °C, respectively. Such values can be compared with the calculated boiling points of HFPO oligomers from the Woolf relationship,51 which are expected to be around 279 and 400 °C for oligo(HFPO) of 8 and 15 monomer units, respectively. For oligomers terminated with 3-amino(propyl) triethoxysilane (APTES), their onset temperature was further shifted to about 30 °C. Note also a tailing of HFPO8-SiOEt3 and HFPO15-SiOEt3 curves at around 270 and 340 °C, respectively (see also DTGA curves, Figure S3). Such behavior is ascribed to some premature hydrolysis/condensation reactions, as previously observed by NMR, giving rise to a small content of larger oligomers with less volatility. Other evidence of such a statement arises from the TGA curve of the oligomer functionalized with a monoethoxysilane that did not show a tailing at high temperature (Supporting Information, Figure S4). Characterization of Activated and Functionalized Silica. Solid-state 29Si cross-polarization magic-angle-spinning (CP/ MAS) NMR characterizations were systematically performed
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to assess the structure and content of silanes grafted onto silica.52 Figure 3 displays the 29Si CP/MAS NMR spectra obtained for bare silica and for functionalized silica after reaction with three different alkoxysilanes (SiO2-SiOx-HFPO, SiO2-NH2, and SiO2-SH). The spectra of the untreated silica shows three signals centered at -91, -100, and -110 ppm that correspond to geminal silanols (Q2), free silanols (Q3), and siloxane groups (Q4), respectively (Scheme 4). After the modification of silica with various functionalized oligomers, the decreasing intensity of Q2 and Q3 signals, relatively to that of Q4 signals, demonstrates that alkoxysilanes indeed reacted on the silica surface. For treated silica, a second group of signals appears in the -50 to -80 ppm range corresponding to the grafted or condensed entities. (Trialkoxysilanes have three functions that can condense with the silanols on the surface or with neighboring molecules, respectively.) In all spectra, the absence of a signal centered at around -45 ppm (T1) indicates that trialkoxysilanes are linked through at least two covalent bonds (Scheme 4). The signal intensities at -59 (T2) and -68 (T3) ppm show that a majority of the grafted molecules are linked through three siloxane bridges. These molecules have been grafted onto the surface but not exclusively. Rather than a triplicate anchoring on the silica surface,53,54 it is more likely that polymerization between neighboring silane groups from HFPO oligomers occurs (especially for the aminosilane55). Evaluation of the Efficiency of HFPO Tethering and Grafting. The different tentative grafting reactions carried out in this study are depicted in Scheme 3, namely, (i) hydrolysis-condensation of HFPO-Si(OEt)3 onto SiO2; (ii) condensation of HFPO-COOMe onto SiO-NH2; and (iii) radical addition of HFPO-allyl onto SiO2-SH.56 In the following text, we first characterized the raw final nanocomposites on which HFPO oligomers are tethered (i.e., adsorbed and/or grafted). Then, after washing these extensively with hexafluoroxylene (HFX), a good solvent for HFPO oligomers, mostly grafted chains remain attached to the surface, as shown by some (but not all) of the characterization techniques developed here. Infrared spectra of silica and HFPO-tethered silica (Figure 4) were compared to perform a preliminary qualitative analysis. The bare silica spectrum presents the peak characteristic of Si-O-Si bonds at around 1070 cm-1. Specific wavelength absorptions of HFPO appear at 1000 and 1220 cm-1 in the three HFPO-grafted silica spectra. These two bands are assigned to ether (C-O-C) or fluoroalkyl (C-F)57 bonds, respectively. The spectra also exhibit a frequency of weak intensity at 1380 cm-1 assigned to C-N bond distortion,58 a small peak at 3300 cm-1 related to the N-H bond, two characteristic amide peaks, carbonyl groups (CdO) at 1700 cm,-1 and an N-H stretching vibration at 1540 cm-1.42,59 The three different routes led to modified silicas with HFPO oligomers tethered to their surfaces. The spectrum of HFPO-COOMe tethered onto silica exhibits a frequency of around 1800 cm-1, which is assigned to unreacted carbonyl ester groups (CdO). After the modified silica was washed with HFX, intensity decreased for both amide peaks, tethered especially in HFPO8-COOMe and HFPO8allyl silica-modified spectra. Such specific observations show that for these two tethering methods some free HFPO chains are simply adsorbed to the functionalized surface. To quantify the total content of oligomers present at the surface and the proportion of covalently grafted oligomers, elemental and thermogravimetric analyses were carried out, 4061
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Table 1. Elemental Analysis, TGA, and BET Results for Bare Silica, Organomodified Silicas, and Functional Oligo(HFPO)Tethered Silica Nanoparticles run no.
