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Degradation of Polymers under Ultrasound And Microwave A Report Submitted in Partial Fulfillment of the Requirement for the Degree of Μαστερ οφ Ενγινεερινγ in the Faculty of Engineering By Jayanta Chakraborty Department of Chemical Engineering INDIAN INSTITUTE OF SCIENCE BANGALORE-560012, INDIA JUNE 2004 ABSTRACT The ultrasonic degradation of polybutadiene and isotactic polypropylene in solution was studied at different temperatures and in different solvents, degradation of poly (ethylene oxide) of different initial molecular weights was studied at a fixed temperature. The time evolution of molecular weight distribution was determined experimentally through gel permeation chromatography. Degradation rate coefficients were determined from a model based on continuous distribution kinetics and assuming mid-point chain scission. The variation of rate coefficients for polybutadiene and isotactic polypropylene with vapour pressure and kinematic viscosity was investigated. The rate coefficient was calculated based on a previous model. To model the effect of initial molecular weight, the ultrasonic rate coefficient, κ, is assumed to be of the form, κ = kd ( x − xlim )λ , where x and xlim represent the molecular weight and limiting molecular weight, respectively. A continuous distribution model assuming midpoint chain scission and two different expressions for the rate coefficient, where either kd or λ is assumed to be independent of the initial molecular weight, was developed to satisfactorily model the experimental data. For λ =1, the value of kd increased with increasing molecular weight while for constant kd, the value of λ decreased with increasing initial molecular weight. The microwave assisted degradation of polyethylene oxide in the presence of persulfate was investigated at different heating cycle times and with different persulfate concentrations. A model based on continuous distribution kinetics with time dependent rate coefficients was used to obtain the kinetic parameters by non- linear regression of experimental data. The change in molecular weight distribution with time was followed by gel permeation chromatography. The highest degradation ii was observed during 20 sec heating time and with 1 kg/m3 of persulfate concentration. The activation energy was found to be 10.3 kcal/mol for the microwave-assisted oxidative degradation of polyethylene oxide. iii CONTENTS 1. Introduction 1.1 A Short Overview on Polymers 1 1.2 Polymer Degradation 1 1.3 The Present Work 2 References 3 2. Ultrasonic Degradation of Polymers 2.1 Introduction 4 2.2 Experimental 2.2.1 Degradation Reaction 6 2.2.2 Polymer Analysis 8 2.3 Model Used 9 2.4 Result and Discussion 10 2.5 Conclusion 13 References 14 3. Microwave Assisted Oxidative Degradation of Polyethylene Oxide 3.1 Introduction 27 3.2 Experimental Section 3.2.1 Materials 29 3.2.2 Microwave Reactor 29 3.2.3 Degradation Reaction 29 3.2.4 GPC Analysis 30 3.3 Results and Discussion iv 3.3.1 Theoretical Model 30 3.3.2 Microwave degradation 33 3.4 Conclusion 35 References 36 4. Conclusion and scope for future work 4.1 conclusion 43 4.2 future work 44 v 6 LIST OF FIGURES 2.1a. Evolution of molecular weight of isotactic poly (propylene) in ODCB 17 2.1b. Evolution of molecular weight of poly (butadiene) in ODCB 18 2.2. Variation of the limiting molecular weight with temperature in ODCB 19 for isotactic polypropylene and polybutadiene. 2. 3a. Ln(H) vs. sonication time for isotactic poly(propylene) in ODCB 20 at various temperature. 2.3b. Ln (H) vs. sonication time for poly(butadiene) in ODCB at various 21 temperature 2.4. Vapor pressure of solvent vs. rate coefficient(k) 22 2.5. Kinematic Viscosity of the solvent vs. rate coefficient(k) 23 2.6. Variation of the molecular weight with time, based on Eq 2.4. 24 2.7. Variation of the non-dimensional number-average molecular weight 25 with time(experimental data and model fit). Inset: variation of the molecular weight with time showing the limiting molecular weight. 2.8. Variation of λ and kd with the initial molecular weight of the polymer. 26 3.1a. Variation of Mn0/Mn with time with 0.2 kg/m3 of persulfate 38 3.1b Variation of Mn0/Mn with time with 0.4 kg/m3 of persulfate 39 3.1c Variation of Mn0/Mn with time with 1.0 kg/m3 of persulfate 40 3.2a Theoretical prediction of persulfate concentration 41 3.2b Theoretical prediction of radical concentration 42 6 7 CHAPTER 1 INTRODUCTION 1.1 A SHORT OVERVIEW ON POLYMERS Synthetic polymers are synthesized mainly from petroleum fractions like naptha, natural gas etc. Polymers are one of the most important materials in the modern world with a wide range of applicability. It is used as a substitute for metal and as an artificial organ in human body. Polymeric materials are very popular because those are light in weight, tough, relatively inert in environmental conditions and cheap. Chemically polymers are (poly=many; mar=unit) large molecules of high molecular weight consists of repetitive units [1]. If the repetitive units are same then it called homopolymer and if there is more than one set of unit repeats, then it is called co-polymers. Generally the long chain consists of carbon atom, although it is possible to synthesize a polymer that contains non-carbon backbone. Different orientation of the chain or different side groups in synthetic organic polymers gives rise to different useful properties. Another important concept in polymer physics is its molecular weight distribution. Unlike other chemicals, polymers do not have a unique molecular weight, but the molecular weight is distributed over a range. That means a given sample of a specified polymer will always contain molecules of very different molecular weights. Therefore, a polymer is always has a molecular weight distribution. 1.2 POLYMER DEGRADATION 7 8 Depending on the application, polymer may need to be degraded. In the study of degradation, different methods of degradation are available, such as thermal and catalytic degradation. Unconventional methods like ultrasound[2], UV, microwave[3], photo catalytic are also important. 1.3 THE PRESENT WORK In this thesis, degradation by ultrasound and microwave has been considered. Some typical features of ultrasound and microwave degradation have been discussed in the following chapters. Experiments have been conducted to explore the effect of different parameters on ultrasonic degradation. The effect of initial molecular weight has been explored through experimental studies and a new model has been proposed. The kinetics of microwave degradation has been studied through experiment and the rate parameters have been determined. 