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Synthesis and characterization of nanostructural polymer–silica composite: Positron annihilation lifetime spectroscopy study
Journal of Colloid and Interface Science 358 (2011) 268–276
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Synthesis and characterization of nanostructural polymer–silica composite: Positron annihilation lifetime spectroscopy study Radosław Zaleski a, Agnieszka Kierys b, Marta Grochowicz c, Marek Dziadosz b, Jacek Goworek b,⇑ a
M. Curie-Sklodowska University, Institute of Physics, Department of Nuclear Methods, M. Curie-Sklodowska sq. 1, 20-031 Lublin, Poland M. Curie-Sklodowska University, Faculty of Chemistry, Department of Adsorption, M. Curie-Sklodowska sq. 3, 20-031 Lublin, Poland c M. Curie-Sklodowska University, Faculty of Chemistry, Department of Polymer Chemistry, Gliniana 33, 20-614 Lublin, Poland b
a r t i c l e
i n f o
Article history: Received 24 January 2011 Accepted 1 March 2011 Available online 8 March 2011 Keywords: Polymer–silica composite Positron annihilation lifetime spectroscopy (PALS) Poly(TRIM) TEOS Porosity
a b s t r a c t The swelling of poly(TRIM) spherical particles in TEOS is assessed as a potential way for obtaining polymer–silica nanocomposite materials. Silica deposition was achieved by simply stirring of swollen polymer particles in acidic hydrochloric-water solution. This procedure leads to spherical composite particles with dispersed silica gel within the polymer matrix. The resulting material exhibits the same morphology as the initial polymer. Nanocomposite particles are silica rich (about 17 wt.%). Characterization of the nanocomposites was performed using scanning electron microscopy, FT-IR spectroscopy, 29Si CP MAS NMR spectroscopy and thermogravimetry. Moreover, the use of positron annihilation lifetime spectroscopy PALS to characterize the structural properties of the nanocomposites is presented. This technique gave more realistic pieces of information about the pore structure of the investigated samples in contrast to nitrogen adsorption studies. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction The importance of obtaining information on textural properties of micro- and mesoporous solids increases with the rapid increase of synthesis routes of new composite materials. Recently, inorganic–organic composites have garnered much interest as a new class of porous solids with potential application in gas separation, catalysis, chemical sensing or drug delivery systems. Various synthesis routes have been developed for preparation of these nanocomposites (see e.g. [1–3]). The techniques necessary for obtaining composite materials containing a wide variety of silica and organic species arranged in different ways are well documented. Silica components may be introduced as a preformed particles, colloidal silica, metal alkoxide or TEOS as silica precursor into monomer solution followed by polymerization [4–10]. Another way for synthesis of nanocomposites with well defined composition and morphology is adsorption of silica particles or in situ silica mineralization onto preformed polymer particles [11–14]. Silica deposition may be achieved by adding a silica precursor like TEOS or TMOS to aqueous solution of copolymer [15–18]. Polymer microgels are also used as templates for inorganic nanoparticles or ⇑ Corresponding author. Fax: +48 81533 3348. E-mail addresses: [email protected] (R. Zaleski), agnieszka.kierys @umcs.lublin.pl (A. Kierys), [email protected] (M. Grochowicz), jacek. [email protected] (M. Dziadosz), [email protected]. pl (J. Goworek). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.03.008
combined materials of these particles with a microgel of unique properties [19–23]. Recently, we suggested an alternative preparation way of a polymer–silica composite consisting in swelling of preformed polymer particles in TEOS, and next condensation of the silica precursor by wetting the swollen beads in aqueous solution of adjusted pH [24]. Similar approach for synthesis of uniform silica microspheres is presented in Ref. [25]. Chen et al. reported also the method to prepare inorganic microspheres using polymer particles as a template [26]. The pore size network for polymer/silica materials is of complex character. Conventional method for characterization of porous solids is nitrogen adsorption. However, characterization of composite materials by nitrogen adsorption method is related to some problems. The main reason of uncertainties of adsorption method is the irreversibility of adsorption. For materials containing polymer component the adsorption and desorption branches of isotherm do not overlap in the whole pressure range. It may indicate some kinetics restrictions of the adsorption, the presence of irregularities or even closed pores which are inaccessible for adsorptive. New insight in the porosity of complex pore systems is possible by measurements of positronium lifetime spectra. Positronium annihilation lifetime spectroscopy is a promising technique which allows investigation of any free volumes in the range of diameters from angstroms to tens of nanometers [27–30]. The PALS technique is particularly suitable for determination of size of pores in solids, including closed pores and space between fine particles
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themselves. Determination of pore size on the basis of o-Ps lifetime for radii larger than 1 nm became possible in the last decade due to Extended-Tao-Eldrup model [31]. For small pores bellow 1 nm, the Tao-Eldrup model is commonly used [32,33]. The ETE model was successfully applied for characterization of several model silicas or glasses of controlled porosity [34–38]. However, is rarely used for more complicated pore systems. In the present paper the ETE model was applied to characterize porosity, of polymer/silica nanocomposite synthesized by swelling of porous polymer poly(TRIM) in TEOS. The aim of this study is to evaluate the correlation between structural parameters derived from PALS spectra and conventional nitrogen adsorption. 2. Experimental 2.1. Sample preparation Trimethylolpropane trimethacrylate (TRIM) and a,a0 azoisobisbutyronitrile were obtained from Merck (Darmstadt, Germany), decan-1-ol, and poly(vinyl alcohol) were from Fluka AG (Buchs Switzerland), toluene was from POCh (Gliwice, Poland). Suspension polymerization of TRIM was made to obtain porous poly (TRIM) microspheres. The following procedure was used: 195 cm3 of redistilled water and 6.65 g of poly(vinyl alcohol) (PVA, stabilizer) were stirred for 3 h at 80 °C in a three-necked flask fitted with a stirrer, a water condenser, and a thermometer. Then, the mixture containing 15 g of TRIM, 0.1 g of a,a0 -azoisobisbutyronitrile (AIBN, initiator) in solution of pore-forming diluents (19 cm3 of toluene and 3 cm3 of decan-1-ol) was added while stirring to the aqueous medium. Polymerization was performed for 20 h at 80 °C. The porous beads formed in this process were sucked off, washed with distilled water, dried and extracted in a Soxhlet apparatus with boiling acetone, toluene, and methanol. The obtained microspheres were of the diameter ranging from 0.1 to 0.2 mm. A poly(TRIM)–Si composite sample was prepared by swelling poly(TRIM) beads in tetraethoxysilane (TEOS, Sigma–Aldrich, 98%) followed by condensation of the silica precursor. Initially, the poly(TRIM) beads were wetted with TEOS. The amount of TEOS was adjusted so that the beads started to stick together preserving a