Langmuir 1998, 14, 1905-1910
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Biodegradable and Biocompatible Inorganic-Organic Hybrid Materials. 3. A Valuable Route to the Control of the Silica Porosity Dong Tian,† Silvia Blacher,‡ Jean-Paul Pirard,‡ and Robert Je´roˆme*,† Center for Education and Research on Macromolecules (CERM), University of Lie` ge, Sart-Tilman, B6, 4000 Lie` ge, Belgium, and Service de Ge´ nie Chimique, University of Lie` ge, Sart-Tilman, B6, 4000 Lie` ge, Belgium Received July 29, 1997. In Final Form: December 15, 1997 Porous silica has been successfully prepared from poly(-caprolactone)-silica hybrid materials on the basis of the template approach. The final texture of the porous silica has been analyzed by the nitrogen adsorption-desorption technique and by small-angle X-ray scattering (SAXS). The porosity of silica can be tailored by the poly(-caprolactone) (PCL) template, particularly molecular weight, molecular weight distribution, content, and type and number of reactive groups per chain. Increasing the molecular weight, content, and chain polydispersity of the PCL template and decreasing the reactivity and number of reactive groups per PCL chain result in a more heterogeneous microporosity and, as a rule, in larger pores, a broader pore size distribution, a larger specific surface area, and a larger total and microporous volume.
Introduction 1-3 or polymeric foams,4,5
Porous materials, such as silica are emerging as a new area of great technological and scientific interest.6 Materials with tailor-made pore shapes and sizes are particularly important in any application based on molecular recognition, such as shapeselective catalysis, molecular sieving, chemical sensing, and selective adsorption.7,8 In addition, porous materials have been used as nanosized reaction vessels9 or hosts for the assembly of semiconductor clusters,10 organic molecules,11 and even molecular wires.12 Moreover, porous materials of a low dielectric constant and a high modulus are extremely important for microelectronic packaging.3 The drying of highly cross-linked gels prepared by the sol-gel process, that is, xerogels, aerogels (supercritically dried gels), and so forth, is the most usual technique for producing porous silica.3 However, the final texture of these materials depends on several independent experimental parameters, such as the amount of catalyst, the gel dilution, the conditions used for the drying process, and so forth.3 This technique is however not very well suited to the fine control of the shape, size, and size distribution of the pores, which is so desirable for several applications. An alternative method to the preparation of porous silica relies on a template-based approach.2,14 A * To whom correspondence should be addressed. † Center for Education and Research on Macromolecules (CERM). ‡ Service de Ge ´ nie Chimique. (1) Kresge, C. T.; Leonowicz, E. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Raman, N. K.; Anderson, M. T.; Brinker, C. J. Chem. Mater. 1996, 8, 1682. (3) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The physics and Chemistry of Sol-Gel Process; Academic Press: London, 1980. (4) Hedrick, J.; Labadie, J.; Russell, T.; Hofer, D.; Wakharker, V. Polymer 1993, 34, 4717. (5) Khemani, K. C.; McConnell, R. L. ACS Polym. Prepr. 1996, 37 (2), 793. (6) Schaefer, D. W. MRS Bull. 1994, 19, 14. (7) Behrens, P. Adv. Mater. 1993, 5, 127. (8) Davis, M. E. Nature 1993, 364, 391. (9) Ozin, G. A.; Gil, C. Chem. Rev. (Washington, D.C.) 1989, 89, 1749. (10) Stucky, G. D.; MacDougall, J. E. Science 1990, 247, 669. (11) Cox, S. D.; Gier, T. E.; Stucky, G. D. Chem. Mater. 1990, 2, 609. (12) Bein, T.; Enzel, P. Angew. Chem., Int. Ed. Engl. 1989, 28, 1692.