8 4 REFERENCES: 1. Billmeyer, F. W., Textbook of Polymer Science, Wiley, New York(2003). 2. Price, G. J., “The use of ultrasound for the controlled degradation of polymer solutions,” Advances in Sonochemistry, Jai Press. (1990). 3. Caddick, S., “Microwave Assisted Organic Reactions,” Tetrahedron, 51, 10403(1995). 5 CHAPTER 2 ULTRASONIC DEGRADATION OF POLYMERS 2.1 INTRODUCTION Polymers can be degraded thermally by pyrolysis [1] or in solution [2] but the processes are energy intensive. The need for alternative techniques that would reduce energy requirements for the degradation process is important. The use of ultrasound, photo and chemical methods is less energy-intensive polymer degradation. Further, the mechanism by which they interact with the polymeric systems can help in gaining insight into the degradation pathways or mechanisms. Polymers undergo degradation when they are subjected to ultrasound irradiation of high intensity. Several workers have investigated the ultrasound degradation of polymers, which has been summarized by Price [3]. The breaking of the chemical bonds is due to the cavitation in the medium. Cavitation is the formation and violent collapse of small bubbles or voids in the liquid as a result of pressure changes in the medium. This leads to shearing forces of sufficient magnitude that they can cause the rupture of bonds. The sound waves do not directly interact with the polymer but they act on the solvent causing the growth and rapid collapse of micro-bubbles resulting in high shear gradient. It has been shown that this shear force is roughly similar to the force required to break chemical bonds in polymers [4-5]. Another unique feature of ultrasonic degradation is the fact that, in contrast to all chemical and thermal decomposition reactions, the ultrasound depolymerization is a non-random process which produces fragmentation at the mid-point of the chain [6]. The existence of limiting molecular weight, below which degradation by ultrasound 6 does not take place, has the additional effect in that the molecular weight distribution initially broadens and before narrowing during degradation. The effect of various parameters like temperature, ultrasound intensity, dissolved gases [7] and polymer concentration [8] on the ultrasonic degradation of polystyrene has been investigated. A primary factor in the ultrasonic degradation of polymers is the effect of solvent in the degradation rate. While some workers [9,10]report no effect of solvent on the degradation of cellulose nitrate and polyisobutylene, several investigators have found an effect of solvent on the ultrasonic degradation of dextran [11], poly(ethylene glycol) [12], hydroxypropyl cellulose [13], poly(alkyl methacrylate)s[14], poly(vinyl acetate) [15] and polystyrene [8]. While the ultrasonic degradation is influenced by polymer solvent interaction parameters like Florye-Huggins and Huggins constant [8], the major factors that influence the degradation are the kinematic viscosity and vapour pressure of the solvent [9].Though polybutadiene and polypropylene are important commercial polymers, there are no studies on the ultrasonic degradation of these polymers. There is no study on the ultrasonic degradation of isotactic polypropylene and polybutadiene over an extensive temperature range or the effect of solvents on degradation of these polymers. The importance of the initial molecular weight for the degradation kinetics is well known because it provides insights into degradation mechanisms for macromolecular reactions. The effect of molecular weight on the thermal degradation kinetics has been investigated and a non-linear dependence of the degradation rate on molecular weight was observed depending on the change in the molecular weight and the initial molecular weight [22]. Thus it is apparent that a determination of the scaling exponent is essential to a molecular understanding of the chain scission mechanism 7 [23]. The influence of molecular weight on the rate of ultrasonic degradation is not clear. For example, a direct proportionality between the rate of ultrasonic degradation and the molecular weight, x, was established for the degradation of dextran [24]. However, a quadratic dependence, κ= kd x2, was established for the ultrasonic degradation of synthetic polymers like polystyrene [25-27]. Experimental data [23] for both ultrasonic and elongational flow also showed that the degradation rate depends non-linearly on molecular weight. The objective of this study is to present new experimental data for the ultrasonic degradation of these polymers and determine the degradation rate coefficients using a model developed earlier [16]. The variation of the rate coefficients with temperature is attributed to the change in the kinematic viscosity and vapour pressure. The other objective of the current investigation is to study the effect of the initial molecular weight on the ultrasonic degradation of polyethylene oxide in water. A continuous distribution model is developed to model the degradation kinetics and it is shown that experimental results and theory demonstrate that the degradation rate of ultrasonic degradation of polymers do not follow a linear dependence of the initial molecular weight. 2.2 EXPERIMENTS 2.2.1 Degradation reaction: The reaction was carried out in a 100-ml beaker. Approximately 80 ml of polymer solution (2 g/l) was taken each time and the beaker was held with a clamp- stand assembly in a constant temperature water bath ( ±10 C ). Ultrasound was coupled directly to the reaction system by a horn type sonicator (Vibronics, India) with voltage and frequency of 180 V and 25 kHz, respectively. The beaker was wrapped with 8 aluminum foil to the sonic horn to minimize solvent evaporation. The experiments were conducted by covering the beaker to ensure that no air entered the system during degradation. No crosslinking was observed because the Gel Permeation Chromatography (GPC) chromatograph did not show any molecular weight above the initial molecular weight. Calculated amounts of solvents were added to maintain the concentration of polymer solution constant inside the reaction system. Samples were drawn at regular intervals through a syringe