Synthesis and Characterization of Polyurea Microcapsules by Interfacial Polycondensation Techniques

Transcription

1 5(1), (January-June 2012), ISSN: , pp RESEARCH SCIENCE PRESS (INDIA) I J S C Synthesis and Characterization of Polyurea Microcapsules by Interfacial Polycondensation Techniques K. T. Jadhav* a and P. V. Vijaybabu b a Department of Chemical Engg., D. Y. Patil College of Engg. & Tech., Kolhapur, Maharashtra, India b Department of Chemical Engg., Dr. B. A. Technological University, Lonere, Raigad, Maharashtra, India Abstract: Interfacial polycondensation reactions, in which the two different types of monomers are brought together at liquid-liquid interface between two immiscible phases. This is important because it provides the possibility of fast reaction and production of high molecular weight polymers under normal temperature and pressure conditions. This paper described the polyurea microcapsules production by an interfacial polycondensation technique at different conditions of preparation. Generally these reactions are very fast as both the monomers are highly reactive and carried out at ambient conditions. Release of controlled quantities of active agents such as drugs, pesticides, herbicides and dyes play important role in various applications. Interfacial polycondensation method is used to prepare the dense and thin film composite membranes which are used for reverse osmosis, microfiltration and nanofiltration. The polymer formed is thermoplastic and semi crystalline in nature which having melting point of about C. The rate at which the polycondensation takes place has an important influence on the crystalline structure of microcapsules. It was generally assumed that the reaction rate is diffusion controlled. The formation of aqueous core polyurea microcapsules by interfacial polycondensation of a 2, 4-Toluene Diisocynate (TDI) in organic solvent and Ethylene Diamine (EDA) in water is studied. The aim was to determine the optimal conditions which allow the formation of microcapsules. Partition coefficients of various solvents are experimentally determined. The optimization of the system was considered in terms of monomer mole ratio, surfactant concentration, ph of the aqueous phases, solvent proportion and reaction time. The chemical structure of the microcapsules was studied by Fourier Transform Infra Red and Mass Spectrometry. Thermal properties of the polyurea microcapsules were also studied by Thermal Gravimetric Analysis. The morphology was observed by Scanning Electron Microscopy. Finally, Differential Scanning Calorimetry and X-ray Diffraction are carried out to know the properties and composition of polyurea microcapsules. Keywords: Interfacial polycondensation, encapsulation, microcapsules, crystallinity, optimization. INTRODUCTION Interfacial polycondensation is one of the most important techniques used for preparation of polyurea microcapsules. Various parameters, such as monomer concentration, diffusion and interfacial reaction, influence on the formation of microcapsule and polymer wall structure [3]. Encapsulation of drugs, pesticides, herbicides and dyes is finding applications in various fields. Interfacial polycondensation is an important method to prepare the dense and thin film composite membranes which used for reverse * correspondence ktjadhav1@gmail.com osmosis, microfiltration and nanofiltration. It can also be used for synthesis of ultra-thin polymeric films having catalytic functions [11]. The objective of this paper is to study the effects of various parameters on an interfacial polycondensation reaction between Toluene Diisocynate and Ethylene Diamine. Characterization of the chemical structure and properties of microcapsules prepared by the method of interfacial polycondensation has been the subject of extensive research in the past few years. In general, the process of interfacial polycondensation is complex in nature and involves multiphase transport [2]. Microcapsules