template is actually dispersed throughout the silica network, and it fingerprints the final porosity that will be created as a result of the template elimination. The template-mediated synthesis and processing of porous silica may be divided into two fundamentally different techniques, that is, the surfactant-templated technique, leading to mesoporous silica, and the organic ligandtemplated one that allows microporous silica to be produced.2 In the surfactant-templated technique, the organic and the inorganic precursors organize themselves cooperatively (but independently of each other), in such a way that strong but noncovalent interactions, for example electrostatic and van der Waals interactions, ensure the precise shaping of the network around the surfactant assembly. In ligand-templated materials, the organic templates are ligands covalently bonded to the silica precursors. The covalently bound template however alters the chemical reactivity of the inorganic precursor and imparts hydrophobicity to an otherwise hydrophilic network. To the best of our knowledge, only one note has reported on organic polymer-templated porous silica and its texture without however giving much detail.15 Recently, we have reported on the synthesis of novel biodegradable and biocompatible inorganic-organic hybrid materials prepared by the sol-gel process.16-19 Poly(-caprolactone) (PCL), which is well-known for biocompatibility, permeability, and biodegradability, has actually been incorporated into silica networks. The organic (PCL) and the inorganic (SiO2) constitutive components can be associated either by covalent bonding (in the case of endreactive PCL) or by hydrogen bonding (between the carbonyl groups of PCL and the residual OH groups on (13) Martin, J. E.; Hurd A. J. J. Appl. Crystallogr. 1987, 20, 61. (14) Beck, J. S.; Vartuli, J. C.; Kennedy, G. L.; Kresge, C. T.; Roth, W. J.; Schramm, S. E. Chem. Mater. 1994, 6, 1816. (15) Chujo, Y.; Matsuki, H.; Kure, S.; Saegusa, T.; Yazawa, T. J. Chem. Soc., Chem. Commun. 1994, 635. (16) Tian, D.; Dubois, Ph.; Je´roˆme, R. Polymer 1996, 37, 3983. (17) Tian, D.; Dubois, Ph.; Grandfils, Ch.; Je´roˆme, R.; Viville, P.; Lazzaroni, R.; Bre´das, J. L.; Leprince, P. Chem. Mater. 1997, 9, 871. (18) Tian, D.; Dubois, Ph.; Je´roˆme, R. J. Polym. Sci., Polym. Chem. 1997, 35 (11), 2295. (19) Tian, D.; Blacher, S.; Dubois, Ph.; Je´roˆme, R. Polymer 1998, 39, 885.
S0743-7463(97)00841-X CCC: $15.00 © 1998 American Chemical Society Published on Web 02/24/1998
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silica).18 “In vitro” cell culture and biodegradation tests have shown that these new inorganic-organic hybrid materials are biomaterials endowed with biodegradable and biocompatible properties.17 The dynamic mechanical properties and phase morphology have also been reported elsewhere,19 and they have led us to conclude that the on-purpose removal of the organic component might be a valuable way to produce silica with a well-controlled porosity. This paper aims at reporting on the preparation of porous silica from the aforementioned inorganicorganic hybrid materials and on the final texture of the porous material as studied by the nitrogen adsorption method and small-angle X-ray scattering (SAXS). The main experimental variables that will be considered in this study are the molecular weight and content of the PCL template and the nature and number of reactive groups per PCL chain. Experimental Section Materials. The synthesis of R,ω-dihydroxyl-PCL (Mn ) 2000 and 4000) and R,ω-bis(triethoxysilyl)-PCL (Mn ) 2000) was detailed elsewhere.18 The PCL diol (Mn ) 1250) and the PCL triol (Mn ) 900) (Aldrich) were used as received. [C(O)(CH2)5O] was the repeat unit of all these polymers. Tetraethoxysilane (Janssen), hydrochloric acid (12 N) (Lab Chemistry), tetrahydrofuran (Janssen), and ethanol (Riedel-de Haen) were also used as received. Preparation of Silica-Poly(E-caprolactone) Hybrid Materials. PCL/TEOS mixtures of various compositions were dissolved in THF (20 wt %) and hydrolyzed with a stoichiometric amount of water with respect to the alkoxide functions. HCl was used as a catalyst in a 0.05/1 HCl/TEOS molar ratio. A representative synthesis was as follows: 1.5 g of TEOS was added to the R,ω-bis(triethoxysilyl)-PCL (0.5 g, Mn ) 2000) solution in THF (10.0 mL) and thoroughly mixed until a homogeneous solution was formed. Then deionized water (0.54 mL), ethanol (0.80 mL), and