and analyzed by GPC. The ultrasonic degradation of polybutadiene was investigated at 32, 50, 60 and 0 70 C with o-dichlorobenzene as solvent and in chlorobenzene, toluene, p-xylene, 0 chloroform, tetrahydrofuran and benzene at 32 C. The ultrasonic degradation of 0 isotactic polypropylene was investigated at 80, 90, 113, 133 and 155 C with o- dichlorobenzene as solvent. Because the initial molecular weight of polypropylene was 0 only 125,000, it was completely dissolved even at 70 C. This was confirmed by dissolving the polymer at 70 0 C and 155 0 C. Both the solutions showed the exact same GPC chromatogram confirming that polypropylene remained soluble in all experimental conditions. Polyethylene oxide of different molecular weights was degraded in an aqueous solution at a fixed temperature of 30 oC. The model to determine the degradation rate coefficient requires the limiting molecular weight for the degradation of the polymer. Several experiments were conducted for 10 h and no detectable change in the molecular weight was noticed after 6 h. Further, the experimental data indicated that the ratio of the limiting molecular weight to the molecular weight after 120 min of degradation was approximately 0.8 in all cases. This ratio determines the limiting molecular weight of the polymer for the experiments conducted for only 3 h. The variation of the limiting molecular weights for 9 the polymers with reaction temperature was measured and the plot was linear (Figure 2.2). This observation was consistent with the observations reported in the literature [7] for the variation of the limiting molecular weight with temperature for the ultrasonic degradation of polystyrene in toluene. For the ultrasonic degradation of polyethylene oxide, a constant limiting molecular weight of 16000 was observed experimentally. 2.2.2. Polymer analysis The molecular weight distributions of the polypropylene samples were measured by Gel Permeation Chromatography(GPC). The HPLC-GPC system consists of an isocratic pump (Waters, USA), mixed B columns(Polymer Labs, UK) and evaporative light scattering detector (Polymer Labs, UK). The columns were 0 maintained at 150 C using a column heater (Eldex, USA) and the system was calibrated with polystyrene standards. The flow rate of the eluent, o-dichlorobenzene, was maintained at 0.7 ml/min and the flow rate of nitrogen to the light scattering detector was maintained at 0.8 ml/min for enhanced sensitivity. The molecular weight distributions of the polybutadiene samples were measured by Gel Permeation Chromatography (GPC system consisted of an isocratic pump (Waters, Model R410), an injector (Rheodyne, 200microliter sample loop) and three GPC columns. (HR 4, HR3 and HR 0.5 size: 300 mm 7.5 mm) of varying pore sizes. Columns kept inside column heater (Waters, USA) maintained at 50 C. A 0.5 micrometer filter (Supelco) was placed at the column inlet to prevent any dirt particle to entry. A differential refractive index (RI) detector (Waters, USA) was connected at the outlet of the last column. Tetrahydrofuran (THF) was distilled twice and filtered through 0.2 micrometer membrane filter (Millipore) and was eluent for the system and pumped at a constant flow rate of 1.0 ml/min through the system to obtain the 10 chromatogram. These were converted to MWD with the help of calibration curve made using six polystyrene standards of narrow distribution. The molecular weight distribution of polyethylene oxide was monitored by Gel Permeation chromatography. The GPC consisted of Waters Ultrahydrogel linear SEC column measuring 7.8x300 mm maintained at 50°C. Double distilled deionised water was used as eluent at a flow rate of 0.5 ml/min. The refractive index was monitored continuously with a Waters 401 Differential Refractometer. About 800 µl of sample was injected into the system to obtain a chromatogram and converted to molecular weight by using narrow-distribution polyethylene oxide calibration standards. 2.3 MODEL USED Because the polymer breaks at the midpoint, → P( x) 2 P( x / 2) k ( x) (A) P ( x) represents the polymer species and p ( x, t ) is the molecular weight distribution of the species. No repolymerization of the degraded species was observed in the experiments. This was confirmed by the GPC chromatogram that showed no molecular weight products higher than the initial distribution. The population balance equation for the above reaction (A) is [16] = − k ( x) p ( x, t ) + 2 ∫ k ( x) p ( x ' , t )δ ( x − )dx ' ∞ ∂p ( x, t ) x' (2.1) ∂t x 2 The degradation is assumed to be first order with the polymer concentration p ( x, t ) and the degradation rate, k ( x ) , is assumed to be of the form k ( x) = k ( x − xl ) where the xl represents the limiting molecular weight[16]. This ensures that the rate coefficient, k , 11 is independentof x and becomes zero when x = xl and no further degradation takes ∫ ∞ place. Applying the moment operation x n []dx to the above equation yields 0 dp ( n ) = kp ( n +1) (21− n − 1) − kp ( n ) xl (21− n − 1) (2.2) dt p (0) and p (1) represent the molar and mass concentrations of the polymer, respectively and can be obtained by putting n = 0 and 1 in Eq. (2.2). For n = 0 dp (0) = kp (1) − kp (0) xl (2.3) dt Solving equation (2.3) with initial condition p (0) (t = 0) = p0 , gives (0) ( p (1) − p0 xl ) ¡ (0) = e kxl t (2.4) ¢ £ ( p (1) − p (0) xl ) ¤ ¥ The number average MW, Mn is defined as p (1) / p (0) , and the above equation reduces to −1 ( xl − M n 0 ) ¦ § ln = ln( H ) = kxl t (2.5) ¨ © − ( xl − M n 1 ) 2.4 RESULTS AND DISCUSSION Figure 2.1a and 2.1b shows the molecular weight dynamics during the sonication reaction. The sharp fall in molecular weight during the beginning of the process is typical characteristics of ultrasonic degradation. The molecular weight curves steadily approaches a limiting value depending on the nature of the reacting system. The limiting molecular weight is found to be linear with temperature (Figure 2.2). Figure 2.3a and 2.3b shows the variation of H with time for the ultrasonic degradation of isotactic polypropylene and polybutadiene, respectively. The semi-log plots are nearly linear, confirming the validity of Eq.