loosely packed structure. The polymer beads rapidly swelled to more than three times their dry volume. The polymer/TEOS w/w ratio was 0.33. Next, the poly(TRIM) particles saturated with TEOS were transferred to acidic aqueous solution (260 cm3 of 2.8 M HCl per 6 g of sample) and kept at room temperature for 24 h for gelation and aging. The obtained organic/silica material was filtered, washed with water and dried at 80 °C under vacuum. The presence of separate silica gel particles was not evidenced. It suggested that the total amount of TEOS was condensed into polymer particles. The drying procedure was performed over 12 h. The characterization of the pore structure for composite components separately is impossible, but after removal of polymer from the composite particles the porosity of silica filler becomes accessible. In order to prepare pure silica, being the inorganic component of the composite, part of initial sample was calcined in air at 600 °C for 8 h. TG measurements demonstrated that above 250 °C starts fast degradation of polymer and at 600 °C this process is completed. The remaining incombustible residue was assumed to be pure silica dioxide SiO2. Thermogravimetric analysis indicated that the composite particles were silica rich the content of which was about 15 wt.%. 2.2. Experimental methods Textural characterization of the samples of initial poly(TRIM), the poly(TRIM)–Si composite and silica gel obtained by calcinations of
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the composite SiO2 was carried out by the low-temperature nitrogen adsorption–desorption method. Nitrogen adsorption–desorption measurements were made at 77 K using a volumetric adsorption analyzer AUTOSORB-1CMS (Quantachrome Instruments, USA). The specific surface areas, SBET, of the investigated samples were evaluated using the standard Brunauer–Emmett–Teller (BET) method for the nitrogen adsorption data in the range of a relative pressure p/po 0.05–0.25. The total pore volume was estimated from a single point adsorption at a relative pressure of 0.985. The pore size distributions were obtained from the desorption branch of the isotherm using the Barrett–Joyner–Halenda (BJH) procedure [39] and the DFT method. For the calculation of DFT pore size distribution the following assumptions were taken into account: the nitrogen adsorption on carbon/silica, cylindrical pore shape, NL DFT equilibrium model. 29 Si magic-angle spinning (MAS) NMR spectra of the solid samples were obtained at the resonance frequency of 59.6 MHz on a Bruker Avance – 300 spectrometer. For 29Si NMR 4 mm circonia rotors spun at 8 kHz were used. About 8000 scans were applied until a satisfactory signal-to-noise ratio was achieved. The spectra were recorded by using the CP pulse program. The chemical shifts are given in ppm and referred to Q8M8 as standard material. SEM studies were conducted on a Tesla BS-301 microscope operating at 15.0 keV. Thermogravimetric analysis was performed with Setaram Setsys 16/18 instrument. The nanocomposite beads were treated to 1000 °C at a heating rate of 10° min1 in air. Positronium annihilation measurements (PALS method) were done using a combined fast–fast (start signal branch) and fast– slow (stop signal branch) delayed coincidence spectrometer. The annihilation radiation detectors were equipped with BaF2 scintillators. The samples were formed in two 2 mm thick layers of powder. The positron source (22Na) enclosed in a Kapton envelope was placed between them. A sample-source-sample sandwich was kept inside a vacuum chamber during the measurements. The chamber vacuum was about 105 Pa in order to avoid ortho-para conversion of the positronium on paramagnetic atmospheric oxygen and keep the sample surface free of adsorbents as much as possible. The coincidence counting rate was about 2.4 106 per hour; about 4.4 107 counts per spectrum were collected during 18 h.
3. Results and discussion SEM studies provided direct evidence that spherical milkywhite particles of the nanocomposite poly(TRIM)–silica preserved the initial shape of the polymer material (Fig. 1). It indicates that when TEOS is reacted under acidic conditions the SiO2 domains are well dispersed on the surface and within the bulk of the polymer. The silica seems to be uniformly distributed in interior and on the surface of the polymer particles. During composite sample drying and shrinkage of polymer/TEOS mesophase on the external surface of the particles deep cracks of the nanometer scale appeared. After calcination of the poly(TRIM)–Si, the silica particles preserve spherical shape of the initial particles, however their dimensions are smaller. Calcination and polymer removal causes the 20% shrinkage of the bulk material. The composite sample under study contains a relatively high amount of silica species. In order to characterize the form of dispersed silica particles (various structural kinds of silicon atoms) the composite sample was investigated using 29Si CP-MAS NMR spectroscopy. The spectrum shown in Fig. 2 exhibits three resonance peaks, at 110 ppm which can be assigned to Q4 units corresponding to four siloxane bridges and those at 100 and 90 ppm are assigned to Q3 and Q2 units, respectively representing Si atoms coordinated with one and two hydroxyl groups. The
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a
b
Fig. 1. SEM micrographs of the poly(TRIM)–Si beads obtained after swelling in TEOS.
Fig. 2.
29
Si MAS NMR spectrum of poly(TRIM)–Si composite.
proportion of the silica species can be estimated after deconvolution of the spectrum using Gaussian–Lorentzian fitting. The Q4 species can be ascribed to the silica core of the primary silica particles, whereas Q3 and Q2 units characterize rather the external surface of the particles. Thus, a relative proportion of Q3 + Q2/Q4 is the measure of the degree of dispersion and condensation of the silica units in silica deposit. A high proportion Q4/Q3/Q2 = 1.3/5.3/1 for the composite sample but higher than that (Q4/Q3/Q2 = 8.8/5.7/1) for standard silica gel [40] suggests a high hydroxylation of the silica component entrapped in the polymer matrix and a small concentration of Q4 sites. Hence, one can assume that dispersed silica species are small and probably in the form of nanoparticles or wires. The standard characterization of the porosity of pure polymer sample, the composite material and pure SiO2 obtained by calcination of the composite was evaluated on the basis of nitrogen adsorption–desorption isotherms at 195 °C (Fig. 3). The shape of these isotherms is characteristic of mesoporous material.
Fig. 3. Adsorption/desorption isotherms of nitrogen at 195 °C of (d) poly(TRIM), (j) poly(TRIM)–Si and (N) SiO2.
Comparing the presented isotherms one can observe a significant drop of adsorption for the modified sample (almost twice) and the increase for pure silica particles (almost triple) compared to the poly(TRIM). As regards the composite sample in the range of p/po = 0.5–1.0 the desorption isotherm is practically flat and pore emptying takes place in a very narrow range of relative pressure. In the case of initial polymer desorption from pores takes place within a wide pressure range. For both samples containing polymer adsorption irreversibility can be observed in the whole pressure range. The shape of nitrogen adsorption/desorption isotherms for pure silica SiO2 is somewhat similar to these of the composite. Unlike the composite and polymer samples, for SiO2 adsorption of nitrogen is reversible in the whole pressure range.
