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In past few years using of clay with nanocomposite polymers is frequently used as in developing of new materials which posses' good properties like improvement in thermal, electrical, mechanical and optical properties. This development has enhanced to use this combination of polymer nanocomposite and clays. The idea behind this combination is that organophilic substance is placed into interlayer gaps of layered silicates to weaken interlayer interaction. This increasing interlayer space allows macromolecules to enter into them during processing, causing uniform distribution.
Mostly clay used for this process is generated by ion exchange method in which exchange between metal cation present within clay with organophilic cation such as quaternary ammonium salts are used. An important characteristic of long hydrophobic chain is that it has more action against broad spectrum of microorganisms.
Aim of project
In this project, clay polymer will be used to develop polymer nanocomposite with antimicrobial properties. Clay polymers nanocomposite will be prepared using commercially available organic clays, and then they will be dispersed in quaternary ammonium polymer matrix which is active against both gram positive and gram negative bacteria. This will help in development of organoclays nanocomposite and will open a new method to prepare polymer materials with antimicrobial activity. The nanocomposite product formed by this method can be used as a barrier for oxygen and this can be used in juice bottles, dairy foods, beer and carbonated drinks and an also be used to enhance shelf life in food articles like cheese, processed meat, cereals, confectionary and boil in bag foods.
Preservation of materials from microbes is a major consideration now a day, to prevent its biodegradation and bio deterioration. Common methods which were used earlier were tanning, painting its surface and impregnation. But now this prevention of growth of microbes on surface had crossed boundaries of health related products in everyday life. Now due to increase in plastic industry it is challenging to protect its surface from microbial contamination. Global consumption of biocides in plastics was 15,500 tonnes in 2007. Plastic industry mainly uses surface protection method, which involves incorporating of biocide into its surface which will pass through material surface and kill microbes. But there is a problem of leaching there, keeping this in mind many permanent or till shelf life of material non leachable materials are being developed. The most suitable approach for self sterilising non leaching surface had come out from using cationic antimicrobials.
Clay polymer nanotechnology is one of the expanding fields in polymer nanocomposite. This concept was first developed late 1980s, and it was first commercialised by Toyota but it was in 1960's that research on clay nanocomposite start publishing. In this process two layer of silicates are used usually they are two dimensional usually 1nm thick and various microns in length, which usually depends on silicate which is to be used. This technique so known as organoclays or nanoclays, in which there is an ion exchange between layers of clay and organic cation. During this exchange surfactants which are used to improve dispersibility gets trapped into clays inner voids. Cationic surfactants are multifunctional in action, this trapping of surfactants in clay's voids increases distance between clay platelets and thus weakening the interaction in between the layers of clay. Also hydrophobic tails of surfactants improve its compatibility with polymer matrix. All these properties help in improving clay interaction with polymer molecules in a polymer matrix. This clay nanocomposite had excellent barrier properties which are due to increase in tortuosity of diffusion pathway for molecule to penetrate.
The commonly used clay filler is montmorillonite (MMT), which is hydrated alumina silicate layered clay, possessing octahedral layer of aluminium hydroxide in between two layers of silicate tetrahedral sheets. The imbalance between negative charges on surface is balanced by exchangeable cation. There is weak electrostatic force in between the parallel layers. Cation exchange capacity (CEC), which is moderate negative charge on surface, is an important parameter to define the clay type we are using. Also it helps in maintaining the equilibrium in layer spacing. Also this charge is not same, it varies from layer to layer and average of it should be taken for consideration.
Due to hydrophilic nature of clay surfaces it has become difficult to get homogenous distribution of clays in organic polymers. But the use of organoclays is an important improvement in the field of polymer chemistry. Proper organophilization is an important process to get clay distributed equally in polymer matrix. The process of organophilization helps in reduction of energy levels and thus improving compatibility of clays with the polymer. Also a compared with other nanomaterials, organic clays are cheaper because they are obtained from naturally available materials and can be produced easily in large quantities.
The clay layer forms a barrier to gases and water molecules thus pressuring them to pass through tortuous pathway. Also many researches had proved the effectiveness of these nanocomposites in reducing oxygen levels and water vapour permeability.
