Effect of polyvinylpyrrolidone on the ammonia

Colloids and Surfaces A: Physicochem. Eng. Aspects 305 (2007) 97–104

Effect of polyvinylpyrrolidone on the ammonia-catalyzed sol–gel process of TEOS: Study by in situ 29Si NMR, scattering, and rheology Yao Xu a,∗ , Dong Wu a , Yuhan Sun a , Wenxue Chen b , Hanzhen Yuan b , Feng Deng b , Zhonghua Wu c a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China State Key Laboratory of Magnetic Resonance & Atomic & Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China c Laboratory of Synchrotron Radiation, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China b

Received 21 December 2006; received in revised form 17 April 2007; accepted 18 April 2007 Available online 24 April 2007

Abstract Ammonia-catalyzed sol–gel process of polyvinylpyrrolidone/tetraethoxysilane (PVP/TEOS) system in methanol was studied by in situ liquid 29 Si nuclear magnetic resonance (NMR), dynamic light scattering (DLS), small angle X-ray scattering (SAXS) and rheology. A scientific connection was established between the reaction composition and the microstructures of sol particles. It was found that the concentration of PVP played an important role in the hydrolysis kinetics of TEOS and the SiO2 sol–gel process. The results of in situ 29 Si NMR showed that PVP addition reduced both hydrolysis rate and condensation rate of TEOS, and the condensation rate decreased more seriously than the hydrolysis rate. The particle size distributions obtained from DLS and SAXS indicated that PVP held back the growth of SiO2 clusters by hydrogen-bonding the silanol groups on particle surface with the electronegative inner amide of PVP side chain. Thus, gelation was retarded and the rheology of sol was largely modified by PVP. Rheology study showed typical Newtonian fluid behavior before gelation and shear thinning fluid behavior after gelation point, respectively. The changes in fractal dimensions of PVP/SiO2 sols, obtained by SAXS, explained the different microstructures of SiO2 sol particles in the presence of PVP. The changes of microstructure and macroscopic properties should be pre-determined by the very early stages of sol–gel process. © 2007 Elsevier B.V. All rights reserved. Keywords: Reaction kinetics; Siloxane; 29 Si NMR; SAXS; Rheology

1. Introduction Organic–inorganic hybrid materials are now of great scientific and technological interest due to their novel properties. These hybrid materials afford a direct connection between inorganic and organic worlds. Sol–gel process has been proved to be a impactful method for mild synthesis of these hybrid materials with high purity and homogeneity [1–5]. In them, a large family of organic–inorganic hybrid materials is polymer/silica nanocomposites, where low molecular weight polymers can be embedded in inorganic silica matrices. These hybrid materials show far more improved properties, such as higher thermal stability and mechanical strength than conventional inorganic

∗

Corresponding author. Tel.: +86 351 4049859; fax: +86 351 4041153. E-mail address: [email protected] (Y. Xu).

0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.04.047

materials or polymeric materials [6]. Up to now, a large amount of such sol–gel derived hybrid materials have been prepared using various inorganic precursors [7–12] and polymers [13–20], aiming at the applications in optics [16], biosensing technology [19], biomaterials [20], and coating technology [21,22]. Among the wide applications of polymer/SiO2 sols, the application in optical materials is very important. For example, polyethyleneglycol (PEG), polyvinylpyrrolidone (PVP), or polytetrahydrofuran (PTHF) modified SiO2 sols were used to prepare refractive index adjustable and porosity adjustable optical coatings [23–25]. Sermon et al. [26] enhanced the laserinduced damage threshold of SiO2 anti-reflective films using PEG-modified SiO2 sol. Chen and Wei [27] prepared crack-free Al2 O3 –SiO2 films using PVP as modifier. The properties of polymer/SiO2 hybrid materials are directly related to the interfacial interaction between polymer and silica species in sol–gel process. The evolution of sol–gel process can

