Developing Membranes with Water - Literature review Example

Developing Membranes with Water Name: Institution: Tutor: Date: 1. Chitosan membrane Chitosan membrane is defined as a plasma membrane perturbing compound that consists of ß-1, 4 linear chains-linked glucosamine residues that become positively charged at acidic pHs and it is mostly used as an antimicrobial compound (Ringbom, 1963). 1.1 Preparing chitosan Chitosan and chitin are obtained from shells of crustaceans e.g. crabs, lobsters prawns and shrimps i.e. the exoskeleton of insects and also from the cell walls of fungi e.g. mucor and aspergillus that provide strength and stability. Shrimp and crab shell waste are mostly used as the major industrial biomass source for chitin and chitosan large scale production. Processing of waste from the marine food factories usually helps to recycle the waste products and make the by-products for use. Crustacean shell wastes are usually composed of chitin, proteins, inorganic salts and lipids as the main structural components (Kaplan & Dordick, 1998). Therefore, the extraction of chitosan and chitin was mainly used by stepwise chemical techniques. Chitin production was linked to food industries e.g. shrimp canning in the first stage and in the second stage it was linked to fermentation processes which are similar to those used in citric acid production from Mucor Rouxii, streptomyces and Aspergillus niger which alkali treatment was involved to yield chitosan. Through grinding, the shells were reduced to smaller sizes and the minerals, which was mainly calcium carbonate were then removed by extraction i.e. demineralization or decalcification by using dilute hydrochloric acid and then stirred at ambient temperature.Deprotenisation, i.e. extraction of proteins was done from the residual material by treating it with dilute aqueous sodium hydroxide and thus preventing contamination of chitin products from the proteins (Kaplan & Dordick, 1998). The resultant chitin was then deacetylated in 45% sodium hydroxide at a temperature of 120 degrees Celsius for about three hours with the exclusion of oxygen and the purification procedures were followed to form chitosan that was of a cationic nature. The protein and deacetylated chitin were simultaneously removed by the alkali. This process shortens the length of the chain of chitin molecule and finally leaves behind a polymer called a chitosan with a complete amino group (Kaplan & Dordick, 1998). 1.2. Structure of chitosan The unit formula of chitosan is C6H11O4N.It has a primary amine and two hydroxyl groups, which are free, for each monomer.Chitosan, which is a natural biopolymer, is a glucosaminoglycan and it consists of two common sugars, N-acetylglucosamine and glucosamine which are both parts of mammalian tissues (Kaplan & Dordick, 1998). 1.3. Solubility of chitosan Chitosan is a weak base, a semi-crystalline polymer that is not soluble in alkali or aqueous, water and common organic solvents because of its rigid and stable crystalline structure (Kaplan & Dordick, 1998). Chitosan is polydispersed and can dissolve in certain organic and inorganic acids e.g. hydrochloric acid, lactic acid,phorsphoric acid,succinic acid,proponic acid, tartaric acid, citric acid, formic acid and acetic acid at certain pH values only after stirring for a long time. According to Ringbom (1963) chitosan could dissolve in nitric acid after which a white gelatinous precipitate would form. Chitosan does not dissolve in sulphuric acid because a reaction occurs in which chitosan sulphate, a white crystalline solid, forms. The solubility of chitosan depends on the concentration of these acids and their pKa values. 1.4 Functionalization of Chitosan Chitosan versatility enables it to be modified easily when changing its properties depending on the field that it has to be applied. Chitosan can be physically modified into beads, membranes and gels. Chemical modification of chitosan is the most common and it involves either cross-linking or grafting on to the chitosan backbone a specific group. This is done to improve the polymer selectivity or increase metal sorption capacity for a certain species (Dittrich & Bernhardt, 2006). When sorption is done in acidic solutions and soluble chitosan isn’t required, modification is also performed to prevent the polymer from dissolving. According to Dittrich & Bernhardt (2006), cross-linking is done by using cross-linking agents e.g. cyclodextrin, epichlorohydrin and glutaraldehyde.In the case of glutaraldehyde however, the uptake efficiency of chitosan might be reduced by cross-linking. Reaction of glutaraldehyde with chitosan amine groups leads to imine functional groups formation thus reducing the number of amine groups. Ethylene glycol and epichlorohydrine can also be used because they react with –OH chitosan groups, the residual amine functional groups used for binding should not therefore be affected. The sorption