Nielsen theory based on tortuous path which is formed along clay molecules, making gas molecules to travel longer part for diffusion is widely accepted worldwide to explain barrier properties of clay molecules. The length of pathway depends upon ratio of clay filler and percentage of filler present in the polymer. Nielsen models forecasts that permeability of system in clay is less than 1% but the experimental results also show higher percentage in some polymers. According to Bharadwaj if we increase length of silicate sheets, permeability will increase itself due to increase in tortuosity. In 2000 Beall introduced a new model which helps in prediction of permeability of polymer nanocomposites. This model provides a correlation factor to Nielsen model and also focuses on polymer clay interface in addition to tortuous path. This model mainly defines three areas which are formed around clay molecule; the surface modifier region, constrained polymer region and unconstrained polymer region. The surface modifier region which is 1-2 nm in area helps in binding of clay with polymer. This region does not affect so much on permeability of nanocomposite. The other region which is unconstrained polymer region does not affect at all to the permeability of nanocomposites. Last region which is constrained region is not so well defined; this area may be up to 50-100 nm from clay surface and posses low diffusion coefficient. There is also improvement in mechanical strength in polymers due to use of clays in it, making them more feasible to use. Park lee in 2003 used thermoplastic starch and clay to produce nanoparticle which has increased mechanical strength and decrease in water vapour permeability. Other benefits like increased in glass transition had also been reported in performance of polymers where clays are used.
Companies like Nanocor Inc. and Southern clay products Inc. are working through out incorporating MMT in nanocomposite products. Making products stronger, heat resistant and having improved barrier against gases and moisture. Nylon 6 which is fluid in nature and penetrates easily in spaces between layers is commonly used by most of companies with clays to form polymer. Nylon 6 nanocomposite has four times oxygen transmission rate than simple nylon 6. Nanocor and Mitsubishi gas chemicals used nylon to form a polymer named nylon MXD6 which has enhanced barrier properties and is used in films and pet bottles.
The other method, known as layer by layer self assembly in which multilayer coatings which are nanometre thick are formed on a solid support by sequential adsorption of opposite charge. In 2008 Jang et al manufactured a multilayer film by deposition on sodium MMT and cationic polyacrylamide on pet substrates. Also there was decrease in oxygen transmission rate due to use of bi layers, thus can be more useful in packaging of food as compared with aluminium foils.
In spite of decades of this polymer clay nanotechnology, it is observed that organoclays based nanocomposite really perceiving antimicrobial nature, cationic surfactants and really has a potential to be used as an antimicrobial material. Some of the recent studies had shown the antimicrobial properties of clay polymer nanocomposite which are made from commercially available organoclays. Commercially available organoclays were used in various polymer matrixes like polyamide, polyurethanes, polylactide, whey proteins and quaternised chitosan, but nanocomposite which were not so efficient. Antimicrobial activity was obtained but to certain limit only, also there were experimental confirmation about the displacement of surfactants from nanocomposite polymer. However, the reason for decrease in antimicrobial activity is due to contact of composites with the solid surface. To overcome this issue, non leaching antimicrobial substance using clay polymer technology were developed. The new innovation in this process was usage of macro molecules containing quaternary ammonium salts in clay polymers. In this process during ion exchange multiple ionic bonds are formed between modifiers and clay particles. This bond formation prevents displacement of polymeric biocide from nanocomposite as well as organoclays. There is also decrease in viable count after coming in contact with the polymer surface and it is achieved as early as within 4 hours. It is active against wide spectrum of bacteria both gram negative well as gram positive bacteria and also there is decrease in biocide leaching.
Mechanism of action
Authors publishing papers so far had adopted cautious statement its mechanism of action is not interrupted, also there are not strong evidences to prove the hypothesis.
Generally, these are membrane bursting agents, which are being absorbed by the cell wall of microbes. On absorption they change the nature of cell wall due to penetration of long chain of alkyls inside membrane, due to hydrophobic interactions leading to destruction of membrane structures, finally causing cell lysis. Thus a broad spectrum of microbes gets affected by these polymers. Such deformation of cell morphology implies that integrity of plasma membrane is compromised upon contact with cationic surface. Fluorescent staining method is used to differentiate between live and dead cells and there are several studies which had used this method to differentiate between cells. Also after coming in contact with cationic surface there is loss of cellular components like DNA/RNA. These all observations are strong enough to prove the action of nanocomposites. Thus, positive charge on solid surface plays an important role in the antimicrobial activity of nanocomposite. The electrostatic force which occurs in between positively charged antimicrobial surface and negative charge of cell membrane helps in adsorption of microbes, thus helping in its antimicrobial action.
