Introduction:
The way of introducing chemical functional groups of inorganic particles such as TiO2 nanoparticles over a polymer core got much research attention than before. The main advantage of using nanoparticles as fillers is due to their unique size dependent characteristics such as electrical, magnetic, chemical and optical properties. These properties may vary with the particles' bulk phase. More importantly, nanoparticles aggregates more rapidly and the level of adhesion is more when compared to that micron size particles. Therefore, it is very important to develop techniques to control the dispersion of nanoparticles in the solvents to use them extensively in various applications. However, controlling the stability of highly concentrated nanoparticles is quite challenging in an organic media. Many researchers around the world had accepted surface modification of nanoparticles is one among the best methods to improve the dispersion stability of nanoparticles. Motoyuki Iijima and Hidehiro Kamiya had done two types of surface modification to maintain the stability of nanoparticles in an organic solvent. They are post synthesis surface modification, in which surface modification is done on manufactured particles and in-situ surface modification, where particles are modified during the synthesis(Kamiya 2009). Ma, H., M. Luo, et al.prepared a shell-core nano-composite using polystyrene/PNIPAAm stabilized by silica nano particles to treat cancer cells(Ma, Luo et al. 2010). Zhang, Wu et al. entrapped magnetic particles in an inner polymer shell and the outer shell is formed by SiO2. This double shelled capsules gives mechanical strength to the particles inside (Zhang, Wu et al. 2009).Yuyang Liu et.al formed polymer microspheres stabilized by titania nanoparticles using two stage pickering emulsion polymerization process. The emulsion formed has potential applications for photo-catalyst, water and air purification(Liu, Chen et al. 2006).
1.1 Project Objective:
In this research project, TiO2 nanoparticles are used to stabilize the latex particles. For this purpose some preliminary modifications has to be done on the surface of the TiO2 particles. At first, the surface of TiO2 particles is modified using a polar solvent and a long chain carboxylic acid, to make the particles disperse well in a non-polar solvent. Subsequently the particles in the nonpolar solvent are added to a polymer-initiator mixture. Polymerization is then carried out at around 60-700C overnight. To get an evenly distributed particle size and optimal properties the dispersion stability of the nano-particles in the emulsion has to be enhanced(Motoyuki Iijima 2009).
2. Literature Review:
2.1 TiO2 nanoparticles:
Titanium dioxide (TiO2) has three different crystalline forms namely, anatase, rutile and brookite. Practical applications involve degradation of organic, gaseous pollutants and removal of carbonaceous compounds. All the three polymorphs have various applications in different industries. The anatase type is widely used in photo catalytic applications such as antifouling, water treatment, anti-bacterial coating and aircleaning, because it oxidises when exposed to UV radiation. It is also non toxic and has good chemical stability. The rutile TiO2 is extensively used in paints, plastics, pharmaceuticals, food, papers and widely used for surface coatings. Rutile titania has high refractive index and therefore it is used for optical lenses and as a dielectric material. The brookite TiO2 is not widely used but it has similar chemistry of rutile and also has some similar properties like density and hardness. Brookite structure will relapse to rutile structure at higher temperature e.g.750 °C. To analyse the phase stability of rutile and anatase thermodynamically Banfield et al. conducted the synthesis of nano size TiO2 particles of anatase and brookite. The particles got changed into rutile structure upon coarsening,after attaining a certain particle size. They concluded that anatase is more stable than rutile when the particle size is less than14 nm. Hwu et al. in their work analysed whether TiO2 was rutile or anatase based on the particle size. The results obtained showed that at smaller particle size (less than 50 nm) anatase seemed to be more stable. The structure was changed to rutile as the particle size grew and the temperature is increased to 7000 C. So we can conclude that, the temperature at which anatase crystalline form changes to rutile depends on particle size(Lee, Wang et al. 2006).