DPna %C(%)b %F (%)b η(C) (μmol/m2)c η(F) (μmol/m2)c onset temp (°C)d Δm1 (%)e Δm2 (%)e τcov (%)f SBET (m2/g)g
SiO2 SiO2-NH2 SiO2-SH
2.17 2.32
4.2 4.6
0.5
150
5.3 6.7
120 130
SiO2-SiOx-HFPO8
8
15.98
36.78
10.5
6.5
400
65
1
64
60
SiO2-SiOx-HFPO15
15
17.64
25.08
9.4
1.6
400
68
18
50
20
SiO2-NH-HFPO8
8
14.08h
35.88
9.0
5.6
190
71
62
9
5
SiO2-NH-HFPO15
15
16.36h
54.23
8.0
10.3
265
85
81
4
1
8
18.34h
54.56
16.5
24.5
220
84
79
5
1
SiO2-S-HFPO8 a
DPn: average experimental degree of polymerization (assessed by 19F NMR). b %X: weight percentage of X atoms (determined by elemental analysis). c η(X): tethering density of silanes calculated according to eq 1. d Onset temperatures in parentheses are those measured for HFPO oligomers solely. (See also Figure 2.) e Δm1 and Δm2 are tethered and physisorbed oligomer contents, respectively, as determined by weight loss from DTGA. See the text for details. f The percentage of covalently grafted oligomer content was calculated as Δm1 - Δm2. g SBET: Specific surface area of silica as measured by BET. h The percentage of carbon in the first silica grafting (APTES or MPTES) has been presubtracted.
Figure 3. Solid-state 29Si cross-polarization/magic-angle-spinning (CP/MAS) NMR spectra of bare silica (SiO2), amine-silica (SiO2-NH2), mercapto-silica (SiO2-SH), and oligo(hexafluoropropylene oxide) silane end-group grafted silica (SiO2-SiOx-HFPO8).
Scheme 4. Nature of Covalent Bonds around Silicon Atoms and Corresponding Chemical Shifts in Solid-State 29Si NMR
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Figure 4. FTIR spectra of bare silica (SiO2), the oligo(hexafluoropropylene oxide) silane end group (HFPO8-Si(OEt)3), the oligo(hexafluoropropylene oxide) ester end group (HFPO8-COOMe), and the oligo(hexafluoropropylene oxide) allyl end group (HFPO8-allyl) grafted onto silica, amino-silica (SiO2-NH2), and mercapto-silica (SiO2-SH), respectively. HFX stands for hexafluoroxylene.
