Ind. Eng. Chem. Res. 2008, 47, 2623-2629
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Effect of Particle Aggregation on the Mechanical Properties of a Reinforced Organic-Inorganic Hybrid Sol-Gel Composite Saran Poovarodom, Dariush Hosseinpour, and John C. Berg* Department of Chemical Engineering, Box 351750, UniVersity of Washington, Seattle, Washington 98195-1750
Sol-gel materials are often reinforced through the addition of particulate fillers to improve their mechanical properties, such as toughness or stiffness. Related studies of filled polymeric materials suggest that the effectiveness of added reinforcements may depend sensitively on the state of particulate aggregation in the composites. In some systems and situations, aggregates are found to enhance properties, while in others they are deleterious. The objective of the present study was to determine the effect of state of aggregation of alumina nanoparticle fillers on the mechanical properties of a GPS sol-gel hybrid composite. Aggregates of different sizes were produced at loadings of 3.6, 7.5, and 16.8 volume percent in the GPS sol-gel monoliths by inducing aggregation through the addition of varying amounts of tetrasodium pyrophospate (TSPP). The small amounts of TSPP required to induce varying states of aggregation did not affect the properties of the gel in the absence of the particles. Unaggregated particles or small aggregates were found to impart significant improvements in the mechanical properties of the gel, but at a particular critical aggregate size and beyond, the cured material cracked and crumbled. The critical size was found to correspond roughly to an effective aggregate volume fraction of 0.64 in the final composite, suggestive of a randomly close-packed aggregate structure. 1. Introduction The sol-gel process is a material synthesis method that involves a transformation of small particulate dispersion (sol) to a three-dimensional solid skeleton with an interpenetrating liquid phase (gel) followed by removal of the liquid. The size of the particulates can range from less than 1 nm up to a few micrometers. Sols are typically synthesized in aqueous media from dissolved precursor molecules, with metal alkoxides, M-(OR)n, being the most common. Hydrolysis and condensation reactions of the precursors lead to precipitation of the particles from the solution. However, methods such as salt spraying, flame oxidation, and pyrolysis using high-temperature furnace, laser, or plasma, have been used to prepare the sols as well. Aggregation or “polymerization” of the particles leads to growing solid clusters, which eventually link with one another to form a network. The sol-gel process is a flexible and versatile method that has been used to prepare ceramic materials, powders, coatings, fibers, membranes, and monoliths, among others.1,2 While the initial focus of sol-gel science was on the synthesis of inorganic metal oxide materials, the sol-gel process also presents an exciting method for producing organic-inorganic hybrid materials. Such materials may combine the desired properties of polymers (flexibility, easy processing, specific chemical functionality, etc.) and inorganic ceramics (thermal stability, mechanical integrity, etc.).3 A common type of organic-inorganic hybrid is synthesized using organo-alkoxysilanes such as glycidoxypropyltrimethoxysilane (GPS) and aminopropyltrimethoxysilane (APS). Via hydrolysis and condensation reactions in water, the silane molecules cross-link to form a network. Organo-alkoxysilane hybrids have many applications, such as adhesion promoting layers,4 corrosion protecting layers,5 or passivation layers for micro-electronics.6 They are also used for micro-optics in manufacturing of microlens arrays3 and photonic purposes, including preparation of * To whom correspondence should be addressed. Tel.: (206) 5432029. Fax: (206) 543-3778. E-mail:
[email protected].