(2.5). The degradation rate 12 coefficients were determined from the regressed slopes. The rate coefficients, k (×108 mol g −1 s −1 ) , for isotactic poly propylene decreased from 0.94 to0.28 as the temperature increased from 80 to 155 0 C. The rate coefficients, k (×108 mol g −1 s −1 ) for polybutadiene decreased from 0.76 to 0.25 as the temperature increased from 32 to 70 0 C. As the rate coefficients decrease with an increase in temperature, an Arrhenius plot would yield negative activation energies and would not have any physical meaning. The decrease of degradation rate with an increase of temperature is similar to that observed for mechanical breakage of polymers. As the temperature of the solution increases, a large quantity of the solvent vapour enters the cavitation bubbles during their expansion and exerts a cushioning. effect [3] during the collapse leading to diminishing of the intensity of the shock wave, reducing the jet velocity [19]leading to reduced degradation at higher temperatures. The same phenomena can be used to explain the decrease of the degradation rate coefficients with an increase in the vapour pressure of the solvent (Figure 2.4). While kinematic viscosity of the solvent was assumed to play a role in the ultrasonic degradation [20,21], a more detailed study [16]indicated that this could be a crucial parameter. Better transmission of the shock waves in solution of higher kinematic viscosity [16] is a probable reason that explains the increase of degradation rate coefficients with increasing kinematic viscosity (Figure 2.5).The limiting molecular weight obtained for the degradation of polybutadiene and polypropylene is similar to the limiting molecular weight obtained for the ultrasonic degradation of polystyrene [7,8], poly(vinyl acetate) [15] and poly(methyl methacrylate) [22].However, the degradation rate coefficients for polybutadiene and polypropylene are higher than the degradation rate coefficients of polystyrene [8], 13 poly(vinyl acetate) [15] and poly(methyl methacrylate) [22].The variation of the rate coefficients with increase in temperature, vapour pressure and kinematic viscosity is consistent with the observations for other polymers in the literature [3,7,15,22]. The effect of initial molecular weight on the degradation rate of polyethylene oxide was investigated. Figure 2.6 shows this plot and the lines are obtained by simple linear regression. The rate coefficient, kd, is directly obtained by dividing the slope of the regressed line with the limiting molecular weight. The variation of the rate coefficient, kd, with the initial molecular weight is shown in Figure 2.8. Though the rate coefficient, kd, should be independent of the initial molecular weight, it is dependent on the initial molecular weight and increases linearly with increasing initial molecular weight (Figure 2.8). Another model is proposed in which the degradation rate, κ(x), is assumed to be of the form κ(x) = kd (x-xl)λ but λ is not assumed to be a constant. Instead, the rate coefficient, kd, is assumed to be independent of the initial molecular weight and equal to 10-10 mol g-1 s-1. The governing population balance equation (2.2) is solved numerically using the finite difference technique, as discussed in details in the literature [28]. The value of is determined from the non-linear regression of the experimental data with the numerical solution of the theory. The theoretical fitting of the experimental data indicates that the model fit is very good (Figure 2.7). The value of is found to be dependent on the initial molecular weight and varies from 1.4 to 1.34 as the initial molecular weight of the polymer varies from 105 to 106, as shown in Figure 2.3. The decrease of λ with increasing initial molecular weight is consistent with the observation of experimental data for ultrasonic and elongational flow [23], wherein it was observed that λ approaches zero for very large initial molecular weights. The assumption of λ ¡ 1 for modeling the degradation kinetics for ultrasonic degradation is 14 consistent with the modeling the degradation kinetics for thermal degradation [22]. The non-linear dependence of the degradation rate on the initial molecular weight can thus be directly accounted by varying the exponent (scaling factor), λ. 2.5 CONCLUSION The ultrasonic degradation of two commercially important polymers, polybutadiene and isotactic polypropylene, has been investigated. The evolution of the molecular weight was determined by gel permeation chromatography and a model based on continuous distribution kinetics was used to determine degradationrate coefficients. The degradation rate decreased with increasing temperatures, increasing vapour pressure and decreasing kinematic viscosity of the solvents. The ultrasonic degradation of an aqueous solution of poly (ethylene oxide) of different initial molecular weights was investigated at 30 oC. The ultrasonic rate coefficient, κ, was assumed to be of the form, κ = kd ( x − xlim )λ , where x and xlim represent the molecular weight and limiting molecular weight, respectively. For λ=1, expressions were derived analytically and the experimental data was fitted to the theory by linear regression. This indicated that the degradation rate coefficient, kd, was linearly dependent on the initial molecular weight. Another approach was envisaged wherein kd was assumed to be independent of the molecular weight and the value of λ was determined by non-linear regression by solving the population balance equation numerically. The value of λ exponentially decreased with increasing initial molecular weight. 15 REFERENCES 1. Jellinek, H. H. G., Editor. Degradation of vinyl polymers. New York: Academic Press (1955). 2. Price, G. J., “The use of ultrasound for the controlled degradation of polymer solutions,” Advances in Sonochemistry, Jai Press. (1990). 3. Thomas, G. R., “Sonic degradation of high polymers in solution” J. Phys. Chem., 63,1725 (1959). 4. Okuyama, M., and Hirose, T. J., “Physicochemical approach to ultrasonic cavitation: Dynamics on ultrasonic cavitation from the viewpoint of sonochemical reactions,” Appl. Polym. Sci., 7, 591 (1968). 5. Pritchard, N. J., Hughes, D.E., and Peacocke, A. R., “Ultrasonic degradation of biological macromolecules under conditions of stable cavitation. I. Theory, methods, and application to deoxyribonucleic acid,”Biopolymers, 4, 259 (1966). 6. Peacocke, Arthur R.; Pritchard, N. J., “Ultrasonic degradation of biological macromolecules under conditions of stable cavitation. II. Degradation of deoxyribonucleic acid”.Biopolymers, 6, 605 (1968). 7. Thomas, B.B., and Alexender, W.J., “Ultrasonic Degradation of Cellulose NitrateII: effect of temperature, solvent and other process variables”, J Polym. Science.,25, 285 (1957). 