R. Zaleski et al. / Journal of Colloid and Interface Science 358 (2011) 268–276 Table 1 Parameters characterizing porosity of the investigated samples obtained from nitrogen adsorption/desorption at 195 °C. Sample name
SBET (m2/g)
Vp (cm3/g)
Dp (nm)
Poly(TRIM) Poly(TRIM)–Si SiO2
638 371 1197
0.72 0.37 1.12
4.51 3.99 3.74
SBET – specific surface area, Vp – total pore volume, Dp – mean pore diameter Dp = 4Vp/SBET.
The numerical values of the parameters characterizing the investigated materials derived from N2 adsorption data are listed in Table 1. As it is seen both samples containing poly(TRIM) have a quite different specific surface area and total pore volume which are
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much smaller for the composite one. Also pore dimensions are differentiated. Calcination of the composite produces silica of very high specific surface area and total pore volume. For the polymer–silica composite pores of various dimensions exist probably in the internal part of composite beads. Small pores being, for example, the pore openings of larger pores which determine the desorption process are more regular as compared to the pure polymer sample. Because the desorption is controlled by the smallest size of pore structure on the desorption isotherm of sharp step at p/po = 0.5 appears. This porosity model for polymer–silica material correlates in some way with the results of grand canonical Monte Carlo (GCMC) studies of the capillary condensation in mesoporous media [41–44]. It should be noted that the composite sample contains a relatively small amount of the polymer component as compared to
Fig. 4. Positron annihilation lifetime spectra (dependence between the number of annihilation events and the time between positron formation and annihilation) for the investigated samples.
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Fig. 5. Histograms of intensities per unit lifetime (normalized contribution of the number of events with the particular lifetime s) for the investigated samples.
the initial sample. However, the number of narrow mesopores evidenced by the presence of a small step at p/po = 0.5 on the isotherm for the initial sample substantially increases as regards the composite sample. Hence, one can assume that porosity of both components, i.e. polymer and silica component within smaller mesopores is similar. It may be the result of structural transformation of the polymer during the swelling process and simultaneous creation of the silica phase of similar porosity. If we assume that the porosity in poly(TRIM)–silica composite represents the volumes between spherical subparticles (microspheres), the important factor determining the pore dimensions is the packing of these microspheres [45,46]. Diameter increase of microspheres due to swelling and simultaneous transformation of their array may produce smaller interparticle pores. In as-synthesized polymer sample rather an irregular arrangement of primary microspheres should be assumed. Only a part of pores exhibit a weak ordering represented by the first step on the condensation segment of the adsorption isotherm. Therefore, if we take into account the calculated pore dimensions for both samples and the substantial decrease of the specific surface area and pore volume after TEOS modification, this
type of internal rearrangement of the pore structure seems to be quite probable. More information to complete the study on the pore structure of poly(TRIM), the composite material and pure silica can be obtained from their pore size distribution curves (PSD).Generally, it should be mentioned that interpretation of hysteresis of ‘‘triangle’’ type is difficult. For the composite sample the steep desorption step at p/po = 0.5 indicates a high uniformity of the pore structure. On the other hand, a continuous and almost linear increase of adsorption within a wide pressure range suggests rather irregularity of pore dimensions. Thus, the PSD derived from desorption differs considerably in comparison to PSD from the isotherm adsorption branch which does not exhibit well marked peaks. In order to reach a better understanding of the origin of the observed differences between adsorption and desorption isotherm the porosity of studied materials was evaluated using another technique i.e. the positron annihilation lifetime spectroscopy. This method uses small hydrogen-like pseudo-atom (positronium or Ps, i.e. bound state of positron and electron) as a probe which is formed randomly in the sample bulk, where positrons emitted
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from the radioactive source encounter free volumes. Thus, PALS provides information about any free volumes present in the solid. Positronium is an unstable particle due to annihilation of positron (e+) with one of electrons in its neighborhood: originating from Ps itself (intrinsic annihilation) or from the surrounding medium (pick-off process). Observing the positron lifetime spectrum, i.e. the histogram presenting the number of annihilation events as a function of time between positron creation in the source and annihilation in the medium for a large population of positrons (Fig. 4), one can see that, despite the statistical nature of positron annihilation, the number of annihilations decreases with time increase since positron creation is followed by exponential curves. The coefficient determining the curve slope is called the lifetime. A positron lifetime spectrum is almost always the sum of such curves (called components) with various lifetimes originating from various fractions of positrons. Long-lived parts of the spectrum originating from annihilation of Ps triplet state (ortho-positronium or o-Ps) are particularly interesting for characterization of porous materials. Its lifetime is related to the size of free volume where o-Ps is trapped. The distribution of o-Ps lifetimes can be obtained from the positron lifetime spectra using the numerical methods. One of the most sophisticated techniques based on the Bayes’ theorem and maximum entropy method is implemented in MELT program [47]. The result of the analysis by MELT for a polymer and composite sample is given as a histogram of intensities, i.e. normalized contribution of the number of events with a particular lifetime to the spectrum. Fig. 5 shows the values of the lifetimes presented in the histogram, which are taken from a priori assumed dense grid of quantized lifetimes (1500 values are in the presented example). One can see that the distribution of the intensities is formed in a series of bell shaped peaks instead of very narrow delta functions, which could appear if strictly exponential curves would be present in the positron lifetime spectra. Such a result reflects dispersion of the sizes of free volumes in the medium; however, the width of the peaks is determined by Bayesian methods in terms of probability. The currently presented distributions are the most probable ones. The peaks will be referred as components in further considerations. The intensity histograms presented in Fig. 5 contain a much more legible information than the rough positron lifetime spectra (Fig. 4). Both in the spectra and intensity histograms it is seen that the lifetime of the longest-lived component is shorter in the case of poly(TRIM)–Si than poly(TRIM), however, the numerical values of its mean value and the distribution width are directly visible only in the intensity histograms. Also the complex nature of this component can be determined. It manifests itself as a tail from the side of short lifetimes. The information originating from the short-lived part of the spectrum can be hardly determined from the spectra. This part of the spectra looks almost the same for poly(TRIM) and poly(TRIM)–Si. The intensity histograms however show that there is a set of a few components forming the short-lived part of the spectra. Moreover, a characteristic triplet of components is shifted towards longer lifetimes in the case of poly(TRIM)–Si, whilst a remnant of the first component of the triplet observed in poly(TRIM) is still present. The histogram for pure silica material is of quite different character. Only one intensive peak is observed for relatively short lifetimes. The longer lifetimes are represented by two small peaks of very low intensity. The knowledge of the relation between the lifetime and the size of free volume is necessary to use PALS for characterization of solids. This relation is well approximated by the extended Tao-Eldrup (ETE) model [31,48]. This model requires two assumptions: the value of the empirical parameter D characteristic for the investigated material (e.g. D = 0.166 nm for organic materials) and the geometrical shape of free volumes. In order to make the PALS results
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comparable to those obtained from BJH and DFT calculations, a cylindrical shape of free volumes is assumed. The relation between the lifetime and size of free volumes provided by the ETE model calculated by using EELViS routine [49] is shown in Fig. 6. Besides the above presented method for calculating the size of free volumes (e.g. pores), their distribution has to be derived from the intensities histogram in order to obtain PSD similar to that provided by BJH procedure. Following the previous assumption of the cylindrical shape of pores and taking into account the relation between the intensity for the particular lifetimes I(s) and concentration of pores of a corresponding size D [50], the relation of pore volumes between their diameters can be approximated by the relation:
dV ds / IðsÞ dD dD
ð1Þ
Fig. 6. The relation between the size of free volume and ortho-positronium lifetime provided by the ETE model assuming cylindrical pore geometry and D = 0.166 nm.
