MSE. Steric Stabilization. Department Materials Science & Engineering Group Inorganic Materials Science Literature Review.

Transcription

1 enter for Industrial Sensors and Measurements Department Materials Science & Engineering Group Inorganic Materials Science Literature Review Steric Stabilization MSE Jingyu Shi The hio State University Group Inorganic Materials Science 2041 ollege Road, 281 Watts Hall olumbus H USA September August /ISM/jys August 29, 2002

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3 1 Abstract Steric stabilization by polymers adsorbed on inorganic particle surfaces is gaining more and more attention in both industry and academia because it plays an important role in stabilizing colloidal dispersions. In this review, basic concepts and some related topics about steric stabilization are introduced. In addition the application of steric stabilization in dispersing ceramic particles in non-aqueous media is summarized. It is found that functional groups, such as carboxyl, hydroxyl, amine, and ester groups in the molecular polymer structure generally play an important role in steric stabilization. Polymers containing carboxyl groups turn out to be the most effective steric stabilizers because carboxyl groups are supposed to interact strongly with basic sites, often present on the particle surface. n the other hand, the long chain hydrocarbons in the molecular structures extend from the surface into nonaqueous solvent and act as good moieties in nonaqueous media. The longer the hydrocarbon chains, the better the stabilization effect. It is also shown in the literature that copolymers are usually more effective in steric stabilization than homopolymers because copolymers consist of more than one type of repeated unit. ne type of repeated unit can act as anchor; the other type can act in the moieties extending into a nonaqueous solution. Steric stabilization in the modified emulsion precipitation method to prepare ceramic nanoparticles is discussed at the end of this review. Poly(octadecyl methacrylate) (PDMA) is used as the steric stabilizer in that method. It is suggested that the ceramic nanoparticles are stabilized by a bilayer of polymer (PDMA) and surfactant (DiDAB). The exact stabilization mechanism still needs further study.

4 2 Table of contents Abstract 1 Table of contents 2 1 Introduction olloid stability Methods for stabilizing colloids Electrostatic stabilization Polymeric stabilization 6 2 Steric stabilization Advantages of steric stabilization Steric stabilizers Some definitions The best steric stabilizers Steric stabilization mechanism Stability of the colloidal particles in steric stabilization The critical flocculation point (FPT) Factors affecting FPT Adsorption of polymers from solution 13 3 Flocculation by bridging 16 4 Application of steric stabilization in dispersing ceramic particles in nonaqueous media 17 5 Steric stabilization in preparing nanoparticles via modified emulsion precipitation The structure of PDMA The selection of PDMA as steric stabilizer The role of PDMA 23 References 24 A rganic component data 26 A.1 poly(dimethyl siloxane) 26 A.2 Polyamides 26 A.3 PMMA 27 A.4 Dioxane 27 A.5 rganomethoxysilanes [ 18 H 37 Si(H 3 ) 3 ] 28 A.6 rganomethoxysilanes [ 18 H 37 (H 3 ) 2 Si(H 3 )] 28 A.7 Styrene 28

5 Table of contents 3 A.8 Methyl ethyl ketone 28 A.9 Alkyl poly(oxyethylene) 29 A.10 Ethylene oxide 29 A.11 Polyacrylate 29 A.12 leic acid 30 A.13 glycerol trioleate 30 A.14 KD-2 31 A.15 melamine/linseed oil 31 A.16 Adipic acid 34 A.17 Neopentyl glycol 34 A.18 Poly(octadecyl methacrylate) 34 A.19 Poly(vinyl butyral) (PVB) 35 A.20 Methacrylate 35 A.21 Methacrylic acid 36 A.22 Acrylamide 36 A.23 Pyridyl 36 A.24 Perchlorethylene 36 A.25 Butadiene 37 A.26 Polyvinylpyrrolidone (PVP) 37 A.27 Poly (vinyl acetate) (PVA) 38 A.28 IPA, isopropylalcohol 38 A.29 L-7500 (silwet surfactants; butoxy terminated polypropylene oxide; 3000 D) 39 A.30 L-7604 (hydroxyl terminated polyethyleneoxide; 4000MW) 40 A.31 Arkopal 40 (Nonylphenol tetraethyleneglycol ether) 40 A.32 DiDAB (Didodecyldimethylammonium) 41 Symbols 42 Distribution list 43