capacity of the chitosan depends on the cross-linking extend and it usually decreases with the increase in cross-linking extend. Nonofibrous scaffold supports which are used in forming membranes of the present discovery may be: applied to a substrate to form membranes; utilized to form membranes by themselves; coated with a functionalized nanofiller/polymer to form membranes; or used in combination with both a functionalized nanofiller/polymer coating and a substrate to form a membrane (Dittrich & Bernhardt, 2006). In some cases, fluid soluble polymers may be cross-linked e.g. water soluble polymers like,polyalkylene oxides, polysaccharides e.g. chitosan, polyvinyl alcohol and their derivatives in order to make these polymers suitable to be used as hydrophilic nanofibrous scaffold (Dittrich & Bernhardt ,2006). Cross-linking is conducted using techniques that are within the range of those who are skilled in the art, including cross-linking agents. Suitable cross-linking agents are C 2 -C 8 monoaldehydes which has an acid functionality, C 2 -C 9 polycarboxylic acids and C 2 -C 8 dialdehyde. These compounds are able to react with a minimum of two hydroxyl groups of a polymer that is soluble in water. Other suitable cross-linking techniques include photo-cross linking and conventional thermal-radiation. The two important criteria used in selecting cross-linking method or agents are such that: the nanofibrous scaffold layer should not be dissolved in the cross-linking method or agent; and large dimensional change should not be induced by the cross-linking method or agent Dittrich & Bernhardt (2006) argue that it may be advisable to add another solvent-miscible liquid or a surfactant to the polymer solution in order to create the nanofibrous scaffold so as to reduce the surface tension of the solution which stabilizes the polymer solution during the process of electro-blowing, electro-spinning, e.t.c. Suitable surfactants include Triton X-100, glycerol monostearate, polyoxyethylene, dimethyl alkyl amines and methyl dialkyl amines, e.t.c. Where it is utilized, the amount of surfactant present is usually from about 0.001 to about 10% by weight of the total polymer solution. A solvent mixture is formed by the solvent miscible with the solvent and the mixture can dissolve the polymer but it changes the rate of evaporation of the solvent mixture and the polymer solution’s surface tension. Where it is utilized with both a substrate and a functionalized nanofiller/polymer coating, the nanofibrous scaffold forms the middle layer of the membrane which is a three-tier membrane and it possesses structures that are similar to the structures of melt-blown substrates except the fiber diameters. The interconnected void volume and the smaller pore sizes that these membranes possess, is used as scaffold in supporting thinner membrane layer i.e.,nano-filtration,for ultra-filtration and coating with a lot of improved throughput (Dittrich & Bernhardt ,2006). According to Dittrich & Bernhardt (2006), the nanofibrous scaffold can be fabricated by electro-blowing, electro-spinning and solution blowing technologies. Electro-blowing uses gas-blowing shear forces and electric force and in order to achieve the required spin-draw ratio, the gas-blowing force is the dominating factor while the electric force usually enables further elongation of fiber. Electro-spinning processes only use electric force without gas flow assistance. In contrast, solution blowing processes utilize only the gas flow with no use of electric force. In one particular case, the middle layer e.g. PVA of PAN can be electrospun on a substrate like a non-woven PET micro-filter using several methods. Solution blowing is a similar method to melt blowing only that a polymer solution is used to fabricate the scaffold instead of a polymer melt. Such methods includes the formation of a blowing agent and a polymeric material in a single phase, usually a liquid, and then sprayed using conventional equipment that is similar to the one used in electro-blowing but the difference is that an electrical field is not used in spraying the liquid (Dittrich & Bernhardt ,2006). Dittrich & Bernhardt (2006) argue that the nanofibrous scaffold may be put under plasma treatment in order to increase its adherence to a coating layer and/or a substrate in forming a membrane. Plasma treatment methods include, e.g., atm. pressure plasma treatment of woven fabrics. This method is an effective means for improving the wettability and also fiber surface affinity for dyeing, substrate adhesion and chemical grafting. Plasma activation produces free radicals and/or functional groups on the surface of the fiber which may react with other molecules. Suitable materials used in coating of the membranes include chitosan grafted with polymers e.g. polyethylene glycol in order to produce PEG-grafted chitosan, cross-linked PVA,cellulose derivatives, cross-linked PEO,copolymers