HCl (0.01 mL) were added under rapid stirring at ambient temperature for approximately 10 min. The clear solution was then cast into a plastic Petri dish and covered with Parafilm. On the basis of a preliminary series of experiments, it was shown that, after several days, depending on the PCL end groups (hydroxyl or triethoxysilyl), gelation had occurred and the Parafilm could be removed.14 The gelified material was then dried under ambient condition for 1 week and finally cured at 100 °C for 2 days prior to testing or pyrolysis. These specific conditions were detailed elsewhere14,16 and proved to be efficient for preventing cracking phenomena and thermal degradation of PCL from occurring. The usual film thickness was 0.1-1 mm. Preparation of Porous Silica. Porous silica was prepared by pyrolysis of the silica-PCL hybrid materials at 400 °C with air as a supporting purge gas (100 mL/min) until no weight loss was observed anymore. The whole pyrolysis process was analyzed by thermogravimetric analysis (TGA). Characterization. Nitrogen adsorption isotherms were measured at the boiling temperature of liquid nitrogen (77 K) with a computer-driven Sorptomatic Carlo Erba 1900. Nitrogen of a high purity (99.98%) was used. Small-angle X-ray scattering (SAXS) was carried out at the “Laboratoire pour l’Utilisation du Rayonnement Electromagnetique” (LURE, Orsay, France) on DCI (D24 station). The size of the X-ray beam (λ ) 1.488 Å) at the sample was smaller than 1 mm2, so that desmearing of the data was not required. The scattered X-rays were detected with an argon-CO2 gas-filled, one-dimensional position-sensitive detector (with a resolution of 0.4444 mm). The sample-to-detector distance (1151 mm) allowed SAXS data to be recorded in the 0.02-0.8 nm-1 s range. These data were plotted as relative intensity versus the scattering vector s, after correction for parasitic scattering and sample absorption. The background scattering was corrected in the standard manner.
Tian et al. Table 1. Main Characteristics of the PCL Template and the Parent Hybrid Materials samples H1 H2 H3 H4 H5 H6 H7 H8
type and no. of reactive end groups of PCLa R,ω-hydroxyl R,ω-hydroxyl R,ω-hydroxyl R,ω-hydroxyl R,ω-hydroxyl R,ω-triethoxysilane PCL triol R,ω-methyl
Mn PCLb
Mw/Mn PCLb
PCL wt %c
1250 2000 4000 4000 4000 2000 900 4000
1.45 1.50 1.50 1.50 1.50 1.50 1.40 1.25
46.5 46.5 46.5 27.8 15.5 45.8 46.5 46.5
a no. stands for number. b M , number average molecular weight; n Mw, weight average molecular weight; Mw/Mn, molecular weight c distribution. PCL content of the PCL-silica hybrid material calculated in the case of a quantitative sol-gel reaction.
marized by eqs 1-3 (eq 3 is unbalanced). This process is basically a two-step hydrolysis-condensation reaction of tetraethoxysilane [Si(OC2H5)4, TEOS] and ethoxysilane end-capped PCL. In a first step, the alkoxysilane groups are hydrolyzed with formation of tSi-OH groups (eqs 1 and 2), which are then condensed into a silica oxide network (tSi-O-Sit) (eq 3)
In this scheme, hydrolysis of the ethoxysilane groups whatever their origin (TEOS or PCL end groups) is assumed to be complete before polycondensation occurs. The actual network-forming process is certainly much more complex due to the interplay of hydrolysis and condensation. It has been shown that the inorganic (SiO2) and the organic (PCL) constitutive components can be associated by either covalent bonding (in the case of endreactive PCL) or hydrogen bonding between the residual OH groups on silica and the carbonyl groups of PCL.18 The extent of PCL incorporation into the silica network and the final structure and morphology of PCL-silica hybrid materials actually depend on PCL content and molecular weight and on the reactivity of the PCL end groups and their number per chain.18,19 On the basis of these preliminary results, a more systematic study has been undertaken, and a series of silica samples have been prepared by the sol-gel process by using various amounts of PCL templates of different molecular weight and bearing different molecular end groups (i.e., one or two hydroxyl or triethoxysilyl groups per chain). HCl was systematically used as a catalyst in a 0.05/1 HCl/TEOS molar ratio, and water was used in a stoichiometric amount with respect to the alkoxide functions. The compositions of PCL-silica hybrid materials prepared in this study are listed in Table 1. They were pyrolyzed at 400 °C in an air flow until constant weight.