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7 5 1 Introduction olloidal systems are dispersed phases finely subdivided in a dispersion medium. This subdivision results in a very high interfacial surface area which determines largely the physical properties of the system [1]. In most common colloidal systems, the continuous phase (dispersion medium) is a liquid. A system exhibits properties of a colloidal character when the particles (the dispersed phase) have a diameter between 1 and 1000 nm. This review will be concerned primarily with liquid droplets or solid particles in a liquid. 1.1 olloid stability olloidal particles in a dispersion medium always show Brownian motion and hence collide with each other frequently. The stability of colloids is thus determined by the interaction between the particles during such a collision. There are two basic interactions: one being attractive and the other repulsive. When attraction dominates, the particles will adhere with each other and finally the entire dispersion may coalesce. When repulsion dominates, the system will be stable and remain in a dispersed state [2]. Van der Waals forces are the primary source of attraction between colloidal particles. These forces are always present between particles of similar composition. Therefore, a colloidal dispersion is said to be stable only when a sufficiently strong repulsive force counteracts the van der Waals attraction [2]. There are three different possible origins for van der Waals forces: permanent dipole-permanent dipole (Keesom) forces, the permanent dipole-induced dipole (Debije) interactions and transitory dipole-transitory dipole (London) forces. The first two are very short range interactions, but the London forces are longer range attractions. Since only London forces contribute to the long-range attraction between colloidal particles, the magnitude and range of the van der Waals- London (VDWL) attraction are decisive in determining strategies for stabilizing colloid particles. For many colloid systems, the range of significant VDWL attraction is between 5 and 10 nm. 1.2 Methods for stabilizing colloids Since there are always strong, long-range attractive forces between similar colloidal particles, it is necessary to provide a long range repulsion between the particles to impart stability. This repulsion should be at least as strong as the attractive force and comparable in range of the attractive interaction. Stability can be obtained by surroundig colloidal particles: With an electrical double layer (electrostatic or charge stabilization). With adsorbed or chemically attached polymeric molecules (steric stabilization). With free polymer in the dispersion medium (depletion stabilization). ombination of the first two stabilization mechanisms lead to electrosteric stabilization. The latter two types of stabilization are often realized by the addition of polymers to stabilize dispersions and are called polymeric stabilization.

8 6 Steric Stabilization Electrostatic stabilization An effective way to counterbalance the VDWL attraction between colloidal particles in polar liquids is to provide the particles with oulombic repulsion. In liquid dispersion media, ionic groups can adsorb to the surface of a colloidal particle through different mechanisms to form a charged layer. To maintain electroneutrality, an equal number of counterions with the opposite charge will surround the colloidal particles and give rise to overall charge-neutral double layers. In charge stabilization, it is the mutual repulsion of these double layers surrounding particles that provides stability. The thickness of the double layer depends, amongst others, on the ionic strength of the 1 2 dispersion medium. The ionic strength can be expressed as I = z i c i, where z is the charge number of ions, i, and c is the molar concentration of the ions. For 1:1 electrolytes, the ionic strength is proportional to the concentration. Here we will use the concentration c to represent ionic strength. At low ionic strengths (electrolyte c=10-3 M), the thickness of the double layer is about 5-10 nm, which is of the same order as the VDWL attraction. This explains the observation of charge stabilization in dispersion media of low ionic strength. The thickness of the double layer is reduced significantly with increasing the ionic strength. At ionic strengths for electrolyte c>10-1 M, the thickness of the double layer is less than 1 nm. In that case, the range of double layer electrostatic repulsion is usually insufficient to counterbalance the VDWL attraction. This accounts for the fact that most charge-stabilized dispersions coagulate when increasing the ionic strength of the dispersion medium [3]. Hence, one great disadvantage of charge stabilization of particles is its great sensitivity to the ionic strength of the dispersion medium. In addition it only works in polar liquid which can dissolve electrolytes. However, due to the advantages in simplicity and cost price, charge stabilization is still widely used in stabilizing dispersions in aqueous media Polymeric stabilization For polymers with molecular weights >10000 D, the chain dimensions are comparable to, or in excess of, the range of the VDWL attraction. Hence, as long as they can generate repulsion, these polymer molecules can be used to impart colloid stability [3]. There are two different mechanisms accepted for polymeric stabilization of colloidal dispersion: steric stabilization and depletion stabilization. 2 i Figure 1.1: Schematics of steric stabilization Figure 1.2: Schematics of depletion stabilization

9 1 Introduction 7 Steric stabilization Steric stabilization of colloidal particles is achieved by attaching (grafting or chemisorption) macromolecules to the surfaces of the particles (figure 1.1). The stabilization due to the adsorbed layers on the dispersed particle is generally called steric stabilization. Depletion stabilization Depletion stabilization of colloidal particles is imparted by macromolecules that are free in solution (figure 1.2). The study of this type of stabilization is still in its initial stage. Electrostatic and steric stabilization can be combined as electrosteric stabilization. The origin of the electrostatic component may be a net charge on the particle surface (figurefigure 1.3a) and/or charges associated with the polymer attached to the surface (i.e. through an attached polyelectrolyte) (figurefigure 1.3b). It is also possible to have combinations of depletion stabilization with both steric and/or electrostatic stabilization. The combination of depletion and steric stabilization is very common when there are high concentrations of free polymer in the dispersion medium. Figure 1.3: Schematics of electrosteric stabilization: (a) charged particles with nonionic polyme rs; (b) polyelectrolytes attached to uncharged particles.