and their derivatives (Dittrich & Bernhardt ,2006).Also, cross-linking agents that may be used to crosslink the water soluble polymers like PEO and PVA include,glyoxal,glutaraldehyde,glyoxylic acid,formaldehyde,nitric acid and oxydisuccinic acid (Dittrich & Bernhardt ,2006). A three-tier composite filter has several advantages as compared to commercially available filters (Dittrich & Bernhardt, 2006). The top layer which is usually very smooth and thin can reduce the accumulation of surfactant molecules, dirt particles and oil at the filter surface and it also facilitates these contaminants removal by pure water washing and by solution. According to Dittrich & Bernhardt (2006), with proper matching of the nano-structural and mechanical properties among the membrane, melt-blown substrate and electro-spun scaffold, low-fouling and high throughput filters have already been designed, constructed and then successfully tested. The present filtration membranes based on nano-fibrous scaffolds have showed major flux improvement of 5 to 10 times more flux than the commercial ultra-filtration devices. 1.5 Chitosan membrane using for treatment water For about three decades, chitosan has been used in the process of water purification. Chitosan holds oil mass together when it is spread over oil spills thus making it easier to clean up the spill (Clesceri & Eaton, 1998). Chitosan is used throughout the world by water purifying plants to remove grease, heavy metals, fine particulate matter and oils which cause turbidity in wastewater streams. Chitosan carries a positive charge because it has free amino groups available in it and therefore it reacts with many surfaces that are negatively charged. Chitosan reacts spontaneously with ionic polyelectrolytes to form water insoluble complexes. Chitosan is therefore an effective coagulating agent that is used for processing waste effluents and for the conditioning of sludge that is produced from the biological treatment of water. According to Clesceri & Eaton (1998), chitosan is also a decolourisation method used for all wastewater streams. Since chitosan has a unique molecular structure, it has a high affinity for many different classes of dyes, including direct, dispersed,acid,naphthol,reactive and sulphur dyes.Diffussion rate of dyes in chitosan is the same as that in cellulose. Heavy metal ions are removed through chelation by chitosan.Chitosan, the deacetylated product, has an amine functional group that reacts very strongly with metal ions and as a result research has been initiated into chitosan use in metal uptake. The degree of deacetylation will control glucosamine content and therefore free amine group fraction available for metal binding. These groups react more than acetamide groups that are present on chitin (Clesceri & Eaton, 1998). Also chitosan being soluble, the solubility of these groups differ in acidic solutions.Chitosan’s physio-chemical properties depends on so many parameters such as polymer weight, degree of deacetylation, e.t.c. According to Clesceri & Eaton (1998), the metal ions interact with the reactive amine groups in different ways e.g. by the total composition of the solution and electrostatic attraction or chelation depending on other parameters like pH.In the non-protonated form or at pH values that are close to neutrality, coordination transition metals e.g. Zn, Cu, Ni e.t.c form donor bonds with the free electron doublet which is on nitrogen. At low pH, where amine protonation takes place, the polymer will attain cationic groups that can bind the anions by electrostatic interactions. Chitin is used to decontaminate wastewater that contains plutonium and also methyl-mercury acetate contained in water. Application of chitosan removes arsenic drinking water that is contaminated. Chitosan is also effective in removing petroleum products and petroleum from wastewater. The deacidifying property of chitin is used in coffee industry and in the clarification of beverages e.g. beer, wine and fruit juices. Regenerated chitosan and chitin could be used for processes such as osmosis, reverse osmosis, dialysis, haemodialysis, desalination and microfiltration. Purification of portable water can also be done by using beds of flaked chitosan. The high sorption capacity for metal ions by modified chitosan can be useful in the treatment of effluents that are contaminated or in the recovery of valuable metals. A lot of chitosan derivatives are obtained with the intention of absorbing metal ions whereby new functional groups are included into the chitosan backbone. The new functional groups are included into the chitosan in order to change pH range for metal sorption, change sorption sites and to increase density of sorption sites. Grafting of carboxylic functions is regarded as a process by which the sorption property of chitosan is increased (Clesceri & Eaton, 1998). The main intention of these modifications is designing chelating derivatives to be used for the sorption of metal cations. 