Results and Discussion
Effect of the PCL Molecular Weight on the Final Texture of Porous Silica
Preparation of Porous Silica. Synthesis of silica templated by poly(-caprolactone) (PCL) can be sum-
Figure 1a shows the nitrogen adsorption isotherms for samples H1P, H2P, and H3P, which contained PCL
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Table 2. Main Characteristics of PCL-Templated Porous Silicas samplesa H1P H2P H3P H3 H4P H5P H6P H7P H8P
SBETb (m2/g)
VTc (cm3/g)
WDAd (cm3/g)
E0 DAe (kJ/mol)
nf
rg (nm)
Rgh (nm)
Pi
CBETj
844 876 1017 295 515 94 653 870 872
0.795 0.634 1.132 0.457 0.536 0.390 0.58 0.625 0.640
0.44893 0.45214 0.66762
8.05 7.90 6.50
1.6 1.6 1
0.97 ( 0.22 0.93 ( 0.30 1.49 ( 0.48
0.23249 0.04325 0.30916 0.41541 0.54442
11.60
3
0.72 ( 0.16
6.70 6.50 5.05 >25 3.60
2.90 2.80 2.60 2.40 2.50
8.93 7.12 8.46
3 3 1
8.28 ( 0.21 0.67 ( 0.33 0.89 ( 0.21
4.95 6.40 6.17
2.70 2.80 2.60
102.51 72.38 50.30 2.67 2.65 >>> 266 94.35 282.62
a The samples are designated as HxP after pyrolysis, where Hx is the symbol used in Table 1. b BET specific surface area. c Total porous volume. d Microporous volume calculated from the DA equation. e Characteristic free energy. f Exponent of the DA equation. g r, average pore size calculated by Brunauer’s method.23 h Particule radius of gyration. i Fractal dimension. j BET constant.
Figure 1. Nitrogen adsorption isotherm for the samples (a) H3P, H2P, and H1P and (b) H3 and H3P.
Figure 2. t-plots for the samples (a) H3P and H3 and (b) H3P, H2P, and H1P.
templates of a different molecular weight before pyrolysis (see Tables 1 and 2). The isotherms for the H1P and H2P samples can be clearly identified as type I isotherms according to the IUPAC classification.20 A sharp initial increase in the absorbed gas volume is followed by a plateau over a large p/p0 range.This type of isotherm indicates a small tendency to multilayer adsorption on a small external surface and is thus characteristic of microporous materials (pore width w < 2 nm). The isotherm for the sample H3P is compared to the isotherm of the precursor H3 sample (thus before pyrolysis) in Figure 1b. Before pyrolysis, the isotherm cannot be identified with one typical isotherm of the IUPAC classification. The absence of a “knee” at low pressure or an inflection point characteristic of monolayer adsorption indicates weak interaction between the solid and the gas.20 This is confirmed by a low BET constant (CBET ) 2.67; Table 2). The typical t-plots20 of sample H3 (Figure 2a) show an upward deviation with respect to the straight line passing through the origin, which is characteristic of a meso- or a macroporous structure.
