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To study the flocculation properties of the dispersed poly-NIPAM microgel having multivalent electrolyte solutions and to identify whether they are stable and reversible overtime.
Microgels are discrete polymer particles which can respond to external stimuli like temperature, pH, ionic strength, etc. The most common microgel is the poly-N-isopropylacrylamide, which has the ability to be swollen and de-swollen in the presence of a solvent such as water.
The microgels have inter as well as intra cross linked structures. The poly (NIPAM) is formed mainly by surfactant free emulsion polymerisation (SFEP) technique. Normally the particle sizes of the microgels are varying from 250nm to 500nm in diameter.
1.1.1 Microgel differs from hydrogel.
Usually there occurs a common misunderstand between the microgel and hydrogel. Microgel is a derivative of hydrogel, that means hydrogel can be characterised into macrogel and microgel which is shown in the fig.1.1.
Fig.1.1 The major difference between macrogel and microgel (Kausar et al, 2007).
According to Kausar et al (2007); even though both the substance have the same polymer chemistry but different in their molecular orientation. Microgels are seen as discrete gel like particles while the macrogels are seen as a bowl of jelly. This discrete gel like property makes the microgels very useful in the various fields like paint industry, ink jet printing, oil recovery, etc.
1.1.2 History of microgels.
The microgels are almost new in the modern chemistry. According to Pelton (2000) the microgel was first discovered by a high school student called Philip Chibante in the year 1978. He prepared the cross linked poly N-isopropyacrylamide. But according to Saunders and Vincent (1999) the term 'microgel' was firstly given by W.O. Baker. More studies were done during the recent years about the physical properties, chemical structures, mechanism of synthesis and particle size of microgels.
1.1.3 Importance of microgels.
Microgels have a great importance in the modern chemistry, because of its intelligent nature. The microgels have the ability to change their conformation with temperature. As the poly (NIPAM) contains temperature sensitive monomers, when the volume phase transition temperature (VPTT) is reached a change in volume is seen. That means, as the temperature changes it can make or break the hydrogen bonds with water and become swells or de-swells, (fig 1.2). Thus water becomes a poor solvent in high temperature say above VPTT.
Fig.1.2 The swelling and de-swelling of a microgel particle with change in tempe-rature. (Kausar et al 2007).
Similarly the intelligent property is also seen as the pH of the solution changes. Kausar et al (2007) has reported that the poly (NIPAM) contains acrylic acid groups show high pH sensitivity. As the pH of the solution is greater than the pKa value (pKa = -log10 Ka, where Ka is the acid dissociation constant), then the hydrodynamic diameter is increased and vice versa, (fig.1.3). These intelligent properties of the microgels makes useful in different fields in the modern world.
Fig.1.3 The swelling and de-swelling property of a microgel particle with change in pH. (Kausar et al, 2007).
1.2 Synthesis of microgels.
Normally microgels are synthesised by using a monomer, cross linker and an initiator. The synthesis of poly-NIPAM is done by surfactant free emulsion polymerisation (SFEP) technique, using N-isopropylacrylamide as monomer, N,N'- methylenebisacrylamide as cross linking agent and potassiumperoxodisulphate as cationic initiator.
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Fig 1.4. Structure of N-isopropylacrylamide.
The N-isopropylacrylamide (C6H11NO) has three parts; a vinyl group (CH=CH2), a hydrophilic group (NH-C=O) and a hydrophobic group (CH3-CH-CH3) as shown in the figure 1.4. The vinyl group which help in the formation of a back bone for poly N-isopropylacrylamide polymer by breaking the double bond in the presence of the initiator potassiumperoxodisulphate (KPS) and the hydrophilic group help in the formation of H-bonds in the presence of aqueous solution while the hydrophilic group increases the polymer length by combining with other polymers.
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Fig 1.5. Structure of N,N'-methylenebisacrylamide.
The N,N'methylenebisacrylamide (C7H10N2O2), is the cross linker having two vinyl group on both sides as shown in the figure 1.5 which helps in the bridging of two adjacent NIPAM chains. This helps in the formation of continuous cross linked structures.
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Fig 1.6. Structure of potassiumperoxodisulphate.