2.0. Carbon nanotube Carbon nanotubes are carbon structures that are crystalline with extraordinary chemical, electrical and mechanical properties that make them potentially valuable in a broad range of end use applications (Johnson, 1983). 2.1. Improving the mechanical properties and controlling the pores size of carbon nanotube sheets by intercalation of polymeric adhesives Both Single-walled carbon nanotubes SWNT and multiwalled nanotubes MWNTs are used as reinforcing agents in epoxy and polymer composites. Any load that is applied to polymer matrix is usually transferred to the nanotubes. This load transfer is dependent on the effective interfacial stress transfer that is at the polymer nanotube interface and which usually tends to be polymer reliant. This reinforcement method has had some success by providing increases in both Young’s modulus and hardness (Berhan, et al, 2004). Through soaking of the sheets in polymer solutions, organic polymers like polyvinyl pyrrolidone, polystyrene and polyvinyl alcohols are intercalated into single-walled carbon nanotube sheets. Significant polymer intercalation is observed even for short soak times. According to Coleman, et al (2003), if tensile tests is carried out on the intercalated sheets it will show that the Young’s modulus and the strength increases by a factor of 3 and 9 respectively while the toughness increases by a factor of 28 thus signifying that the intercalated polymer improves load transmission between nanotubes. The reverse process of polymer intercalation can also be used in reinforcement of bulk nanotube materials. Binding agents like organic polymers are intercalated into the porous internal formation of nanotube materials like SWNT sheets Buckypaper.Carbon nanotube sheets Buckypaper can be used as a model system investigate the reinforcement effects of polymer intercalation on nanotube structures by soaking Buckypaper in solutions of polyvinylpyrrolidone PVP which is a polymer solution(Yu,Files,Arepalli and Ruoff, 2000). Zhu, Kim, Peng, Barrera, et al, 2003 argue that the mechanical properties of carbon nanotubes Buckypaper significantly improved after soaking the sheets in different organic polymer solutions. Increases of approximately 3 and 9 in Young’s modulus and tensile strength respectively were obtained. This is important since polymer intercalation either postproduction or in situ maximizes both the toughness and strength of high performance nanotubes fibers. Studies of polymer intercalation for two molecular weights as a function of time show that the diffusion mechanism for high and low molecular weight polymers. Mechanical studies also show that reinforcement is more efficient for high molecular weight polymer and this is shown to be connected to polymer conformation during diffusion and adsorption process. Single-walled carbon nanotubes are extraordinary materials with excellent mechanical properties. Exceptional Young’s modulus values of up to 1500 GPa and 50 GPa tensile strength have been proved.However, macroscale applications have been stalled as bulk nanotube materials comprise mainly of aggregated bundles which are bound together by van der Waals interactions which are weak. This results in considerable reduction in bulk mechanical properties as compared to those of individual tubes. Of late, progress has been made by use of polymer solution based processing in order to organize the nanotube containing powder into fibers and films in the form of functional macroscale composite materials.However, these improvements in the properties of nanotube based materials depend on dispersing nanotubes in a matrix that is polymer based (Choi, et al, 2003). Generally, the reinforcement nature is thought to be as result of the adsorption of polymer strands on internal surface of free volume inside the sheet. The thickness of polymer coating increases as more polymer intercalates. As this process continues, strands begin filling the space where adjacent ropes meet. Interfacial interaction is expected since polymer strands usually tend to crystallize on the nanotube. This according Johnson (1983) leads to high polymer-mediated inter-nanotube stress transfer as compared to polymer-free case, and also improved macroscopic mechanical properties. Since the minimum inter-rope separation is in the range of 0.34 nm and the average rope diameter is approximately 35 nm,the amount of polymer required to start reinforcing the narrowest inter-rope junction is estimated to be around 3%.This demonstrates that even at very low level of intercalation, the mechanical reinforcement in these systems is possible. It has been shown by Coleman, et al, 2003 that SWNT Buckypaper contains a good fraction of 60-70% free volume in form of branched pores which are one-dimensional. These pores have diameter distribution which varies from on to over a hundred