Upon pyrolysis of the sample H3, a short initial increase in the adsorbed volume is observed, at low relative pressures (Figure 1b), which is an indication that a microporous structure has been formed, which favors the adsorbent-adsorbate interaction (CBET ) 50.30) and the occurrence of a micropore-filling adsorption mechanism prior to the monolayer-multilayer formation observed in the original sample. In contrast to the case for H3, the t-plot for the sample H3P (Figure 2a) shows a downward deviation which is characteristic of microporous or slitshaped porous materials. The deviation in the t-plot of sample H3P is observed at higher adsorbed volumes and in a more gradual manner compared to those of the H2P and H1P samples (Figure 2b). The micropore size distribution has been analyzed for the three samples by Brunauer’s method.21 From the mean size of the pores (r) and the standard deviation (Table 2), it appears that the pore size distribution is broader for H3P than for the two other samples H2P and H1P. It is worth noting that only sample H3 has a porous structure before pyrolysis, although its origin is still unclear.
(20) Gregg S. J.; Sing K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982.
(21) Mikhail, R. S.; Brunauer, S.; Bodor, E. E. J. Colloid Interface Sci. 1968, 26, 45 and 54.
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Figure 3. DR plots for the samples (a) H1P, (b) H2P, and (c) H3P.
Adsorption data relative to microporous structures are generally described by the Dubinin-Radushkevich (DR) equation (eq 1):22
ln W ) ln WDR -
( ) A E0β
2
(4)
where W is the amount adsorbed at the relative pressure p/p0, WDR is the volume of the micropores, A ) RT ln(p/p0) is the adsorption potential, T is the absolute temperature, β is a constant for a given adsorbent, and E0 is a characteristic adsorption free energy. The linear fitting of the experimental data by the DR equation over the whole range of pressure is characteristic of a homogeneous microporous structure, that is, of a narrow Gaussian pore-size distribution. It has been shown that, in the case of slit-shaped and cylindrical model micropores, the free energy of interaction E0 is inversely proportional to the pore radius.23 Convex and concave deviations with respect to the ln W versus (A/β)2 relationship are usually accounted for by a broad non-Gaussian pore size distribution.24-27 In this case, the use of the generalized Dubinin-Astakhov (DA) equation20 is recommended, in which the exponent n ) 2 of the DR equation is substituted by an adjustable constant n. This exponent n reflects the width of the energy distribution in relation to the pore size distribution. According to Figure 3, the DR plot for the H1P, H2P, and H3P samples shows a convex deviation with respect to the abscissa, which seems to increase with the molecular weight of the PCL template. Convex deviations have been observed for strongly activated heterogeneous carbons24-27 with a wide pore size distribution, that is, micropores of various sizes and supermicropores whose size exceeds twice the molecular diameter of the probe molecule (0.70 nm < w < 2.00 nm). The exponent n that better fits a straight line for the ln w versus (A/β)n dependence has been determined by application of the DA equation in the whole range of relative pressures (Table 2). The low value for the H3P sample (n ) 1) compared to those for H1P and H2P (n ) 1 and 1.6) suggests that the average micropore (22) Dubinin M. M. In Progress in Surface and Membrane Science; Cadenhead, D. A., Ed.; Academic Press: New York, 1975; Vol. 5, pp 1-70. (23) Dubinin, M. M. Carbon 1985, 23, 593-598. (24) Stoeckli, F. Carbon 1990, 28, 18. (25) Rand, B. J. Colloid Interface Sci. 1976, 56, 337. (26) Masters, K. J.; McEnaney, B. J. Colloid Interface Sci. 1983 95, 340. (27) Ehrburger-Dolle, F. Langmuir 1994, 10, 2052.
Figure 4. SAXS profiles for the samples H3P, H2P, and H1P.