2 8 K. T. Jadhav and P. V. Vijaybabu formation is affected by many of the variables such as composition of organic and aqueous phases, partition coefficient of aqueous phase monomer, rate of stirring, surfactant concentration and the mole ratio of the reactants. EXPERIMENTAL WORK The monomers used in the interfacial poly condensation experimentations were Ethylene Diamine (EDA) of purity > 99 %, 2, 4 Toluene Diisocynate (TDI) obtained from Merck (Switzerland) of purity > 99%. These monomers are used without further purification. Cyclohexane is used as the solvent for the oil soluble monomer (TDI) and distilled water is used as the solvent for water soluble monomer (EDA). The other solvents used were Toluene, P-Xylene, Benzene, etc. Span 80 and Tween 85 obtained from Fulka, Switzerland is used as surfactant to stabilize the emulsions. Sodium Lignosulfonate is also used as emulsifying agent in few experiments to obtain better granular shape of the microcapsules. EDA is used as limiting monomer in all the experiments. All the chemicals purchased are of reagent grade [14]. The reaction is carried out in two steps; in the first step oil in water emulsion was prepared. In second step, an appropriate volume of this emulsion, depending on required phase volume ratio, was added to the aqueous phase of EDA contained in beaker. Reaction is carried out under mild stirring. ph probe is inserted in the beaker for data collection. EDA is alkaline in nature so addition of emulsion decreases value of ph preferably from 11.8 to 7. The reaction is considered to be complete when ph value becomes constant. After that stirring was stopped and microcapsules are separated. All reactions are carried out at temperature of C. The size distribution of microcapsules was determined in few of the experiments [12]. The reproducibility of the experiments was checked by repeating few experiments completely. Experiments were conducted for different values of phase volume ratio at 0.10 and Monomer mole ratio is to be varied from 1 to 10. The chemical structure of the microcapsules was studied by Fourier Transform Infra Red and Mass Spectrometry. Thermal properties of the polyurea microcapsules were also studied by Thermal Gravimetric Analysis. The morphology was observed by Scanning Electron Microscopy. Finally, Differential Scanning Calorimetry and X- ray Diffraction are carried out to know the properties and composition of polyurea microcapsules. In few experiments reproducibility of analysis steps are carried out separately for each of duplicates. PREPARATION PROCEDURE OF MICROCAPSULES The general two step procedure is carried out as follows by the preparation of polyurea under preferred conditions. In the first step, oil in water emulsion is prepared by using Toluene Diisocynate (TDI), cyclohexane, Tween 85 as the emulsifying agent and distilled water. The emulsification was carried out in a beaker for minutes by stirring at speed of approximately 3000 rpm with four bladed pitched turbine impeller (Remi Motors Ltd. Model with a 1 hp motor). By keeping the above conditions fixed, similar procedure has followed for all experiments to obtain same emulsion drop size distribution. In the second step, the required amount of this emulsion (depending on ratio of dispersed phase to continuous phase) was added to a beaker containing aqueous solution of EDA. Thus initiating the polycondensation reaction between EDA and TDI. The reaction mixture kept agitating using a magnetic stirrer. ph is measured on line to study conversion of product and for data collection [11]. The reaction leads to the formation of polyurea microcapsules at the interface. After that stirring is stopped and the final product formed is taken out from the beaker. The product is then washed with warm distilled water and methanol alternatively for 3-4 times and dried with the help of filter paper. The product is then sent to dryer for controlled drying temperature up to 60 C then product hence obtained is then sent for analysis. RESULT AND DISCUSSIONS Effects of various parameters are studied to obtain an optimal condition which allows formation of polyurea microcapsules. The whole the system was studied by considering various parameters

