Effect of Oxidizers on Microwave-Assisted Oxidative Degradation of

Sep 13, 2008 - Department of Chemical Engineering, Indian Institute of Science, ...... (23) Mahalik, J. P.; Madras, G. Effect of alkyl group substitue...
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Ind. Eng. Chem. Res. 2008, 47, 7538–7544

Effect of Oxidizers on Microwave-Assisted Oxidative Degradation of Poly(alkyl acrylates) A. Marimuthu and Giridhar Madras* Department of Chemical Engineering, Indian Institute of Science, Bangalore-12, India

The effect of oxidizers on the microwave-assisted oxidative degradation kinetics of poly(alkyl acrylates), namely, poly(methyl acrylate) (PMA), poly(ethyl acrylate) (PEA), and poly(butyl acrylate) (PBA), was studied. The molecular weight distributions were measured by gel permeation chromatography, and continuous distribution kinetic models were used to determine the degradation rate coefficients. The effect of alkyl group substituents on the microwave-assisted oxidative degradation of poly(alkyl acrylates) was also investigated. The degradation rate of poly(alkyl acrylates) decreased with an increase in the number of carbon atoms of the alkyl substituents and thus followed the order PMA > PEA > PBA, while the activation energy increased with the length of alkyl group substituents. The rate coefficients of hydrogen abstraction and oxidative random chain scission were found to be independent of the oxidizer and dependent only on the nature of the polymer. The differences in the overall degradation rate of poly(ethyl acrylate) in the presence of different oxidizing agents were only dependent on the rate of oxidizer dissociation. This is the first study that shows that the degradation rate of the polymer in the presence of any oxidizer can be predicted by knowing only the thermal dissociation rate constant values of the oxidizer, which can be easily obtained from existing literature. Introduction The use of microwaves as a nonconventional method for many chemical reactions1,2 has been extensively investigated. Microwave heating has many advantages over conventional heating including better control over the heating process.3 The microwave heating occurs in materials containing polar molecules having an electrical dipole moment. The time taken by the microwave electric field to change direction is comparable to the time of the orientation polarization of dipoles.1 The microwave heating is due to the alignment and reorientation of the molecules in the applied microwave field by rotation of the molecules and the successive rotations at the molecular level. This leads to molecular movement and subsequent heat generation.2 The ability to convert the microwave energy into thermal energy depends mainly on the dielectric constant of the material. Experimental observations on the microwave-assisted reaction show excellent efficiencies (approximately 85%) for conversion of electrical energy into heat.3,4 There are several reports that show an increase in reaction rate when exposed to microwave radiation compared to the rate obtained thermally.5,6 Microwaves are also reported to show product selectivity in some Diels-Alder reactions and confirm the specific activating effect of microwaves under homogeneous conditions.1 Peng et al.7 investigated the synthesis of hydrazides under simultaneous ultrasound and microwave irradiation and reported the enhancement in the reaction rate. In the past few years, using microwave energy to heat and drive chemical reactions has become increasingly popular in the medicinal chemistry also.8 Reviews8,9 on microwave-assisted drug discovery and chemical synthesis have been published. Numerous observations have been reported of enhanced mass transport10 and reaction rates during microwave heating or processing of polymer materials. For monomers containing polar groups that favor the absorption of microwaves, microwaveassisted polymerization has been proven to be more rapid and * To whom correspondence should be addressed. Tel.: 091-8022932321. Fax: 091-80-23600683. E-mail: giridhar@ chemeng.iisc.ernet.in.