Fig. 7. Derivative of the relation between the lifetime and the size of free volume provided by the ETE model assuming cylindrical pore geometry and D = 0.166 nm.
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Fig. 8. PSDs derived from the nitrogen adsorption/desorption isotherms: BJH – broken line, DFT – dotted line and distributions of free volumes from PALS – solid line.
where ds/dD is the derivative of s(D) function given by the ETE model. One can see from Eq. (1) that the relation between the volume per unit size dV/dD is linear when using the model. However, the proportionality factor ds/dD depends on D. In order to present the range of the best sensitivity of PALS, ds/dD values for various D’s are shown in Fig. 7. The factor ds/dD reaches its maximum for D 1.95 nm, which means that the unit intensity corresponds to the largest value of the specific volume for pores of this size. The proportionality coefficient ds/dD is two times smaller for D 0.91 nm with regard to small pores, and D 3.65 nm with regard to big ones. For example, the model predicts that if there are two kinds of pores in the same sample: one of size D 3.65 nm, the other of size D 1.95 nm, and the specific volume of both kinds of pores is the same, the intensity of o-Ps will be two times larger for the former kind of pores because the factor ds/dD is two times smaller for them.
The result of applying Eq. (1) to the data presented in Fig. 5 is shown in Fig. 8. The PSDs derived from PALS spectra measured at 302 K for the studied samples indicate that post-synthesis modification of poly(TRIM) with TEOS substantially influences its pore structure. In the initial poly(TRIM) sample two groups of mesopores represented by not completely resolved peaks 4 and 5 are present. Their maxima are centered at about D = 3.22 and D = 4.72 nm, respectively. Moreover, in the polymer sample much smaller pores are present represented by peaks 1, 2 and 3 with their maxima centered at D = 0.37 nm, 0.77 nm and 1.32 nm, respectively. These pores may be classified as micropores according to IUPAC recommendations. Taking into account the nature of the investigated samples these micropores may be mainly ascribed to the polymer component. In the composite sample micropores are also present, but their dimensions are more differentiated than those in the pure polymer sample. After TEOS modification in poly(TRIM)–Si sample
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there are still present mesopores of a dimension slightly smaller than for the initial polymer sample. However, PSD becomes narrower and exhibits a monomodal character. It testifies the fact that within this range of pore diameter the porosity becomes more homogeneous and uniform. All three peaks 1, 2 and 3 in the micropore range are differentiated and shifted towards higher D values. It means that silica loading causes the change of the free volumes dimensions in polymer matrix. The PSD for mesopores derived from PALS spectra for pure silica material is of bimodal character. There are also present two groups of pores of dimensions lower than 1 nm which represent the free volumes in silica network. It is reasonable to assume that these pores are rather inaccessible to nitrogen molecules. In Fig. 8 are also shown pore size distributions derived from the nitrogen adsorption data. The micropore size analysis on the basis of the nitrogen adsorption data was performed using computational DFT procedure which is incorporated in Quantachrome Autosorb processing software. In the same figure are also shown PSDs for mesopores calculated BJH procedure. The mean pore diameters at the peak of PSDs derived from the long lived component of PALS experiment are similar but not identical to those calculated by DFT and BJH from the nitrogen adsorption data. Comparing these data one can say, that the results obtained by different techniques reflect in general the character of the pore system for investigated materials. However, the PALS measurements provide information about porosity in very wide range of pore dimensions. The analysis of the results obtained from PALS experiment confirms that poly(TRIM) swelling in the presence of TEOS is related to the total penetration of the silica precursor within the polymer particles. After condensation of TEOS the mesopores present in the initial sample become smaller and more uniform in size. Simultaneously, micropores in the polymer matrix become slightly larger. After calcination of the composite the pores of D < 2 nm practically disappear.