2.2 Surface Modification of TiO2 nanoparticles:
The nanoparticles as mentioned earlier have the ability to conjugate more rapidly. The degree of adhesion of these particles are tend to be more than compared to that of the micron size. TiO2 nanoparticles are widely used in optical materials, therefore it is important to disperse the particles in the organic solvent without aggregating. Until now, several works had been done to modify the surface of TiO2 particles. For instance, Nakayama and Hayashi had effectively modified TiO2 with propionic acid and some variety of amine ( 2 step Process), the TiO2 dispersed well in the organic solvent compared to hexanoic acid or n-hexylamine (one-step method)(Nakayama and Hayashi 2008). Wu X et al. had modified TiO2with stearic acid by sol-gel methods and then characterized using X-Ray photoelectron spectroscopy. The results revealed that a chemical bond is formed between inorganic nuclei and organic surface layer(Wu X 2000). Tada et al. modified TiO2with SiOx monolayer and the results obtained showed good dispersion stability in neutral water with no change in the optical properties (Tada, Nishio et al. 2007). Similarly Weller et.al modified TiO2 particles with oleic acid and Mishra and Liu prepared C11-resorcinarene-capped anatase TiO2 nanoparticles to overcome the poor visibility factor of anatase TiO2 (Misra and Liu 2007). In all the above examples the dispersion stability of the TiO2 nanoparticles are improved with long chain alkyl surfactants. In some cases when the organic solvents are added the dispersion stability may be good but the refractive index of the particles will be lost. So the solvent used for modification should not change the properties of TiO2 nanoparticles as much as possible.
2.3. Polymer grafted TiO2 nanoparticles:
The surface of the nanoparticles can be modified using either 'grafting to' or 'grafting from' technique. The former needs pre-prepared polymer chains which should have an anchoring group. Nawaz Tahir et al. in their work used chelating dopamine anchor groups which is incorporated by TiO2 functionalized reactive polymeric ligands. These anchor groups allows the long polymer chain to attach onto the surface of the particles which eventually stabilizes the TiO2 particles in organic solvents. However, the surface chemistry of titania particles requires new surface initiated anchoring groups. Therefore highly reactive groups like phosphoric acid, carboxylic acid and catechol derivatives are needed to form strong covalent bond. Three methods have been proposed by Demetra S. Achilleos and Maria Vamvakaki for attaching long chain carboxylic acids groups on the TiO2 surface namely, the monodentate, the chelating and the bridging coordination mode. The selection of coordination method depends on the particle size(Vamvakaki 2010).
Fig. 1 Source: Behnaz hojjati et al. Synthesis and Kinetics of Graft Polymerization of Methyl Methacrylate from the RAFT Coordinated Surface of Nano-TiO2
2.4 Polymer Brushes:
The formation of polymer chains which are attached to a surface or interface at one end is known as polymer brushes (Milner, S. T, 1991). This methodology is adopted more nowadays because it has wide range of applications in stabilizing colloidal solutions. Polymer brushes can be formed using two methods namely, physical absorption and covalent bonding. In physical absorption one block of the polymer get absorbed on to the surface of the particle and the other block forms the brush. This method has some limitations such as thermal instabilities i.e the polymer chains formed are not well organised and leads to reaction instability. Thus in this project much focus is given on the covalent bonding techniques and hence they are discussed in detail.
2.4.1 Covalent bonding using 'grafting to' technique:
It is quite difficult to achieve high grafting density using 'grafting to' technique. This is mainly due to the steric crowding of already adsorbed polymers on the reactive sites. Therefore it is extremely difficult for the other monomer molecules to reach the reactive sites. Also the film thickness is low and it depends on the molecular weight of the polymer (Steve Edmondson,2003)
2.4.2 Covalent bonding using 'grafting from' technique:
In 'grafting from' technique an initiator layer is formed on the surface of the TiO2 particles. Since the monomer can diffuse easily through the polymer chain reactive sites, high graft density thick brushes can be formed. Generally an initiator is anchored to the surface of the nanoparticles by covalent bonding or physical absorption techniques. Subsequently the initiators are activated and the monomers are added under given polymerization conditions. Finally polymer chains are formed on the surface of the nanoparticles(Xiaowu Fan 2006).
This technique is also known as surface-initiated polymerization. In this technique the in-situ chains will not create any hindrance for the monomer molecules. Prucker, O. and J. Ruhe reported polymerization of styrene by means of self assembled single layers of initiators covalently bound to silica gels. The results showed that to get high draft density and controlled polymer layers 'grafting from' technique is a good option(Prucker and Rühe 1998). Zhao B et al. justified that atom transfer radical polymerization can form wide range of brushes with exact molar mass, composition and architecture (Zhao B et al.,2005). Xiaowu Fan et al. prepared core-shell polymer nanocomposites by grafting PMMA from TiO2 nanoparticles using surface initiated polymerization technique. These results showed a successful path to prepare polymer functionalised metal oxide nanoparticles(Fan, Lin et al. 2006).