planar waveguides.7 One of the most common applications for this class of hybrid materials, especially for GPS-based systems, is as a protective coating for surfaces such as those of eyeglass lenses.3,8 It has been reported that many properties of the silane-based organic-inorganic hybrids could be improved by the incorporation of oxide nanoparticles to form nanocomposites. Such properties include electrical conductivity, optical transparency, catalytic activity, and mechanical toughness or strength.9 For example, oxide particles have been used to adjust the refractive index9 and increase UV protection10 of hybrid materials. Such oxide nanoparticles are also used as carriers of corrosion inhibiting compounds, such as cerium salts, in self-healing corrosion protection coatings.11 They have also been used to increase the maximum allowed thickness of a sol-gel coating before cracking occurs, as well as improving hardness and abrasion resistance.9,10 It is well known that the properties of composites depend on the nature of the matrix and the fillers, their proportions, the matrix-filler interaction, and the state of dispersion of the fillers. For example, the automotive tire industry has extensively studied the impact that the state of aggregation of carbon black or silica particle has on the reinforced elastomer’s properties. It has been claimed that aggregated fillers are more effective than primary particles in enhancing the elastic modulus and tensile strength of the elastomer. The aggregates absorb most of the stresses when the composite is subjected to an external tensile force, since they have a much higher elastic modulus than the matrix, resulting in an increased stiffness of the reinforced elastomer. Additionally, at large strains, the deformation and irreversible breakdown of aggregates absorb energy, allowing the composite to tolerate higher amounts of stress. It is also known that the size and fractal characteristics of the aggregates had to be carefully considered. In this system, small, rigid aggregates were optimal, since larger and looser aggregates are easier to deform and less effective for reinforcing the elastomeric matrix.12-15 In other systems, the aggregation of particulates is not preferred. For instance, in most polymer-clay nanocomposites, such as clay-Nylon 6 composite, full exfoliation of the layered
10.1021/ie071563n CCC: $40.75 © 2008 American Chemical Society Published on Web 03/13/2008
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clay fillers is essential to property enhancement.16-18 Exfoliated clays provide better composite tensile strength, higher thermal stability, and improved barrier properties than intercalated clay particles, since the former results in a more homogeneous composite and allows stronger interactions between the matrix and the fillers due to its greater clay-matrix contact area.19 Additionally, aggregation of mineral fillers in a thermoplastic polymer matrix can actually deteriorate properties of the composite by producing defects and stress concentration centers leading to crack initiation and poor mechanical properties.20,21 In the surface coatings industry, it is also known that the aggregation of pigments in the polymer matrix can result in poor color, lower gloss, and inferior coating durability.22 In some applications, the percolation of the fillers is desired. For electrically conductive composites, the fillers must percolate throughout the volume of the composite to form an effective conduction path. While the aggregation of the conductive fillers, such as metal particles or carbon nanotubes, may worsen the composite mechanically, it enhances conductive properties.23 Contrary to the reinforced elastomer case, loose aggregates are preferred, as they can percolate the network at a lower concentration.24 It is clear from above that the optimal state of aggregation of fillers is system-specific and application-specific. The preferred aggregation state depends on the relevant properties and varies from one matrix-filler system to another. For the most commonly encountered sol-gel composites, it appears that optimum properties are achieved when the particles are well-dispersed in the matrix. Some examples have been reported showing that aggregation of the reinforcing particles may lead to more porous and brittle sol-gel structure.8,25 No systematic study appears to have been made, however, delineating the effect of different degrees of particle aggregation on the properties of hybrid sol-gel composites. The objective of the present study is to determine the effect of state of aggregation of alumina nanoparticle fillers on the mechanical properties of a GPS sol-gel hybrid composite. Aggregates of different sizes will be produced at various loadings into the GPS sol-gel monoliths. 2. Materials and Methods 2.1. Materials. (3-Glycidoxyproply)trimethoxysilane (GPS) (97%) was purchased from Gelest, Inc., Morrisville, PA. Nyacol Al20 (Nyacol Nano Technologies, Inc., Ashland, MA), a boehmite alumina dispersion (20% w/w in water) was used as received to reinforce the GPS monoliths. The Nyacol Al20 dispersion had a pH of 4.0, since it contained nitric acid used to stabilize the alumina particles. Their effective diameter as measured by a Brookhaven ZetaPALS (Brookhaven Instrument, Holtsville, NY) was 90 ( 40 nm. The dispersion state of the alumina particles in GPS gel precursor mixtures was controlled by adding tetrasodium pyrophosphate (TSPP) (Na4P2O7‚10H2O) from Mallinckrodt, Inc. (Hazelwood, MO). Nitric acid from EMD Biosciences, Inc. (San Diego, CA) and sodium hydroxide from VWR, Inc. (West Chester, PA) were used to control the system pH. Prior to its use, water was passed through a Millipore DirectQ3 purification system (Millipore Inc., Billerica, MA) with a 0.22 µm filter unit and had a resistivity of 18.2 MΩ‚cm. 2.2. Preparation of Sol-Gel Precursor Mixtures. GPS, water, and the Nyacol alumina dispersion were combined at room temperature (23 ( 1 °C). The ratio of GPS to total water was 30:70 by weight. The loadings of the particles studied were 8.0%, 16.0%, and 32.0% of the GPS by weight, equivalent to 3.6, 7.5, and 16.8 volume percent, respectively, in the finished monoliths. Typically, the precursor was made in a batch of ∼1 kg. Initially, GPS and water were immiscible. However, the
Figure 1. Schematic of the inducement of aggregation and curing procedure for reinforced GPS monoliths.