respectively. The results of elemental analysis on raw nanocomposites, expressed in weight percent of the studied atom (carbon or fluoride), are presented in Table 1. From this data, the tethering density (μmol/m2) was assessed from eq 1 (Experimental Section)44 and is reported in Table 1. From TGA measurements, the covalent grafting percentage (τcov, Table 1) can be simply assessed using eq 5 τcov ¼ Δm1 - Δm2
ð5Þ
where Δm1 represents the overall weight loss percentage of oligomers (between room temperature and 600 °C) and Δm2 represents the weight loss percentage of adsorbed chains (between room temperature and the temperature corresponding to the end of oligomer volatilization as given in Figure 2) assigned to the different silicas used to graft HFPO oligomers. Results based on the carbon content may be overestimated because of the presence of remaining alkoxy groups or trace solvent. To evaluate HFPO tethering, it is thus better to consider the fluorine content only. Silicas activated with APTES and MPTES have tethering densities of 4.2 and 4.6 μmol/m2, respectively, from the carbon content. From the fluorine content, the direct reaction of HFPO8-SiOEt3 (DPn = 8) on silica leads to a higher tethering density (around 6.5 μmol/m2) than those obtained with functionalized APTES and MPTES silanes. As expected, the grafting density decreased by increasing the molecular weight of HFPO15-SiOEt3 oligomers probably because of the steric hindrance induced by the longer chains. On the contrary, with HFPO8-COOMe oligomers, the average tethering density is around 5.6 μmol/m2, whereas with HFPO15-COOMe oligomers the density is almost twice this value (10.3 μmol/m2). For HFPO8-allyl oligomers, the tethering density is at least 5 times higher than that of MPTES. Therefore, the three routes led to different tethering densities and thus presumably to different proportions of physisorbed and covalently grafted HFPO oligomers (Discussion section). Thermogravimetric analyses were also carried out to quantify the proportion of physically adsorbed and chemically linked HFPO on silica (Figure 5). Temperatures at the maxima of the peaks in the differential TGA curves, which correspond to the main volatilization and/or degradation temperature of the samples, are listed in Table 1. (DTGA thermograms are given in Figure S6.) The covalent
tethering between the oligomer and the silica surface is estimated by comparing the temperatures for the maximum weight loss rate before and after the grafting reaction. One-step grafting routes led mainly to covalent grafting because the onset temperature increases to at least 100 °C. The other two methods led predominantly to fast volatilization of the chains in a temperature range similar to the one observed for the original oligo(HFPO) free oligomers. Note that the surface of the silica modified by a monoethoxysilane-functionalized oligo(HFPO) (HFPO8-Si(CH3)2OEt) was covered with 2 times less polymeric material than the surface of the silica bearing triethoxysilane oligo(HFPO) (Figure S4). Such an observation confirms the possibility of the surface polymerization of HFPO8SiOEt3 onto both silica silanols and adjacent ethoxysilane groups of two HFPO chains. These three nanohybrids were then observed and compared by transmission electron microscopy (TEM) (Figure 6). The average diameters were measured directly on TEM images before treatment with HFX to give 14 ( 3, 23 ( 4, 34 ( 3, and 54 ( 6 nm for bare silica (SiO2), oligo(hexafluoropropylene oxide) silane end-group grafted silica (SiO2-SiOx-HFPO8), an ester end group (SiO2NH-HFPO8), and an allyl end group (SiO2-S-HFPO8), respectively. When the same modified silica were deposited on copper grids after their dispersion in HFX, we could observe a significant decrease in particles diameter (Figure 6).60,61 In Table 2, the contact angle values and surface tensions are presented on four different solid substrates made from bare and modified silica after various washing steps for HFPO oligomers with HFX. (Corresponding photographs of drops are given in Figure S7.) The contents of tethered and grafted oligomers, calculated from values given in Table 1, are also reported.62 The water droplet has a contact angle (CA) of around 33 ( 1° on bare silica. For grafted silicas SiO2-NH-HFPO8 and SiO2-SHFPO8, CAs are around 110 ( 3°, and for silica SiO2-SiOxHFPO8, they are 130 ( 2°. Larger differences between grafted SiO2-NH-HFPO8 and SiO2-S-HFPO8 silicas were apparent using diiodomethane droplets with contact angles of 98 ( 6 and 55 ( 1°, respectively. From the surface tension measurements, it can be noted that both dispersive and polar components decrease dramatically with an increasing quantity of tethered oligomers. Note that the saturation of the surface by grafted molecules occurs at about 35 wt %; indeed, monoethoxy-modified silica was 4063
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Figure 5. TGA thermograms of silica nanoparticles grafted by oligo(hexafluoropropylene oxide)s using three different grafting methods. HFX stands for hexafluoroxylene.
Figure 6. Transmission electron microscopy (TEM) images of (a) bare silica (SiO2), (b) HFPO8-SiOEt3 oligomer grafted silica (SiO2-SiOxHFPO8), (c) HFPO8-COOMe oligomer grafted silica (SiO2-NH-HFPO8), and (d) HFPO8-allyl oligomer grafted silica (SiO2-S-HFPO8). (e, f) The same samples dispersed in HFX (hexafluoroxylene). The scale bar is 100 nm (inset 10 nm).