8. Nelkenbaum, Y.Y., Prokofrev, I.K., and Sangalore., Y.A., “Ultrasonic Degradation Of Poly Iso-Butylene,” Vyskomolek Soedin Ser A., 29, 2395 (1986). 9. Basedow, A.M., and Ebert, K.H., “Mechanism of Degradation of Polymers in Solution by Ultrasound,” Makromole. Chem., 176, 745 (1975). 16 10. Basedow, A.M., Ebert, K.H., and Fosshag, E., “Ultrasonic Degradation of Polymers in Mixed Solvents,’ Makromole. Chem.., 179, 2565 (1978). 11. Malhotra, S.L., “Ultrasonic Degradation of Hydroxy propyl cellulose solutions in Water, Ethanol and tetrahydrofuran,” J Macromolec. Sci. Chem., A17, 601(1982). 12. Price, G.J., Smith, P.F., “Ultrasonic Degradation of Polymer Solutions: III. Effect of Changing solvent and Solution Concentration, ” Polym. J., 29, 419 (1993). 13. Scmid, G. Z., “Effect of ultrasonic vibrations on the magnetic properties of nickel,” Phys. Chem., 186, 113 (1940). 14. McCoy, B. J., Madras, G., “Degradation Kinetics of Polymers in Solution: Dynamics of Molecular Weight Distributions”. AIChE. J., 43, 802 (1997). 15. McCoy, B. J., Madras, G., “Oxidative Degradation Kinetics of Polystyrene in Solution,” Chem. Eng. Sci., 52, 2707 (1997). 16. Perry, R.H., and Chilton, C.H., Editors, Chemical Engineers Handbook, McGraw-Hill, New York (1973). 17. Brandrup J, Editor, Polymer handbook, John Wiley, New York (1975). 18. 'The properties of liquid and gas' Reid; R.C, Sherwood; T.K, Second Edition, McGraw -Hill Book Company 1965. 19. Jellinek, H.H. J., “Degradation of long chain molecules by ultrasonic waves. VII. Effect of viscosity and surface tension on cavitation” Polym. Sci., 22 149 (1956) 17 20. Basedow, A. M., Ebert, K. H., “Determination of molecular weight distributions and mean values of the molecular weight of clinical dextrans,” Makromol. Chem., 176 745 (1975). 21. Madras, G., Chottopadhyay, S., “Effect of solvent on the ultrasonic degradation of poly(vinyl acetate),” Polymer Degradation and Stability , 71, 273 (2001). 22. Madras, G., Chung, G. Y., Smith, J. M., McCoy, B. J., “Mol. Wt. effect on the dynamics of polystyrene degradation,” Ind. Eng. Chem. Res., 36, 2019 (1997). 23. Nguyen, T. Q., Liang, Q. Z., and H. H. Kausch, “Kinetics of Ultrasonic and Transient Elongational Flow Degradation: A Comparative Study”, Polymer, 38, 3783(1997). 24. Ederer, H. J., Basedow, A. M. and Ebert, K. H., in Modelling of Chemical Reaction Systems, ed. K. H. Ebert, P. Deuflhard and W. Jagger. Springer- Verlag, Berlin, 1981, p. 189. 25. Price, G. J., “The Use of Ultrasound for the Controlled Degradation of PolymerSolutions”. Mason T. J. Editor, Advances in Sonochemistry Vol I, Cambridge Jai Press, p 231 (1990). 26. Florea, M., “New use of size exclusion chromatography in kinetics of mechanical degradation of polymers in solution”, J. Appl. Polym. Sci, 50, 2039(1993). 27. Mason, T. J. and Lorimer, J. P., Sonoehemistry: Theory, Applications and Uses of Ultrasound in Chemistry. Ellis Horwood, New York, 1989, Ch. 2 28. Madras, G., McCoy, B.J., “Molecular Weight Distribution Kinetics for Ultrasonic Reactions of Polymers,” AIChE. J 47, 2341(2001). 27 140000 120000 100000 80000 Mn 60000 40000 20000 0 0 20 40 60 80 100 120 140 160 Time(min) Figure 2.1a. Evolution of molecular weight of isotactic poly (propylene) in ODCB ¡ Legends: 80°C 90°C 113°C 133°C 155°C ¢ £ ¤ 28 110000 100000 90000 80000 70000 Mn 60000 50000 40000 30000 0 20 40 60 80 100 120 140 160 180 200 Sonication time(min) Figure 2.1b. Evolution of molecular weight with time for poly (butadiene) in ODCB ¡ Legends: 80 C 70 C 30 C 60 C 50 C ¢ £ ¤ 29 4 5.0x10 4 4.8x10 4 4.6x10 4 4.4x10 4 4.2x10 4 4.0x10 4 3.8x10 Mlim 4 3.6x10 4 3.4x10 4 3.2x10 4 3.0x10 4 2.8x10 4 2.6x10 20 40 60 80 100 120 140 160 0 Reaction temperature( C) Figure2.2. Variation of the limiting molecular weight with temperature in o- dichlorobenzene for the ultrasonic degradation of isotactic polypropylene ( ) and polybutadiene ( ). 30 2.0 1.5 log(H) 1.0 0.5 0.0 0 20 40 60 80 100 120 Time(min) Figure2. 3a. Ln(H) vs. sonication time for isotactic poly(propylene) in ODCB ¡ at various temperature. Legends: 80 C 90 C 113 C 133 C 155 C ¢ £ ¢ 31 3.0 2.5 2.0 Ln(H) 1.5 1.0 0.5 0.0 0 20 40 60 80 100 120 140 160 180 200 Time(min) Figure 2.3b. Ln(H) vs. sonication time for poly(butadiene) in ODCB at various ¡ temperature. Legends: 32°C 50°C 60°C 70°C ¢ £ 32 -7 6x10 -7 5x10 -7 Rate coefficient (mol g min ) -1 5x10 -7 4x10 -1 -7 4x10 -7 3x10 -7 3x10 -7 2x10 -7 2x10 -7 1x10 -8 5x10 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Vapor pressure(bar) Figure 2.4. Vapor pressure of solvent vs. rate coefficient(k) Legends: PP(ODCB) at ¡ different temp. PB(ODCB) at different temp PB (Chlorobenzene) at 32°C ¢ £ PB( Benzene) at 32°C ¤ PB(THF) at 45°C ¡ PB(Toluene) at45°C ¢ PB(Chloroform) at 45°C £ PB(Xylene) at 45°C. 33 -7 6x10 -7 6x10 -7 5x10 min ) -1 -7 5x10 -7 4x10 -1 Rate Coefficient (mol g -7 4x10 -7 3x10 -7 3x10 -7 2x10 -7 2x10 -7 1x10 -8 5x10 0 0.000 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 2 Kinematic viscosity(cm /sec) Figure 2.5. Kinematic Viscosity of the solvent vs. rate coefficient(k)Legends: ¡ PB(ODCB) at different temp. PB(Chlorobenzene) at 32°C. PB(Toluene) at45°C . £ PB(Xylene) at 45°C. PB(Chloroform) at 45°C. ¤ £ PB(THF) at 45°C. PP(ODCB) at different temp. ----- Linear fit for PB . —— Linear fit for PP. ¡ 34 1.4 1.2 1.0 0.8 ln H 0.6 0.4 0.2 0.0 0 50 100 150 200 Time (min) Figure2.6. Variation of the molecular weight with time, based on Eq 2.5. The lines are obtained from linear regression and the slope corresponds to the degradation rate coefficient, kd. See Figure 1 for legend 35 1.0 10 5 M n x 10 0.8 1 0.6 Mn/Mn0 0 50 100 150 200 250 0.4 Sonication Time(min) 0.2 0.0 0 50 100 150 200 250 Sonication Time(min) Figure 2.7. Variation of the non-dimensional number-average molecular weight with time. The inset of the figure shows the variation of the molecular weight with time. Legends Mn0 = 105; ♦Mn0 = 2 x 105; ¡ Mn0 = 6 x 105; ¢ Mn0 = 106; _______ Model 36 5.4 1.40 5.2 -1 kd (x 10 ) mol g s 1.39 -1 5.0 1.38 4.8 -9 1.37 λ 4.6 1.36 4.4 1.35 4.2 1.34 4.0 0 2 4 6 8 10 5 Initial Molecular Weight (x 10 ) Figure 2.8. Variation of λ and kd with the initial molecular weight of the polymer. The lines in the graph represent exponential decay and linear fit for the variation of λ and kd with the initial molecular weight, respectively 37 CHAPTER 3 MICROWAVE ASSISTED OXIDATIVE DEGRADATION OF POLYETHYLENE OXIDE 3.1 INTRODUCTION