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Y. Xu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 305 (2007) 97–104

be controlled by the type and the amount of polymer and catalyst, nature of solvent, reaction temperature, and ratio of water to silica. The addition of polymer remarkably affects the hydrolysis and condensation behavior of inorganic precursor, the evolution of sol–gel process, and the physical properties, such as particle size distribution, cluster structure, rheology of sol. The scientific connection between process parameters, structure, and properties can be very helpful for the design of materials with desired property. So, it is very important to make a deep understanding about the effect of polymer on the sol–gel process. Though several literatures have been published about the structure and forming mechanism of hybrid materials at various levels of sophistication [10,15,28–33], to best of our knowledge, understanding in the underlying reaction kinetics and the interaction between polymer and silica species is still far from abundance because of the complexity of sol–gel process. Therefore no general rule can be used to guide the control of sol–gel process so far. So the purpose of our present work is to disclose the effect of PVP on hydrolysis kinetics of TEOS, and then on sol–gel process and microstructure, finally to find out the relationship between them. 2. Experiment 2.1. Materials and synthesis Reagent grade aqueous ammonium hydroxide (12 mol L−1 ), anhydrous methanol, deionized and distilled water, TEOS (99%, Acros), PVP (MW = 1300000, Acros) were used as received. The preparation compositions were listed in Table 1. PVP/SiO2 sols were prepared by mixing two solutions A and B at the temperature of 25 ◦ C. Solution A: PVP was dissolved in one half of the total methanol and vigorously stirred for 20 min, then TEOS was dissolved into the PVP solution. Solution B: water and ammonia hydroxide were dissolved in another half of the total methanol and was stirred for 5 min. The hydrolysis reaction of TEOS was initiated by mixing solutions A and B. 2.2. In situ liquid 29 Si NMR experiments After 10 min stirring of the mixture of solutions A and B, the reaction solution was transferred to a NMR sample tube (5 mm o.d.) and analyzed immediately. Chromium (III) acetylacetoTable 1 Preparation compositions of PVP/SiO2 sols

nate, Cr(acac)3 (1 wt%) was added as a spin relaxation agent. Many studies [34,35] have proved that Cr(acac)3 had little effect on reaction rate and product distribution. All in situ liquid-state 29 Si NMR experiments were carried out in duplicate on a UNITY INOVA-500 Spectroscopy. To achieve sufficient signal intensity, 128 scans were acquired for each spectrum with 3 s pulse delay using 90◦ pulse angle. The spectral frequency of 29 Si was 99.351 MHz. The resulting spectra were internally referenced to tetramethylsilane (TMS) standard. The resonance peaks were well resolved and could be integrated quantitatively. During the experiments, gelation did not occur and the transesterification was negligible [34]. All these NMR experiments were conducted at 25 ◦ C and the temperature was controlled to an error range of ±0.1 ◦ C. As reference experiment, hydrolysis of TEOS without PVP addition was carried out under the same reactive condition as mentioned above. 2.3. DLS, SAXS, rheology, and 13 C NMR experiments After the synthesis according to the parameters listed in Table 1, solutions PSi0–PSi8 were kept airproof. After 100 min of aging, the sols PSi0–PSi3 were subjected to particle size distribution (PSD) analysis by DLS (N4 plus, Coulter). The sols PSi4–PSi8 were subjected to particle size distribution analysis by DLS after 500 min of aging. The resultant sols PSi0–PSi8 were also subjected to SAXS study. The SAXS experiments were performed on 4B9A beam line at Beijing Synchrotron Radiation Facility with a longslit collimation system. The incident X-ray wavelength was 0.154 nm. The scattering angle 2θ was approximately 0–3◦ . The fractal dimensions of PVP/SiO2 particles were calculated as well as the particle size distribution. The rheological measurement was performed on a programmable rheolometer (RVDV-III + CP, Brookfield) for all the sols PSi0–PSi8 at a time interval of 30 min. In every measurement, the rotate speed was continuously changed from 5 to 250 rpm. All these measurements were conducted at 25 ◦ C and the temperature was controlled to an error range of ±0.1 ◦ C. To prove the interaction between PVP and Si–OH groups of silica particles, the 13 C NMR spectra were investigated to compare the ethanol solution of PVP and the sample PSi3. 13 C NMR spectra were acquired in CDCl3 on a UNITY INOVA-500 Spectroscopy, using cross-polarization (CP) with a relaxation delay of 1 s and a cross-polarization time of 2 ms. The spectral frequency of 13 C was 74.475 MHz.