10 8 2 Steric stabilization 2.1 Advantages of steric stabilization Steric stabilization has several distinct advantages over electrostatic stabilization [3]: Relative insensitivity to the presence of electrolytes. For instance, for 1:1 electrolytes 2 ( I = z c ), a charge-stabilized dispersion will not be stable and coagulate when the concentration of electrolytes exceeds the 10-1 M limit. The dimensions of polymer chains display no such dramatic sensitivity and sterically stabilized dispersions are relatively insensitive to the presence of electrolyte. Equal efficacy in both aqueous and nonaqueous dispersion media. harge stabilization is less effective in nonaqueous dispersion media than it is in aqueous media. This is primarily due to the low relative dielectric constant (<10) of most nonaqueous media. In contrast, steric stabilization is effective in both nonaqueous media and aqueous media. This explains why steric stabilizion is usually preferred for nonaqueous dispersion media. Equal efficacy at both high and low solids content. In charge stabilization in nonaqueous media, the thickness of the double layers can be so large, (due to the low dielectric constant of the dispersion medium), that the mere preparation of high solids dispersions forces the particles too close together which then leads to coagulation. In aqueous dispersion media, the preparation of charge-stabilized particles at high solids dispersions is often difficult because of the gel formation induced by the interactions between the double layers surrounding each particle. Reversibility of flocculation. The coagulation of charge-stabilized particles (induced by the addition of electrolyte) is usually irreversible by subsequent dilution. In contrast, flocculation of sterically stabilized dispersions (induced by the addition of a nonsolvent for the stabilizing moieties) can usually be reversed spontaneously by mere dilution of the nonsolvent concentration to a suitably low value. This difference is due to the fact that sterically stabilized dispersions may be thermodynamically stable while charge stabilized dispersions are only thermodynamically metastable. As a consequence, for charge stabilized dispersions, the coagulated state represents a lower energy state and the coagulation can be reversed only after input of work into the system. Another important consequence of the thermodynamic stability of sterically stabilized dispersions is that they can redisperse spontaneously after drying. 2.2 Steric stabilizers Some definitions Polymer: a polymer molecule is defined as a molecule of relatively high molecular weight consisting of regularly repeating units, or chemically similar units, connected by primary covalent bonds. The units are called monomer.

11 2 Steric stabilization 9 opolymer: opolymer refers to a polymer having two different monomers incorporated into the same polymer chain [4]. opolymers may be of the random (statistical), block or graft (comb) type: Figure 2.1: Structure of random, block or graft copolymer. Random copolymer: In random polymers, the units of one monomer and of the other have no definite order or arrangement along any given chain and generally composition or ratio of one monomer to the other may slightly differ between chains [4]. Block copolymer: Block polymers have a long segment or block of one monomer followed by a block of a second monomer. The result is that different homopolymers chains are joined in a head-to-tail configuration [4]. So a block polymer is a linear arrangement of blocks of different monomer composition. A diblock copolymer is poly- A-block-poly-B, and a triblock copolymer is poly-a-block-poly-b-block-poly-a [1]. If A is a hydrophilic group and B is hydrophobic group, the result can be regarded as polymeric surfactant. Graft copolymer (comb copolymer): A graft polymer is a type of copolymer in which the chains of one monomer are grafted onto main chains of the other monomer.

12 10 Steric Stabilization The best steric stabilizers It is found that the best steric stabilizers are amphiphilic 2-block or graft copolymers, as shown in figurefigure 2.2. Polymer A has a strong affinity for the adsorbent (particle surface) and serves to anchor the copolymer to the particle surface. Polymer B is much more compatible with the dispersion medium and has little affinity for the surface. Hence, it is extended into the dispersion medium and provides a steric barrier [2]. Random copolymers are usually not so effective in steric stabilization as block or graft copolymers. However, some amphiphilic homopolymers can be effective if their structure is based on monomers having the same role as polymer A and polymer B in block or graft copolymers. Figure 2.3 illustrates the reason why amphiphilic molecules are such effective steric stabilizers. The part of polymer that is insoluble in the dispersion medium and shows affinity to the particle surface would attach itself to the particles by either physi- or chemi-sorption or even by incorporation in a growing particle. This insoluble part of polymer serves to anchor the soluble part of polymer to the colloidal particles and is accordingly referred to as the anchor polymer. The role of the soluble part of polymer is to impart steric stabilization and, for this reason, such chains are termed the stabilizing moieties [3]. It can be imagined that amphiphilic polymer molecules can serve as effective steric stabilizers if a second identical particle approaches the one in figurefigure 2.3. The stabilizing moieties that reach out into the dispersion medium must be mutually repulsive to effectively keep the particles at a distance from each other. They have to be attached to the particle strongly enough as not to be desorbed from the surface when particle undergoes Brownian collisions. omplete surface coverage also helps preventing escape. Figure 2.2: Examples of (a) block copolymers and (b) graft copolymers used as stabilizers. A is the anchor group, and B is the soluble group providing the steric barrier. Figure 2.3: Schematic representation of the steric stabilization of a colloidal particle by an amphipathic block copolymer (only some of the stabilizing chains are shown). Generally, any polymer that is appropriately soluble in the dispersion medium is effective as a stabilizing moiety and any polymer that is insoluble in the dispersion medium and also shows affinity to the particle surface at the same time is effective as an anchor

13 2 Steric stabilization 11 polymer. Some typical stabilizing moieties and anchor polymers for aqueous and nonaqueous dispersion media are shown in table 2.1 [3]. Table 2.1: Typical stabilizing moieties and anchor polymers for sterically stabilized dispersions. 2.3 Steric stabilization mechanism When two particles with adsorbed polymer layers approach each other at a distance of less than twice the thickness of the adsorbed layer, interaction of the two layers takes place. The degree of stabilization can be defined quantitatively in terms of the energy change occurring upon the interaction of the adsorbed layers. The Gibbs free energy change G of the overlap interaction of the adsorbed layers is expressed as G = H T S. If G is negative upon the overlap of the adsorbed layers, flocculation or coagulation will result, and if G is positive, stabilization will result. Under isothermal conditions, the stability is then a function of the enthalpy change, H and the entropy change, S. Many theories for explaining the steric stabilization mechanism have been proposed and many theoretical equations for calculating the energy change with the overlap of the adsorption layer have been devised. Here we only introduce the generally accepted entropic stabilization theory. In one entropic stabilization theory, it is assumed that a second surface approaching the adsorbed layer is impenetrable. Thus, the adsorbed layer is compressed and the polymer