size increases with the molecular weight of the PCL template by analogy with results reported for activated carbons.25,27 Table 2 lists the BET specific surface area (SBET) measured by the BET technique20 in the classical range of pressure (0.05-0.20), the porous volumes (VT) calculated from the adsorbed volume at saturation, the microporous volume (WDA), the characteristic free energy E0, the exponent n of the DA equation, and the mean pore size and standard deviation calculated by Brunauer’s method. These results show that pyrolysis of the H1, H2, and H3 samples results in a very heterogeneous microporosity, whose main characteristics, that is, the mean pore size, the pore size distribution, the specific surface area, the micropore volume, and the total pore volume, increase with the molecular weight of the PCL template. The pyrolyzed samples have been studied by smallangle X-ray scattering (SAXS). The scattered intensity, I, has been measured over a range of scattering vectors s ) 2λ-1 sin(θ/2), where λ is the wavelength of the incident beam and θ is the scattering angle. Figure 4 shows the log-log dependence of the scattered intensity I (in arbitrary units) on s. Beaucage et al.28 have proposed a unified equation to describe the scattering curves of systems that contain multiple length scales separated by power law regimes. The structure of the samples under consideration can be easily described by a single length scale, so that the simplest equation that expresses the scattering intensity as the sum of two components (eq 2) can be used.28
I(q) ≈ G exp
(
) [
]
-q2Rg2 (erf(qRg/x6))3 +B 3 q
P
(5)
where q ) 2πs, G is the classic Guinier prefactor, B is a prefactor specific to the type of power-law scattering, and Rg is the large particulate radius of gyration. In reference to the Porod’s law that considers the scattering between sharp surfaces, 1 < P < 3 gives information on the mass/ pore fractal dimension (Dm/p ) P), and 3 < P < 4 gives information on the surface fractal dimension (Ds ) 6 P).13 The limiting values, P ) 3 and P ) 4, are characteristic of uniformly dense structures and smooth surfaces, respectively. SAXS data for the sample H3 (before pyrolysis) have been reported elsewhere.19 For this sample a correlation length of 3.7 nm was observed, in agreement with a microscopic phase separation of PCL and silica. Figure (28) Beaucage, G. J. Appl. Crystallogr. 1996, 29, 134.
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4 compares the SAXS curves for pyrolyzed PCL-silica hybrid materials that originally contained PCL of different molecular weight (samples H1P, H2P and H3P). When the molecular weight of the PCL template is increased, Rg decreases from 7 to 5 nm. For high s values, a linear regime is observed. This region is large enough to permit a reliable evaluation of the exponent P (eq 5), which decreases from 2.90 to 2.60 when the molecular weight of the PCL template is increased. These P values are consistent with a mass/pore fractal structure. Nevertheless, this behavior has been measured over less than one decade. Usually, a cutoff is observed at high s values, which indicates a smooth or a surface fractal regime for the building blocks of the mass/pore fractal system. This type of transition is not observed in this study. The elementary unit of the observed mass/pore fractal structure is thus not accessible in the considered range, which goes up to (1/s) ) 1 nm. Moreover, the possible mass/pore fractal structure might be related to the important microporosity shown by the adsorption measurements. Effect of the PCL Content on the Final Texture of Porous Silica Figure 5a shows the adsorption isotherms for the H3P, H4P, and H5P samples that contain 46.5, 27.8, and 15.5 wt % PCL (Mn ) 4000) before pyrolysis. The total volume adsorbed by the porous silica decreases with the content of the PCL template. In the 0.1 < p/p0 < 1 range of relative pressure, the adsorption increases linearly for the sample H3P, whereas it is essentially independent of pressure for samples H4P and H5P, being however close to zero for the latter sample. The t-plot shows a sharper deviation from the straight line in the case of the sample H4P, which contains less PCL template than sample H3P (Figure 5b) and whose average pore size is smaller (Table 2). A convex deviation with respect to the abscissa is observed in the DR plot of the sample H4P (Figure 5c). This type of downward deviation has also been reported in the DR plot of slightly activated carbons with micropores of a very narrow size distribution or with a restricted N2 diffusion (sieve effect).26 In that case, the data were fitted to the DA equation with n > 2. From the DR plots for samples H3P (Figure 3) and H4P (Figure 5c) and knowing that the sample H5P is nonporous, it appears that decreasing the content of the PCL template results in a microporous material that contains less micropores of smaller size (E0 increases) and of a narrow size distribution. This conclusion is consistent with the SBET, VT, and WDA values, which decrease with the PCL content (Table 2). SAXS profiles confirm the adsorption data (Figure 6), since both Rg and the fractal dimension (P) decrease with the PCL content.
Figure 5. (a) Nitrogen adsorption isotherm for the samples H3P, H4P, and H5P. (b) t-plots for the samples H3P and H4P. (c) DR plot of sample H4P.