nanometers. A non conjugated and amphiphillic polymer like PVP can be easily intercalated into these pores by simply soaking the Buckypaper in polymer solution at very high and also low molecular weights. The polymer diffusion coefficient can be calculated for each molecular weight through monitoring intercalated mass uptake as a function of time. After comparing the diffusion coefficients as a function of Mw, it is found that the larger polymer moves by reputation while the low weight polymer normally diffuses through the pores. This is important as it shows that the low molecular weight strands move as a random coil while the high molecular weight strand move in an extended state. As it is expected that this conformation will be retained on adsorption, it will be important for proper ties of the resultant composite (Smith, et al, 2000). Mechanical measurements show that the toughness, strength and Young’s modulus of the Buckypaper are all improved by polymer intercalation.However, improvement of both strength and modulus is more efficient for papers that are intercalated with high molecular weight polymer. In contrast, the scenario observed for toughness is opposite. This paradox in the mechanical properties is explained by the differences in polymer conformation i.e. “coil” for low Mw polymer and “extended chain” for high Mw polymer and also their capability to unravel and to bind under applied stress. This shows the significance of polymer conformation for reinforcement in nanotubes which contain composites. These results can be used to improve the properties of modern polymer nanotube fibers (Yu, Files, Arepalli and Ruoff, 2000). In conclusion, in near equilibrium conditions, polymer chains could be intercalated into porous nanotube sheets by soaking in polymer solutions. Significant intercalation has been shown for soak times which are as short as 2 h.SEM and density measurements also show that the intercalated polymer is adsorbed onto internal surface of the existing free volume that is within the sheets. In all the cases, it is shown through Raman spectroscopy that polymer strands have partly diffused between individual nanotubes inside the ropes (Berhan et al, 2004). Overally, intercalation results in the improvement of inter-rope stress transfer. This strongly modifies the mechanical properties of carbon nanotubes and provides increase in toughness, Young’s modulus and strength by factors of 28, 3 and 9 respectively. References A.Ringbom. Complexation in Analytical Chemistry. New York: Wiley-Interscience.1963. B.W.Smith,Z.Benes,D.E.Luzzi,J.E.Fisher,et al. Structural anisotropy of magnetically aligned single wall carbon nanotubes films. Appl. Phys. Lett. 77, 663–665.2000. E.S.Choi,J.S.Brooks,D.L.Eaton,et al. Enhancement of thermal and electrical properties of carbon nanotube polymer composites by magnetic field processing. Journal of Applied Physics.;94(9):6034-9:2003. J.N.Coleman, et al, Improving the mechanical properties of single walled carbon nanotube sheets by intercalation of polymeric adhesives. Appl. Phys. Lett. 82, 1682–1684.2003 J.Zhu,J.Kim,H.Peng,E.V.Barrera,et al, Improving the Dispersion and Integration of Single-Walled Carbon Nanotubes in Epoxy Composites through Functionalization. Nano Letters; 3(8):1107-13, 2003. L.Berhan, A.M.Sastry, E.Munoz, R.Baughman, et al, Mechanical properties of nanotube sheets: alterations in joint morphology and achievable moduli in manufacturable materials. J. Appl. Phys. 95, 4335–4345, 2004. L.D, Kaplan and S.J. Dordick. Enzymes in Polymer Science-An Introduction, Washington DC: American Chemical Society, 1998. L.S.Clesceri and A.D.Eaton. Standard methods for the examination of water and wastewater.Washington: American Public Health Association,1998. M.F.Yu, B.S.Files, S.Arepalli and R.S.Ruoff, Tensile loading of ropes of single wall Carbon nanotubes and their mechanical properties. Physical Review Letters; 84(24):5552-5,2000. P.R.Johnson, The most probable pore size distribution in fluid filter media. J. Test. Eval. 11, 117–121.1983. R .Dittrich and A.Bernhardt.Mineralized Scaffolds for hard tissue engineering by ionotropic gelation of alginate. Zurich, Switzerland: Trans Tech Publications Inc.2006. W.J.Boo, L.Sun, J.Liu, E.Clearfield, et al. Effect of nanoplatelet dispersion on mechanical behavior of polymer nanocomposites. Journal of Polymer Science Part B: Polymer Physics: 45(12):1459-69, 2007. Read More

1.3. Solubility of chitosan Chitosan is a weak base, a semi-crystalline polymer that is not soluble in alkali or aqueous, water and common organic solvents because of its rigid and stable crystalline structure (Kaplan & Dordick, 1998). Chitosan is polydispersed and can dissolve in certain organic and inorganic acids e.g. hydrochloric acid, lactic acid,phorsphoric acid,succinic acid,proponic acid, tartaric acid, citric acid, formic acid and acetic acid at certain pH values only after stirring for a long time.