Sample

CSiO2 (wt%)

CPVP (10−4 mol L−1 )

CNH3 (10−6 mol L−1 )

[H2 O]/ [TEOS]

3. Results

PSi0 PSi1 PSi2 PSi3 PSi4 PSi5 PSi6 PSi7 PSi8

5 5 5 5 7 7 7 7 7

0 0.81 1.62 3.24 0.81 0.81 0.81 0.81 0.81

0.75 0.75 0.75 0.75 1.18 11.8 7.5 7.5 7.5

2 2 2 2 2 2 2 4 8

3.1. Hydrolysis kinetics 3.1.1. Hydrolysis-condensation mechanism and kinetics model Under basic conditions, it is likely that water dissociates to produce nucleophilic hydroxyl anions in a rapid first step. The hydroxyl anion then attacks the silicon atom. Iler [36] and Keefer [37] propose an SN 2-Si mechanism (Eq. (1)) in which OH−


displaces OR− with inversion of the silicon tetrahedron.

kc

2Si(OEt)3 (OH)−→(EtO)3 SiOSi(OEt)3 + H2 O.

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(9)

(1) This mechanism is affected by both steric and inductive factors, and steric factors are more important because silicon acquires little charge in the transition state. The most widely accepted mechanism for the condensation reaction involves the attack of a nucleophilic deprotonated silanol on a neutral silicate species as proposed by Iler [36] to explain condensation in aqueous silicates system. Pohl and Osterholtz [38] and Voronkov et al. [39] proposed essentially the same mechanism (Eqs. (2) and (3)) to account for deuteroxide (hydroxyl) anion and general base-catalyzed condensation of silicon alkoxide. Fast :

k1

R(R)Si(OEt)OH + OH− R(R)Si(OEt)O− + H2 O k−1

(2)

This is a total model of hydrolysis and condensation kinetics, not a detailed rudimentary reaction model. 3.1.2. Hydrolysis and condensation rate constants Fig. 1a and b show the typical time-dependent 29 Si NMR spectra of samples PSi0 (without PVP) and PSi3 (with PVP) during the initial hydrolysis reaction. From the comparison between Fig. 1a and b, no serious difference can be found in the positions of NMR peaks except for the different resonance intensity. In Fig. 1, four peaks were detected by 29 Si NMR. In order to assign the 29 Si NMR chemical shifts for different silicon species, the traditional notation was adopted [1]. The symbol Qnm denotes the products of hydrolysis or condensation of TEOS, where m and n are the number of Si–O–Si bridges and the number of

(3) In above-mentioned hydrolysis and condensation mechanism at a rudimentary level, it is ignored how the various functional groups, (OR), (OH), and (OSi), are distributed on the silicon atoms. At this level only three reactions and three rate constants are necessary to describe the functional group kinetics (Eqs. (4)–(6)), where h is hydrolysis, cw is condensation of water, and ca is condensation of alcohol: kh

SiOR + H2 O−→SiO + ROH k /2

cw + H2 O 2SiOH −→2(SiO)Si

kca /2

SiOH + SiOR−→2(SiO)Si + ROH

silanol surrounding Si atom, respectively. The resonance peak at −81.3 ppm corresponds to Q00 (TEOS, Si*(OEt)4 ) in methanol. The two resonance peaks at −80.4 and −78.3 ppm correspond to Q10 (Si*(OEt)3 (OH)) and Q30 (Si*(OEt)(OH)3 ), which are

(4) (5) (6)

In practice, hydrolysis and condensation occur concurrently, and at the nearest functional group level there are 15 distinguishable local chemical environments. Assink and Kay [40,41] have represented the 15 silicate species in matrix forms. At this more sophisticated level, there will be 165 distinguishable rate coefficients: 10kh , 55kcw , and 100kca , considering only the forward reactions. If to go further to the next-to-nearest functional group level, the number of rate constants will be terribly big. So based on the above consideration, the precise sol–gel kinetics (including all of the determined rate constants) is impossible to be obtained via experimental methods. Therefore, a simple sol–gel kinetic model proposed by Lee [42] was adopted in the following kinetics calculations: kh1

Si(OEt)4 + H2 O−→Si(OEt)3 (OH) + EtOH kh2

Si(OEt)3 (OH) + 2H2 O−→Si(OEt)(OH)3 + 2EtOH

(7) (8)

Fig. 1. Time-dependent liquid 29 Si NMR spectra of (a) single TEOS system (PSi0) and (b) TEOS/PVP system (PSi3).