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PSDs for samples of different pore system one can say that pore dimensions derived from PALS spectra correlate quite reasonably with structural parameters calculated on the basis of the nitrogen adsorption data using conventional analytical procedures. Interestingly, the best correlation of PSDs in the mesopore range occurs in the case of PALS and BJH distribution. Application of positron annihilation lifetime spectroscopy allowed an assessment to be made of the complex pore structure. Thus ‘‘positron porosimetry’’ is a convenient tool for the characterization of polymer/silica composites. Acknowledgment This work was supported by the Polish Ministry of Science Grant No. N N204 131137. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
4. Conclusions
[19] [20]
The post-synthesis modification of poly(TRIM) with TEOS was shown to influence the porosity of the polymer sample. The investigations of the free volumes within the composite material illustrate that the TEOS loading followed by its condensation causes decrease of the specific surface area and pore volume in addition to the framework transformation. The amount of adsorbed nitrogen decreases after introducing silica into the polymer matrix. Simultaneously, the porosity of the composite sample becomes more regular. From this study one can conclude that after polymer swelling in TEOS the packing of polymer microspheres becomes denser, and the internal pore system of the resulting nanoparticles becomes simultaneously of a mixed character, in which silica particles are dispersed homogenously in the whole bulk phase of the composite. Calcination of composite particles produces silica particles of spherical morphology. The resulting silica gel exhibits relatively uniform mesoporosity of high pore volume and very high surface area. The three porous materials studied in the present work are highly mesoporous but contain also micropores. Pore size distribution for micro- and mesopores determined by the conventional adsorption method and PALS exhibits small but noticeable differences. These differences may be justified if we take into account that a part of pores in the investigated samples are effectively blocked for nitrogen molecules. On the other hand PALS can be used to obtain useful information on the pore size without assumptions concerning the physisorption mechanism of surface coverage, pore filling and hysteresis phenomena. Comparing the
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Synthesis and characterization of nanostructural polymer–silica composite: Positron annihilation lifetime spectroscopy study Radosław Zaleski a, Agnieszka Kierys b, Marta Grochowicz c, Marek Dziadosz b, Jacek Goworek b,⇑ a
M. Curie-Sklodowska University, Institute of Physics, Department of Nuclear Methods, M. Curie-Sklodowska sq. 1, 20-031 Lublin, Poland M. Curie-Sklodowska University, Faculty of Chemistry, Department of Adsorption, M. Curie-Sklodowska sq. 3, 20-031 Lublin, Poland c M. Curie-Sklodowska University, Faculty of Chemistry, Department of Polymer Chemistry, Gliniana 33, 20-614 Lublin, Poland b
a r t i c l e
i n f o
Article history: Received 24 January 2011 Accepted 1 March 2011 Available online 8 March 2011 Keywords: Polymer–silica composite Positron annihilation lifetime spectroscopy (PALS) Poly(TRIM) TEOS Porosity
a b s t r a c t The swelling of poly(TRIM) spherical particles in TEOS is assessed as a potential way for obtaining polymer–silica nanocomposite materials. Silica deposition was achieved by simply stirring of swollen polymer particles in acidic hydrochloric-water solution. This procedure leads to spherical composite particles with dispersed silica gel within the polymer matrix. The resulting material exhibits the same morphology as the initial polymer. Nanocomposite particles are silica rich (about 17 wt.%). Characterization of the nanocomposites was performed using scanning electron microscopy, FT-IR spectroscopy, 29Si CP MAS NMR spectroscopy and thermogravimetry. Moreover, the use of positron annihilation lifetime spectroscopy PALS to characterize the structural properties of the nanocomposites is presented. This technique gave more realistic pieces of information about the pore structure of the investigated samples in contrast to nitrogen adsorption studies. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction The importance of obtaining information on textural properties of micro- and mesoporous solids increases with the rapid increase of synthesis routes of new composite materials. Recently, inorganic–organic composites have garnered much interest as a new class of porous solids with potential application in gas separation, catalysis, chemical sensing or drug delivery systems. Various synthesis routes have been developed for preparation of these nanocomposites (see e.g. [1–3]). The techniques necessary for obtaining composite materials containing a wide variety of silica and organic species arranged in different ways are well documented. Silica components may be introduced as a preformed particles, colloidal silica, metal alkoxide or TEOS as silica precursor into monomer solution followed by polymerization [4–10]. Another way for synthesis of nanocomposites with well defined composition and morphology is adsorption of silica particles or in situ silica mineralization onto preformed polymer particles [11–14]. Silica deposition may be achieved by adding a silica precursor like TEOS or TMOS to aqueous solution of copolymer [15–18]. Polymer microgels are also used as templates for inorganic nanoparticles or ⇑ Corresponding author. Fax: +48 81533 3348. E-mail addresses: [email protected] (R. Zaleski), agnieszka.kierys @umcs.lublin.pl (A. Kierys), [email protected] (M. Grochowicz), jacek. [email protected] (M. Dziadosz), [email protected]. pl (J. Goworek). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.03.008
combined materials of these particles with a microgel of unique properties [19–23]. Recently, we suggested an alternative preparation way of a polymer–silica composite consisting in swelling of preformed polymer particles in TEOS, and next condensation of the silica precursor by wetting the swollen beads in aqueous solution of adjusted pH [24]. Similar approach for synthesis of uniform silica microspheres is presented in Ref. [25]. Chen et al. reported also the method to prepare inorganic microspheres using polymer particles as a template [26]. The pore size network for polymer/silica materials is of complex character. Conventional method for characterization of porous solids is nitrogen adsorption. However, characterization of composite materials by nitrogen adsorption method is related to some problems. The main reason of uncertainties of adsorption method is the irreversibility of adsorption. For materials containing polymer component the adsorption and desorption branches of isotherm do not overlap in the whole pressure range. It may indicate some kinetics restrictions of the adsorption, the presence of irregularities or even closed pores which are inaccessible for adsorptive. New insight in the porosity of complex pore systems is possible by measurements of positronium lifetime spectra. Positronium annihilation lifetime spectroscopy is a promising technique which allows investigation of any free volumes in the range of diameters from angstroms to tens of nanometers [27–30]. The PALS technique is particularly suitable for determination of size of pores in solids, including closed pores and space between fine particles