mixture was vigorously stirred at room temperature for 1.5 h to allow hydrolysis of the silanes, following which the GPS dissolved. At this point, the pH of the precursor mixtures ranged from 2.7 to 3.2, based on the amount of the Nyacol alumina dispersion added to achieve the desired particle loading. Then, the precursor solution was divided into 100 g samples. The particles were stable in the gel precursors at this stage. 2.3. Inducement of Particle Aggregation. The state of aggregation of the reinforcing particles in the precursor was changed by adding a small amount of 0.10 or 0.20 M TSPP solution, depending on the desired final TSPP concentration, to a 100 g sample. Since TSPP solutions are basic, they increased the pH of the mixtures. An aliquot amount of 1 M NaOH or 1 M HNO3 was added to the system in order to achieve a final pH of ∼5.0. If the amount of TSPP solution added was sufficiently high, the aggregation of the alumina particles occurred immediately and could be observed easily by the change in turbidity of the dispersion. The negatively charged pyrophosphate ions specifically adsorbed onto the positively charged alumina particles and electrostatically destabilized them at low ion concentrations, of the order of 1-10 mM. The properties of the matrix were not measurably affected by TSPP additions at this level as will be discussed in Section 3.3. 2.4. Monolith Sample Preparation. The inducement of particle aggregation and preparation of the monolith are illustrated in Figure 1. A GPS precursor (30 g) was poured into a polystyrene weighing dish, which was then placed in a Thermo Fisher Scientific StableTemp convection oven (Pittsburgh, PA) at 48 °C for 36 h to cure. The evaporation of water accelerated the condensation reaction and cross-linking of the silane molecules. The cure at 48 °C resulted in solid monoliths weighing ∼9 g with a density ranging between 1.39 and 1.59 g/cm3, depending on the particle loading. If a coherent solid specimen was obtained, the monolith sample was cut into rectangular strips approximately 9.0 mm wide, 55 mm long, and 1.6 mm thick. These strips were post-cured at 113 °C for 12 h. 2.5. Measurement of Mechanical Properties. The mechanical properties of the monolith strips were measured using an Instron mechanical tester (Instron, Inc., Norwood, MA) with a three-point bend tool in accordance with the ASTM D790-03 standard. For each TSPP concentration, six to eight sample strips
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Figure 2. Electrophoretic mobility of reinforcing alumina particles, measured within 30 min after aggregation was induced.
were tested to obtain the standard deviation of the measured properties. The load cell used was a 5 lb load cell (MDB-5) from Transducer Techniques Inc. (Temecula, CA). The span of the three-point bend fixture was set to 40 mm. The flexural strength (σf) of the material was found using eq 1 below, and the Young’s modulus (E) of the specimen was calculated using eq 2:26
σf ) E)
PfL 2bh2
PL3 4dbh3
(1)
(2)
where L is the span of the fixture, b is the width of the specimen, and h is the specimen’s thickness. d is the deflection of the specimen, P is the load measured by the load cell, and Pf is the load at the break point. 2.6. Precursor Dispersion Characterization. The aggregate size and the electrophoretic mobility of the reinforcing particles were measured by photon correlation spectroscopy (PCS) using the Brookhaven ZetaPALS. The rheological behavior of the precursors was studied using an Anton Paar MCR 300 rheometer (Anton Paar, Ashland, VA). A shear sweep was performed by ramping the shear rate from 100 to 4000 s-1 and back using a cone (CP50-1) and plate (TEK 150P-C) configuration with a gap of 0.05 mm. 3. Results and Discussion 3.1. Electrostatic Destabilization of Reinforcing Particles by TSPP. The reinforcing alumina particles were electrostatically destabilized by the addition of TSPP as described above. Figure 2 shows the electrophoretic mobility, a representation of the electrostatic stability, of the reinforcing particles in the precursor mixtures within 30 min after addition of TSPP. The mobility of the 32 wt % loading samples could not be measured directly because the particle concentration exceeded the instrument measurement range. The alumina particles in the precursor mixture without any TSPP, as expected, had positive electrophoretic mobility, since the system pH was lower than the point of zero charge (PZC) of ∼8-9.27 As its concentration increased, the specifically adsorbed, negatively charged pyrophosphate ion gradually reduced the electrophoretic mobility, until the particles became negatively charged. It took approximately double the concentration of TSPP to reduce the mobility of the 16 wt % dispersions to the same level as the 8 wt % samples because there was twice the particle surface area to be covered. The
Figure 3. Effective diameter of the aggregates more than 2 months after aggregation was induced as a function of TSPP concentration.