14 12 Steric Stabilization segments present in the interaction region lose configurational entropy. That is, the polymer segments occupy fewer possible configurations in the compressed state than in the uncompressed state. This reduction in entropy increases G, producing the net effect of repulsion between the particles and thus preventing the particles from flocculating. In the entropic stabilization theory, the enthalpic interaction between the adsorbed molecules and the dispersion medium is neglected so that G = T S [2]. 2.4 Stability of the colloidal particles in steric stabilization The critical flocculation point (FPT) The point at which flocculation is first detected on decreasing the affinity of the dispersion medium for the stabilizing moieties is termed the critical flocculation point. The transition from long-term stability to catastrophic flocculation occurs abruptly at the FPT. Thus, FPT can be used to evaluate the stability of the colloidal particles in steric stabilization. The FTP can be influenced by the following system parameters: particle number concentration, the particle size, the nature of the anchor polymer, the surface coverage of the particles, the nature of the dispersed phase, the nature and molecular weight of the stabilizing moieties and the solvent Factors affecting FPT Particle concentration effect. For high molecular weight stabilizing chains, the FPT is relatively insensitive to the particle number concentration. For low molecular weight stabilizing moieties and large colloidal particles, dilute dispersions display greater stability than more concentrated systems. Particle size effect. The FPT is independent of the particle size when the stabilizing moieties are of high molecular weight and the particle size is not too large. For low molecular weight stabilizing moieties, which give rise to thin steric layers, the dispersions become progressively easier to coagulate as the particle size increases. This is ascribed to the VDWL attraction increasing with increasing particle size. Anchor polymer nature effect. Provided the anchor polymer is attached sufficiently strong to the colloidal particles, its chemical nature has no significant effect on the observed FPT. Insolubility in the dispersion medium is often sufficient to ensure effective anchoring, if the molecular weight of the anchor polymer is sufficiently high. Surface coverage effect. Any decrease in surface coverage will lead to an increase in the ease of coagulation and thus decrease in stability. Particles only partially coated by stabilizer may undergo irreversible coagulation caused by VDWL attraction between the core particles, followed by irreversible chemical bonding. It seems likely that, under the stress generated by a Brownian collision, well-anchored stabilizing chains can move laterally on the surface of colloidal particles ( surface migration ). This creates bare patches on the surfaces, allowing their close approach to proceed further. Dispersed phase nature effect. It is believed that with thin steric layers, the nature of the dispersed phase could exert an influence on stability if its chemical nature is drastically different (e.g. polymer, an inorganic salt or a metal). In this case, the

15 2 Steric stabilization 13 VDWL attraction between the core particles will be different according to the nature of the dispersed phase. Molecular weight and adsorbed layer thickness effect. The FPT is found to be independent of the molecular weight of the stabilizing moieties provided that their molecular weight is sufficiently high (e.g. at least several thousands). For low molecular weight stabilizing moieties, limiting stabilities are controlled by the VDWL attraction between the core particles. A molecular weight dependence of the flocculation point is then observed. The higher the molecular weight of the stabilizing moieties, the thicker the adsorbed layer and the further apart are the core particles. As the VDWL attraction decreases with increasing distance of separation, particles stabilized by higher molecular weight stabilizers appear to be more stable near the FPT. Solvent affinity effect. Solvent affinity is a very important factor in steric stabilization. The better solvent the dispersion medium is for the stabilizing moities, the more extended the chains become and the bigger their sphere of influence. In a good solvent, repulsive interactions between colloidal particles occur if the grafting density is high enough. If the solvent affinity is poor, attractive interactions between particles can lead to flocculation. In summary, if the colloidal particles are small in size and fully coated by well-anchored, high molecular weight stabilizing moieties, the observed FPT is essentially independent of the volume fraction and the chemical nature of the disperse phase, the chemical nature of the anchor polymer, the particle size, and the molecular weight of the stabilizing moieties. Flocculation at the FPT is reversible. However, the FPT is dependent on the surface coverage by the stabilizing chains. n the other hand, if the stabilizing moieties are of low molecular weight and the particle size relatively large, stability appears to be affected by the VDWL dispersion attraction between the core particles. The FPT becomes a function of the particle size and larger particles are easier to flocculate. In addition, the FPT becomes a function of the particle number concentration and dilute dispersions are more stable [3]. 2.5 Adsorption of polymers from solution Before discussing adsorption of polymers from solution, two terms, adsorbent and adsorbate will be defined first. Adsorbent is the substance where another substance is adsorbed on and adsorbate is the substance that is adsorbed on the surface of the adsorbent. A polar surface is supposed to have acidic and/or basic sites and the polar groups of the polymers refer to the acidic or basic functional groups in the molecular structure of polymers. It is found that both poly(dimethyl siloxane) [A.1] and polyamides [A.2] are significantly adsorbed onto strongly polar particles but not onto the weakly polar ones [2]. Ellersterin and Ullman [5] also showed that strong polar groups in a polymer molecule enormously enhance the adsorption of the polymer because of the interaction of functional groups of polymer with the acidic and/or basic sites on particle surfaces. When low-molecular-weight molecules having strong basic or acidic groups are used to stabilize powders with opposite acid-base characteristics in nonaqueous media, an electrostatic stabilization can be present along with steric stabilization. When high-