3. Research Methodology:
3.1 Preparation of pigments:
The TiO2 nanoparticles are polar in nature, therefore when they are dispersed in the non-polar solvents like dodecane, they get agglomerated. So in order to disperse them well in hydrophobic solvents like dodecane, the particles' surface has to be modified. The reactive group (OH) present in the particle is therefore reacted with any long alkyl chain carboxylic acid or long chain silane derivatives in the presence of a polar solvent(e.g. DMSO). The reaction is usually carried out overnight at around 75-800 C. This makes the particles less polar and can disperse well in the hydrophobic solvent. In order to find the right combination for the particle modification, different carboxylic acids and solvents have to be examined. The good modifier is the one which disperses the particles well and should have minimum particle diameter.
3.2. Initiation of the Monomer:
The monomer will be chosen based on two aspects,
Solubility in the monomer phase
Immiscible with the oil-continuous phase.
The monomer will be initiated by means of an initiator. Initiator is a compound which forms free-radical entities (I*) when decomposed. During the initiation process the species I*adds to the monomer unit (M) and forms IM*. Subsequently other monomers will get added to the chain. Jui Hung Chenet et al. used AIBN and KPS initiator to synthesis ZnO/ polystyrene composite particles. When the results were analysed, the crystal structure was the same but the morphology was different, this was due to the difference of degree of hydrophilicity. This shows the initiator plays a vital role in defining the morphology, so initiator will be chosen with respect to the morphology required at the end of the process.
3.3. Emulsion formation:
The monomer phase with initiator dissolved will be dispersed into the oil-continuous phase. As soon as the monomer is dispersed, the pigments particles self assembles at the interface. This process will be effective only if the formed monomer droplets have enough pigment particle surface coverage. Chaoyang Wanget al. used XPS (X-ray photoelectron spectroscopy) technique to determine the surface coverage. Similarly, field-emission gun scanning electron microscope (FEG-SEM) is used by Stefan A. F. Bon and Patrick J. Colver to determine the surface coverage when Laponite RD clay particles are used to stabilize styrene(Colver 2007). If the surface coverage is found to be inappropriate, experiments will be performed with different concentration of monomer and pigment until the expected result is achieved. This process is performed at the room temperature (250C).
3.4. Polymerization:
Polymerization occurs subsequently after the emulsion is formed. Polymerization proceeds in the polymer particles as the monomer concentration in the particles is maintained at the equilibrium level by diffusion of monomer from solution. The system is usually degassed using nitrogen and it will be carried out at a suitable temperature between 50-700C overnight. The process will be repeated with different monomer concentration to get optimum result.
Fig.2 Schematic representation of TiO2 stabilized organic-inorganic hybrid hollow spheres using Pickering emulsion polymerization. (Tao Chen 2007)
3.5. Characterization:
There are many techniques used to characterize the polymer colloids. The properties like nature of dispersion of solid content, colloidal stability, surface charge of polymer colloid and particle size distribution are measured using Dynamic Light Scattering technique (DLS), Capillary Hydrodynamic fractionation (CHDF) and electron microscopy (TEM & SEM). The particle morphology is measured via electron microscopy and Nuclear Magnetic Resonance spectroscopy (NMR). Thermal properties like glass transition temperature and crystalline melting temperature are measured using Differential Scanning Calorimetry (DSC). Mechanical behaviour is measured by tensile testing or dynamic mechanical spectroscopy. Molecular weight and molecular weight distribution is measured by gel permeation chromatography (GPC).(Eric S. Daniels 2001). In this project characterization is based on the size of the particle, therefore Zeta- Sizer is used to determine the diameter of the particle.
Experimental Section - Surface Modification
The dispersion stability of TiO2was studied using various solvents like DMSO (dimethyl sulfoxide, chloroform, dichloromethane and IPA (isopropyl alcohol). The dispersed solution was characterised using Malvern zetasizer. When the particle diameter was analysed only DMSO and IPA showed good dispersion stability. Therefore the surface modification was done using DMSO and IPA.