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Table 2 Reaction rate constants obtained by in situ 29 Si NMR Sample

kh1

PSi0 PSi1 PSi2 PSi3

7.31 6.44 5.84 4.95

kh2 ± ± ± ±

0.4 0.3 0.3 0.3

42.97 42.56 43.00 42.14

kc ± ± ± ±

2.1 2.1 2.1 2.1

151.0 143.7 123.2 100.7

± ± ± ±

7.6 7.2 6.2 5.1

The errors of kh and kc are estimated from the 5% error of integration area, and subscripts h and c indicate the hydrolysis and the condensation, respectively.

those partially hydrolyzed intermediate species of TEOS. Q20 was not observed probably because its signal intensity is below the detective limit of the liquid 29 Si NMR spectrometer due to the fast condensation of intermediate Q20 under basic conditions [34]. The resonance peak at −88.4 ppm corresponds to Q01 ((EtO)3 Si*OSi(OEt)3 ), the dimer of Q10 . The relative concentrations of intermediate soluble Si species were determined by integrating the resonance peak at fixed individual frequency in NMR spectra. Using the calculation method proposed in our previous work [43], two hydrolysis rate constants, kh1 and kh2 , and the condensation rate constant, kc , were calculated and the results were collected in Table 2. With PVP additive, the hydrolysis rate constant kh1 decreased, but no obvious influence was found with kh2 . The condensation rate constant kc obviously decreased from 151.0 to 100.7 with PVP addition increased from 0 to 3.24 × 10−4 mol L−1 . These results show that PVP prevents the hydrolysis and the subsequent condensation of TEOS. 3.2. Particle size distributions Fig. 2 shows the effects of PVP content, ammonia content, and water content on the SiO2 particle size distribution obtained by DLS. Fig. 2a shows the particle size distributions of sols PSi0–PSi3 after same aging time of 100 min. All the four sols present bimodal particle size distribution. It is found that even a very small amount of PVP can largely change the particle size distribution: both distribution peaks are left-shifted, that is, the mean size of particles decreases with the increase in PVP content. Additionally, the major peak becomes narrower and stronger and the minor peak is gradually weakened with PVP content, which indicates the decrease in bigger aggregates in size and amount. So a high content of PVP benefits higher uniformity of silica particles. Fig. 2b shows the effect of water content (PSi6, PSi7, PSi8) on the particle size distribution in the presence of PVP. Similar to the effecting trends of PVP content (see Fig. 2a), water content has also an obvious effect on the particle size distribution. The distribution peak is left-shifted indicating decreased mean size of particles with water content (See PSi6, PSi7, PSi8). The distribution peak becomes narrower and stronger with water content. Interestingly, the effect of ammonia content on particle size distribution is very close to that of water content (the PSD curves are not shown here). So, it is clear that PVP plays an important preventing role in the growth of SiO2 particles, which should be

Fig. 2. The effects of (a) PVP content and (b) water content on the particle size distributions (DLS method) in PVP/SiO2 sols.

consistent with the result of hydrolysis kinetics mentioned in Section 3.1. With SAXS experimental data, the particle size distribution of PVP/SiO2 sols was calculated using Shell-Rose method [44]. In calculation, the SiO2 particles were assumed to be spherical and possessed a continuous Gaussion distribution. The parameters of particle size distribution function were determined by fitting the experimental SAXS data. Fig. 3 shows the SAXS particle size distribution of SiO2 particles in the presence of PVP. It can be found that the particle size distribution curve all centered at 3.6 nm no matter how the content of PVP, ammonia or water changes. This result seems to be contrary to that obtained from DLS experiments. It should be noticed that SAXS detects the single primary particles but DLS detects mainly the clusters. So the difference between these two results is reasonable. By comparing the particle size distributions from SAXS and from

Fig. 3. Particle size distributions of PVP/SiO2 sols (SAXS method).