TSPP concentration required to reduce the mobility to zero for both particle loadings was less than 10 mM. Once the particles surface was saturated with the pyrophosphate ions, the mobility remained constant with further increase in the TSPP concentration up to 17 mM, as ionic strength increases compensated for increases in the Stern potential. Complete coverage of the particle’s surface requires ∼3.5 TSPP ions/nm2, assuming the radius of the ion of 0.3 nm (MS Modeling simulation software, Accelrye, Inc., San Diego, CA). This is equivalent to ∼3 mM TSPP for the 8 wt % loading case. However, saturation was not reached until a TSPP concentration of 6 mM because a fraction of the ions remained in the solution. Also, some of the ions became protonated, producing OH-, as evidence by the increase in pH mentioned earlier. 3.2. Aggregate Size and Effect on Rheological Behavior of the Reinforced Sol-Gel Precursor Mixtures. The different TSPP concentrations in the dispersion resulted in various degrees of particle stability and aggregate size. Figure 3 shows the resulting effective diameter of the aggregates in the uncured precursor mixtures as a function of TSPP concentration for 8 and 16 wt % loaded samples, as measured using PCS. The measurement was done by diluting by ∼30 fold the 8 and 16 wt % dispersion mixtures in a blank GPS precursor solution with a comparable TSPP concentration in order to bring the particle volume fraction down to an acceptable level for dynamic light scattering measurements. The aggregate size increased slowly with time, but reached a final value within 24 h. The data shown here were obtained 2 months after the aggregation was induced by the addition of TSPP. Figure 3 shows that at low TSPP concentrations, the dispersion remained as primary particles. As the TSPP concentration was increased, the aggregate size grew larger, up to a few micrometers. The variation in TSPP concentration resulted in various degrees of electrostatic destabilization and aggregation rates, leading to the different aggregate sizes. The termination of the aggregate growth at an early stage was unexpected and will be further discussed in Section 3.4. The change in the turbidity of the precursor mixture was also an effective indication of the presence of the aggregates of different size, as shown in Figure 4, which was taken more than 2 months after aggregation inducement. Turbidity increased with the aggregate size, until the latter exceeded an average size of ∼600 nm in diameter and settled to the bottom, as shown in the rightmost sample. The sedimentation of aggregates occurred at a much slower rate than the gelation of the systems due to (1) an increase in viscosity of the samples during cure and (2) the “crowding” of aggregates due to the high particle volume fraction used here. Therefore, sedimentation of aggregates did not result in inhomogeneity of the sol-gel samples.
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Figure 4. Different appearance of the uncured precursors with 8 wt % particle loading more than 2 months after the aggregation was induced. Figure 7. Young’s modulus of blank and reinforced GPS monoliths.
Figure 5. Viscosity of the uncured precursors with 8 wt % reinforcing particle loading, measured within 30 min after aggregation was induced.
Figure 8. Comparison of (a) a coherent reinforced monolith sample and (b) a “dry lakebed” sample.
Figure 6. Flexural strength of blank and reinforced GPS monoliths.