16 14 Steric Stabilization molecular-weight polymeric molecules having acidic or basic functional groups are used to stabilize the dispersions, the predominant dispersion mechanism is steric stabilization. Usually, besides the adsorbate, solvent molecules can also adsorb to the adsorbent. The competition between adsorbate and solvent adsorption is determined by the chemical nature of the adsorbent surface. Thus, in addition to the acid-base strength of polymer functional group and the particles, the solvent also plays an important role. If the functional group of the polymer is less acidic (or basic) than the solvent, the polymer will not absorb on the surface of particles, resulting in poor dispersion stability. The absorption of PMMA (basic) [A.3] in various solvents illustrates the effect of solvents in dispersions. PMMA is effectively absorbed from l 4, a neutral solvent, but is not absorbed from Hl 3, an acidic solvent which dissolves the polymer too well for it to be taken from the completely dissolved state. PMMA is poorly absorbed in silica from dioxane [A.4] because this basic solvent preempts the acidic surface of silica so successfully that the basic polymer is excluded [6]. Purely steric stabilization of particles can be obtained by chemically bonding the dispersant to the particle. rganoethoxysilanes ( 8, 16, 18 ) were covalently bonded to silica in hexane to give improved dispersion stability [7]. The 18 silane gave the best dispersion stability because of the longer chain. rganomethoxysilanes 18 H 37 Si(H 3 ) 3 [A.5] and 18 H 37 (H 3 ) 2 Si(H 3 ) [A.6] showed little difference in their effect on steric stability. High-molecular-weight polystyrene [A.7] was grafted onto silica to obtain very good steric stabilization of the powder in toluene, whereas homopolystyrene was ineffective [8]. These examples suggest that powders in nonaqueous dispersion can be stabilized by a purely steric mechanism and a wide range of dispersant molecular weights can be effective. It is clear that the most effective way to stabilize inorganic powder in a nonaqueous medium is to chemically bond a polymer soluble in the medium to the particles. Polymers with relatively strong acid or base groups opposite in polarity to those on the particle surface generally work well by chemisorption to the particles. However, the effectiveness of polymers with weaker acidic (or basic) strength depends to a large degree on the acidic or basic strength of the solvent. Homopolymers without functional groups such as polystyrene [A.7] are the least effective. Block polymers are more effective when one block has interacting functional groups, but can be effective in some cases without interacting functional groups with the proper choice of solvent [6]. Since water molecules are always present on the surface of hydrophilic adsorbents even if they are carefully dried, the presence of water may play an important role in the adsorption process in a nonaqueous solution. For polymers covalently bonded to the particle surface, the presence of water may reduce the adsorption process. For example, in the adsorption of poly(dimethyl siloxane) [A.1] onto iron powder and powdered glass [9], traces of water in solution significantly reduced the amount of polymer adsorbed. And, in the adsorption of polystyrene [A.7] onto charcoal [10], traces of water in methyl ethyl ketone [A.8] caused anomalous delay in the adsorption process. n the other hand, the presence of water may also assist the interaction of polar groups of polymers with the acidic or basic sites on the particle surfaces and additional electrostatic stabilization may result in this case. For example, in the presence of traces of water, the carboxyl groups of polymers will ionically bound to the particle surfaces and enhance the stabilization.

17 2 Steric stabilization 15 Alkyl poly(oxyethylene) [A.9] surfactant molecules adsorb on the solid surface by the ethylene oxide [A.10] (hydrophilic) chain and leave the hydrophobic hydrocarbon (alkyl) chain in the dispersion medium. This is a typical case of the adsorption of nonionic surfactants on inorganic pigments in an organic dispersion medium [2]. The general formula is n H 2n+1 (H 2 H 2 ) x H or n E x. Van der Waarden [11] found that this stabilizing activity increases with the length of the aliphatic chain ( 8 to 16 ) and with the number of chains per molecule. It is supposed that the adsorbed molecules are able to prevent the particles from approaching each other to such a distance where they enter each other s sphere of attraction.

18 16 3 Flocculation by bridging For a better understanding of stabilization mechanisms and a proper design and use of stabilizing agents, it is helpful to discuss the behavior of polymeric materials as flocculants. Particles that are stabilized by polymers can flocculate by the bridging mechanism in two ways [12]. ne is flocculation by the bridging of two or more particles by one polymer molecule, as shown in figurefigure 3.1(A); the other is bridging by the interaction of polymer chains adsorbed onto different particles, as shown in figurefigure 3.1(B). Figure 3.1: Bridging flocculation: (A) two particles by one polymer molecule; (B) two particles by two separately adsorbed polymer molecules. Type A flocculation occurs when: The polymer molecule has more than two adsorbable segments. The chain is long enough to adsorb onto more than one particle. The surface coverage by adsorption of polymer is low, so that there are more chances for adsorption of polymer extending from one particle to another particle. This bridging flocculation occurs only at low polymer concentrations where the surface coverage of the particles is less than half of the saturation value. Type B flocculation occurs when the polymer chains are very long and the surface coverage by adsorbed polymer is so high that adsorption sites are scarce, and the probability for polymer extending between the particles is low. Another condition for this type of flocculation is that the affinity between the interacting chains should be large enough to overcome the repulsion cause by steric stabilization. In summary, very long polymer molecules contribute to bridging flocculation. However, if the polymer molecules are made too short to avoid this effect, they will form a dense thin layer on the particle surface and can not act as effective steric stabilizers. Whether the polymer acts as a stabilizer or as a flocculant also depends on many other factors. Further study is needed in this field.