Table 2. Contact Angle Values and Surface Tensions for Bare Silica and Functional Oligo(HFPO)-Tethered Silica Nanoparticles after Several Washings with HFX run no.
Δm10 (%)a
θH20(deg)b
θCH2I2 (deg)b
γds (mN/m)c
γps (mN/m)c
γs(mN/m)c
SiO2 SiO2-SiOx-HFPO8
64
33 ( 1 130 ( 2
36 ( 2 113 ( 2
41.5 4.7
52.8 0.2
94.3 4.9
SiO2-NH-HFPO8
33
110 ( 3
98 ( 6
9.4
1.8
11.2
SiO2-S-HFPO8
21
110 ( 3
55 ( 1
31.4
0.1
31.5
Δm10 : weight loss percentage of the tethered oligomer content after several washings with HFX. b θH2O and θCH2I2: average experimental values of contact angle measurements assessed by the sessile drop method. c γds , γps , and γS are dispersive (eq 2) and polar (eq 3) components of the total surface tension (eq 4), respectively (Experimental Section). a
similarly hydrophobic and oleophobic to triethoxylated silica, although the amount of surface grafting was twice as small (Table S1).
’ DISCUSSION In the results section, different routes for HFPO attachment onto functionalized or bare silica surfaces were described. Different techniques, both qualitative (FTIR, TEM, and contact angle measurements) and quantitative (elemental analysis and TGA), have shown that the content of effectively grafted chains was very different from one chemistry to another. Washing the functional silica with HFX confirmed the high content of chains that is merely physisorbed on the surface, in some instances. The discussion below proposes a deeper analysis of the data and interpretation, in agreement with those in the literature.
Physisorption versus Covalent Grafting Discrimination through Different Characterization Techniques. Elemental
analyses showed tethering densities of 4.2, 4.6, and 6.5 μmol/m2 for silicas activated in a one-step silanization reaction with APTES, MPTES, and HFPO8-SiOEt3, respectively. The maximum grafting density that one could achieve at a silica surface corresponds to the maximum number of silanols available on a fully hydroxylated silica surface (i.e., 7.6 μmol/m2 or 4.6 SiOH/nm2) according to the Zhuravlev model.63 However, it is well known that the surface of pyrogenic silica is far from being fully hydrated, according to its preparation process (gas-phase production at very high temperature of around 1200 °C). Morrow and McFarlan64 quantified the different types of silanols (H-bonded, isolated, and ready to be trimethylsilated) on precipitated and fumed silicas by using different analytical tools such as the combination of H/D exchange and 4064
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Langmuir chemical grafting. They concluded that the total silanol density on a pyrogenic silica surface is about 5.15 μmol/m2 (3.1 SiOH/nm2) and that the number of silanols ready to react to become trimethylsilated is about 2.5 μmol/m2 (1.5 SiOH/nm2). The widely hydrated precipitated silica from Rhone-Poulenc, with a much larger overall content of silanol groups (especially vicinal ones), showed reactive silanol groups in the same range of concentration. In the present study, we used this density of 2.5 μmol/m2 (1.5 SiOH/nm2) to consider the maximum grafting density for monolayer grafting. Grafting densities obtained with the three different trialkoxysilanes are quite high, which indicates polymerization between the alkoxysilane groups. For silicas prepared in a two-step strategy, densities increased to 5.6 and 24.5 μmol/m2 for HFPO8-COOMe and HFPO8-allyl oligomers, respectively, whereas by considering the first step in silica functionalization by various silanes, the grafting density should at best be worth 4.6 μmol/m2. This indicates that in both examples part of the oligomers did not react on the functional groups linked to the silica surface whatever the chain length (i.e., some oligomers were physisorbed). From Δm1 values, the total quantities of oligomers tethered on the surface were compared. The two-step grafting routes led to a slightly higher tethering density, although this difference may not be significant. The reason for important physisorption of the HFPO oligomers onto silicas, whatever their surface groups, is difficult to establish clearly. It is anticipated that the bad solvent for HFPO (ethanol/water) used here forces the oligomers to adsorb onto any surface, particularly on silica with its large surface area. Besides, the grafting reaction takes advantage of oligomer physisorption to put the different functions of oligomers and the surface that is driving the reaction into contact with one other. However, its extent may depend only on the reactivity and availability of reactive groups. Such a finding is very similar to that obtained from the study of oligonucleotides/latex complexes by surface grafting reactions, where it was shown that adsorption and grafting were uncorrelated, although grafting would not occur without prior adsorption.65 A two-step reaction led to a poor proportion of covalently grafted oligomers. Indeed, the