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Fig. 5. Effects of PVP content, NH3 content and water content on the rheology of PVP/SiO2 sols.

Fig. 4. Fractal curves of PVP/SiO2 sols.

DLS, a conclusion can be drawn that PVP only prevents the growth of particle clusters but have little effect on the formation of primary particles. 3.3. Fractal characteristics of PVP/SiO2 sols All the microstructure information of colloidal particles is included in the scattering curve of I(q) versus q, where I(q) is the scattering intensity, q = (4πλ)/sin(θ/2) is the scattering wave vector, λ is the wavelength of incident X-ray, and θ is the scattering angle. The interface characteristics of sol particles can be represented by the fractal structure that can be obtained by analyzing the SAXS experimental data [45–47]. The SAXS of typical fractal system generally has an exponential decrease in scattering intensity in term of I(q) ∝ q−Dm (1 ≤ Dm < 3) or I(q) ∝ qDs−6 (2 < Ds < 3) for mass-fractal system (fractal dimension is Dm) or surface-fractal system (fractal dimension is Ds) [48], respectively. The fractal dimension Dm or Ds can be determined by the slope of linear range in fractal curve ln[I(q)] versus ln(q). According to Craievich [49] and Marliere et al.’s [50] study, it is possible for the existence of double-fractal structure, that is, a complex fractal structure including two fractal dimensions. Fig. 4 shows the fractal curves of PVP/SiO2 sols and the detailed fractal dimensions are collected in Table 3. In all the fractal curves shown by Fig. 4, there are no flat linear regions in small q range. According to Rosa-Fox’s study [51], the scatters are multidispersive (or not uniform in size and shape) and generally possess fractal microstructure at some spatial scale. The spatial scale in which fractal structure exists can be easily decided by the qmin and qmax values corresponding to the begin-

ning point and the ending point of linear range in fractal curve [49,50]. In Fig. 4, except for PSi7 and PSi8, the other curves all have two linear regions corresponding to two fractal dimensions: Ds (surface fractal of primary particles) in large q range (small spatial scale) and Dm (mass fractal of particle clusters) in small q range (large spatial scale). Compared the fractal curves of PSi0 with PSi2 and PSi3, it can be easily found that Ds and Dm all decrease with the additive PVP. In the presence of PVP, the effect of ammonia content on the fractal dimension is shown by the curves of PSi4 (CNH3 = 1.18 × 10−6 mol L−1 ), PSi6 (CNH3 = 7.5 × 10−6 mol L−1 ), and PSi5 (CNH3 = 11.8 × 10−6 mol L−1 ) in Fig. 4. Ds increases and Dm decreases with the ammonia content, which is different from the effect of PVP content. As to the effect of water content on fractal dimension, Ds increases from 2.63 of PSi6 ([H2 O]/[TEOS] = 2) to 2.71 of PSi8 ([H2 O]/[TEOS] = 8) with the water content. It must be noticed that PSi7 and PSi8 haven’t Dm. So, there aren’t any fractal cluster in these two sols. 3.4. Rheology of PVP/SiO2 sols Fig. 5 shows the viscosity of PVP/SiO2 sol as a function of time and shear rate. Generally the gelation point is defined as the time when the viscosity of sol has a sharp increase. At the beginning stage of sol aging, all the sols show typical Newtonian flow characteristic, and viscosity is independent of shear rate and go through a smooth and slow increase before the gelation point. After gelation, the viscosity of sol increases rapidly and deviates obviously from Newtonian fluid. Furthermore, after gelation, the viscosity was lowered with the shear rate, indicating a shear thinning flow behavior. Accordingly, different gelation time were defined: 100 min for PSi0, 200 min for PSi3, and 700 min for PSi4–PSi8. So, the additive PVP can delay the gelation. Analyzing the viscosities of PSi4–PSi8 at a same shear

Table 3 Fractal dimensions of PVP/SiO2 sols obtained by SAXS Sample

PSi0

PSi1

PSi2

PSi3

PSi4

PSi5

PSi6

PSi7

PSi8

Ds Dm

2.80 2.13

2.75 1.90

2.69 1.67

2.58 1.43

2.52 1.43

2.70 1.07

2.63 1.20

2.67 –

2.71 –

The calculation error is 4% from the SAXS experimental data.