efficient than conventional polymerization. An increased polymerization rate of ε-caprolactone11,12 was reported over microwave irradiation. The enhanced reaction rate for the microwave polymerization of poly(methyl acrylate)13 compared to the thermal method was also reported. The enhancement rate was 138, 220, and 275% when the microwave power used was 200, 300, and 500 W, respectively. This indicates a significant correlation between the enhancement in reaction rate and the microwave power. The bulk polymerization of styrene was investigated by Chia et al.,14 and the comparison of thermal and microwave polymerization under similar conditions showed a reaction rate enhancement of 120 and 190% for 300 and 500 W, respectively. The enhanced polymerization rates were also reported15,16 for the emulsion polymerization of styrene under pulsed microwave irradiation. Correa et al.15 reported that the emulsion polymerization of styrene could be carried out more rapidly with significant savings of energy and time when compared to conventional methods. Microwave irradiation has also been used for the manufacture of joining of composite structures and microwave-assisted curing material that resulted in enhanced shear strength.17 Other than these systems, a few studies have also investigated polymer degradation,18-22 suggesting that the microwaveenhanced degradation can occur in polymeric systems. Krzan et al.18 investigated the use of microwave irradiation as the energy source in polyethylene terephthalate (PET) solvolysis reactions and reported that the short reaction times needed for complete PET degradation compared with conventional heating methods. Microwaves were also used to increase the degradation rate of high-density polyethylene and aluminum polymer laminates,3 lignin,19 and cellulose.20 The microwave-assisted oxidative degradation of polystyrene21 and poly(ethylene oxide)22 in solution were also reported to be more efficient than the thermal-assisted process. The overall mechanism in the microwave-assisted oxidative degradation of polymer mainly consists of the oxidizer dissociation, hydrogen abstraction, and depropagation of polymer chain by β-scission.21 Poly(alkyl acrylates) (PAA) with short side chains are relatively polar, and

10.1021/ie7017349 CCC: $40.75  2008 American Chemical Society Published on Web 09/13/2008

Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7539 23,24

recent studies have investigated the effect of alkyl substituents on the thermal, ultrasonic, and enzymatic degradation of these polymers. In the present study, the effect of different oxidizing agents on the degradation kinetics of poly(alkyl acrylates) was investigated. A continuous distribution kinetics model was used to determine the time-dependent kinetic parameters. The activation energies were determined from the Arrhenius temperature dependency of the rate coefficients by nonlinear regression of the experimental data. The results indicated that the oxidative degradation of polymer under microwave radiation could be predicted in the presence of any oxidizer by knowing the dissociation rate constants of just the oxidizer. Experimental Section Materials. Methyl acrylate was obtained from Merck Chemicals. The monomers ethyl acrylate and butyl acrylate, the oxidizers benzoyl peroxide (BPO) and dicumyl peroxide (DCP), and the solvents benzene, dichlorobenzene, and tetrahydrofuran were obtained from S. D. Fine Chemicals. The monomers were purified by washing with 5% caustic solution followed by washing with distilled water and distilling. The initiator azobis(isobutyronitrile) (AIBN) was obtained from Kemphasol. purified by precipitating in acetone, and recrystallized. The solvents were distilled and filtered through 0.2 µm nylon filter paper prior to use. Polymer Synthesis. The solution polymerization technique was used to synthesize the polymers at 60 °C in benzene with benzoyl peroxide as an initiator. An initiator concentration of 2 g/L in the mixture of 60% monomer and 40% solvent (by volume) was used to synthesize the polymers. A 10 ml aliquot of the reaction mixture was taken in culture tubes with screw caps. The temperature was maintained by a water bath, and the variation in temperature was (1 °C. After 12 h of polymerization the unreacted monomer was separated from the polymer by precipitation. Chloroform was used as a solvent and methanol as a nonsolvent. The precipitated polymer was dried in a oven at 110 °C and used for experiments. The number average molecular weights of poly(methyl acrylate), poly(ethyl acrylate), and poly(butyl acrylate) were experimentally determined by gel permeation chromatography to be 157000, 197000, and 181000 with polydispersities of 1.67, 1.34, and 1.41, respectively. Degradation Experiments. A domestic microwave oven with a magnetron source was used (Essentia; 2.45 GHz). A constant power of 700 W was employed for all experiments. The degradation of poly(alkyl acrylates) was conducted at a constant polymer concentration of 5 g/L in a 100 mL (7.0 cm × 4.5 cm) glass beaker. The oxidizer concentrations were varied from 10 to 30 g/L, and the heating cycle time was varied from 40 to 100 s. The volume of the solution taken was 50 mL for all of the experiments. The sample was placed at the center of the oven directly below the magnetron source, and it was rotated on a turntable to avoid the temperature gradients in the reaction mixture. For comparing the effect of oxidizer, experiments were conducted for the degradation of poly(ethyl acrylate) in the presence of three different oxidizerssBPO, DCP, and AIBNs under the same experimental conditions. The oxidizer concentration of 20 g/L was used for the degradation studies in the presence of benzoyl peroxide and dicumyl peroxide. Because AIBN has lower solubility in many organic solvents, an initiator concentration of 5 g/L was used for the degradation studies in the presence of AIBN. All the experiments were conducted in cyclic operation, and each sample was irradiated for 10 cycles. The time for each cyclic operation is τ ()th + tc), which consists

Figure 1. Variation of temperature with heating time.