1.2.2 Preparation of Microgels.
There are mainly 4 types of microgel synthesise.
126.96.36.199 Emulsion polymerisation.
Emulsion polymerisation is a special technique for the production of nano particles. The emulsion polymerisations are of two types.
Formation of microgels in the presence of surfactants.
This technique is called conventional emulsion polymerisation (EP). Normally surfactants like sodium dodecyl sulphate (SDS) are used. This technique helps to produce microgel particles having less than 150nm diameter. The main problem in this technique is the difficulty to remove the residual surfactant completely.
Formation of microgels in the absence of surfactants.
This technique is called surfactant free emulsion polymerisation (SFEP) or precipitation polymerisation. This technique overcomes the difficulty in emulsion polymerisation technique with surfactant (Dobie and Boodhoo, 2010; Yeole et al, 2010).
188.8.131.52. Inverse emulsion polymerisation (water in oil).
According to Gracia and Snowden (2007) Neyret and Vincent developed a new technique in the preparation of microgels called inverse polymerisation. In this technique, they added the oil phase consisted of anionic 2-acrylamide-2-methylpropanesulfonate and cationic 2-methylacryloxyethyltrimethy-ammonium-(MADQUAT) monomer along with the cross linker BA.
In the inverse emulsion polymerisation method (Gracia and Snowden, 2007), the co-polymerisation is initiated by the UV radiation and the products gets removed. This product is re-dispersed to get poly ampholyte microgel particles in an aqueous electrolyte media. Due to the attractions between the neighbouring chains (electrostatic), the particles become swollen.
The radiation polymerisation technique is used for the preparation of various microgels by the irradiation of polyacrylic acid (El-Rahim, 2005). This results in the production of PAA free radicals. These PAA free radicals which facilitate inter linking process of the polymer molecules and hence produce the microgels.
Biocompatible microgels are produced by the irradiation of high purified pluronic acid aligate (PHG) co-polymer by using γ radiation (Kausar et al, 2007). Since the γ radiations have an ability of sterilisation process; these microgels are highly used in the medical field.
Another example of radiation polymerisation is given by Ji et al (2005) in the irradiation of a mixture of acrylamide monomer, Fe3O4 nanoparticle dispersion using BA as cross linker under UV radiation. This reaction is carried out in room temperature results in the production of magnetic core-shell nano- particles having Fe3O4 as the core.
Living free radical polymerisation.
Living free radical polymerisation is a newly developed technique for the controlled polymerisation of vinyl monomers (Kausar et al, 2007). This technique is seen in the beginning of 1990's after the discovery of chain carrier radicals á·‰ R- (Korolev and Mogilevich, 2003). The importance of this technique is, this allows synthesise of almost all kind of polymers. This technique is very helpful for the synthesis of statistical microgels and star microgels using divinyl monomer than the traditional free radical polymerisation (fig.1.7).
Statistical microgel. Star microgel.
Fig.1.7 The schematic diagram illustrates the structural differences between statistical and star microgels (Kausar et al, 2007).
1.2.3 Surfactant free emulsion polymerisation (SFEP).
Surfactant free emulsion polymerisation is an emulsion polymerisation technique in which no surfactants are used. This is the standard method for the formation of poly-NIPAM microgels. Goodwin et al (2007) worked on SFEP to produce non-swollen polystyrene latex particle (Kausar et al, 2007).
In this method mainly monomers (NIPAM), cross linker (BA) and an initiator (KPS) are used. This technique normally gives a microgel particle size of 100 to 1000nm. The solvent having high dielectric constant like water is used.
The SFEP can be divided into 5 steps (Kausar et al, 2007).
In this process, the initiator gets decomposed into free radicals at about 60°c.
K2S2O8 2 (-SO4¯) + 2K+
After the formation of the free radicals, the vinyl monomers (M) combines with the free radicals to form oligomers.
M + -SO4¯ -MSO4¯
Formation of oligomers and particle nucleation.
More vinyl groups come joined to form oligomers which exceed the solubility limit of the solvent which act as a surfactant.
M + -MSO4¯ -M(x+1) SO4¯
Thus formed oligomers then undergo limited aggregation, thereby increasing the surface charge until the electrostatic stabilisation is achieved (Goodwin et al, 1973).