3 Synthesis and Characterization of Polyurea Microcapsules by Interfacial Polycondensation... 9 which includes ph of the aqueous phases, solvent concentration, stirring rate and reactant concentration [7]. Effect of ph Value of Aqueous Phase on Formation of Microcapsules The ph value is a one of the most important variable in the formation of polyurea microcapsules system, because it varies throughout during formation of microcaspules. At the beginning of the reaction, the ph is relatively high due to the presence of diamine in the aqueous phase. Then as the reaction proceeds, diamine get consumed, which decrease ph of aqueous solution. Ethylene Diamine is alkaline in nature so addition of emulsion decreases value of ph preferably from 11.8 to 7. The reaction is considered to be complete when ph value remains constant. Graph of ph and conversion of diamine vs. time is shown in fig. 1 for polyurea microcapsule sample which showed that conversion of diamine increases with decrease in ph of an aqueous phase [7, 11]. common organic solvents which are inert and immiscible with water are generally preferred in interfacial polycondensation systems. Aromatic and chlorinated hydrocarbons are particularly useful. Some experiments are conducted in various solvents to study certain aspects of process kinetics. Interfacial reaction involves various steps like diffusion of diamine into organic phase, reaction of diamine and diisocynate to form oligomers, precipitation of initially formed oligomer molecule then the growth of polymer membrane by diffusion of monomer. The precipitation of polycondensate and formation of primary membrane around the droplets is mostly controlled by organic solvent [1, 7, 10]. Partition coefficients of various solvents are determined by shake and flask method. The aqueous and organic phases are separated and titrated against HC l to determine the concentrations. Methyl orange is used as indicator to determine concentration. Different sets of experiments have been carried with various solvents like Cyclohexane, Xylene, Toluene and benzene. We found that Cyclohexane has highest partition coefficient with EDA solution. Therefore, cyclohexane is used as solvent for organic phase monomer for all the experiments. Figure 1: Graph of ph and conversion of diamine vs. time for S25 polyurea sample. (R = 9.0, Vd/Vc = 0.25) Effect of Organic Solvent on Formation of Microcapsules The selection of the organic solvent is one of the important parameter since it affect several other factors such as the partition coefficient of the reactants between the two phases, the diffusion of the reactants, reaction rate, and the solubility of polymer [8]. Partition coefficients of various solvents are experimentally studied. The solvent must be insoluble in the microcapsules formed as it may influence the physical properties of the product separated from the experiment. The Figure 2: Partition coefficient values of EDA in various solvents Effect of Stirring Rate on Formatio n of Microcapsules Stirring is one of the important variable in the formation of polyurea microcapsules by interfacial polycondensation technique. It is essential to maintain required stirring speed during emulsion formation and microcapsules

4 10 K. T. Jadhav and P. V. Vijaybabu formation. Four pitched blade turbine is used for uniform mixing. Since the strength of initially formed polymers is not so high, the stirring rate has to be reduced to minimize breakage of polymer film. If the stirring speed is reduced, the droplet size increases by small extent. The average diameter of microcapsules decreased as the stirring speed increases. Addition of Tween 85 to aqueous solution leads to an increase in microcapsule diameter and uniform distribution of microcapsules due to the changes in interfacial tension [9]. Effect of Monomer Concentration on Formation of Microcapsules Monomer ratio (R) is ratio of monomers present in organic phase to monomer in aqueous phase. It plays very important role interfacial polycondensation reactions. Generally monomer ratio, R varies between 1 to 10. The monomer ratio is influenced by the properties of the monomers, the organic solvent, and rate of stirring. If microcapsule formation is at concentrations of 10% and above, all of the liquid may be absorbed and then the mass cannot be stirred. This can leads to incomplete reaction so it should be less than 5% on the basis of combined weights of water and organic solvent [8]. The experimental result shows that as the amount of monomer increases, the size of the microcapsules also increases [11]. This phenomenon is may be due to the permeability of the polyurea microcapsules to the hydrophilic monomer. Operating variables used for preparation of polyurea microcapsules are given in Table 1. Table 1 Operating Variables used for Preparation of Polyurea Microcapsules Sample No. R Vd/Vc N Initial Rx. Rate S *10-7 S *10-6 S *10-6 S *10-7 S *10-7 CHARACTERIZATION PROCEDURES OF PRODUCTS Various methods are used to characterize the polyurea microcapsules samples, Fourier Transform Infrared Spectroscopy (FTIR) To identify and characterize the polyurea microcapsules Fourier s Transform Infrared Spectroscopy is used. It gives information of the vibrational and rotational energy of molecules. The infrared spectrum of microcapsules provides unique information, which is different from the absorption patterns of all other compounds; only optical isomers absorb in exactly the same way. Figure 3 shows infrared spectra for S10 polyurea microcapsules sample. Infrared Spectroscopy of microcapsules shows presence of various functional groups like N-H, C-H and C=O. It showed that reaction between TDI and EDA produced many more hydrogen bonded N H groups. From these characteristics, it is proved that the polyurea microcapsules were successfully formed [6]. Figure 3: FTIR for polyurea sample S10 X- Ray Diffraction (XRD) The crystalline structure of polyurea microcapsules was studied by X Ray Diffraction methods. Figure 4 shows the XRD of sample (S19) prepared under concentrated emulsion conditions (Vd/Vc = 0.25). The monomer ratio, R was 5.0 used for preparation of emulsion. The XRD results showed a semi crystalline nature of microcapsules and most of the samples showed a crystalline peaks in the region 20 < 2 < 30 [12, 14] Thermo Gravimetric Analysis Differential Scanning Calorimetry (TGA-DSC) Thermal behavior is one of the important physical properties of polyurea microcapsules, which gives