19 17 4 Application of steric stabilization in dispersing ceramic particles in nonaqueous media In nowaday s ceramic processing, less expensive homopolymers are almost universally used to stabilize ceramic particle dispersions. Unfortunately, these may be subject to desorption, which is a complex process depending on several different solvent and particle parameters. Homopolymers such as polyacrylates [A.11] are fairly simple molecules that can provide both electrostatic and steric stabilization, provided the polymer adsorbs on the surface to give full coverage in a good solvent. Strong attraction to the surface can occur when the polymer has several segments which have an affinity to the particle surface. However, in cases where very strong adsorption of segments occurs, the polymer may stick to the first area of contact and prevent further polymers from adsorbing in the vicinity, producing poor overall coverage. Weaker adsorption generally allows mobility resulting in a surface rearrangement and hence better packing. Also, there must be sufficient polymer present in solution to give a high steric layer density. In cases of low polymer concentration in a poor solvent, it is possible for adsorption to occur at more than one particle surface at a time, forming an extend link, which can cause bridge flocculation [13]. Generally, the most effective and economic stabilizers for ceramic oxide systems in nonpolar media are small molecules such as fatty acids, amines, and esters. Early studies by Koelmans [14] showed that good dispsersions of Al 2 3 and Fe 2 3 in xylene could be obtained with a monomolecule layer of oleic acid [A.12]. Doroszkowski and Lambourne [15] have shown how dispersant properties depend on the molecular architecture of the fatty acid. They found that a variety of acids bind to titania surfaces through the carboxyl groups, with the molecular chains roughly perpendicular to particle surfaces. More polar solvents gave a lower surface coverage of dispersant and a greater degree of flocculation. Branched chains and increasing chain length both lead to reduced flocculation. Fish oil is universally used in dispersing alumina or ferrite powders in tape casting. It has been demonstrated by alvert [16] that fish oil which can be converted to carboxylic acids by natural oxidation is a superior stabilizer for α-al 2 3 in toluene compared with pure glyceryl trioleate [A.13] (fish oil s major component). The reason is that carboxylic acid impurities resulting from oxidation of the oil turn out to be a better stabilizer than glyceryl trioleate. Although both the ester group in glyceryl trioleate and the carboxylic group in acid can adsorb strongly to α-al 2 3 in toluene, the latter can force the hydrocarbon chains to extend into solvent more effectively and act as a better steric stabilizer. Thus, he suggested that a suitable synthetic analog to fish oil should be a multiple-branched polyester with a molecular weight of a few thousand and several acid groups per molecule. ther studies have shown that additive KD-2 [A.14] which consists of a basic ethoxylated amine stabilizes oxidized silicon particles in ethanolic media [17]. Mikeska and annon [18] studied the stability of tape casting BaTi 3 in an ethanolmethyl ethyl ketone solution and found that a phosphate ester, a fish oil, a fatty acid and an ethoxylate give the best performance. In addition, many other homopolymers are used in stabilizing ceramic particles. The study of Romo [19] shows that a dispersion of Ti 2 in xylene can be stabilized by the

20 18 Steric Stabilization presence of melamine/linseed oil [A.15]. Sato[20] studied the stability of dispersions of red iron oxide in cycohexanone in the presence of fatty polyamides [A.2], and found that there is a close relationship between the adsorption of the polyamides and the stability of the dispersion. The greater the adsorption, the better the stability. He also found that the stabilizing effect of polyamides on dispersing Ti 2 and Fe 2 3 increases with an increasing amine value of the polyamides, namely, the number of anchoring segments. rowl and Malati [21] studied the dispersion of Ti 2 and Fe 2 3 in benzene stabilized by a polyester of adipic acid [A.16] and neopentyl glycol [A.17]. They found that the stability of dispersions prepared with hydroxyl-terminated polyesters is extremely low compared with those with the carboxy-terminated polyesters. Adsorption of the hydroxylterminated polyesters by pigments is also lower. rowl and Malati attributed this to the fact that the carboxy-terminated polymer anchored more strongly to the solid surface. Joppien and Hamann [22] found that the rate of sedimentation of alumina particles decreased as the fraction of carbonyl groups in adsorbed polyesters decreased, suggesting that the resultant increase in film thickness slowed down the flocculation rate. Yin et.al. [23] suggested that an Al 2 3 powder could be packed to high density (58.7vol.%) during slow sedimentation in heptane by use of a poly(octadecyl methacrylate) (PDMA) [A.18] polymer. From above, it is clear that functional groups in polymers, such as carboxyl, hydroxyl, amine, and ester group play an important role in the steric stabilization. These functional groups can interact with the particle surface and act as good anchors. It is also found that hydroxyl and acetate groups in Poly(vinyl butyral) (PVB) [A.19] are important sites for interaction with alumina [24]. Poly(alkylmethacrylate) [A.20] polymers improved the dispersion of alumina in heptane and paraffin oil because poly(alkylmethacrylate)s can interact with the alumina by hydrogen bonding, as determined by Diffusive Reflection Infrared Fourier Transform spectroscopy [23]. As mentioned in section 2.2.2, copolymers are supposed to be the best steric stablizers. A large number of copolymers containing functional groups have been used to stabilize inorganic powders in nonaqueous media. opolymers of methyl methacrylate and small amounts of methacrylic acid [A.21] (3.2% or less) gave very good dispersion of Ti 2 powder in methyl ethyl ketone where methyl methacrylate homopolymers have been shown to be ineffective [25]. Howard and Ma [25] studied the adsorption of methyl metharylate homopolymer and its copolymers on surface-modified titanium dioxide (pigments are coated to impart various degrees of acidity to their surfaces) in polar and nonpolar dispersion medium. They found that homopolymers adsorb poorly from methyl ethyl ketone (polar) and do not stabilize the dispersions of the coated pigments; however, they are better adsorbed from their solutions in toluene (nonpolar) and are then effective in dispersion stabilization. This difference may be due to the poor solubility of methyl methacrylate in toluene or due to the competition for surface sites by the basic methyl ethyl ketone. Incorporation of small quantities of carboxylic acid polar groups into the macromolecular chain increases the adsorption and has a beneficial effect on dispersion ability. opolymers with small amounts of nitrile, acrylamide [A.22], pyridyl [A.23], and hydroxyl groups also show enhanced adsorptions but do not, in general, improve dispersion stability.

