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'Gels are semisolid systems comprising small amounts of solid dispersed in relatively large amounts of liquid ,possessing more solid-like than liquid like character'. The gel is the one which is easier to recognise than to define1. 'Hydrogels are cross-linked network of hydrophilic polymers'. They possess the ability to absorb large amounts of water and swell, while maintaining their three-dimensional (3-D) structure1. They may be described as aqueous gels.
Polymers are both natural and synthetic, occur in different forms. They are classified into 4 type based on their macroscopic structure. They are
- linear type polymers
- branched macromolecular
- macroscopic network
Microgels intermediate between the branched and macroscopically cross-linked systems2. Because of their sponge like structure they shows swelling nature in the suitable solvents, this swelling is limit and depend on degree of cross-linking. Microgels are insoluble and do not form solutions as such but considered as colloidal dispersions3. Microgels are intra molecularly cross linked polymers. Microgels exhibit swelling nature, this nature in turn depends on the nature of solvent and degree of crosslinking. The IUPAC nomenclature recommended 'micronetwork' as it's synonym for microgels. Microgels are in the range of nanometres, even though micro represents the dimensions above one micron.
Pelton suggested microgels dispersion in the range of having diameter between 50 nm to 5µm4.
The first attempt to prepare microgels was made in 1930's. Staudinger and Husemann were tried to prepare microgels and succeeded, they polymerised divinylebenzene in very diluted solution at 60 c temp. For several days and expected for colloidal molecule of globular shape, but unfortunately they got solution of low viscosity and high molecular weight ,finally concluded that the product consists of strongly branched three dimensional polymer molecule4.
Structure of N-isopropylacrylamide [poly(NIPAM)]
Poly(NIPAM) is the polymer of N-isopropylacrylamide. It is the major building block used for temperature sensitive microgels. It's structural properties resembles the acrylamide. This polymer shows critical volume phase transition temperature in water around 32-340c.Above the VPTT temperature polymer shows phase separation and below VPTT it soluble in water.
METHODS FOR PREPARING MICROGELS:
- Emulsion Polymerization
- Inverse Emulsion Polymerization
- Radiation Polymerization
- Living Free-Radical Polymerization
Emulsion polymerisation is the most efficient way to prepare microgels. This method yields microgel particles with diameters of less than 150nm5. But there is a problem in this method i.e. the complete removal of residual surfactant is not possible. This problem can overcome by SFEP (surfactant free emulsion polymerisation) method. SFEP does not suffer from residual surfactant contamination. The particle nucleation period is very short in the SFEP, which ensures a narrow particle size distribution. So SFEP method is ideal technique to the preparation of poly (NIPAM). SFEP method is standard synthesis method for NIPAM based microgels and obtained microgels were rather spherical and exhibited reversible thermo-sensitive behaviour. The size, morphology, swellability and phase transition behaviour of the microgels were dependent in the initial co polymer or precipitation temperarture, monomer concentration.
This reaction includes 3 steps
Initiation: sulphate radicals(I*) reacts with a molecule of the monomer (M), produce new radical
I0 +M ?M0
Propagation: monomer molecules then add on to this free radicals
This process will be continued until the number of units in the chain (or) is several hundred (or) in several thousand
Termination: the above step terminates when the free radical is destroyed in one way or another. Here two un paired electrons become paired.
INVERSE EMULSION POLYMERIZATION
This technique was developed by neyret and Vincent for the synthesis of microgel particles. The cationic 2- methyl -acryloyloxy-ethyl-trimethylammonium monomers and oil phase consisted of anionic 2-acrylamid-2-methyl propane sulphate an addition to the cross linker N,N-methylene bis -acrylamide(BA).The copolymerization reaction was initiated using UV irradiation and the product isolated and redispersed in aqueous electrolyte solution to yield poly microgel particles. Water swellable microgels i.e. monomer whih dissolves only in aqueous media can be prepared by this technique. Poly (di methyl acrylamide-co-2-acrylamide-2-methyl-1-propane-sulphonic acid) cross -linked micogel particles prepared by this technique.
Radiation has been used to synthesis microgels. It has two advantages, additive-free initiation and easy process control. An aqueous solution of linear poly(acrylic acid) (PAA) was exposed to pulse irradiation produced by fast electrons. The irradiation energy facilitates the formation of PAA radicals, and these radicals undergo a major reaction path of intramolecular recombination. This reaction led to an interlinking process within the polymer molecules, and hence the formation of nanogel particles5.
UV radation has been widely used in the preparation of magnetic microgels. Acrylamide monomers are mixed with Fe3O4 nano particle dispersion. BA was added as the cross-linking agent. The solution was then exposed to UVlight.The reaction carriedout at room temperature.The final product contains magnetic core-shell nanoparticles containing Fe3O4 as the core.
Free radical polymerisation:
Nitroxide living free radical polymerization methodology is used to prepared microgels which are soluble cross linked polymer networks. When heated in the presence of 1, 4-divinyl benzene and tetra-butyl styrene. The alkyl amine derived from 2, 2'-azo isobutyronitrile (AIBN) offered a high molecular weight soluble polymer which was shown by size exclusion chromatography in conjugation with an online multi angle laser light scattering (MALLS) detector to have microgel properties.
