Photocatalytic Oxidative Degradation of Poly(alkyl acrylates) with

Thermal degradation of polymers occurs by random chain scission or by chain ... and mechanism.28-30 These studies report that photooxidative degradati...
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Photocatalytic Oxidative Degradation of Poly(alkyl acrylates) with NanoTiO2 A. Marimuthu and Giridhar Madras* Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India

The degradation of poly(alkyl acrylates) was investigated in the presence of two catalysts: commercial Degussa P-25 and TiO2 synthesized by combustion synthesis (CS-TiO2). The molecular weight distributions of the polymers were measured by gel permeation chromatography. The effect of alkyl group substituents on the photodegradation of poly(alkyl acrylates) was investigated. The degradation rate of poly(alkyl acrylates) decreased with increase in the number of carbon atoms of the alkyl substituents, and the degradation of the polymers in the presence of CS-TiO2 was significantly higher than that obtained with Degussa P-25. The effect of oxidizer on the UV-assisted oxidative degradation kinetics of poly(ethyl acrylate) was studied. The degradation was also investigated at various temperatures without UV, and enhancement in the oxidative degradation rate under UV exposure was observed compared to that under thermal conditions. Theoretical models for the degradation in the presence and absence of oxidizers using continuous distribution kinetics were developed to determine the kinetic parameters. The hydrogen abstraction and oxidative random chain scission rate coefficients were found to be independent of the oxidizer and were dependent only on the polymer. The overall polymer degradation rate in the presence of different oxidizers was predicted without any adjustable parameters. Introduction The study of polymer degradation is important in characterizing its structure, for understanding its stability, and for largescale plastic recycling.1 Degradation of polymers can occur due to the application of heat, light, or chemical reagents or by mechanical means such as ultrasound. Polymer degradation studies2-6 in the presence of oxidizing agents indicate faster degradation rates compared to ordinary thermal degradation. Scott et al.7 demonstrated the photoinitiated oxidative degradation of cross-linked polymers by introducing radicals via photodecomposition of the residual photoinitiator in the polymer matrix. This results in plasticity in the polymers through the chain transfer of functional groups. Mathematical models for degradation kinetics and time evolution of molecular weight distributions (MWDs) are needed for the better understanding of polymer degradation. Thermal degradation of polymers occurs by random chain scission or by chain end scission, and earlier experimental results confirm that oxidative degradation of polymers in solution occurs only by random chain scission.3,4 The thermooxidative degradation of styrene-b-butadiene diblock copolymer films was studied,8 and a kinetic model was developed by accounting for both oxidative chain scission and cross-linking reactions. A mechanistic scheme for polymer oxidation in which the source of radicals is the unimolecular decomposition of hydroperoxides has been proposed.9 It was stressed that the scientific community should endeavor to determine exact mathematical solutions for this kind of complicated system. Reich and Stivala10 discussed various schemes to interpret the complexities in oxidative degradation, and a detailed review11 on the kinetics and mechanism of oxidative degradation has been published. The free radicals of initiators can be generated in a number of ways, and the commonly used methods to produce radicals are the thermal and photochemical decomposition of peroxides.12 The generation of free radicals by the thermal decomposition method has some disadvantages, such as the difficulties in * To whom correspondence should be addressed. Tel./fax: 91-08022932321. E-mail:[email protected].