Growing interest in non-conventional methods of polymerization- depolymerization has lead to research on applications of microwaves for these processes. Microwave heating offers increased reaction rates and reduced electrical consumption, compared to thermal heating, as observed in several organic synthesis reactions [1-2]. There is a better distribution of heat in the sample and hence better control over the process. Moreover, the efficiency of converting electrical energy to thermal energy is over 85 % and the rate of heating is many times faster than thermal heating [3]. Microwaves are known to enhance the solid-state polymerization of PET and nylon 66 by increasing the overall diffusion rates and reducing the activation energy for the diffusion process [4]. Improved polymerization of caprolactone initiated by carboxylic acids was observed on microwave irradiation [5]. However, lower Mw was obtained with increasing acidity of the initiator due to simultaneous degradation of the formed polymer. Microwave polymerization of methyl methacrylate showed marked dependence on microwave power. The reaction rate increased with microwave power because of increased heating rate at comparable temperature with thermal heating [6]. The emulsion polymerization of styrene was studied in a microwave reactor. The results showed that the monomer-water ratio, microwave power, volume of reaction mixture and geometry of vessel used were important factors to be considered to accelerate heating rates [7]. 38 The pyrolysis of lignin and cellulose could be manipulated to improve product selectivity by rapid homogenous heating by microwaves. The rate enhancement during pyrolysis was also observed [8-9]. The surface oxidation of polyethylene with permanganate was studied by FT IR when microwaves were made to impinge on solid samples [10]. Starch degradation in microwaves was studied by Khan et.al. [11]. The hydrolysis of starch occurred at high temperatures and pressures produced in a sealed glass ampule, with increase in total acidity and a concurrent decrease in pH. The thermo-oxidative degradation of edible fats by microwave and conventional heating was studied. It was found that greater amount of alterations occurred on microwave-heated samples as quantified by chromatographic techniques [12]. The microwave heating effect arises when polarizable molecules tend to reorient and align themselves in an applied microwave field. The ability to convert microwave energy to thermal energy depends on the dielectric constant and dielectric loss associated with a material. Greater effects are observed when the material involved has a greater dielectric constant at a given microwave frequency and power [13]. The heating effect by microwaves is unique because heat is generated internally within the material. Therefore, it heats the material from inside. There is instantaneous heating, which is volumetric and specific to the material. Also it has been found that microwave heating does not alter the composition of products appreciably when compared to those produced by conventional heating techniques [7]. Water couples with microwaves to a greater extent because of its high dielectric constant. Hence reactions with water are expected to proceed at faster rates by microwave heating over conventional thermal heating. Polymer degradation in 39 aqueous solutions is a viable approach to study the kinetics of the degradation process by the application of microwave energy. In the present study we have aimed to look in to the oxidative degradation of polyethylene oxide by persulfate under microwaves by changing cycle times and the number of the cycles of irradiation. The effect of persulfate concentration on the reaction kinetics was also investigated. Continuous distribution kinetics was used to obtain the degradation rate parameters by non-linear regression of the experimental data. 3.2 EXPERIMENTAL SECTION 3.2.1 Materials Polyethylene oxide (Aldrich Co) and potassium persulfate (S. D. Fine Chemicals) were used as obtained. 3.2.2 Microwave reactor A domestic microwave oven with a magnetron source for microwave generation was used (Essentia, India). The maximum power was 700 W with a frequency of 2.45 GHz. All experiments were conducted at the maximum power of the equipment. 3.2.3 Degradation experiments The polymer solution concentration taken was 2 kg/m3. The reaction vessel was a glass beaker of 50 ml capacity. The volume of the solution taken was 40 ml for all the experiments. Three different concentrations of persulfate were used. The required persulfate was added and stirred to dissolve before starting the irradiation of the sample polymer solution. The sample was placed at the center of the oven directly below the magnetron source. Different microwave cycle times (th) were employed like 40 10, 15, 20 s and the number of cycles was fixed at 10 cycles. Therefore, for 10, 15 and 20 s cycles the total irradiated time was 100, 150 and 200 s. To ensure uniform heating and to avoid temperature gradients, the sample was rotated on a turntable. The temperature of the reaction mixture was monitored with a fluoro-optic thermometer with an accuracy of ± 0.5 ºC. The irradiated sample was cooled to 22 ºC by immersing in an ice-bath for a set time of 110 s (tc). 3.2.4 GPC Analysis The molecular weight distribution was monitored by Gel Permeation chromatography. The GPC consisted of Waters Ultrahydrogel linear SEC column measuring 7.8x300 mm maintained at 50°C. Double distilled deionized water was used as eluent at a flow rate of 0.5 ml/min. The refractive index was monitored continuously with a Waters 401 Differential Refractometer. About 800 µl of sample was injected into the system to obtain a chromatogram and converted to molecular weight by using narrow-distribution polyethylene oxide calibration standards. 