The aggregation of the alumina particles also resulted in a change in the rheological behavior of the gel precursor dispersions. Within 30 min after initiating aggregation, a shear sweep was performed on the dispersions. Figure 5 shows the viscosity of the 8 wt % loading samples at different TSPP concentrations as a function of shear rate. For the unaggregated systems, the
viscosity was ∼3 cP and remained constant with shear rate over the range investigated. As expected, the presence of aggregates resulted in an increase in viscosity and shear thinning behavior. As the TSPP concentration increased and the aggregates became larger, the viscosity increased and the shear thinning became more pronounced. The study carried out on the 16 and 32 wt % loading samples followed the same trends. 3.3. Mechanical Properties of Reinforced Sol-Gel Monoliths. The objective of this study was to determine the effect of the aggregation state on the mechanical properties of the monoliths, viz., the flexural strength and Young’s modulus, measured using three-point bend tests. Figure 6 shows the flexural strength of the blank and alumina-reinforced GPS monoliths as a function of TSPP concentration. Young’s modulus data for the same set of samples are shown in Figure
Ind. Eng. Chem. Res., Vol. 47, No. 8, 2008 2627 Table 1. Summary of the TSPP Concentrations and the Aggregate Size below and above the Critical Aggregate Size last coherent reinforced monolith
8 wt % loading 16 wt % loading
first “dry lakebed” monolith
last coherent reinforced monolith
TSPP concn (mM)
aggregate effective diam (nm)
TSPP concn (mM)
aggregate effective diam (nm)
2.98 3.64
880 ( 310 260 ( 110
5.92 4.64
2270 ( 1000 580 ( 330
7. Both mechanical properties exhibited the same trend. The flexural strength and Young’s modulus of the blank sample were 0.5 and 60 MPa, respectively. For blank GPS monoliths, mechanical properties were unaffected by the addition of TSPP up to a concentration of ∼15 mM. Above 15 mM TSPP, a large amount of TSPP precipitated out of the precursor mixture during cure at 48 °C as water evaporated, increasing the electrolyte concentration over the solubility limit (16.8 g in 100 g of water at 50 °C).28 Incorporation of alumina particles above 8 wt % loading improved mechanical properties of the monolith. For instance, at the highest loading studied (32 wt %), the flexural strength and Young’s modulus of the reinforced monoliths were approximately three and five times higher than those of the blank samples, respectively. The properties of the monoliths reinforced with small aggregates of alumina particles produced by intermediate concentrations of TSPP (Regime 2 in Figures 6 an 7) are similar to the properties of the monoliths containing unaggregated particles (Regime 1). However, larger aggregates caused by the highest TSPP concentrations (Regime 3) resulted in complete mechanical failure. These reinforced monoliths had no measurable mechanical strength since they did not form coherent specimens. They had instead a structure resembling a “dry lakebed” (Figure 8), with many cracks across the specimens, which formed during cure at 48 °C. All “dry lakebed” samples are represented in Figures 6 and 7 as having zero strength and modulus, even though no mechanical measurements could be performed on them. The mechanical results suggest that for a given loading there was a critical size of aggregate that produced an abrupt change in the cured monolith structure. Below the critical size, the aggregates enhanced the monolith’s strength compared to those of the unfilled parent systems. Above the critical size, the aggregates destroyed the integrity of the monolith. Table 1 summarizes the TSPP concentrations and the aggregate sizes below the critical aggregate size (the highest TSPP concentration yielding a coherent monolith) and above it (the lowest concentration yielding a “dry lakebed”). The critical aggregate size for the higher particle loading samples is smaller than that for the samples with lower particle loading. This suggests that the enhancement in mechanical properties or the mechanical failure of the monoliths depends not on the aggregate size but on their effective volume fraction in the monoliths, and the approach to percolation. The degree of percolation of the aggregates was estimated by calculating the “effective volume fraction of the aggregates” (φk), defined as the volume fraction of the cured monoliths that would be occupied by the aggregates, defined in terms of their circumscribing spheres. The effective volume fraction for any case is obtained using
φk )
∑1
( ) 4
3
πrk3
Vf
)
Table 2. Summary of the Range of the Effective Volume Fraction of Aggregate below and above the Critical Aggregate Size
() rk
r1
3
φ1 k
(3)
8 wt % loading 16 wt % loading
first “dry lakebed” monolith
aggregate effective diam (nm)
φk (D ) 2.1-1.8)
aggregate effective diam (nm)
φk (D ) 2.1-1.8)
880 ( 310
0.28 - 0.56
2270 ( 1000
0.66 - 1.7
260 ( 110
0.19 - 0.27
580 ( 330
0.40 - 0.71
where r1 is the radius of a primary particle. rk is the average hydrodynamic radius of the aggregates, as measured by PCS, and regarded as the radius of the circumscribing sphere of the aggregate. φ1 is the volume fraction of the monolith occupied by primary particles Vf is the final volume of the monolith after cure. k is the number of primary particles in an aggregate, and can be calculated using