4.1. Surface Modification with Decanoic acid:
Commercial TiO2 (P25), dimethyl sulfoxide (DMSO, (CH3)2 SO) and decanoic acid were purchased and used without further purification.
At first, 0.044 g of TiO2 (P25) was dispersed in 40ml of DMSO. The mixture was sonicated for 10 minutes to disperse the particles well in the solvent. The contents were then transferred to a 250 ml round bottomed flask. Secondly, 1.1 g of decanoic acid was dissolved in 10 ml DMSO and mixed with the contents in the round bottomed flask. The solutionwas purged with nitrogen for 10 min to degas the solution. The mixture was then stirred and heated at 800 C in an oil bath for 24 hours. After the reaction got over, the particle size was determined using Malvern zetasizer. 1 ml of the sample was then centrifuged using Eppendorf centrifuge for 10 minutes at 10000 rpm to separate the particles from the solvent. The excess solvent was dried in a vacuum oven overnight at room temperature. To the dried modified particles 1 ml of dodecane was added and sonicated for about 20 minutes. The particle size in dodecane was then determined using zetasizer.
DMSO
800C , 24 hours
CH2 - (CH2)7 - CH3
O
OO - Ti- OH + OH
O
O- Ti - O
O
CH2 - (CH2)7 - CH3
4.2. Modification with Dodecanoic acid:
Commercial TiO2 (P25), Isopropyl alcohol (IPA, (CH3)2CHOH) and dodecanoic acid were purchased and used without further purification.
At first, 0.031 g of TiO2 (P25) was dispersed in 31.44g of IPA. The mixture was sonicated for 10 minutes to disperse the particles well in the solvent. The contents were then transferred to a 250 ml round bottomed flask. Secondly, 0.393 g of dodecanoic acid was dissolved in 7.86 g IPA and mixed with the contents in the round bottomed flask. The solution was purged with nitrogen for 10 min to degas the solution. The mixture was then stirred and heated at 780 C in an oil bath for 24 hours. After the reaction got over, the particle size was determined using Malvern zetasizer. 1 ml of the sample was then centrifuged using Eppendorf centrifuge for 10 minutes at 10000 rpm to separate the particles from the solvent (IPA). The excess solvent was dried in a vacuum oven overnight at room temperature. To the dried modified particles 1 ml of dodecane was added and sonicated for about 20 minutes. The particle size in dodecane was then determined using zetasizer.
O
IPA
CH2- (CH2)9 - CH3
O
800C , 24 hoursO - Ti- OH + OH
CH2 - (CH2)9 - CH3
O
O- Ti - O
O
4.3. Modification with dodecyl trimethoxysilane:
Commercial TiO2 (P25), Isopropyl alcohol (IPA, (CH3)2 CHOH) and dodecyl trimethoxysilane were purchased and used without further purification.
At first, 1.672 g of TiO2 (P25) was dispersed in 31.44 g of IPA. The mixture was sonicated for 10 minutes to disperse the particles well in the solvent. The contents were then transferred to a 250 ml round bottomed flask. Secondly, 0.379 g of dodecyl trimethoxysilane was dissolved in 7.86 g IPA and mixed with the contents in the round bottomed flask. The solution was purged with nitrogen for 10 min to degas the solution. The mixture was then stirred and heated at 780 C in an oil bath for 24 hours. After the reaction got over, the particle size was determined using Malvern zetasizer. 1 ml of the sample was then centrifuged using Eppendorf centrifuge for 10 minutes at 10000 rpm to separate the particles from the solvent (IPA). The excess solvent was dried in a vacuum oven overnight at room temperature. To the dried modified particles 1 ml of dodecane was added and sonicated for about 20 minutes. The particle size in dodecane was then determined using zetasizer.
OCH3
IPA
800 C, 24 hrs
O O - Ti- OH + H3CO - Si - CH3 (CH2)10 CH2
OCH3
OCH3
O O- Ti - O = H2C - Si - CH3 (CH2)10 CH2
OCH3
5. Project Plan:
6. Concluding Remarks:
Finally the modified nanoparticles in the organic solvent are used to stabilize the latex particles as the result of polymerization process. The chosen non-polar solvent can therefore redisperse TiO2 nanoparticles at their primary particle size. On successful completion of this project, a detail explanation can be given on the best surface modification combination (carboxylic acid and solvent) for dispersing nanoparticles in a non-polar organic solvent.