controlling the rate of generation of free radicals and the comparatively high energy consumption for radical generation. However, the photodecomposition of photoactive initiators can be controlled with high precision by controlling the intensity of the light.12 There have been many investigations on the photopolymerization13 and photodegradation of polymers.14-16 The use of catalysts for enhancing polymerization17,18 and degradation19 has been extensively investigated in recent years. TiO2 and metal-substituted TiO2 are well-known semiconductor catalysts that are extensively used for the photodegradation of dyes, surfactants, toxic organic compounds like phenols, oxalates, pesticides, pollutants, and polymers.19-21 Several studies have reported the TiO2 enhanced photodegradation rates of polymer nanocomposites22,23 and polymers in solution.24,25 The solution combustion method produces nanoparticles of higher surface area, and TiO2 prepared by this method was found to degrade the dyes at a faster rate compared to commercial catalysts.26 Since the poly(alkyl acrylates) and their copolymers have a wide variety of applications such as making textiles, glass fibers, automobiles, shoes, floor coverings, paints, and coatings, investigation into their photochemical behavior is very important.27 There are several studies on the photodegradation of poly(alkyl acrylates) and poly(alkyl methacrylates) using solid samples with a focus on the mode of degradation and mechanism.28-30 These studies report that photooxidative degradation of poly(alkyl acrylates) undergoes extensive random free radical chain scission. It has been difficult to obtain quantitative kinetic information and to deduce reaction mechanisms from such solid sample studies. The main complication associated with the solid state is that it makes the system complex by concurrent cross-linking. This can be greatly minimized by using the liquid phase and thus the cross-linking probabilities can be reduced.31 Solvents such as chlorobenzene and dichlorobenzene appear to be inert solvents for oxidation reactions.31 Other than these systems, a few studies32,33 have investigated the oxidative degradation of polystyrene and poly(R-methylstyrene) initiated by photodecomposition of 2,2′azobisisobutyronitrile (AIBN), suggesting that the random scission of the polymer chain occurs in the system. However,

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no detailed kinetic information is available for these systems. Among the photoinitiators, AIBN, benzoyl peroxide (BPO), and dicumyl peroxide (DCP) are generally used photoactive initiators, and a number of studies have been reported in the literature for the photoinitiated reactions using these initiators.33,34 The objective of this work is to (a) confirm the enhancement in degradation rate of photoinitiated oxidative degradation compared to thermal-assisted degradation in the absence of UV, (b) determine the enhancement in the degradation rate of polymers by combustion-synthesized nanoanatase TiO2 (CS-TiO2) as a photocatalyst under UV and solar exposure, and (c) develop kinetic mechanisms for the photoassisted oxidative degradation of polymers in solution. The effects of different oxidizers and alkyl group substituents on the degradation kinetics of poly(alkyl acrylates) were also investigated. To the best of our knowledge, this is the first detailed kinetic study that attempts to predict the overall polymer degradation rate in the presence of different oxidizers. Materials and Methods Materials. Methyl acrylate was obtained from Merck Chemicals (India). The monomers ethyl acrylate and butyl acrylate, the oxidizers benzoyl peroxide and dicumyl peroxide, and the solvents benzene, dichlorobenzene, and tetrahydrofuran were obtained from S. D. Fine chemicals (India). The monomers were purified by washing with 5% caustic solution followed by washing with distilled water and distillation. The initiator AIBN was obtained from Kemphasol Inc. (India) and purified by precipitating in acetone and recrystallization. The solvents were distilled and filtered through 0.2 µm nylon filter paper prior to use. The precursor for the catalyst titanium isopropoxide (Lancaster) and the fuel glycine (Merck) were used as purchased. Catalyst Preparation and Characterization. The solution combustion method was used for the preparation of anatase CSTiO2 by using titanyl nitrate [TiO(NO3)2] as precursor with glycine [C2H5NO2] as fuel. The titanyl nitrate was synthesized by a two-stage process: the first step involves the hydrolysis of titanium isopropoxide and in the second stage it is converted into titanyl nitrate by nitration with nitric acid. The titanium content of the solution was determined in a UV-visible spectrophotometer by the standard calibration procedure, using potassium titanyl oxalate. Hydrogen peroxide was added to the solution for the estimation to give a yellow color to the titanyl nitrate solution, which causes the maximum absorption for the solution at 410 nm. An aqueous redox mixture of titanyl nitrate and glycine (100 mL) in stoichiometric amount was taken in a Pyrex glass dish and heated to 600 K in a muffle furnace for the catalyst synthesis by the combustion method. The solution underwent dehydration followed by frothing to give TiO2. The overall reaction for the above process can be written as