Figure 2. Temperature profile for the complete cycle.

of different heating time (th) and constant cooling time (tc ) 60 s). A sample of 0.5 mL volume was collected after the first, third, fifth, seventh, and 10th cycles and analyzed in GPC. The temperature of the reaction mixture was measured with a fluoroptic thermometer (Luxtron) with an accuracy of (0.5 °C. The temperature profile of the reaction mixture in the microwave oven mainly depends on the solvent properties and varies linearly in the present study, as shown in Figure 1. The sample reaches the maximum temperature (Tpeak) at the end of the heating period. The irradiated sample was then cooled to Tw (25 °C) by immersing in an ice water bath for a constant set time of 60 s. A linear cooling profile was obtained by adjusting the stirring rate of the reaction mixture. Therefore, the linear heating and cooling period constitutes the triangular temperature profile for each cycle, as shown in Figure 2. Several experiments were conducted in triplicate, and the variation in the rate coefficients was less than 3%. During the microwave degradation of the polymer, no gas-phase product was observed and all products formed were only oligomers of the parent polymer. Sample Analysis. The molecular weight distributions of the polymer samples were determined by gel permeation chromatography (GPC, Waters Inc.). The GPC system consists of an isocratic pump, a sample loop (50 µL), three size exclusion columns of varying pore size (HR 5E, HR 3, and HR 0.5; 300 mm × 7.5 mm), and a differential refractive index detector. Tetrahydrofuran (THF) was used as eluent with a constant flow rate of 1 mL/min through the system, and the columns were

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maintained at 50 °C. The refractive index was continuously monitored and stored digitally using the data acquisition system. The chromatograph was converted to molecular weight distribution using a universal calibration curve determined by using polystyrene standards (Polymer Laboratory). Theoretical Model. The oxidizers used in this study also act as initiators for polymerization and degradation. Thus, the words oxidizer and initiator are used interchangeably in this paper. The homolytic cleavage of initiator (oxidizer) into two radicals can be written as

chain into polymer radical, R*(x′), of molecular weight x′ and polymer, P(x-x′), of molecular weight x - x′. ks(x)

R*(x) 98 R*(x′) + P(x - x′)

The population balance equations for polymer and polymer radicals can be written as21,22 ∂p(x, t) ⁄ ∂t ) -kd(x) c(t) p(x, t) - kh(x) p(x, t) + kH(x) r(x, t) +

∫

∞

kp

C2f 2C*

dcp ) -kpcp dt

(2)

where cp denotes the molar concentration of oxidizer. The hydrogen abstraction of the polymer chain, P(x), of molecular weight x by these radicals can be written as kd(x)

C∗ + P(x) 98 CH + R*(x)

(3)

The population balance equation for the consumption of oxidizer radicals can be written as dc(t) ⁄ dt ) 2kpcp(t) - c(t)

∫

∞

0

kd(x′) p(x′, t) dx′

(4)

To account for the continuous variation of the rate coefficients with time, the temperature profile for each cycle has to be written as a function of time, t. We represent th as the heating time, Tw as the temperature at the end of cooling cycle, and Tpeak as the maximum temperature reached at the end of the heating cycle. The temperature increases linearly with time (as shown in Figure 1) during the heating time, th. The temperature decreases linearly with time (as discussed in the Experimental Section) during the cooling time, tc. Thus the equations for the variation of temperature with time can be written as

{

Tpeak - Tw ∀t ∈ (0, th) Tw + t th T) Tpeak - Tw (t - th) ∀t ∈ (th, τ) Tpeak τ - th

kb

P(x) \ y z R∗(x′) + R∗(x - x′)

(5)

(6)

ka

This step is less frequent compared to the depropagation steps and can be neglected.25 The polymer, P(x), of molecular weight x can reversibly abstract hydrogen to produce polymer radical, R*(x), of molecular weight x. Thus, the reversible hydrogen abstraction from the polymer chain is represented as ∗