According to Ringbom (1963) chitosan could dissolve in nitric acid after which a white gelatinous precipitate would form. Chitosan does not dissolve in sulphuric acid because a reaction occurs in which chitosan sulphate, a white crystalline solid, forms. The solubility of chitosan depends on the concentration of these acids and their pKa values. 1.4 Functionalization of Chitosan Chitosan versatility enables it to be modified easily when changing its properties depending on the field that it has to be applied.

Chitosan can be physically modified into beads, membranes and gels. Chemical modification of chitosan is the most common and it involves either cross-linking or grafting on to the chitosan backbone a specific group. This is done to improve the polymer selectivity or increase metal sorption capacity for a certain species (Dittrich & Bernhardt, 2006). When sorption is done in acidic solutions and soluble chitosan isn’t required, modification is also performed to prevent the polymer from dissolving.

According to Dittrich & Bernhardt (2006), cross-linking is done by using cross-linking agents e.g. cyclodextrin, epichlorohydrin and glutaraldehyde.In the case of glutaraldehyde however, the uptake efficiency of chitosan might be reduced by cross-linking. Reaction of glutaraldehyde with chitosan amine groups leads to imine functional groups formation thus reducing the number of amine groups. Ethylene glycol and epichlorohydrine can also be used because they react with –OH chitosan groups, the residual amine functional groups used for binding should not therefore be affected.

The sorption capacity of the chitosan depends on the cross-linking extend and it usually decreases with the increase in cross-linking extend. Nonofibrous scaffold supports which are used in forming membranes of the present discovery may be: applied to a substrate to form membranes; utilized to form membranes by themselves; coated with a functionalized nanofiller/polymer to form membranes; or used in combination with both a functionalized nanofiller/polymer coating and a substrate to form a membrane (Dittrich & Bernhardt, 2006).

In some cases, fluid soluble polymers may be cross-linked e.g. water soluble polymers like,polyalkylene oxides, polysaccharides e.g. chitosan, polyvinyl alcohol and their derivatives in order to make these polymers suitable to be used as hydrophilic nanofibrous scaffold (Dittrich & Bernhardt ,2006). Cross-linking is conducted using techniques that are within the range of those who are skilled in the art, including cross-linking agents. Suitable cross-linking agents are C 2 -C 8 monoaldehydes which has an acid functionality, C 2 -C 9 polycarboxylic acids and C 2 -C 8 dialdehyde.

These compounds are able to react with a minimum of two hydroxyl groups of a polymer that is soluble in water. Other suitable cross-linking techniques include photo-cross linking and conventional thermal-radiation. The two important criteria used in selecting cross-linking method or agents are such that: the nanofibrous scaffold layer should not be dissolved in the cross-linking method or agent; and large dimensional change should not be induced by the cross-linking method or agent Dittrich & Bernhardt (2006) argue that it may be advisable to add another solvent-miscible liquid or a surfactant to the polymer solution in order to create the nanofibrous scaffold so as to reduce the surface tension of the solution which stabilizes the polymer solution during the process of electro-blowing, electro-spinning, e.t.c.

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