3.3 RESULTS AND DISCUSSION 3.3.1 Theoretical model The homolytic cleavage of persulfate in to two radicals can be written as C2 2C • → p k (3.1) The rate of persulfate disappearance by dissociation is dc p = −k p c p (3.2) dt 41 where cp denotes the molar concentration of persulfate. The hydrogen abstraction from polymer chain occurs through these radicals resulting in the formation of polymer tertiary radicals. This is written as follows C • + P ( x ) CH + R • ( x ) d( ) → k x (3.3) The population balance for the consumption of persulfate radicals can be written as dc ( t ) dt = −2k p c p ( t ) − c ( t ) ∫ kd ( x ' ) p ( x ', t ) dx ' ∞ (3.4) 0 The temperature variation during microwave heating was found to vary linearly with time. The temperature profile for each cycle can be assumed to be Tpeak − Tw Tw + t ∀t ∈ ( 0, th ) ¡ ¡ th T= (3.5) Tpeak − Tw ¢ ¡ ¡ Tpeak − ( t − th ) ∀t ∈ ( th ,τ ) τ − th £ The initiation and termination steps for the polymer can be represented by a reversible reaction P ( x) R• ( x ') + R• ( x − x ') (3.6) ¤ But since these steps are not frequent when compared with the propagation step they can be neglected [14]. Hydrogen abstraction from the polymer chain can also occur through a polymer radical. This is a reversible reaction and can be given by P ( x ) R• ( x ') (3.7) The propagation step occurs by the irreversible β-scission of the polymer chain and can be written as 42 R • ( x ) R • ( x ') + P ( x − x ' ) ks → (3.8) The population balance equations for polymer and polymer radical are ∂p ( x, t ) ∂t = −kd ( x ) c ( t ) p ( x, t ) − kh ( x ) p ( x, t ) + k H r ( x, t ) + ∫ ks ( x ' ) r ( x ', t ) Ω ( x, x ' ) dx ' ∞ (3..9) x ∂r ( x, t ) ∂t = kd ( x ) c ( t ) p ( x, t ) + kh ( x ) p ( x, t ) − k H r ( x, t ) − k s ( x ) r ( x, t ) + ∫ k s ( x ' ) r ( x ', t ) Ω ( x, x ') dx ' ∞ (3.10) x For random chain scission, the stoichiometric kernel is given by 1/x' [14]. Then by applying moment operation we get dp j k = −kd c ( t ) p ( j +1) ( t ) − kh p ( j +1) + k H r ( j +1) + s r ( j +1) (3.11) dt j +1 dr j j ( j +1) = kd c ( t ) p ( n +1) ( t ) + kh p ( j +1) − k H r ( j +1) − k s r (3.12) dt j +1 Quasi-steady state approximation can be applied to equation (3.12) and the radical concentration can be given by k d c ( t ) + kh r ( j +1) = ( j + 1) p ( j +1) (3.13) jk s + ( j + 1) k H The simultaneous solution of equations (3.11) and (3.13) gives the jth moment in terms of known quantities. 43 dp ( j) kd c ( t ) + kh = − ( j − 1) ks p( j +1) (3.14) dt jk s + ( j + 1) k H When j=0, the molar concentration of polymer can be known and equation reduces to dp ( 0) = k0 p (1) (3.15) dt where the overall rate coefficient k0 is given by koxd c ( t ) + ktherm . The oxidative degradation coefficient, koxd , is kd ks k H and the thermal degradation coefficient in the absence of persulfate, ktherm , is kh ks k H . Since experimental observation shows that no degradation occurred in the absence of persulfate, the contribution from thermal degradation can be neglected, and therefore k0 = koxd c ( t ) . Thus equation (3.15) becomes dp ( 0) = koxd c ( t ) p (1) (3.16) dt The simultaneous solution of equation (3.16) along with equations (3.2) and (3.4) gives the time evolution of number-average molecular weight with the initial set conditions and temperature dependency. The rate coefficients and activation energies for hydrogen abstraction and random oxidative chain scission can be found by non- linear regression of experimental data. 3.3.2 Microwave degradation The microwave assisted oxidative degradation of polyethylene oxide was studied at different heating cycle time varying from 10 to 20 sec cycles and at different concentrations of persulfate ranging from 0.2 kg/m3 to 1.0 kg/m3. When the polymer solution is exposed to microwave radiation in the presence of persulfate rapid 44 degradation occurs. No degradation occurred in the absence of persulfate. The variation of Mn0/Mn with time for three different persulfate concentrations is shown in Figure 3.1a, 3.1b and 3.1c. The rate of decrease is greater with greater concentration of persulfate and the degradation was found to be rapid during the initial stages of irradiation. The points are from experiment and the solid line is from theory. As seen from the figures there is good agreement between theory and experimental data. No degradation was observed at 10 sec cycle time with 0.2 kg/m3 concentration of persulfate. The accelerated degradation by microwaves can be attributed to the volumetric heating of the sample and the improved mixing of the solution due to microstirring by the absorption of microwave energy by molecules. The rate of radical formation is also enhanced because of inherently high temperatures produced in shorter times thereby increasing the overall rate of the degradation. Degradation of polyethylene oxide under similar conditions (for a fixed time, temperature and concentration of persulfate) was done by thermal heating and it was found that the end molecular weight obtained at the end of 5 min of thermal degradation was many times higher than the molecular weight obtained after microwave degradation of the sample. This clearly indicates that at comparable temperatures too there exists a definite "microwave effect" which clearly enhances the rate of degradation. The kinetic parameters for the degradation can be obtained by non-linear regression of the experimental data. Dependence of rate coefficient on temperature was assumed to be of Arrhenius form. The rate coefficients for persulfate decomposition, hydrogen abstraction and oxidative random scission are given by − E p / RT k p = k p 0e , kd = kd 0 e − Ed / RT and koxd = koxd 0 e− Eoxd / RT . Since the rate coefficients are temperature dependent the temperature terms in the expressions were replaced by 45 equation (3.5) The decomposition of persulfate is well known and the rate coefficient for the homolytic cleavage is given by k p = 38.4e−33500 / RT [15]. The values are for kd and koxd are obtained by solving equations (3.2), (3.4) and (3.16) by incorporating the above expressions for rate coefficients. The initial conditions are set ( 0) as p ( t = 0 ) = p0( 0) , c p ( t = 0 ) = c p 0 and c ( t = 0 ) = 0 . The non-linearly regressed values of rate coefficients are kd = 19.0e −7700 / T and koxd = 12.5e −5200 / T . The theoretical variation of persulfate and radical concentration with heating and cooling time is shown in Figure 3.2a and 3.2b. The fit is for 0.4 kg/m3 of persulfate concentration at 15 s heating cycle time and 110 s cooling time. The persulfate concentration decreases rapidly during heating and slowly during cooling times. The concentration of radicals is zero initially. It rises during heating cycle and falls during cooling cycle. After the first cycle, the radical concentration variation becomes periodic with time. The highest radical concentration is obtained during heating cycle when the temperature is highest. 