21 4 Application of steric stabilization in dispersing ceramic particles in nonaqueous media 19 Thies [26] studied the effect of molecular structure on stabilization by determining the amount and composition of polymers adsorbed on stabilized and on flocculated silica particles dispersed in perchlorethylene [A.24]. The polymers used were styrene [A.7]/butadiene [A.25] copolymers which varied in composition and molecular weight. Thies found that the copolymer adsorbed on the flocculated silica was richest in styrene. opolymer adsorbed on stabilized silica contained less styrene than the original copolymer blend and the unadsorbed copolymer had the lowest styrene content. These results indicated the importance of a favorable balance between the anchoring unit (styrene polymer) and the extending unit (butadiene polymer), to achieve the optimum stabilizing performance. Parker et.al. [27] studied the sedimentation behavior of alumina powder in the presence of poly-vinylpyrrolidone (PVP) [A.26] and poly (vinylpyrrolidone-co-vinyl acetate) (PVP/VA) [A.27] in thermodynamically good (IPA, isopropylalcohol) [A.28] as well as poor solvents (toluene) for the PVP homopolymer. The steric stabilizing efficiency of PVP in alumina slurries (in IPA) is increased by the incorporation of the VA comonomer because IPA is a good solvent for PVP but a poor solvent for PVA. NMR results suggested that the PVP/VA copolymer is anchored to the alumina powder surface by means of VA (VA is supposed to have strong affinity with the basic groups on Al 2 3 surface), whereas the PVP moieties extend into the continuous phase of the slurry medium. The effect of stabilization is reduced when a solvent poor for PVP but good for PVA (such as toluene) is added. This is ascribed to chain contraction of the PVP mietoies upon the addition of a poor solvent and VA moieties desorbing upon the addition of a good solvent. In Armstrong s study [6] of the stability of alumina dispersions in a 90:10 wt.% tolueneethanol mixed solvent it was found that: High-molecular-weight sulfonated polystyrene (60,000 D) having a low suphonation level is an effective stabilizer. Low-molecular-weight sulfonated polystyrene (10,000 D), silwet surfactants L-7500 [A.29] (butoxy terminated polypropylene oxide; 3000 D) and L-7604 [A.30] (hydroxyl terminated polyethyleneoxide; 4000MW) and homo-polystyrene are ineffective stabilizers. Discussion Generally, functional groups, such as carboxyl, hydroxyl, amine, and ester groups in polymer molecular structure play an important role in the steric stabilization. These functional groups can interact with the particle surface and act as good anchors. Among all the functional groups, carboxyl groups turn out to be the most effective anchors to the ceramic particle surface because they are supposed to interact strongly with the basic sites on the particle surface. Ester groups can interact with the ceramic particle surface by hydrogen bonding. However, this interaction is less strong than that of carboxyl groups. Thus, polymers which only contain ester groups are not as good stabilizers as those including carboxyl groups. The long chain hydrocarbons in the molecular structures may, on the other hand, extend from the surface into nonaqueous solvent and act as good moieties in nonaqueous media. Usually, the longer the hydrocarbon chains, the better the stabilization effect. Sometimes, hydrocarbon chains containing a benzene group are used

22 20 Steric Stabilization as such moieties. The selection of polymer moieties depends on the dispersion media used. Usually, copolymers are more effective in steric stabilization because they consist of more than one type of repeated unit. ne type of repeated units can act as anchor; the other type can act in the moieties extending into the nonaqueous solution. However, if a homopolymer consists of repeated units which contain both the anchor and the moiety function, it can also stabilize the particles effectively. For example, the repeated units of polyacrylic acid contain both carboxyl groups and a long chain hydrocarbon.