9TiO(NO3)2 + 10C2H5NO2 f 9TiO2 + 14N2 + 20CO2 + 25H2O The reaction was conducted at high temperature (623 K) only for a short time and cooled immediately to lower temperature; therefore, pure anatase phase CS-TiO2 was obtained and the other phases of titania such as rutile and brookite were not present. The detailed structure of CS-TiO2 has been reported earlier.35,36 X-ray diffraction patterns showed that CS-TiO2 is more crystalline than Degussa P-25; they also confirm that CSTiO2 is in anatase phase and other phases such as rutile and brookite are absent, whereas Degussa P-25 has only 80% anatase

phase. Transmission electron microscopy (TEM) of the catalyst showed that the particles are homogeneous in size with a mean size of 11 ( 3 nm, while Degussa P-25 is 25-30 nm. The surface area of CS-TiO2 is found to have higher surface area (250 m2/g) than commercial Degussa P-25 (50 m2/g). Thermogravimetric analysis suggested that CS-TiO2 has more surface hydroxyl groups compared to commercial Degussa P-25. The CS-TiO2 catalyst showed 11.4% weight loss, whereas Degussa P-25 showed only 1.2% weight loss. The product analysis from thermal desorption studies under vacuum confirmed that the weight loss is water.35 This is confirmed by the Fourier transform infrared (FTIR) spectra, as described below. The FTIR spectra of the catalysts showed a broad band at 3350 cm-1. This can be attributed to the surface bound hydroxyl species. The corresponding peaks for Degussa P-25 under identical conditions are much lower, indicating that CS-TiO2 has a larger amount of hydroxyl groups, in accordance with thermogravimetric analysis. The UV-vis diffuse reflectance spectra of both catalysts were measured for the dry pressed disk samples using a UV-vis spectrophotometer (GBC Cintra 40, Australia) and used to determine the band gaps.35 Commercial Degussa P-25 TiO2 showed a single peak at 400 nm (which corresponds to a band gap energy of 3.10 eV), while the combustion-synthesized TiO2 showed two optical absorption thresholds at 570 and 467 nm that correspond to the band gap energies of 2.18 and 2.65 eV, respectively. The decrease in the band gap can be due to carbide ion substitution for oxide ion in the TiO2, as suggested by Khan et al.37 This was further confirmed by XPS, which indicated that the combustionsynthesized TiO2 showed substitution of carbon in the form of TiO2-2xCx0x. The lower band gap, higher surface area, and lower size make it an attractive and potential catalyst for use in photodegradation studies. Polymer Synthesis. The solution polymerization technique was used to synthesize the polymers at 60 °C in benzene using benzoyl peroxide as an initiator. An initiator concentration of 2 g/L and monomer concentration of 60 vol % in benzene was used to synthesize the polymers. The number-average molecular weights of the obtained poly(methyl, ethyl, and butyl acrylates) were found to be 15 500, 184 000, and 197 000 with polydispersities of 1.62, 1.37, and 1.45, respectively. Photochemical Reactor. A jacketed quartz tube of 3.4 cm inner diameter, 4 cm outer diameter, and 21 cm length was used as a source of UV energy. A high-pressure mercury vapor lamp of 125 W (Samson lamp) was used as the light source. The lamp was connected in series with a ballast and capacitor to minimize the fluctuation in the input supply. The temperature of the reactor was maintained at 40 °C by circulating cold water through the annulus portion. The reaction vessel was a glass cylinder of 6 cm inner diameter and 16 cm height, and it was placed concentrically over the quartz jacket source. Further details of the photochemical reactor are provided elsewhere.35,36 The polymer solution was stirred uniformly with a magnetic stirrer during the degradation period. The various wavelengths recorded in the emission spectra were 258, 312, 360, 365 (predominant), 395, 442, and 512 nm. The photon flux, determined by chemical actinometry using o-nitrobenzaldehyde at the predominant wavelength, was 5.3 × 10-6 mol of photons/ s. The solar experiments were conducted with direct sunlight in a cylindrical borosilicate glass reactor with 5 cm inner diameter and 8 cm height. To minimize solar intensity fluctuation, the experiments were carried out between 10 a.m. and 3 p.m. and the average solar intensity during this period was 0.757 kW/m2.