P(x) y\z R (x)

ks(x′) r(x′, t) Ω(x, x′) dx ′

(9)

∂r(x, t) ⁄ ∂t ) kd(x) c(t) p(x, t) + kh(x) p(x, t) - kH(x) r(x, t) ks(x) r(x, t) +

∫

∞

x

ks(x ′ ) r(x′, t) Ω(x, x′) dx ′ (10)

The absence of specific products in the GPC chromatograph and the increase of polydispersity due to the broadening of molecular weight distribution, approaching a value of 2 at long times26,27 confirm the random scission of the polymer. For random chain scission, the stoichiometric kernel, Ω(x,x′) is given by 1/x′.25 In the above expressions the rate coefficients, kd, kh, kH, and ks, are assumed to be linearly proportional to the molecular weight, x.28 With application of moment operation on eqs 9 and 10, ks ( j+1) dp( j) ) -kdc(t) p( j+1)(t) - kh p( j+1) + kHr( j+1) + r dt j+1 (11) dr( j) j (j+1) ) kdc(t) p(n+1)(t) + kh p( j+1) - kHr( j+1) - ks r dt j+1 (12) Applying quasi-steady-state approximation to the polymer radicals, eq 12 can be written as r( j+1) ) (j + 1)p( j+1)

kdc(t) + kh jks + (j + 1)kH

(13)

The simultaneous solution of eqs 11 and 13 gives the jth moment as

The polymer, P(x), of molecular weight × can degrade reversibly into two polymer radicals, R*(x′) and R*(x-x′) of molecular weights x′ and x - x′, respectively. Thus, the initiation and termination reactions that occurs during polymer degradation can be written as

kh(x)

x

(1)

The rate of disappearance of oxidizer for the above equation can be written as

(8)

(7)

kH(x)

The depropagation of the polymer radical, R*(x), of molecular weight x can occur by the irreversible β-scission of the polymer

kdc(t) + kh ( j+1) dp( j) ) -(j - 1)ks p dt jks + (j + 1)kH

(14)

For j ) 1, p(1) is constant, indicating that the mass concentration of the polymer is constant throughout the reaction. For j ) 0, the molar concentration of polymer, p(0), is dp(0) ) k0 p(1) dt

(15)

where the overall rate coefficient k0 is given by koxdc(t) + ktherm. The oxidative degradation coefficient,koxd, is kdks/kH, and the thermal degradation coefficient in the absence of oxidizer, ktherm, is khks/kH. Because no polymer degradation was observed in the absence of oxidizer (in the same temperature range), the contribution from thermal degradation can be neglected. Thus, k0 ) koxdc(t). Thus, eq 15 becomes dp(0) ) koxdc(t)p(1) (16) dt The simultaneous solution of eq 16 along with eqs 2 and 4, with boundary conditions Cp(t)0) ) Cpo, C(t)0) ) 0, and p(0)(t)0) ) po(0) and from temperature dependency with time