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themselves. Determination of pore size on the basis of o-Ps lifetime for radii larger than 1 nm became possible in the last decade due to Extended-Tao-Eldrup model [31]. For small pores bellow 1 nm, the Tao-Eldrup model is commonly used [32,33]. The ETE model was successfully applied for characterization of several model silicas or glasses of controlled porosity [34–38]. However, is rarely used for more complicated pore systems. In the present paper the ETE model was applied to characterize porosity, of polymer/silica nanocomposite synthesized by swelling of porous polymer poly(TRIM) in TEOS. The aim of this study is to evaluate the correlation between structural parameters derived from PALS spectra and conventional nitrogen adsorption. 2. Experimental 2.1. Sample preparation Trimethylolpropane trimethacrylate (TRIM) and a,a0 azoisobisbutyronitrile were obtained from Merck (Darmstadt, Germany), decan-1-ol, and poly(vinyl alcohol) were from Fluka AG (Buchs Switzerland), toluene was from POCh (Gliwice, Poland). Suspension polymerization of TRIM was made to obtain porous poly (TRIM) microspheres. The following procedure was used: 195 cm3 of redistilled water and 6.65 g of poly(vinyl alcohol) (PVA, stabilizer) were stirred for 3 h at 80 °C in a three-necked flask fitted with a stirrer, a water condenser, and a thermometer. Then, the mixture containing 15 g of TRIM, 0.1 g of a,a0 -azoisobisbutyronitrile (AIBN, initiator) in solution of pore-forming diluents (19 cm3 of toluene and 3 cm3 of decan-1-ol) was added while stirring to the aqueous medium. Polymerization was performed for 20 h at 80 °C. The porous beads formed in this process were sucked off, washed with distilled water, dried and extracted in a Soxhlet apparatus with boiling acetone, toluene, and methanol. The obtained microspheres were of the diameter ranging from 0.1 to 0.2 mm. A poly(TRIM)–Si composite sample was prepared by swelling poly(TRIM) beads in tetraethoxysilane (TEOS, Sigma–Aldrich, 98%) followed by condensation of the silica precursor. Initially, the poly(TRIM) beads were wetted with TEOS. The amount of TEOS was adjusted so that the beads started to stick together preserving a loosely packed structure. The polymer beads rapidly swelled to more than three times their dry volume. The polymer/TEOS w/w ratio was 0.33. Next, the poly(TRIM) particles saturated with TEOS were transferred to acidic aqueous solution (260 cm3 of 2.8 M HCl per 6 g of sample) and kept at room temperature for 24 h for gelation and aging. The obtained organic/silica material was filtered, washed with water and dried at 80 °C under vacuum. The presence of separate silica gel particles was not evidenced. It suggested that the total amount of TEOS was condensed into polymer particles. The drying procedure was performed over 12 h. The characterization of the pore structure for composite components separately is impossible, but after removal of polymer from the composite particles the porosity of silica filler becomes accessible. In order to prepare pure silica, being the inorganic component of the composite, part of initial sample was calcined in air at 600 °C for 8 h. TG measurements demonstrated that above 250 °C starts fast degradation of polymer and at 600 °C this process is completed. The remaining incombustible residue was assumed to be pure silica dioxide SiO2. Thermogravimetric analysis indicated that the composite particles were silica rich the content of which was about 15 wt.%. 2.2. Experimental methods Textural characterization of the samples of initial poly(TRIM), the poly(TRIM)–Si composite and silica gel obtained by calcinations of
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the composite SiO2 was carried out by the low-temperature nitrogen adsorption–desorption method. Nitrogen adsorption–desorption measurements were made at 77 K using a volumetric adsorption analyzer AUTOSORB-1CMS (Quantachrome Instruments, USA). The specific surface areas, SBET, of the investigated samples were evaluated using the standard Brunauer–Emmett–Teller (BET) method for the nitrogen adsorption data in the range of a relative pressure p/po 0.05–0.25. The total pore volume was estimated from a single point adsorption at a relative pressure of 0.985. The pore size distributions were obtained from the desorption branch of the isotherm using the Barrett–Joyner–Halenda (BJH) procedure [39] and the DFT method. For the calculation of DFT pore size distribution the following assumptions were taken into account: the nitrogen adsorption on carbon/silica, cylindrical pore shape, NL DFT equilibrium model. 29 Si magic-angle spinning (MAS) NMR spectra of the solid samples were obtained at the resonance frequency of 59.6 MHz on a Bruker Avance – 300 spectrometer. For 29Si NMR 4 mm circonia rotors spun at 8 kHz were used. About 8000 scans were applied until a satisfactory signal-to-noise ratio was achieved. The spectra were recorded by using the CP pulse program. The chemical shifts are given in ppm and referred to Q8M8 as standard material. SEM studies were conducted on a Tesla BS-301 microscope operating at 15.0 keV. Thermogravimetric analysis was performed with Setaram Setsys 16/18 instrument. The nanocomposite beads were treated to 1000 °C at a heating rate of 10° min1 in air. Positronium annihilation measurements (PALS method) were done using a combined fast–fast (start signal branch) and fast– slow (stop signal branch) delayed coincidence spectrometer. The annihilation radiation detectors were equipped with BaF2 scintillators. The samples were formed in two 2 mm thick layers of powder. The positron source (22Na) enclosed in a Kapton envelope was placed between them. A sample-source-sample sandwich was kept inside a vacuum chamber during the measurements. The chamber vacuum was about 105 Pa in order to avoid ortho-para conversion of the positronium on paramagnetic atmospheric oxygen and keep the sample surface free of adsorbents as much as possible. The coincidence counting rate was about 2.4 106 per hour; about 4.4 107 counts per spectrum were collected during 18 h.
3. Results and discussion SEM studies provided direct evidence that spherical milkywhite particles of the nanocomposite poly(TRIM)–silica preserved the initial shape of the polymer material (Fig. 1). It indicates that when TEOS is reacted under acidic conditions the SiO2 domains are well dispersed on the surface and within the bulk of the polymer. The silica seems to be uniformly distributed in interior and on the surface of the polymer particles. During composite sample drying and shrinkage of polymer/TEOS mesophase on the external surface of the particles deep cracks of the nanometer scale appeared. After calcination of the poly(TRIM)–Si, the silica particles preserve spherical shape of the initial particles, however their dimensions are smaller. Calcination and polymer removal causes the 20% shrinkage of the bulk material. The composite sample under study contains a relatively high amount of silica species. In order to characterize the form of dispersed silica particles (various structural kinds of silicon atoms) the composite sample was investigated using 29Si CP-MAS NMR spectroscopy. The spectrum shown in Fig. 2 exhibits three resonance peaks, at 110 ppm which can be assigned to Q4 units corresponding to four siloxane bridges and those at 100 and 90 ppm are assigned to Q3 and Q2 units, respectively representing Si atoms coordinated with one and two hydroxyl groups. The
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a
b
Fig. 1. SEM micrographs of the poly(TRIM)–Si beads obtained after swelling in TEOS.
Fig. 2.
29
Si MAS NMR spectrum of poly(TRIM)–Si composite.
proportion of the silica species can be estimated after deconvolution of the spectrum using Gaussian–Lorentzian fitting. The Q4 species can be ascribed to the silica core of the primary silica particles, whereas Q3 and Q2 units characterize rather the external surface of the particles. Thus, a relative proportion of Q3 + Q2/Q4 is the measure of the degree of dispersion and condensation of the silica units in silica deposit. A high proportion Q4/Q3/Q2 = 1.3/5.3/1 for the composite sample but higher than that (Q4/Q3/Q2 = 8.8/5.7/1) for standard silica gel [40] suggests a high hydroxylation of the silica component entrapped in the polymer matrix and a small concentration of Q4 sites. Hence, one can assume that dispersed silica species are small and probably in the form of nanoparticles or wires. The standard characterization of the porosity of pure polymer sample, the composite material and pure SiO2 obtained by calcination of the composite was evaluated on the basis of nitrogen adsorption–desorption isotherms at 195 °C (Fig. 3). The shape of these isotherms is characteristic of mesoporous material.
Fig. 3. Adsorption/desorption isotherms of nitrogen at 195 °C of (d) poly(TRIM), (j) poly(TRIM)–Si and (N) SiO2.
Comparing the presented isotherms one can observe a significant drop of adsorption for the modified sample (almost twice) and the increase for pure silica particles (almost triple) compared to the poly(TRIM). As regards the composite sample in the range of p/po = 0.5–1.0 the desorption isotherm is practically flat and pore emptying takes place in a very narrow range of relative pressure. In the case of initial polymer desorption from pores takes place within a wide pressure range. For both samples containing polymer adsorption irreversibility can be observed in the whole pressure range. The shape of nitrogen adsorption/desorption isotherms for pure silica SiO2 is somewhat similar to these of the composite. Unlike the composite and polymer samples, for SiO2 adsorption of nitrogen is reversible in the whole pressure range.