k)
() rk r1
D
(4)
where D is the fractal dimension of the aggregate.29 The fractal dimension of the aggregates was not measured. However, the upper and lower limits of the effective volume fraction can be found using the fractal dimension expected for diffusion-limited cluster-cluster aggregation (D ) 1.8) and that of the reaction limited cluster-cluster aggregation (D ) 2.1), respectively. Table 2 summarizes the upper and lower limits of the effective volume fraction at the conditions below and above the critical aggregate size (same as those in Table 1). For filled-composite and coating systems, it is well known that there is a critical pigment volume concentration (CPVC) of the filler particles above which properties of the composite change abruptly. The CPVC is the volume fraction beyond which there is not enough matrix material to completely wet out the pigment, leading to air entrapment inside the composite. At the CPVC, important properties such as gloss, permeability, and strength decline precipitously.22 Similarly, we believe that the loss of structural integrity of the reinforced GPS monoliths was due to the effective volume fraction of aggregates (φk) exceeding a certain critical value. As water evaporated from the system, the volume fraction of the aggregates increased, and the spacing between them was reduced. For a given particle loading, sufficiently large aggregates would come into contact with one another resulting in a continuous network, filling the whole space of the cured monolith, i.e., percolation.29 This approach to percolation is gauged by the aforementioned effective volume fraction of aggregates. The relevant critical effective volume fraction (φc) is the volume fraction of the randomly packed circumscribing spheres at which percolation of aggregates becomes possible. If the circumscribing spheres are assumed to be uniform in size and randomly close-packed, φc should be 0.64. At φc, the aggregates would be percolated, as all the circumscribing spheres would be in contact with one another. Figure 9 illustrates the physical meaning of φk and the relationship it has with the degree of percolation of the aggregates. When the aggregates were small (φk < φc, Figure 9a), they would be not percolated at any point during and after the volume reduction caused by curing. When φk ) φc (Figure 9b), the aggregates reached percolation when the sol-gel volume was reduced to the final volume (Vf). Large aggregates, caused by highest TSPP concentration, resulted in φk > φc (Figure 9c). This suggests that percolation of the aggregates occurred before the final monolith volume was reached at the completion of the cure. In other words, for these large aggregates
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Figure 10. Proposed mechanism of aggregation arrest.
Figure 11. Qualitative picture of two competing kinetic processes. In case 1, aggregate growth reached the critical size causing the “dry-lakebed” structure. In case 2, the silane encapsulation arrested the aggregate growth before the critical size is reached resulting in improved mechanical properties.
Figure 9. Illustration of the relationship between φk and the degree of percolation of the aggregates. (a) φk < φc, (b) φk ) φc, (c) φk > φc.
to be packed into the final volume, the aggregates must interpenetrate, jam, or break down. These processes impeded the volume reduction of the gel from the percolation point to the final volume, resulting in internal stresses that led to crack formation and the “dry lakebed” structure. It should be noted that since φk does not represent the actual packing of large aggregates in the monoliths, an indicated φk > 1 is possible. Table 2 shows that for our system, φc is around the 0.64 value anticipated for random-packed spheres. Coherent GPS monoliths with improved mechanical properties could be obtained when the φk is less than 0.64 because the small aggregates could rearrange themselves within the shrinking volume. However, when the upper and lower bounds of φk approached or exceeded the critical value, the reinforced monoliths failed mechanically due to crack formation during shrinkage from the percolation point to the final volume. 3.4. Competing Kinetic Processes and Termination of Aggregation. Normally when metal oxide particles in water are made to aggregate by reducing their electrostatic protection, the aggregates continue to grow irreversibly until they become large entities that can no longer be dispersed in the medium and settle out to form a sediment cake. However, in this study, we found that not all aggregation events resulted in large aggregates. In fact, at intermediate TSPP concentrations, aggregation stopped when the aggregates were only a few times larger than the primary particles and, in some cases, constituted less than 20 primary particles (Figure 3). Therefore, there must have been another process that terminated the aggregation. We believe that the aggregation process was stopped by the formation of silane layers encapsulating the aggregates as