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from eq 5, gives the number average molecular weight for any time, t. Results and Discussion The degradation of poly(ethyl acrylate) was investigated in the presence of three different oxidizerssBPO, DCP, and AIBNsunder the same experimental conditions. The kinetic parameters in the model can be determined by the nonlinear regression of the experimental data. The relation between the rate coefficients and temperature was assumed by Arrhenius equation and can be written as kp ) kp0 exp(-Ep/RT), kd ) kd0 exp(-Ed/RT), and koxd ) koxd0 exp(-Eoxd/RT). The rate coefficients for the dissociation of oxidizers, kp, are given by ln kp ) 32.37 - 14900 ⁄ T for benzoyl peroxide29 ln kp ) 35.00 - 15900 ⁄ T for AIBN29 ln kp ) 38.38 - 19100 ⁄ T for dicumyl peroxide30 where kp is in s-1 and T is in K. Since the temperature of the system varies with time, the temperature in the rate coefficients can be substituted as an expression in time from eq 5. The kinetic parameters for hydrogen abstraction and oxidative random chain scission were used as model fitting parameters. The rate coefficients were substituted in the governing eqs 2, 4, and 16 and were solved using Mathematica. In all cases, the nonlinear regression coefficient was greater than 0.95. The nonlinearly regressed values obtained for kd p(1) and koxdCp0 are exactly the same for the three oxidizers, and the values are ln(kd p(1)) ) 16.0 - 6000/T and ln(koxdCp0) ) 16.0 - 7500/T, where kd p(1) is in s-1, koxdCp0 is in mol g-1 s-1, and T is in K. Thus, it is apparent that the hydrogen abstraction and oxidative random chain scission rate coefficients are independent of the oxidizer and depend only on the polymer. Since the rate coefficient for eq 3, kd p(1), is independent of the oxidizer, the number of polymer radicals formed from any of these three oxidizer radicals is also independent of the nature of the oxidizer radicals for the microwave-assisted oxidative degradation. The differences in the overall degradation rate of poly(ethyl acrylate) in the presence of different oxidizing agents are only dependent on the rate of oxidizer dissociation. Thus, the degradation rate of poly(ethyl acrylate) in the presence of any other oxidizers can be predicted by only knowing its dissociation rate constant values. The results are discussed as follows. Figures S1-S3 (see Supporting Information) represent the variation of the concentration of the oxidizers and its free radicals with time as predicted by the model. Figures 3-5 show the variation of the molecular weight of the polymers with time for different heating cycles. The effect of various oxidizers is shown in Figure 3, and Figure S1a-d are used to interpret the results. Similarly, the effect of the initial concentration of the oxidizer is shown in Figure 4 and interpreted on the basis of the model predictions shown in Figure S2. The effect of the alkyl chain length on the degradation of the polymer is shown in Figure 5, while discussed in conjunction with the results shown in Figure S3. Figure 3a shows the variation of the number average molecular weight of poly(ethyl acrylate) with time for different heating times and at constant oxidizer initial concentration of 20 g/L for benzoyl peroxide, dicumyl peroxide, and 5 g/L of AIBN. Though the figures and the discussions are based on the mass concentration of the oxidizer, the calculations are carried out with the molar concentration. Each point in the figure represents 10 cycles of different heating times (th) and a constant

Figure 3. Variation of the number average molecular weight of poly(ethyl acrylate) (PEA): (a) with heating time, 10τ, (b) with time for constant heating time of 100 s, and (c) with time for constant heating time of 60 s in the presence of different oxidizers. Experiment: (2) BPO; (0) AIBN; ([) DCP. Model: (-) BPO; ( · · · ) AIBN; (s) DCP.

cooling time of 60 s. Parts b and c of Figure 3 show the variation of the number average molecular weight of poly(ethyl acrylate) with time for constant heating times of 100 and 60 s, respectively. The model predictions are in good agreement with the experimental values. From the figure it can be seen that, for the same initial oxidizer concentration of 20 g/L, the degradation of poly(ethyl acrylate) is faster in the presence of

7542 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008

Figure 4. Variation of the number average molecular weight of PEA with heating time, 10τ, for different benzoyl peroxide initial concentrations (g/ L). Experiment: (0) 10; (2) 15; ([) 20; (2) 25; (b) 30. Model: (s) 10; (- -) 15; (--) 20; (- · -) 25; ( · · · ) 30.

benzoyl peroxide than that observed in the presence of dicumyl peroxide. The difference in the degradation rate in the presence of different oxidizers can be interpreted on the basis of the concentration of oxidizer and oxidizer radical (see the Supporting Information). The concentrations of the oxidizer and oxidizer radicals with time at constant heating times of 100 and 60 s for the three oxidizers (see Supporting Information, Figure S1a-d) indicate that benzoyl peroxide dissociates faster as compared to dicumyl peroxide for the same initial oxidizer concentration resulting in more free radicals. Thus, the consumption of oxidizer and the availability of the oxidizer free radicals follow the order benzoyl peroxide > AIBN > dicumyl peroxide. Because the degradation in the presence of dicumyl peroxide is negligible even after 10 cycles of 60 s heating time, the variation of molecular weight is not plotted for this case in Figure 3c. This is also in accordance with the plot (Figure S1b,d.ii) of the concentration of dicumyl peroxide and its free radical concentration with time at constant heating time of 60 s, which show negligible DCP consumption. For 100 s heating time, Figure 3b shows a significant reduction in the molecular weight of the polymer at the end of the first cycle. The degradation rate is much slower in subsequent cycles. However, for the 60 s heating rate, Figure 3c shows that there is a continuous reduction in the molecular weight with time. This is in accordance with the variation of the free radical concentration with time. For 100 s heating time, the maximum availability of oxidizer free radicals is at the end of the first cycle. However, for the 60 s heating time, the concentration of free radicals available is nearly invariant with the number of cycles. To study the effect of oxidizer concentration on the degradation rate, the degradation of poly(ethyl acrylate) was investigated at five different initial concentrations of benzoyl peroxide and at different heating times. Figure 4 shows the variation of the number average molecular weight of poly(ethyl acrylate) with time (10τ) for different heating times and at different initial concentrations of BPO. The model predictions show a better fit in the higher molecular weight regime (small conversion) and deviate in the low molecular weight regime. A significant decrease in the number average molecular weight of the polymer is observed with an increase in oxidizer concentration. The plots (Figure S2a,b) of benzoyl peroxide concentration and its radical