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rate of 384 s−1 , it can be found that ammonia content and water content have no effect on the gelation time of PVP/SiO2 sol but influence the viscosity. In detail, the viscosity decreases with the increase in both ammonia content (see PSi4, PSi6, and PSi5) and water content (see PSi6, PSi7, and PSi8). 4. Discussions Polymer-modified silicates are of very important in sol–gel chemistry and organic–inorganic hybrid materials due to its wide applications. However, it has not been described sufficiently so far how polymer affected the reaction kinetics, sol–gel process, and the microstructure of the resultant hybrid materials. To understand the interaction between polymer and the hydrolyzed and condensed species of TEOS, Brus and Kotlik [28] investigated the effect of polyacrylate on TEOS polycondensation in a considerable detail. They utilized 29 Si NMR sensitivity enhancement technology to detect the numerous intermediates produced in the TEOS hydrolysis and condensation process in acidic environment. Based on the model described by Eqs. (4)–(6), they studied the influence of polyacrylate on the condensation degree, the extent of cyclization, and the average polymerization degree of polycondensation product. Under acidic reaction condition, the complex condensation species, such as linear or cyclic oligomers described in Ref. [28], can be easily detected. Due to the much faster hydrolysis than condensation of TEOS in acidic solution [1], there is enough time for the hydrolysis intermediates to condense into various oligomers. Therefore, the exact extent of condensation could be obtained. However, it may be noticed from Fig. 1 that no NMR signals of more complex condensation species than Q01 were detected (not a lose of NMR spectra). This should be resulted from the much slower hydrolysis than condensation of TEOS in basic solution [1]. In this case, the produced hydrolysis intermediates quickly participate in the further nucleation, leading to the difficult accumulation of condensation species. Therefore, unlike the result of Ref. [28], it is difficult to detect the condensation species in our experiments. Consequently, it was impossible for us to adopt the same kinetic model as Brus and Kotlik [28] used. However we obtained the precise hydrolysis rate constants, kh1 and kh2 , and the total condensation rate constant, kc . From these results, it can be found that the additive PVP could reduce the condensation rate of TEOS hydrolysis system similar to polyacrylate reported by Ref. [28]. The difference lies in that PVP can also affect the initial hydrolysis rate constant, kh1 , but polyacrylate cannot. It may be because the too fast hydrolysis of TEOS in acidic condition covered the effect of polyacrylate on it. The scheme of PVP molecule is shown in Fig. 6. The side chains of PVP molecule, pyrrolidone groups, contain C O groups and N atoms as electronic donor. Thus, PVP molecules can strongly hydrogen-bond with water molecules to form socalled “hydration layer”. Because of this interaction, water in methanol solution can be divided into three forms: free water, bound water and bond-combined water, in which the free water is the most effective for TEOS hydrolysis. The hydration effect of PVP reduces remarkably the water amount available

Fig. 6. Scheme of enwrapped SiO2 particle by PVP molecules.