Kugler et al. during their study of surface density of quaternary ammonium compounds on poly vinyl chloride had observed that it show anti bacterial effect only at a certain content of pyridinium group which should be sufficient enough to get charge density threshold to show antimicrobial activity and also showed how ion exchange mechanism is also important.
Also the length of alkyl substitute in polymer also affects the antimicrobial activity. Penetration of macromolecular chain through cell membrane is dependent on the chain length and thickness of protective layers of cell surface, for this reason chain length of 50 nm is used which can easily penetrate through bacterial cell wall. The influence of molecular weight on antimicrobial activity is also shown in many studies. However, bonding of cation with bacterial cell wall results in multiple binding, thus alkyl chain is difficult to penetrate in the surface. Also once chain is attached it undergoes structural transformation, thus long chains helps to improve antimicrobial activity. But sometimes small chains also cause modifications if they are amphiphilic in nature. Action of polymer on the bacterial surface depends on molecular structure and contacting media.
In this project, poly vinyl benzyl chloride will be fully aminated with a tertiary amine which should posses one long alkyl chain. Then this macromolecule will be added in clay and its non leaching polymer nanocomposites will be formed. Organoclays with different quantity of antimicrobial polymer will be prepared and will be used to produce polyamide 6 nanocomposites. These nanocomposites will be checked for antimicrobial action against wide range of bacteria both gram positive and gram negative. Influence of organoclays composition, its antimicrobial action, mechanical properties and morphology of nanocomposite will be studied during this project.
Synthesis of fully aminated poly vinyl benzyl chloride
Synthesis of fully aminated poly vinyl benzyl chloride derivative will be prepared by the reaction between poly vinyl benzyl chloride and quaternary ammonium group in a certain ratio. This reaction will be carried out in round bottom flask with reverse condenser at 60 Í¦C for 24 hours with constant stirring. After the completion of reaction, product formed will be used for preparation of organoclays.
Preparation of polymerically modified organoclays
Commercially available clay will be used for this process. Clay suspension will be formed by addition of water to it with constant stirring at ambient temperature for overnight. This suspension will then be added to diluted solution of fully aminated poly vinyl benzyl chloride solution with constant stirring; there will be precipitates formation due to this addition. To complete the reaction process water is added to it. Then the slurry will be kept for 24 hours at ambient temperature. After the reaction is completed repeated centrifugation will be performed with constant washings. After final washing the material obtained will be dried in freeze driers.
Polyamide nanocomposite will be prepared by melt extrusion using twin screw extruder. This process will be carried out at fixed temperature, with a fixed length and diameter ratio, at a constant screw speed and feeding rate. Different series of nanocomposites will be produced using pre dried and pre mixed clay polymer keeping ration of clay and aminated poly vinyl benzyl chloride different in each series. After formation they will be pelletized and dried in vacuum oven. After drying they will be moulded; in bone shaped coupons for checking its tensile strength, in bars for dynamic mechanical analysis, in square samples for antimicrobial activity and XRD test will be done using injection modelling.
Evaluation of final products
XRD characterisation of organoclays and nanocomposite
The X ray diffraction characterisation of organoclays and nanocomposite will be carried out using Philips XPert Pro XRD.
Mechanical test of nanocomposite
The tensile strength of nanocomposites will be measured with the help of Monsanto Tensiometer. Due to limitation if sample size it is difficult to fit stain gauge to the sample while loading, so only strength data will be reliable. The stiffness and other mechanical properties will be studied using dynamic mechanical testing.
Assessment of antimicrobial activity
The antimicrobial activity of nanocomposites will be done according to the Japanese Industrial standard test method JIS Z 2801:2000 (antimicrobial product test for antimicrobial activity and efficiency). This method was specially formulated to for characterisation of antimicrobial properties of solid surface of nanocomposites including non leaching substances. For this process a cell suspension of gram positive and gram negative bacteria will be made of particular concentration in a nutrient broth. Samples of different series of nanocomposite and their control will be held in close contact with the help of sterile polyethylene film. The seeded nanocomposite and their controls will be incubated at 35 ÌŠC for 24 hours at saturated humidity. After incubation, the samples will be washed with nutrient broth and bacterial colonies that will be formed in suspension will be counted using pour plate technique.
The antimicrobial activity of prepared nanocomposite will be calculated by difference which will come in between number of colonies in surface with that of no of colonies in control.
Antimicrobial activity = log (C/A)
C = average number of colonies in control
A = average number of colonies in polymer suspension.
Time period for project
The project is to be finished in six months.