Microgels whose diameters range in the nano size have been synthesized by precipitation copolymerization of 4-nitrophenol acrylate (NPA) with methacrylamide (MCAM) and NIPAM to produce poly (NPA-co-MCAM) and poly (NPA-co-NIPAM) microgels.
The collapsed microgel particle swells in good solvent, previously buried segments become accessible to the continuous phase. The other important characteristic feature of the microgel is their rapid swelling and de-swelling kinetics in comparison to macrogels. Bulkgels (macrogels) and microgels have similar polymeric chemical properties, but their physical molecular arrangements differ them from each other. The major differences between microgels and macrogels are illustrated in the following diagram.
- MICROGEL STRUCTURE :
- PARTICLE SWELLING BEHAVIOUR :
- RHEOLOGICAL PROPERTIES :
- OSMOTIC DESWELLING:
- ELECTROPHORETIC MOBILITY :
- EFFECT OF TEMPERATURE :
The significant features of the microgels are their internal structure, which determines their swelling properties, which also have considerable importance. The main structural parameter is the distribution of the crosslinking monomer as a functional of distance from the particle center although swelling theory implies a uniform cross-link density. However microgels produced by SFEP , cross-link density decreases from the centre to of the particle towards the periphery.
The swelling property of the microgel mainly depends on the presence of cross-linker concentration used in the preparation. Large amount of the crosslinker will result in the large microgel particle having a tight compact structure, with the centre being difficult to penetrate. Higher concentration of cross-linker will cause a broadening of the VPTT. As we use low concentration of cross-linker thus produced microgels particle have properties resembling high molecular weight polymers, i.e. macrogels. So we have to be careful while preparing the microgels i.e., the optimum concentration of the cross-linker must be used.
Microgels upon applying stress shows both viscous and elastic properties, these characteristics can be studied by viscoelasticity. This type of viscoelastic properties are commonly observed in melts of polymers, concentrated polymer solutions and concentrated colloidal dispersion. Since aqueous poly(NIPAM) is sensitive to temperature,it would therefore be expected that deformable nature of microgel particle has important for their rheological properties. If we heat the dispersion above the VPTT the dispersion become less elastic and displays the rheological characteristics typical of any particulate dispersion.
Siegalff , was the first person who reported osmotic de-swelling mechanism for microgel particle for polystyrene (microgel)/toluene/polystyrene (free polymer) system.He suggested that 'exclusion shell' for the free polymer would be produced around the microgel particle. Exclusion result when the polymer confirmation required to penetrate te particle interior become entropically unfavourable.
The Electrophoretic mobility and hydrodynamic diameter measurement are temperature dependent. Measuring of these properties leads to study of de-swelling mechanism and structure of poly (NIPAM) microgel particles. The Electrophoretic mobility is an important property of microgel particles that is frequently reported and is temperature dependant. Miller reported Electrophoretic mobility measurements for an organic swellable microgel system. They measured mobilities of approximately -4 x 10-10 m2 s-1 v -1 for polystyrene (ps) microgel particle dispersed in toluene using phase analysis light scattering (PALS) . Snowden as reported mobilities for poly(NIPAM) microgel particles dispersed in water in the range -1 to -3 x 10-8m2 s-1 v-1 at 25-400c.
The swelling nature of the poly(NIPAM) microgels is temperature dependant. Rapid swelling and de-swelling kinetics has received a lot of attention for both microgels and macrogels. These swelling natures will also sensitive to the size of the gels. Microgels reaches steady state swelling in less than a second when the temperature is changed whereas macrogels can take a long time. Because shrinking of the exterior layer prevents water transport from the interior. Microgels shows swollen confirmation below the VPTT as a result of van der walls forces and hydrogen bonding. Poly (NIPAM) microgels shows good solubility at low temperature, because of the hydrogen bonds formed above co-operatively form a stable shell around the hydrophobic groups. As the temperature increased these interactions become weak, the polymer solvent interactions become weak, the particle start to contract and entrapped water molecule released. When temperature reaches the VPTT or above these interactions are weakened or even destroyed and the hydrophobic interactions among the hydrophobic groups are strengthened which may lead to collapse of polymer chain. When particles are in collapsed state, they remain dispersed as a result of electrostatic repulsion between the charged groups on the microgel surface. The de-swelling of microgel particles is a reversible process on cooling and the microgel particles return back to their original structure.
An increase in temperature above the VPTT leads to a drastic shrinkage of the previously highly polymer gel. As a result, the physically bound water is released. The dependence of swelling transition rate on particle size for spherical poly (acrylamide) gel have been investigated by Tanaka. These swelling properties of the microgel made them to be used in various fields potentially.
Microgel particles have a high degree of sensitivity and are capable of undergoing rapid changes in their physical properties including their particle size and surface charge density. The rapidity of change comes about as result of microgel particle having high surface area to volume ratio which allow rapid diffusion of a given stimuli. This unique ability has allowed microgels to have wide range of application in various fields.
Thus microgel particles can act like "nanosponges" and offer many potential applications in medicine, environmental science, and in industries like paint industry, cements, oil recovery, molecular separation, enzyme immobilization.
Microgels are present in the natural rubber.
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