This is the final step of the microgel synthesis, which occurs by the absorption of monomer/or oligomers which results in the reduction of oligomers below the critical value for particle formation. Depending upon the concentration of the solvents, the size of the microgels varies. If the concentration of the solvent is high then form swollen particles and vice versa.
The different steps of a microgel particle growth are shown in the following diagram, fig .1.8.
Fig.1.8 Diagrammatic representation of the formation of microgel particles (Kausar et al, 2007).
Stability of microgels.
184.108.40.206 Structural and colloidal stability.
The stability of the microgels depends upon the balance between two forces (Saunders et al, 1999; Panayiotou et al, 2006; Pelton, 2000).
Van der Waals force
Steric and/or electrostatic repulsion.
When the medium with microgels comes below the critical temperature (VPTT), then the microgels become swollen and have water in it. Such a state the Van der Waals force has only a little influence and the microgel particle core is highly cross linked and a hairy diffuse like periphery. The charge group in the core as well as in the surface helped in the colloidal stability through electrostatic repulsion below the VPTT (Saunders et al, 1999).
In the case of above VPTT, the water inside the microgels comes out, which results in the increase Van der Waals force and hence the size gets reduced. These results in the increase charge density and produce the electrostatic repulsion which makes them colloidally stable, fig (1.9) (Panayiotou et al, 2006).
Fig.1.9 The schematic illustration of microgel colloidal stability below and above the VPTT (Pelton, 2000).
1.3 Characteristic features of microgels.
1.3.1 Swelling and de-swelling characters of microgels.
The microgels are mainly temperature sensitive particles. As the difference between the temperatures, the physical size as well as the behaviour of the microgel particles gets varied.
As the temperature changes the volume of the microgels get changes and this change occurs in a specific temperature. This particular temperature is called the volume phase transition temperature (VPTT). The VPTT is shown in the below graph fig (1.10).
Fig.1.10. The Volume phase transition Temperature.
Decrease the particle size of microgels with increase in temperature in VPTT. At low VPTT, the particle size is quite high, i.e. the microgel particles get occupied with solvent, here water which results weak Van der Waals force.
When the temperature increase above VPTT then the water moves out, increases the Van der Waals force and the fibres become more aggregate, fig (1.11).
Fig.1.11 The swells and de-swells of a microgel particle with change in temperature (Kausar et al; 2007).
As the microgel keeps in low temperature with the solvent, it becomes cloudy. As the temperature is going on increasing then the microgel becomes insoluble. Thus the temperature at which the phase separation of the microgel solution takes place is called the lower critical solution temperature (LCST) fig (1.12). Below the LCST there will be a strong hydrogen bonding between the solvent and the microgel particles. This results in the formation of microgel solution. This process is reversible.
Fig.1.12 The phase diagram of a system exhibiting LCST (Kausar et al, 2007).
1.3.2 Dynamic light scattering (DLS).
Dynamic light scattering is a technique used to determine the particle size, shape and interactions of colloidal dispersions like polymer in a solution. This technique is also called Quasi-elastic light scattering or Photon correlation spectroscopy.
In a solution, the particles are always in Brownian motion i.e.; the particles perform elastic type of movement and bombard each other (Villani, 2003). When a laser beam falls on the solution, some of the lights get absorbed and others get scattered and the scattered lights get fluctuates with rates of the bombardment of the particles and move more rapidly. By studying the fluctuations can find out the velocity of the Brownian motion thus find out the particle size (hydrodynamic diameter) using the Stokes-Einstein equation. This theory works in the DLS to find out the particle size of the dispersed particles in a solution.
The Stokes-Einstein equation is the derivative of Stokes law and Einstein law of diffusion. Thus Stokes-Einstein law can be represented as
Where; D is the diffusion coefficient, k is the Boltzmann constant, T is the absolute temperature, η is the continuous phase viscosity and RH is the particle hydrodynamic diameter (Sifaoui et al, 2007).
1.4 Techniques for the analysis of microgels.
Atomic Force Microscopy (AFM).
Fluorescence resonance energy transfer (FRET).
Differential scanning colorimetry (DSC).
Thermodynamic properties, VPTT.