R. Zaleski et al. / Journal of Colloid and Interface Science 358 (2011) 268–276 Table 1 Parameters characterizing porosity of the investigated samples obtained from nitrogen adsorption/desorption at 195 °C. Sample name
SBET (m2/g)
Vp (cm3/g)
Dp (nm)
Poly(TRIM) Poly(TRIM)–Si SiO2
638 371 1197
0.72 0.37 1.12
4.51 3.99 3.74
SBET – specific surface area, Vp – total pore volume, Dp – mean pore diameter Dp = 4Vp/SBET.
The numerical values of the parameters characterizing the investigated materials derived from N2 adsorption data are listed in Table 1. As it is seen both samples containing poly(TRIM) have a quite different specific surface area and total pore volume which are
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much smaller for the composite one. Also pore dimensions are differentiated. Calcination of the composite produces silica of very high specific surface area and total pore volume. For the polymer–silica composite pores of various dimensions exist probably in the internal part of composite beads. Small pores being, for example, the pore openings of larger pores which determine the desorption process are more regular as compared to the pure polymer sample. Because the desorption is controlled by the smallest size of pore structure on the desorption isotherm of sharp step at p/po = 0.5 appears. This porosity model for polymer–silica material correlates in some way with the results of grand canonical Monte Carlo (GCMC) studies of the capillary condensation in mesoporous media [41–44]. It should be noted that the composite sample contains a relatively small amount of the polymer component as compared to
Fig. 4. Positron annihilation lifetime spectra (dependence between the number of annihilation events and the time between positron formation and annihilation) for the investigated samples.
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Fig. 5. Histograms of intensities per unit lifetime (normalized contribution of the number of events with the particular lifetime s) for the investigated samples.
the initial sample. However, the number of narrow mesopores evidenced by the presence of a small step at p/po = 0.5 on the isotherm for the initial sample substantially increases as regards the composite sample. Hence, one can assume that porosity of both components, i.e. polymer and silica component within smaller mesopores is similar. It may be the result of structural transformation of the polymer during the swelling process and simultaneous creation of the silica phase of similar porosity. If we assume that the porosity in poly(TRIM)–silica composite represents the volumes between spherical subparticles (microspheres), the important factor determining the pore dimensions is the packing of these microspheres [45,46]. Diameter increase of microspheres due to swelling and simultaneous transformation of their array may produce smaller interparticle pores. In as-synthesized polymer sample rather an irregular arrangement of primary microspheres should be assumed. Only a part of pores exhibit a weak ordering represented by the first step on the condensation segment of the adsorption isotherm. Therefore, if we take into account the calculated pore dimensions for both samples and the substantial decrease of the specific surface area and pore volume after TEOS modification, this
type of internal rearrangement of the pore structure seems to be quite probable. More information to complete the study on the pore structure of poly(TRIM), the composite material and pure silica can be obtained from their pore size distribution curves (PSD).Generally, it should be mentioned that interpretation of hysteresis of ‘‘triangle’’ type is difficult. For the composite sample the steep desorption step at p/po = 0.5 indicates a high uniformity of the pore structure. On the other hand, a continuous and almost linear increase of adsorption within a wide pressure range suggests rather irregularity of pore dimensions. Thus, the PSD derived from desorption differs considerably in comparison to PSD from the isotherm adsorption branch which does not exhibit well marked peaks. In order to reach a better understanding of the origin of the observed differences between adsorption and desorption isotherm the porosity of studied materials was evaluated using another technique i.e. the positron annihilation lifetime spectroscopy. This method uses small hydrogen-like pseudo-atom (positronium or Ps, i.e. bound state of positron and electron) as a probe which is formed randomly in the sample bulk, where positrons emitted
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from the radioactive source encounter free volumes. Thus, PALS provides information about any free volumes present in the solid. Positronium is an unstable particle due to annihilation of positron (e+) with one of electrons in its neighborhood: originating from Ps itself (intrinsic annihilation) or from the surrounding medium (pick-off process). Observing the positron lifetime spectrum, i.e. the histogram presenting the number of annihilation events as a function of time between positron creation in the source and annihilation in the medium for a large population of positrons (Fig. 4), one can see that, despite the statistical nature of positron annihilation, the number of annihilations decreases with time increase since positron creation is followed by exponential curves. The coefficient determining the curve slope is called the lifetime. A positron lifetime spectrum is almost always the sum of such curves (called components) with various lifetimes originating from various fractions of positrons. Long-lived parts of the spectrum originating from annihilation of Ps triplet state (ortho-positronium or o-Ps) are particularly interesting for characterization of porous materials. Its lifetime is related to the size of free volume where o-Ps is trapped. The distribution of o-Ps lifetimes can be obtained from the positron lifetime spectra using the numerical methods. One of the most sophisticated techniques based on the Bayes’ theorem and maximum entropy method is implemented in MELT program [47]. The result of the analysis by MELT for a polymer and composite sample is given as a histogram of intensities, i.e. normalized contribution of the number of events with a particular lifetime to the spectrum. Fig. 5 shows the values of the lifetimes presented in the histogram, which are taken from a priori assumed dense grid of quantized lifetimes (1500 values are in the presented example). One can see that the distribution of the intensities is formed in a series of bell shaped peaks instead of very narrow delta functions, which could appear if strictly exponential curves would be present in the positron lifetime spectra. Such a result reflects dispersion of the sizes of free volumes in the medium; however, the width of the peaks is determined by Bayesian methods in terms of probability. The currently presented distributions are the most probable ones. The peaks will be referred as components in further considerations. The intensity histograms presented in Fig. 5 contain a much more legible information than the rough positron lifetime spectra (Fig. 4). Both in the spectra and intensity histograms it is seen that the lifetime of the longest-lived component is shorter in the case of poly(TRIM)–Si than poly(TRIM), however, the numerical values of its mean value and the distribution width are directly visible only in the intensity histograms. Also the complex nature of this component can be determined. It manifests itself as a tail from the side of short lifetimes. The information originating from the short-lived part of the spectrum can be hardly determined from the spectra. This part of the spectra looks almost the same for poly(TRIM) and poly(TRIM)–Si. The intensity histograms however show that there is a set of a few components forming the short-lived part of the spectra. Moreover, a characteristic triplet of components is shifted towards longer lifetimes in the case of poly(TRIM)–Si, whilst a remnant of the first component of the triplet observed in poly(TRIM) is still present. The histogram for pure silica material is of quite different character. Only one intensive peak is observed for relatively short lifetimes. The longer lifetimes are represented by two small peaks of very low intensity. The knowledge of the relation between the lifetime and the size of free volume is necessary to use PALS for characterization of solids. This relation is well approximated by the extended Tao-Eldrup (ETE) model [31,48]. This model requires two assumptions: the value of the empirical parameter D characteristic for the investigated material (e.g. D = 0.166 nm for organic materials) and the geometrical shape of free volumes. In order to make the PALS results
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comparable to those obtained from BJH and DFT calculations, a cylindrical shape of free volumes is assumed. The relation between the lifetime and size of free volumes provided by the ETE model calculated by using EELViS routine [49] is shown in Fig. 6. Besides the above presented method for calculating the size of free volumes (e.g. pores), their distribution has to be derived from the intensities histogram in order to obtain PSD similar to that provided by BJH procedure. Following the previous assumption of the cylindrical shape of pores and taking into account the relation between the intensity for the particular lifetimes I(s) and concentration of pores of a corresponding size D [50], the relation of pore volumes between their diameters can be approximated by the relation:
dV ds / IðsÞ dD dD
ð1Þ
Fig. 6. The relation between the size of free volume and ortho-positronium lifetime provided by the ETE model assuming cylindrical pore geometry and D = 0.166 nm.