condensation between SiO2-NH2 and HFPO8-COOMe led to only 9% covalent grafting whereas with HFPO15-COOMe the efficiency was even lower (4%). The same trend was observed when SiO2-SH was used because only 5% of HFPO (DPn = 8) was covalently grafted onto silica. On the contrary, the direct condensation of HFPO-Si(OEt)3 onto silica enabled us to graft almost all and two-thirds of the physisorbed short and long oligomers, respectively. Finally, the grafting reaction of monoethoxy- and triethoxysilane-functionalized HFPO oligomers doubled from one to the other (Figure S4). To corroborate these results, samples with DPn = 8 were washed several times with HFX. Such treatment for SiO2SiOx-HFPO led to almost no change in the particle diameter and showed a decrease in tethered oligomer content from 65 to 64%, which corresponds to the calculated grafted content (compare τcov and Δm10 in Tables 1 and 2, respectively). However, for SiO2-NH-HFPO8 and SiO2-S-HFPO8 the washing steps greatly affected the particle morphology (the diameters decrease from 34 to 16 nm for SiO2-NH-HFPO8 and from 54 to 15 nm for SiO2-S-HFPO8), and the final oligomer weight content (compare Δm1 and Δm10 in Tables 1 and 2, respectively) indicates the difficulty of removing physisorbed oligomers. Indeed, τcov and Δm10 did not match in these two cases (vide supra). Finally, increasing the chain length of the grafting moieties seems to attenuate the extent of chemisorption,
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as observed by TGA for both the one-step and two-step grafting methods (Table 1). The densities and percentage of covalent grafting were in agreement with the specific surface areas assessed by nitrogen adsorption (Table 1). Between SiO2-SiOx-HFPO8 and SiO2NH-HFPO8, it was noted (by elemental analyses) that they had almost the same grafting density (around 6 μmol/m2). However, the former showed 100% covalent grafting whereas the second exhibited only 10% on the surface, which is related to a significant decrease in the specific surface area from 60 to 5 m2/g1. This trend was observed for the five oligo(HFPO)-modified silicas. Finally, contact angle results obtained with silica containing the most important part of chemically grafted HFPO (SiO2-SiOxHFPO) are in line with those presented by Gao et al.57 and Yarosh et al.,38 who found contact angles that ranged between 120 and 130° with water and were around 113 ( 2° with diiodomethane, depending on the overall surface roughness. Moreover, the value of the diiodomethane contact angle (θCH2I2) increased while the covalent grafting density of the fluorinated chain on the surface increased, as previously reported by Shafrin and Zisman66 and Pittman.67 Thus, θCH2I2 SiO2-SHFPO8 < θCH2I2 SiO2-NH-HFPO8 < θCH2I2 SiO2-SiOxHFPO8. Explanation of Depressed Covalent Grafting. The limitation of the total number of covalently grafted chains can be explained by a scaling argument used in polymer brush studies.68 Because under our experimental conditions a poor solvent (90:10 v/v ethanol/ distilled water) for oligo(HFPO) was used, each oligomer adsorbed on the silica surface may spread, thus hiding the remaining functional groups. Apart from this phenomenon, other explanations can be pointed out to explain the wide variation in covalent grafting yields between the different pathways. First, the reaction of functionalized oligomers with silylated silica depends on the availability of the reactive group anchored onto the silica. In the case of SiO2NH2, the literature reports many probable interactions between the amino group and the silica surface, leading to a cyclic conformation of the aminopropyl group.55,69-71 In fact, hydrogen bonds may arise between two neighboring amino groups or between amino groups and residual silanols. Also, ionic interactions due to the zwiterionlike character of the surface coating created by the presence of acidic (silanols) and basic (amine) sites are likely to occur. Such interactions may also explain why HFX washing steps were not able to remove all unreacted oligomers from the silica surface. With SiO2-SH, the thioether group does not play as important a role as with SiO2-NH2 because the electronegativity of sulfur (2.58) is smaller than that of nitrogen (3.04) and is close to that of carbon (2.55).72 However, with this mercaptosilane, the formation of disulfide (S-S) bridges is known to occur,72,73 hence decreasing the available functionality at the modified silica surface. Finally, previous studies also reported that the presence of excess water in the synthesis process may induce the formation of a more disordered layer if the alkoxysilane contains a polar headgroup such as -NH2 or -SH.74,75 On this basis, another possibility for the poor reactivity of the functional headgroup may arise from existing domains at the silica surface where the headgroups are randomly distributed at the surface.