illustrated in Figure 10. The silane molecules formed Si-OAl bonds with the particle’s surface and served as surface modifying agents for the alumina particles. Stabilization of particles using small molecules such as silane compounds, β-diketones, carboxylic acids, and long-chain alkylamines have been reported by many researchers.30,31 Once the layer of silane encapsulated the aggregates, they could no longer aggregate to form larger entities. They were then ultimately trapped in the sol-gel. This has led us to conclude that the final state of aggregation and, consequently, the mechanical properties of the composite monoliths, especially when the reinforcing particles are not stable, are the result of a competition between two kinetic events: the aggregation kinetics and the kinetics of silanization around the particles. This concept is qualitatively illustrated in Figure 11. If the growth of the aggregate (thin curve) reaches the critical size that led to percolation before the encapsulation (thick curve) can arrest the process (Figure 11, Case 1), the resulting composites self-destruct upon curing. On the other hand, if the silanization around the aggregates can arrest the aggregation prior to their reaching the critical size (Figure 11, Case 2), only small aggregates will end up in the composites. These small aggregates still enhance the mechanical properties of the matrix. While the detailed explanation of the aggregation arrest by completing kinetic processes is deferred to a later communication, all experiments preformed thus far suggested that the governing phenomenon is kinetics, rather than thermodynamics. For example, if the TSPP addition was delayed, allowing more time for the silane binding initially, the final aggregate size became smaller. On the other hand, when the particles were made to aggregate first, before silane was added, the aggregate sizes were found to be larger, as a function of the time delay before the silane addition. Both results confirm the theory of
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the competition between the kinetics of silane encapsulation and particle aggregation. 4. Summary This study focuses on the effect that the state of aggregation of reinforcing alumina particles has on the mechanical properties of GPS monoliths in which they are incorporated. Aggregation of alumina particles was controlled by decreasing their electrostatic stability using specifically adsorbing pyrophosphate ions at concentrations low enough not to have an impact on the properties of the GPS matrix itself. With increasing pyrophosphate concentration, the alumina aggregates became larger ranging from only a few times the primary particle size of about 90 ( 40 nm to several micrometers. It was found that there was a critical aggregate size below which the aggregates enhanced the mechanical properties of the monoliths and above which the aggregates destroyed the integrity of the sample, producing a “dry lakebed” structure upon curing. As expected, the critical aggregate size was larger for samples with lower particle loading, suggesting that the approach to percolation of the aggregates was critical to the composite mechanical properties. The approach to percolation can be represented by the “effective volume fraction of aggregates” (φk), which is the volume fraction of the cured monolith that would be occupied by the circumscribing spheres around the aggregates. The critical effective volume fraction of the aggregates (φc), in terms of their circumscribing spheres, was found to be ∼0.64, corresponding to the close-packed spheres of uniform size. Below this volume fraction, the aggregates cannot percolate throughout the monolith volume and are effective in improving its mechanical properties. Above this value, the aggregates are forced to interpenetrate, jam together, or break down, resulting in internal stresses and crack formation upon curing. Termination of the aggregate growth, which led to the different aggregate size, was due to a presence of the competing process of the formation of the silane layers around the particles and aggregates. If aggregation arrest occurs before the aggregates reach the critical size, dependent on the loading, the composite mechanical properties are enhanced. If not, the aggregates become percolated and cause the mechanical failure of the reinforced monoliths. Acknowledgment This work was supported in part by a grant from the Boeing Company and by the Center for Surface, Polymer, and Colloids at the University of Washington. Literature Cited (1) Brinker, J.; Scherer, G. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processings; Academic Press, Inc.: Boston, 1990. (2) Wright, J. D.; Sommerdijk, N. A. J. M. Sol-Gel Materials: Chemistry and Applications; Gordon and Breach Science Publishers: Australia, 2001. (3) Sanchez, C.; Julian, B.; Belleville, P.; Popall, M. Applications of Hybrid Organic-Inorganic Nanocomposites. J. Mater. Chem. 2005, 15, 3559. (4) Liu, J.; Chaudhury, M. K.; Berry, D. H.; Seebergh, J. E.; Osborne, J. H.; Blohowiak, K. Y. Fracture Behavior of an Epoxy/Aluminum Interface Reinforced by Sol-Gel Coatings. J. Adhes. Sci. Technol. 2006, 20, 277. (5) Zheludkevich, M. L.; Serra, R.; Montemor, M. F.; Salvado, I. M. M.; Ferreira, M. G. S. Corrosion Protective Properties of Nanostructured Sol-Gel Hybrid Coatings to AA2024-T3. Surf. Coat. Technol. 2006, 200, 3084.
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ReceiVed for reView November 16, 2007 ReVised manuscript receiVed February 5, 2008 Accepted February 13, 2008 IE071563N