Effect of the Nature and Number of the PCL End Groups on the Final Texture of Porous Silica As reported elsewhere,18 when the PCL content is kept constant, an increase in the reactivity and number of the PCL end groups is favorable to the PCL incorporation into the silica network. After pyrolysis, it is consistently observed that increasing either the reactivity or the number of the PCL end groups has the same effect on the final texture of the porous silica (see samples H2P and H6P and samples H1P and H7P in Table 2). These two pairs of samples show the same behavior, which is illustrated in Figure 7 in the case of the H2P and H6P samples. The adsorption isotherms (Figure 7a), the t-plots (Figure 7b), the DR plots (Figure 7c), and the SAXS
Figure 6. SAXS profile for the samples H3P, H4P, and H5P.
measurements (Figure 7d) indicate that the microporous silica templated with PCL bearing two hydroxyl end groups rather than two triethoxysilyl end groups has a broader pore size distribution, which however rules out small pores with a sieve effect. This effect is also observed in the case of porous silica templated with the PCL diol compared to the PCL triol. Table 2 shows that the
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Figure 7. Nitrogen adsorption isotherm (a), t-plots (b), DR plots (c), and SAXS profiles (d) of the samples H2P and H6P.
experimental sizes are greater for the samples H2P and H1P than for the samples H6P and H7P, respectively, thus indicating a more open porosity. However, the reverse behavior is observed for the samples H3P (R,ωhydroxyl-PCL template) and H8P, the latter (R,ω-methylPCL template) being nonreactive in the sol-gel process (Table 2). Indeed, a wide pore size distribution at the exclusion of small pores (sieve effect) is promoted by the R,ω-hydroxyl-PCL in contrast to the R,ω-methyl-PCL template. This observation indicates that the reactivity of the PCL end groups is not as important as, for example, the molecular weight distribution of the template (see Table 1, samples H3 and H8). As expected, the microporous silica templated with PCL of a narrower molecular weight distribution has a smaller microporous size with a narrow size distribution and smaller SBET, VT, and WAD values, as well (Table 2, samples H8P and H3P). Conclusion Porous silicas have been successfully prepared by pyrolysis of silica-PCL hybrid materials. The final porous texture has been analyzed by a set of characterization techniques and the influence of different experimental parameters as well. The silica microporosity can actually be tailored by the content, molecular weight, and molecular weight distribution of PCL and by the reactivity and number of the PCL end groups. An increase in PCL molecular weight results in a much more heterogeneous microporosity and a systematic increase of the average pore size, the width of the pore size distribution, the specific surface area, the microporous volume, and the total porous volume. Decreasing the content of the PCL template leads to microporous materials with less micropores of a smaller
size and a narrow size distribution and with systematically smaller specific surface area, microporous volume, total porous volume, particule radius of gyration, and fractal dimension. Microporous silica templated with PCL bearing two more reactive triethoxysilyl end groups rather than hydroxyl end groups has a narrow porous size distribution such that a sieve effect can occur and the average pore size, the specific surface area, the microporous volume, the total porous volume, the particule radius of gyration, and the fractal dimension decrease. When the number of functional groups per chain is changed, that is, the PCL triol compared to the PCL diol, the same behavior is observed. The template molecular weight distribution also has an effect on the final texture of porous silica. A narrow molecular weight distribution results in a smaller size microporosity with a narrow size distribution and in smaller specific surface area, microporous volume, and total porous volume. The analysis of the surface morphology of the porous silicas by atomic force microscopy (AFM) and image analysis will be reported in a forthcoming paper. Acknowledgment. The authors are very much indebted to the “Services Fe´de´raux des Affaires Scientifiques, Techniques et Culturelles” for general support to CERM in the frame of the “Poˆles d'Attraction Interuniversitaires: Polyme`res”. They warmly thank R. Sobry, B. Diez, and P. Van den Bosch for SAXS measurements (Laboratory of Experimental Physics, University of Lie`ge, Belgium) and Ph. Dubois (CERM) for his contribution to the development of the hybrid materials. LA970841S