5 Synthesis and Characterization of Polyurea Microcapsules by Interfacial Polycondensation chemical structure on various characteristics. The reaction conditions have been studied by changing a various experimental parameters, such as the monomer mole ratio, R, the phase volume ratio, Vd/Vc, and the moles of limiting monomer per unit volume of the dispersed phase, N/Vd. Figure 4: XRD pattern for polyurea sample S19 (Vd/Vc = 0.25, R = 5.0, N = moles) estimation of components, nature and applications [13]. The behavior of polymer samples in DSC shows some important features. Fig. (5) shows a DSC curve of polyurea microcapsules with an exothermic peak at a temperature of melting at about C. This peak represents a region of cold crystalline transition with a glass transition temperature of the polymer is approximately in the region of 160 to C. This result showed that the high melting thermoplastic polyurea is successfully formed with a high degree of crystallinity [14]. Figure 5: TGA-DSC curve for S6 polyurea microcapsules CONCLUSIONS In this paper, we have discussed about effect of the various reaction conditions on the kinetics and properties of polyurea microcapsules. Polyurea microcapsules were formed by carrying out an interfacial polycondensation reaction in emulsion globules between Toluene Diisocynates and Ethylene Diamine to investigate the effects of the It is observed that, the mean diameter of microcapsules decreases as the stirring speed increases. The suspensions of polyurea microcapsules were most stable when EDA monomer was used. Low crystallinity of the microcapsules is essential for the growth of microcapsules. Infrared Spectroscopy showed that reactive Toluene Diisocynate (TDI) produced many more hydrogen bonded N-H groups than other aliphatic diisocynates. TGA-DSC results showed prominent thermal stabilities of polyurea microcapsules. XRD results prominently showed a semi crystalline nature and a peak lies in given range which showed that polyurea microcapsules are successful prepared. REFERENCES [1] Anna Shulkin, Harald and D. H. Stove, Microcapsules from Styrene maleic Anhydride Copolymers: Study of Morphology and Release Behavior, Journal of Membrane Science 209, 2002, [2] F. Mac Ritchie, Catedra de, Escudadela de, Universi de Chile, Mechanism of Interfacial Polycondensation, 1Oct [3] Jiang Ji, Ronald F. Childs, Mahesh Mehtra, Mathematical Model for Encapsulation by Interfacial Polymerization, Journal of Membrane Science, 192, 2001, [4] I. Montasser, S. Brianc and H. Fessi, The Effect of Monomers on the Formulation of Polymeric Nanocapsules based on Polyureas and Polyamides, Pharmaceutical Nanotechnology, Journal of Membrane Science, 335, 2007, [5] K. Bouchamel, S. Briançon, E. Perrier, H. Fessi, I. Bonnet and N. Zydowicz, Synthesis and Characterization of Polyurethane and Poly(ether urethane) Nanocapsules using a New Technique of Interfacial Polycondensation Combined to Spontaneous Emulsification, International Journal of Pharmaceutics 269, 2004, [6] K. Hong, S. Park, Polyurea Microcapsules with Different Structures: Preparation and Properties, Journal of Membrane Science, 125, 1997, [7] N. Zydowicz, P. Chaumont and M. L. Soto-Portas, Formation of Aqueous Core Polyamide

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