Figure 5. Variation of the number average molecular weight of poly(alkyl acrylates) (a) with heating time, 10τ, (b) with time at constant heating time of 100 s, and (c) with time for constant heating time of 60 s in presence of 20 g/L of initial BPO concentration. Experiment: (0) PMA; (2)PEA; ([) PBA. Model: ( · · · ) PMA; (--) PEA; (s) PBA,

concentration with time at a constant heating time of 100 s for different initial peroxide concentrations (10-30 g/L). This is consistent with higher oxidizer consumption and radical availability at higher initial peroxide concentration, as shown in Figure S2a,b (see Supporting Information). It should be noted, however, that the rate parameters determined (Table 1) are independent of the oxidizer concentration.

Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7543 Table 1. Rate Parameters Obtained for the Effect of Alkyl Substituent on Microwave-Assisted Oxidative Degradation of Poly(alkyl acrylates)a

ln(kd p(1)) ln(koxdcpo) a

PMA

PEA

PBA

16.3 - 5400/T 16.2 - 7200/T

16.0 - 6000/T 16.0 - 7500/T

15.5 - 7200/T 15.6 - 9500/T

kd p(1) is in s-1, koxdCp0 is in mol g-1 s-1, and T is in K.

Table 2. Comparision of Rate Parameters Obtained for Microwave-Assisted Oxidative Degradation of Poly(ethyl acrylate) with Conventional Thermal Oxidative Degradationa polymerPEA

microwave-assisted oxidative degradation

conventional thermal oxidative degradation31

ln(kd p(1)) ln(koxdcpo)

16.0 - 6000/T 16.0 - 7500/T

27.65 -13400/T 6.27 -10300/T

a

kd p(1) is in s-1, koxdCp0 is in mol g-1 s-1, and T is in K.

The effect of alkyl group substituents on the polymer degradation was investigated by studying the degradation of PMA, PEA, and PBA in the presence of 20 g/L of benzoyl peroxide concentration at different heating times (40-100 s). Figure 5a shows the variation of number average molecular weights of PMA, PEA, and PBA with time (10τ) for different heating times. From the figure it is clear that PMA degrades faster than PEA and PBA. Table 1 shows that the rate coefficients for the hydrogen abstraction and the oxidative random chain scission decrease with an increase in the alkyl group and the degradation rate follows the order PMA > PEA > PBA. A similar trend has been reported23,24 for the degradation of poly(alkyl acrylates) by thermal, ultrasonic, and enzymatic degradation. The activation energies (Eoxd) for the microwave-assisted oxidative degradation of PMA, PEA, and PBA were determined to be 14.3, 14.9, and 18.9 kcal/mol, respectively, and thus the activation energy increased with the length of the alkyl group. Parts b and c of Figure 5 show the variation of the number average molecular weight of poly(alkyl acrylates) with time at constant heating times of 100 and 60 s, respectively. For the same dissociation rate as that of benzoyl peroxide, the differences in molecular weight variation of the poly(alkyl acrylates) arise from the differences in the hydrogen abstraction and oxidative random chain scission rate coefficients. The plots (Figure S3a,b) show the variation of the concentration of benzoyl peroxide radical with time at constant heating time of 100 and 60 s for the degradation of three poly(alkyl acrylates) in presence of constant initial peroxide concentration of 20 g/L. These figure panels indicate the availability of the oxidizer radical is higher during the degradation of poly(butyl acrylate) compared to that of poly(methyl acrylate) degradation. This also indicates that the concentration of peroxide radicals produced (eq 1) is the same for the degradation of the three polymers, but the peroxide radical consumed (eq 3) to produce the polymer radical depends on the nature of the polymer. Because poly(butyl acrylate) is more stable than the other two polymers, the peroxide radical consumption for this polymer is less, leading to a smaller concentration of the polymer radical that can degrade. The comparison of the rate parameters obtained in the present study for the microwave-assisted oxidative degradation of poly(ethyl acrylate) with our previous study31 under conventional oxidative thermal degradation confirm the enhancement in the degradation rate under microwave radiation. The comparison of the rate coefficients is shown in Table 2. From Table 2, it is clear that the activation energies required for both hydrogen abstraction and oxidative random chain scission steps are less under microwave radiation as compared to conventional