for TEOS hydrolysis at the initial stages of the sol–gel process. This reduction of available water is important for TEOS hydrolysis in methanol system containing limited water (for example, [H2 O]/[TEOS] = 2 or 4). According to the hydrolysis rate expression in our previous work [43], PVP reduces the concentration of free water, thus reduces the hydrolysis rate of TEOS. So the hydrolysis rate constants, kh1 and kh2 , all decrease with PVP content. After TEOS are hydrolyzed and some particle nuclei are formed, PVP molecules can strongly adsorb the soluble silica species or particle nuclei by hydrogen-bonding the silanol groups (Si–OH). Theoretically the hydrogen-bonds between the C O groups of PVP and Si–OH groups of silica particles will weaken the electronic shielding on C atoms, which corresponds to a low field shift (larger δ) of C O peak in 13 C NMR spectrum. Fig. 7 shows the 13 C NMR spectra of PVP in ethanol and in PSi3 sample. It can be found that the chemical shift of C O groups has shifted from 179.155 ppm in ethanol solution to 181.465 ppm in PSi3 sample. This should be a direct evidence for the hydrogenbonds between the C O groups of PVP and Si–OH groups of silica particles. In addition, the bonds of Si–O− · · ·H+ can be formed, though in small quantities, because of the weak basicity of N atoms in pyrrolidone groups and the weak acidity of Si–OH groups in hydrolyzed monomers. Therefore, the effective concentration of hydrolyzed monomers decreases, which slows down the condensation reaction (the condensation rate constant, kc , decreases accordingly). During the nucleation period, some primary particles are formed rapidly by the condensation of soluble intermediate Si species. These primary particles aggregate with each other to gradually produce resultant sol. PVP molecules interact with these primary particles by the hydrogen-bonds between the

Fig. 7. 13 C NMR of C O group of PVP in (a) ethanol solution and (b) PSi3 sample.


imide groups of PVP and the silanol groups on particle surface (the scheme of interaction is shown in Fig. 6), which sterically inhibits the collision of primary particles and thus prevents the further aggregation between particles during sol–gel process. As result, the added PVP not only makes the mean particle size smaller but also makes the particle size distribution narrower than the system without PVP addition (see Fig. 2a). On the other hand, higher contents of water or ammonia in initial solutions lead to faster TEOS hydrolysis and more intermediate species. The newly formed primary particles are enwrapped immediately by PVP molecules before they can continue the further aggregation to produce larger clusters. Thus, the mean particle size was also reduced in these two cases (see Fig. 2b). From Fig. 3, the mean particle size from SAXS method always centered at about 3.6 nm that differed from the results of DLS shown by Fig. 2. Because PVP molecules loosely surround the SiO2 primary particles and lots of free PVP molecules exist in solvent, SAXS can only detect the primary particles. So PVP didn’t have considerable effect on the primary particle size. Besides the influence of PVP on the particle size distribution, PVP have more detailed effect on the fractal characteristic of sol particles. The strong hydrogen-bonds between the pyrrolidone groups of PVP and the silanol groups of primary particles hold back the surface condensation, so the surface of primary particles become smoother with the increase in PVP content (corresponding to smaller Ds, see PSi0–PSi3 in Fig. 4). Simultaneously the hydrogen-bonds limit the aggregation of primary particles and make clusters looser, leading to smaller Dm (see PSi0–PSi3 in Fig. 4). For PSi4, PSi6, and PSi5, the increase in ammonia content speeds up remarkably the hydrolysis of TEOS, and PVP can’t effectively prevents the rapid formation of primary particles with coarse surface, so Ds increases to some extent with ammonia content. Similarly the limit of hydrogen-bonds to the aggregation of primary particles would result in smaller Dm with ammonia content (see PSi4, PSi6, and PSi5 in Fig. 4). Out of a same consideration, the effect of water content on the Ds is similar to the case of ammonia content (see PSi6, PSi7, and PSi8 in Fig. 4). The fact that there is no Dm in PSi7 and PSi8 also reveals that in some cases it is difficult to form particle clusters in the presence of PVP. All the changes in hydrolysis kinetics, particle size distribution, and fractal microstructure explain different aspects of the PVP/SiO2 sol–gel process, and should be expressed by sol’s macroscopic property, such as rheological behavior. The viscosity of sol is time-dependent and is related to the particle size and the interaction between particles. The larger the particles are and the stronger interaction the particles have, the higher the viscosity of sol will be. At the beginning of synthesis, the interaction between colloidal particles is very weak; hence all the sols show Newtonian fluid behavior. When particles attain a certain size, the interaction between the particles contributes to a high viscosity at low shear rates. But the viscosity decreases at high shear rates because the formed interacting structure is broken down. Therefore, after gelation point it shows a shear thinning fluid behavior. For the sols containing more PVP, the longer gelation time can be explained by the cooperative effect mentioned above. First, PVP sterically stabilizes the sols and hinders the conden-