’ CONCLUSIONS This work described three different methods of grafing onto of oligo(hexafluoropropylene oxide)s onto silica nanoparticles. HFPO 4065
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oligomers were obtained by anionic polymerization followed by chain deactivation with methanol and amine compounds. The modification of these oligomers with alkoxysilanes was carried out by the condensation of primary amine onto ester end groups. The functionalization of the oligomers and silica was qualitatively checked by FTIR and 29Si solid-state NMR spectroscopies. Moreover, the grafting density and physisorption phenomenon were quantified by elemental and thermogravimetric analyses. The highest covalent grafting density was obtained with SiO2-SiOx-HFPO grafts, with an average of 6 μmol 3 m-2 while keeping a large specific surface area (20-60 m2 3 g-1) and a high thermal stability. For silica mainly modified with physically adsorbed oligomers, a higher grafting density can be achieved but oligomers easily volatilize and hide the structure of silica, decreasing its surface area down to 1 m2/g1. Surface tensions on thus-prepared materials were decreasing (giving a surface tension of as low as 5 mN/m) with an increasing content of grafted chains, whereas thermal stabilities were significantly enhanced (onset at 400 °C). Further work will propose similar modification steps from different fillers, including laponite and sepiolite.
’ ASSOCIATED CONTENT F and 1H NMR spectra of oligo(hexafluoropropylene oxide) (HFPO-COOMe). 1H NMR spectrum of hexafluoropropylene oxide oligomers modified with 3-aminopropyl(dimethyl)ethoxysilane (APDMES). Derivative TGA curves of a series of hexafluoropropylene oxide (HFPO) oligomers as a function of chain length. TGA and DTGA curves of hexafluoropropylene oxide (HFPO) oligomers functionalized with amidopropyltriethoxysilane and amidopropyldimethylethoxysilane grafted onto silica nanoparticles and functionalized with a methyl ester. DTGA curves of a series of hexafluoropropylene oxide (HFPO) oligomers grafted onto silica nanoparticles. Images of water and diiodomethane droplets on different substrates. Contact angles values and surface tensions for bare silica, and functional oligo(HFPO)-tethered silica nanoparticles not presented in the main text. This material is available free of charge via the Internet at http://pubs.acs.org.
bS
Supporting Information.
19
’ AUTHOR INFORMATION Corresponding Authors
*Phone: þ33 4 67 14 43 68/þ33 4 67 14 72 96. Fax: þ33 4 67 14 72 20. E-mail:
[email protected]/
[email protected].
’ ACKNOWLEDGMENT We thank Dr. Sergei Kostjuk and Dr. Esteban Ortega for their kind help with the anionic polymerization of HFPO and Pr. Gilles Silly and Dr. Philippe Gaveau for making solid-state 29Si NMR measurements. The French National Reserch Agency is also acknowledged for financial support (NAFEL Project). ’ REFERENCES (1) Zou, H.; Wu, S.; Shen, J. Chem. Rev. 2008, 108, 3893–3957. (2) Achilleos, S. D.; Vamvakaki, M. Materials 2010, 3, 1981–2026. (3) Zhou, Q.; Wang, S.; Fan, X.; Advincula, R.; Mays, J. Langmuir 2002, 18, 3324–3331. (4) Pyun, J.; Kowalewski, T.; Matyjaszewski, K. Macromol. Rapid Commun. 2003, 24, 1043–1059. (5) Ruiz-Hitzky, E. Chem. Rec. 2003, 3, 88–100.
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