Fig. 7. Derivative of the relation between the lifetime and the size of free volume provided by the ETE model assuming cylindrical pore geometry and D = 0.166 nm.
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Fig. 8. PSDs derived from the nitrogen adsorption/desorption isotherms: BJH – broken line, DFT – dotted line and distributions of free volumes from PALS – solid line.
where ds/dD is the derivative of s(D) function given by the ETE model. One can see from Eq. (1) that the relation between the volume per unit size dV/dD is linear when using the model. However, the proportionality factor ds/dD depends on D. In order to present the range of the best sensitivity of PALS, ds/dD values for various D’s are shown in Fig. 7. The factor ds/dD reaches its maximum for D 1.95 nm, which means that the unit intensity corresponds to the largest value of the specific volume for pores of this size. The proportionality coefficient ds/dD is two times smaller for D 0.91 nm with regard to small pores, and D 3.65 nm with regard to big ones. For example, the model predicts that if there are two kinds of pores in the same sample: one of size D 3.65 nm, the other of size D 1.95 nm, and the specific volume of both kinds of pores is the same, the intensity of o-Ps will be two times larger for the former kind of pores because the factor ds/dD is two times smaller for them.
The result of applying Eq. (1) to the data presented in Fig. 5 is shown in Fig. 8. The PSDs derived from PALS spectra measured at 302 K for the studied samples indicate that post-synthesis modification of poly(TRIM) with TEOS substantially influences its pore structure. In the initial poly(TRIM) sample two groups of mesopores represented by not completely resolved peaks 4 and 5 are present. Their maxima are centered at about D = 3.22 and D = 4.72 nm, respectively. Moreover, in the polymer sample much smaller pores are present represented by peaks 1, 2 and 3 with their maxima centered at D = 0.37 nm, 0.77 nm and 1.32 nm, respectively. These pores may be classified as micropores according to IUPAC recommendations. Taking into account the nature of the investigated samples these micropores may be mainly ascribed to the polymer component. In the composite sample micropores are also present, but their dimensions are more differentiated than those in the pure polymer sample. After TEOS modification in poly(TRIM)–Si sample
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there are still present mesopores of a dimension slightly smaller than for the initial polymer sample. However, PSD becomes narrower and exhibits a monomodal character. It testifies the fact that within this range of pore diameter the porosity becomes more homogeneous and uniform. All three peaks 1, 2 and 3 in the micropore range are differentiated and shifted towards higher D values. It means that silica loading causes the change of the free volumes dimensions in polymer matrix. The PSD for mesopores derived from PALS spectra for pure silica material is of bimodal character. There are also present two groups of pores of dimensions lower than 1 nm which represent the free volumes in silica network. It is reasonable to assume that these pores are rather inaccessible to nitrogen molecules. In Fig. 8 are also shown pore size distributions derived from the nitrogen adsorption data. The micropore size analysis on the basis of the nitrogen adsorption data was performed using computational DFT procedure which is incorporated in Quantachrome Autosorb processing software. In the same figure are also shown PSDs for mesopores calculated BJH procedure. The mean pore diameters at the peak of PSDs derived from the long lived component of PALS experiment are similar but not identical to those calculated by DFT and BJH from the nitrogen adsorption data. Comparing these data one can say, that the results obtained by different techniques reflect in general the character of the pore system for investigated materials. However, the PALS measurements provide information about porosity in very wide range of pore dimensions. The analysis of the results obtained from PALS experiment confirms that poly(TRIM) swelling in the presence of TEOS is related to the total penetration of the silica precursor within the polymer particles. After condensation of TEOS the mesopores present in the initial sample become smaller and more uniform in size. Simultaneously, micropores in the polymer matrix become slightly larger. After calcination of the composite the pores of D < 2 nm practically disappear.
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PSDs for samples of different pore system one can say that pore dimensions derived from PALS spectra correlate quite reasonably with structural parameters calculated on the basis of the nitrogen adsorption data using conventional analytical procedures. Interestingly, the best correlation of PSDs in the mesopore range occurs in the case of PALS and BJH distribution. Application of positron annihilation lifetime spectroscopy allowed an assessment to be made of the complex pore structure. Thus ‘‘positron porosimetry’’ is a convenient tool for the characterization of polymer/silica composites. Acknowledgment This work was supported by the Polish Ministry of Science Grant No. N N204 131137. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
4. Conclusions
[19] [20]
The post-synthesis modification of poly(TRIM) with TEOS was shown to influence the porosity of the polymer sample. The investigations of the free volumes within the composite material illustrate that the TEOS loading followed by its condensation causes decrease of the specific surface area and pore volume in addition to the framework transformation. The amount of adsorbed nitrogen decreases after introducing silica into the polymer matrix. Simultaneously, the porosity of the composite sample becomes more regular. From this study one can conclude that after polymer swelling in TEOS the packing of polymer microspheres becomes denser, and the internal pore system of the resulting nanoparticles becomes simultaneously of a mixed character, in which silica particles are dispersed homogenously in the whole bulk phase of the composite. Calcination of composite particles produces silica particles of spherical morphology. The resulting silica gel exhibits relatively uniform mesoporosity of high pore volume and very high surface area. The three porous materials studied in the present work are highly mesoporous but contain also micropores. Pore size distribution for micro- and mesopores determined by the conventional adsorption method and PALS exhibits small but noticeable differences. These differences may be justified if we take into account that a part of pores in the investigated samples are effectively blocked for nitrogen molecules. On the other hand PALS can be used to obtain useful information on the pore size without assumptions concerning the physisorption mechanism of surface coverage, pore filling and hysteresis phenomena. Comparing the
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