3.4. CONCLUSIONS The degradation of polyethylene oxide under microwave irradiation in the presence of persulfate was studied at various heating times and concentrations of persulfate. The kinetic parameters were obtained from model based on continuous distribution kinetics, by non-linear regression of experimental data. The microwave degradation was found to be faster and more efficient than degradation by conventional thermal heating. Higher degradation was found at higher concentrations of persulfate and higher microwave heating cycle times. 46 REFERENCES 1. Gedye, R., Smith, F., Westaway, K., Ali, H., Baldisera, L., Laberge, L., Rousell, J., “The Use of Microwave Ovens for Rapid Organic Synthesis,” Tetrahedron letters, 27, 279 (1986). 2. Giguere, R. J., Bray, T. L., Duncan, S. M., “Application of Commercial Microwave Ovens to Organic Synthesis,” Tetrahedron letters, 27, 4945(1986). 3. Ludlow-Palafox, C., Chase, H. A., “Microwave-Induced Pyrolysis of Plastic Wastes,” Ind. Eng. Chem. Res, 40, 4749(2001). 4. Mallon, F. K., and Ray, W.H., J. “The Effect of the Type of Purge Gas on the Solid-State Polymerization of Polyethylene Terephthalate,” App. Polym. Sci., 69, 1203(1998). 5. Zhang, C., Liu, L. J., and Zhuo, R. X., J. Polym. Sci. Part A: Polym. Chem., 41, 13(2003). 6. Jacob J, Chia LHL, Boey FYC, “Microwave Polymerization of poly(methyl acrylate): Conversion Studies at Variable Power”, J. App. Polym. Sci., 63, 787 (1996). 7. Correa, R., Gonzalez, G., Dougar, V., “Emulsion Polymerization in a Microwave Reactor,” Polymer, 39, 1471(1998). 8. Chan, R. W. C., and Krieger, B. B., “Kinetics of dielectric-loss Microwave Degradation of Polymers: Lignin,” J. App. Polym. Sci., 26, 1533(1981). 47 9. Allan, G.G., Krieger, B. B., and Donald, W., “Dielectric Loss Microwave Degradation of Polymers: Cellulose,” J. App. Polym. Sci., 25, 1839(1980). 10. Mallakpour, S. E., Hajipour, A. R., Mahdavian, A. R., Zadhoush, A., and Hosseini, F. A., “Microwave assisted oxidation of polyethylene under solid-state conditions with potassium permanganate,” Eur. Polm. J., 37, 1199(2001). 11. Khan, A. R., Johnson, J. A., and Robinson, R. J., “Degradation of starch polymers by microwave energy,” Cereal Chemistry, 56,303(1979). 12. Albi, T., Lanzon, A., Guinda, A., Leon, M., Perez-Camino, M. C., “Microwave and Conventional Heating Effect on Thermooxidative Degradation of Edible Fats,” J. Agri. Food Chem., 45, 3795(1997). 13. Caddick, S., “Microwave Assisted Organic Reactions,” Tetrahedron, 51, 10403(1995). 14. Kodera, Y., and McCoy, B., “Distribution Kinetics of Radical Mechanism: Reversible Polymer Decomposition,” AIChE J., 43, 3205(1997). 15. Bandyopadhyay, M., Konar, R. S., “Thermal Decomposition of Persulphate in the Aqueous Media,” J. Indian Chem. Society, 51,722(1974). xlviii 3.5 3.0 2.5 Mn0/Mn 2.0 1.5 1.0 0 30 60 90 120 150 180 210 Time (sec) Figure 3.1a. Variation of Mn0/Mn with time with 0.2 kg/m3 of persulfate xlix 4.5 4.0 3.5 3.0 Mn0/Mn 2.5 2.0 1.5 1.0 0 30 60 90 120 150 180 210 Time (sec) Figure 3.1b Variation of Mn0/Mn with time with 0.4 kg/m3 of persulfate l 7 6 5 Mn0/Mn 4 3 2 1 0 30 60 90 120 150 180 210 Time (sec) Figure 3.1c Variation of Mn0/Mn with time with 1.0 kg/m3 of persulfate li -3 1.5x10 -3 1.5x10 Persulfate conc.(cp),kmol/m3 -3 1.5x10 -3 1.5x10 -3 1.5x10 -3 1.5x10 -3 1.5x10 -3 1.5x10 0 200 400 600 800 1000 1200 Time,s Figure 3.2a Theoretical prediction of persulfate concentration with heating and cooling cycles lii 18 16 Radical concentration (c) kmol/m3 14 12 10 8 6 4 2 0 0 200 400 600 800 1000 1200 Time,s Figure 3.2b Theoretical prediction of radical concentration with heating and cooling cycles liii CHAPTER 4 CONCLUSIONS AND FUTURE WORK 4.1 CONCLUSION The ultrasonic degradation of two commercially important polymers, polybutadiene and isotactic polypropylene, has been investigated. The degradation rate coefficient has been calculated from a modle based on continuous distribution kinetics. The rate coefficient is found to be decreased with increasing temperatures, increasing vapour pressure and decreasing kinematic viscosity of the solvents. The ultrasonic degradation of an aqueous solution of poly (ethylene oxide) of different initial molecular weights was investigated at 30 oC. The ultrasonic rate coefficient, κ, was assumed to be of the form, κ = kd ( x − xlim )λ , where x and xlim represent the molecular weight and limiting molecular weight, respectively. For λ=1, expressions were derived analytically and the experimental data was fitted to the theory by linear regression. This indicated that the degradation rate coefficient, kd, was linearly dependent on the initial molecular weight. Another approach was envisaged wherein kd was assumed to be independent of the molecular weight and the value of λ was determined by non-linear regression by solving the population balance equation numerically. The value of λ exponentially decreased with increasing initial molecular weight. The degradation of polyethylene oxide under microwave irradiation in the presence of persulfate was studied at various heating times and concentrations of persulfate. The kinetic parameters were obtained from model based on continuous liv distribution kinetics, by non-linear regression of experimental data. The microwave degradation was found to be faster and more efficient than degradation by conventional thermal heating. Higher degradation was found at higher concentrations 4.2 FUTURE WORK Experiments and modeling can be done on ultrasonic degradation of polymers. Although the mechanism of degradation is assumed to be mid point and it gives reasonably good fit over experimental data, the nature of degradation is yet to be explored completely. Some more experimentation may give insight into the actual process. Dependence of rate constant on solvent parameters should be studied more thoroughly. Effect of intensity and power on limiting molecular weight is an interesting field of study. In terms of the simulation and theoretical study, ample scope can be found in exploring the most suitable kinetics of ultrasonic degradation. The population balance equation can be solved with different stoichiometric kernel and rate expression that can be physically meaningful. The existence of limiting molecular weight and nonlinear dependence made this problem more interesting. Like microwave assisted degradation, microwave assisted simultaneous polymerization and degradation is an interesting phenomenon to study. That can be studied and the kinetics can be explored. Study of effect of different parameters can also be conducted. lv