thermal heating. The comparison between absolute values of rate coefficients shows 4 orders of magnitude enhancement in hydrogen abstraction rate constant and 7 orders of magnitude enhancement in oxidative random chain scission rate constant. The rate coefficient, koxd, is approximately equal to the β-scission rate constant value (ks). The activation energy required (14.3-18.9 kcal/mol) for the β-scission of poly(alkyl acrylates) radical under microwave radiation is less than (24.7-28.1 kcal/mol) that required for the polystyrene radical under conventional thermal degradation.32 Similarly the values of chain scission rate coefficients under microwave-assisted oxidative degradation are 2-3 orders of magnitude higher than the chain scission rate constant values reported for the ultrasonic degradation of poly(alkyl) acrylates.24 In the ultrasonic degradation of poly(alkyl) acrylates,24 the polymer attains a limiting molecular weight after which no degradation takes place. This is in contrast to degradation by microwave radiation where complete degradation takes place. These comparisons confirm the enhanced degradation rate under microwave-assisted oxidative degradation. Conclusions The microwave-assisted oxidative degradation of PMA, PEA, and PBA was investigated in the presence of 20 g/L of benzoyl peroxide. On the basis of these studies, it is found that the degradability of the polymer in the presence of oxidizer decreases with an increase in the alkyl group chain length of poly(alkyl acrylate) and the activation energy increased with the length of the alkyl group. The polymer shows significant increase in the degradation rate with an increase in the initial oxidizer concentration. The degradation of poly(ethyl acrylate) was investigated in the presence of three different oxidizers. The hydrogen abstraction and oxidative random chain scission rate coefficients were found to be independent of the oxidizer and dependent only on the polymer. The differences in the overall degradation rate of poly(ethyl acrylate) in the presence of different oxidizing agents were dependent only on the rate of dissociation of the oxidizer. Therefore, the degradation rate of polymer can be predicted in the presence of any oxidizer by just knowing the dissociation rate constant of the particular oxidizer. Acknowledgment G.M. acknowledges the Department of Science and Technology, India for financial support and the Swarnajayanthi Fellowship. Supporting Information Available: Figure S1, showing variation of the concentration of oxidizer (model prediction) with time for PEA degradation at constant heating time of (a) 100 and (b) 60 s in the presence of different oxidizers, (c) of the oxidizer radical at 100 s in the presence of different oxidizers, and (d) of the oxidizer radical at 60 s in the presence of (i) BPO and AIBN and (ii) DCP, Figure S2, showing variation of the concentration of (a) BPO and (b) BPO radical (model predictions) with time for PEA degradation at constant heating time of 100 s in presence of benzoyl peroxide of different initial concentration (g/L), and Figure S3, showing variation of concentration of BPO radical (model prediction) with time at constant heating time of (a) 100 s for different polyalkyl acrylates (PAA) degradation and (b) 60 s for (i) poly(methyl acrylate) (PMA) and poly(ethyl acrylate) (PEA) and (ii) poly(butyl acrylate) (PBA) degradation. This material is available free of charge via the Internet at http://pubs.acs.org.

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ReceiVed for reView December 20, 2007 ReVised manuscript receiVed July 13, 2008 Accepted July 25, 2008 IE7017349