Dynamic light scattering (DLS)/Photon correlation spectroscopy (PCS)/Quasi-elastic light scattering (QELS).
Hydrodynamic size, VPTT.
Gel permeation chromatography (GPC)
Weight or number average, molecular weight, poly dispersity.
High sensitivity differential scanning colorimetry (HSDSC).
Thermodynamic properties, VPTT.
Small angle neutron scattering (SANS).
Small angle X-ray scattering (SAXS).
Stability of microgel dispersion, VPTT.
Transmission electron microscopy (TEM).
Particle size, size, diameter.
Scanning electron microscopy (SEM).
Particle size, shape, diameter.
Table 1.1. Different techniques for the analysis of microgels.
1.5 The flocculation properties of microgels.
The stability of the microgels is explained in the section, where the steric repulsive force has a great importance for their stability. According to Kawanguchi (2000) the steric stabilization effect consists of both enthalpy and entropy effect. Since the microgels are very sensitive to temperature, as the temperature reaches to a particular point which affects the steric stabilization results in the aggregation of the polymer particles leads to the flocculation of the microgels. This particular point of temperature is called critical flocculation temperature (CFT). According to Rasmusson and Vincent (2004), the CFT of the microgel is decreased dramatically with increase in the concentration of NaCl. Thus these studies explained that the microgels (poly-NIPAM) having multivalent ions in certain concentrations have a great influence in the flocculation, CFT and the stability of the re-dispersed microgels once it is heated.
1.6 The applications of microgels.
There are so many applications of microgels in the modern world. The microgels are widely used in the field of pharmaceutical, biotechnological and in the medical fields. Some of the applications are listed below. The bio sensitive molecules have much more applications in biotechnology as well (Pichot, 2004).
1.6.1 Microgels for drug delivery.
Microgels have a tremendous role in the pharmaceutical field. Nowadays there are a lot studies are going on to produce target oriented drugs. The main problem in the drug therapy is to maintain the level of the plasma drug concentration within the therapeutic level. Microgels are very suitable for such kind of particular applications. According to Niidome et al (2010), the gold nanorods were entrapped within the thermal sensitive poly-NIPAM which can release the drug by converting the poly-NIPAM to hydrophobic due to the heat produced by the gold nanorods by the aid of laser beam.
The microgels help in the preparation of encapsulated dosage forms. Since the microgels can change their property with change in the pH; helps in the formation of sustained release formulations. They can remain stable for prolonged circulation in the blood stream (Oh, 2008).
The microgels have greater importance in the transdermal mode of drug administration. In the case of applying drugs in large patches may cause skin toxicity. This kind of problems can be avoided by using the smart materials. The microgels, due to the characteristic changes to pH or temperature, is incorporated with drugs can produce a sustained release to the wounded part and more over reduce the systemic intake (Kausar et al, 2007).
The microgels have potential importance in the delivery of protein based drugs and other bio macromolecules through oral administration. As the microgels are hydrophilic nature below the VPTT, can incorporate large amount of protein based drugs and can produce maintained bioavailability of bio macromolecular drugs (Malmsten et al, 2010).
1.6.2 In the oil industry.
The hydrophilic nature of microgels provides potential application in the removal of water content from oil. The study conducted by Nur et al (2008) using microgel in bio diesel has explained about the reduction of water in bio diesel to the acceptable level. The microgels can also use in the reduction of water from other oils as well. Moreover they can be used to control the water mobility from a long distance to the oil wells and thereby improve the efficacy of oil production (Rose et al, 2003).
Mainly microgels are used for the absorption or the entrapping of proteins in the biotechnology field due to its temperature dependent properties. The microgel's surface has greater hydrophobic nature when they come above the lower critical solution temperature and on that point they can adsorb a large amount of protein and vice versa (Kawaguchi et al, 1992). Pelton (2000) investigated that the above explained property of microgels happened when they reach at a temperature of 400C and at low pH.
Further studies are going on about the binding property of enzymes (proteins). The enzymes are active only on their optimum temperature, is above 300C that means normally above the VPTT of the microgels. Hence the microgels can be used for the entrapping of the enzymes, freeze-dried, can maintain the enzymatic reactivity for several months (Rubio-Retama et al, 2005).