23 21 5 Steric stabilization in preparing nanoparticles via modified emulsion precipitation In the modified emulsion precipitation method for preparation of ceramic nanoparticles, metal oxide precursors tend to aggregate during the water removal step. These aggregates can not be destroyed by redispersing the precipitate in decane via ultrasonification. Aggregates with sizes of 5-50 µm quickly lead to sedimentation of the dispersions. Large contact areas between the particles indicate an early aggregation. To prevent aggregation at a high content of the dispersed phase in decane, steric stabilization by adding poly(octadecyl methacrylate) (PDMA) is used. 5.1 The structure of PDMA Molecular Formula: (-H 2 (H 3 )[ 2 (H 2 ) 17 H 3 ]-) n Molecular weight: 170,000 PDMA is a homopolymer whose repeated unit contains an ester group and a 18 hydrocarbon chain. The ester groups are supposed to bind to the hydroxyl groups on the particle surface through hydrogen bonding and act as anchors. The hydrocarbon chains extend into the decane and act as the mioeties. 5.2 The selection of PDMA as steric stabilizer To select a suitable polymer, the following criteria must be fulfilled: the stabilizing moiety of the copolymer should be soluble in decane and should not contain species such as silicon that may end up as an unwanted residual in the final microstructure. The anchoring group should not be soluble in decane. A further requirement is that the total polymer, added before or after emulsification, must be soluble in the decane phase and should not influence emulsion stability. From the literature study, we know that carboxyl groups in polymers could be effective anchors onto the ceramic particle surface in nonaqueous media. And since decane is used as the dispersion medium, long hydrocarbon chains could be good moieties and extend outward into decane. Thus a polymer which contains both the carboxyl group and the long hydrocarbon chain could be a suitable steric stabilizer in this case. Fatty acids satisfy this requirement and contain both the carboxyl groups and the long hydrocarbon chains. However, there is only one carboxyl group in each molecule which can acts as anchor, so the polymer is easy to desorb from the particle surface and cause aggregation. Poly(acrylate)s [A.11] and Poly(methacrylate)s [A.20] contain a carboxyl

24 22 Steric Stabilization group for each repeated unit so that they can strongly adsorb to the particle surface. The long hydrocarbon chain connecting to the carboxyl group can extend into decane and act as stabilizing moieties. Polyester is also a good candidate here. The ester groups are supposed to attach to the particle surfaces by hydrogen bonding and the hydrocarbon chains extend into the decane as the mioeties to stabilize the nanoparticles. Also, polyesters include one ester group in each repeated unit so that the polymer will strongly attach to the particles surface with many anchors. Poly(alkyl methacrylate)s are commonly used to stabilize particles in nonaquous media. They contain one ester group for each repeated units and the long hydrocarbon chains connecting to the ester group can extend into decane and act as stabilizing moieties. Poly(octadecyl methacrylate) [A.18] has (H 2 ) 17 H 3 connected with the ester group in each repeated unit and is used as stabilizer in our study. Experimental results show that the stabilization effect of PDMA is good in our case and it can form dispersions of nanoparticles in decane which are stable for several years. ther poly(alkyl methacrylate)s with longer hydrocarbon chain may also be considered. The longer the chains, the thicker the stabilizing layer outside the particle, possibly resulting in better stabilization. However, we must also keep in mind that longer chains may lead to bridging flocculation and increase the effective particle diameter which may lead to decreased packing densities. As mentioned before, copolymers are usually more effective steric stabilizers than homopolymers. So we can also try some copolymers with a carboxyl group or ester group in one repeated unit and a long hydrocarbon chain in the other repeated unit. For example, poly(methylmethacrylate-styrene) block polymers, poly(methylmethacrylatebutadiene) block polymers on poly(vinyl acetate-styrene) block polymers. However, copolymers are much more expensive than homopolymer. And the selection of the copolymers will also depend on their availability. In the study of Segar [28], various polymers were tested as a dispersant for Zn in decane and also for their effect on emulsion stability. Most technically applied dispersants were found to be insoluble in decane and to destabilize the w/o emulsion. nly PDMA forms dispersions of Zn in decane which are stable for several days and can be added to the emulsion before the extraction of water. The experimental results also show that the addition of PDMA drastically reduces particle aggregation.

25 5 Steric stabilization in preparing nanoparticles via modified emulsion precipitation The role of PDMA To better understand the role of PDMA in the steric stabilization of the nanoparticles in modified emulsion precipitation, it is necessary to understand the roles of other additives used in the entire preparation process and the interactions, if any, between these surfactants and PDMA. In the present study, non-ionic surfactant Arkopal 40 [A.31] is used to prepare w/o emulsions at the initial stage of the process. In addition, the ionic surfactant DiDAB [A.32] is used as co-surfactant to reduce the total concentration of surfactant required to prepare the emulsion. PDMA is added just Figure 5.1: Assumed model for the stabilization of the oxide particles in decane. before azeotropic distillation so that it can stabilize the nanoparticles after the water removal. According to Sager [28] and Woudenberg [29], the particles in decane are stabilized by a combined layer of PDMA and DiDAB, as schematically represented in figurefigure 5.1. In this model, the particles are covered with PDMA and surrounded by a bilayer of DiDAB. In the outer layer the hydrophobic tail of the DiDAB molecules point outward, which makes the particles dispersable in the oil phase. It is suggested that the PDMA/DiDAB layer is formed during water removal. In this model, both PDMA and DiDAB can be regard as steric stabilizers because it is the combined layer of PDMA and DiDAB which prevents the aggregation. Emulsion precipitation without PDMA also leads to a stable dispersion in decane. The yield is, however, small. It may be possible that DiDAB adsorbs with its polar head directly onto the particle surface and stabilize the particle in decane. But the monolayer formed by DiDAB is so thin that the particles are aggregate easier. Emulsion precipitation without DiDAB even can not lead to a stable emulsion since DiDAB is necessary in preparing w/o emulsions. After mixing with Ultra-Turrex, the emulsions become two layers during the process of decomposing HMTA. More experiments and studies are still needed to find out the exact stabilization mechanism.