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sation reaction. Second, PVP reduces the collision possibility of particles in sol that is the cause of aggregation. These two effects result in longer gelation time. In the case of PVP addition, under higher contents of water or ammonia, the particle size become smaller and the interaction between particles is weaker. Therefore, the bad-formed three-dimensional gel network results in smaller viscosity for sample PSi4–PSi8. 5. Conclusion Based upon the experimental results of in situ 29 Si NMR, DLS, SAXS, and rheology, the effect of PVP additive on the SiO2 sol was disclosed. The PVP addition reduced both the hydrolysis rate and the condensation rate of TEOS. PVP held back the growth of SiO2 clusters by hydrogen-bonding the silanol groups on particle surface with the pyrrolidone groups of PVP side chain. Thus the gelation was retarded and the rheology of sol was largely modified by PVP. The changes in fractal dimensions of PVP/SiO2 sols can be used to explain the different microstructure of SiO2 sol particles in the presence of PVP. The changes of microstructure and macroscopic properties should be pre-determined by the very early stages of the sol–gel process. Acknowledgements The financial supports from the National Key Native Science Foundation (No. 20133040 and 20573128), the Natural Science Foundation of Shanxi Province (No. 20051025 and 2006021031) were gratefully acknowledged. References [1] C.J. Brinker, O.W. Scherer, Sol–Gel Science: the Physics and Chemistry of Sol–Gel Processing, Academic, San Diego, 1990. [2] L.L. Hench, J.K. West, Chem. Rev. 90 (1990) 33. [3] A.K. Cheetham, C.J. Brinker, M.L. Mecartney, C. Sanchez (Eds.), Better Ceramics Through Chemistry V, Materials Research Society, Pittsburgh, 1994. [4] J.D. Mackenzie, E.P. Bescher, J. Sol–Gel Sci. Tech. 13 (1998) 371. [5] J.D. Mackenzie, J. Sol–Gel Sci. Tech. 26 (2003) 236. [6] C.C. Sun, J.E. Mark, Polymer 30 (1989) 104. [7] M. Toki, J. Sol–Gel Sci. Tech. 2 (1994) 97. [8] F. Suzuki, K. Nakane, J. Piao, J. Mater. Sci. 31 (1996) 1335. [9] T. Kotoky, S.K. Dolut, J. Sol–Gel Sci. Tech. 29 (2004) 107. [10] C. Chan, I. Chu, C. Ou, Y. Lin, Mater. Lett. 58 (2004) 2243. [11] K. Kim, T. Inakura, Y. Chujo, Polym. Bull. 43 (2001) 351. [12] D. Loy, K. Shea, Chem. Rev. 95 (2001) 1431. [13] C. Oh, C. Ki, J. Chang, S. Oh, Mater. Lett. 59 (2005) 929. [14] J. Martin, B. Hosticka, C. Lattimer, P.M. Norris, J. Non-Cryst. Solids 285 (2001) 222. [15] H.Y. Chang, R. Thangamuthu, C.W. Lin, J. Membr. Sci. 228 (2004) 217. [16] S. Fujihara, S. Kitta, Chem. Phys. Lett. 397 (2004) 479. [17] K. Tanaka, H. Kozuka, J. Sol–Gel Sci. Tech. 32 (2004) 73. [18] R. Tamaki, Y. Chujo, Appl. Organomet. Chem. 12 (1998) 755. [19] A. Kros, M. Gerristen, V.S.I. Sparakel, N.A.J.M. Sommerdijk, J.A. Jansen, R.J.M. Nolte, Sens. Actuators B 81 (2001) 68. [20] A. Paula, V. Pereira, W.L. Vasconcelos, R.L. Orefice, J. Non-Cryst. Solids 180 (2000) 273. [21] J. Brus, M. Spirkova, D. Hlavata, A. Strachota, Macromolecules 37 (2004) 1346.

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