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Degradation Experiments. The degradation reactions were conducted with an initial polymer concentration of 3 g/L in dichlorobenzene. A 80 mL volume of the solution was taken each time and the concentration of the catalyst was fixed at 1 g/L for all the photocatalytic experiments to study the kinetics of degradation. The catalysts employed were commercial nanoTiO2 from Degussa (P-25) and nanoanatase TiO2 phase synthesized by the solution combustion method. For comparing the effect of oxidizer, experiments were conducted for the degradation of poly(ethyl acrylate) in the presence of three different oxidizerssbenzoyl peroxide, dicumyl peroxide, and azobisisobutyronitrile (AIBN)sunder the same experimental conditions with the initial oxidizer concentration of 5 g/L. Samples of solutions were taken at various time intervals, and the molecular weight distribution was obtained by injecting the samples in a gel permeation chromatograph. The samples were centrifuged and filtered to remove catalyst particles before analysis. For the oxidative degradation under thermal conditions, the polymer was dissolved in dichlorobenzene and the concentration was 3 g/L. The initial concentration of the oxidizers was kept constant at 5 g/L. A 15 mL volume of the polymer solution was taken in culture tubes with screw caps. The temperature was maintained by a water bath controlled by a PID controller, and the variation in temperature was (1 °C. The experiments were conducted in the temperature range 50-80 °C. The tubes were placed in the water bath until it reached the desired temperature, and then initiators were added to the polymer solution. A control experiment was also done at each temperature to ensure that no degradation of the polymer occurred without initiators. Several experiments were conducted in triplicate, and the variation in the rate coefficients was less than 3%. Sample Analysis. The molecular weight distribution of the polymer samples was determined by gel permeation chromatography (GPC; Waters, USA). 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 maintained at 50 °C. The refractive index was continuously monitored and stored digitally using a data acquisition system. The chromatograph was converted to the molecular weight distribution using a universal calibration curve determined using polystyrene standards (Polymer Lab, U.K.). Determination of Oxidizer Concentration. To determine the dissociation rate of the oxidizers during photodegradation, experiments were conducted by adding only the initiators in dichlorobenzene and degrading under the same experimental conditions as was done for polymer degradation in the presence of initiators. Samples were taken at regular intervals, and the concentration of AIBN was determined using a UV/visible spectrophotometer (Shimadzu, UV-2100) in the wavelength range 200-500 nm. Calibration based on the Beer-Lambert law was used to quantify the concentration of AIBN. AIBN shows absorption bands in the 310-380 nm regions with maximum absorption at 340 nm. The initiators benzoyl peroxide and dicumyl peroxide were analyzed using high-pressure liquid chromatography (HPLC). The HPLC consists of an isocratic pump (Waters 501), a Rheodyne injector, a C-18 column, a UV detector (Waters 2487), and a data acquisition system. The eluent stream consists of 20% water and 80% methanol (volume percent) pumped at 0.5 mL/min. Samples were injected in a Rheodyne valve with a sample loop of 50 µL, and the UV

absorbance at 227 nm was continuously monitored using an UV detector and stored digitally. The chromatograph was converted to concentration units using calibration with pure compounds. Theoretical Model For the oxidative degradation of the polymer, the oxidizer initiates the reaction by the homolytic cleavage followed by the random hydrogen abstraction from the polymer molecule to produce polymer radicals. MWD results showed no evidence of repolymerizing or cross-linking reactions, which would be indicated by broadening of the MWD due to formation of high MW compounds. Further, specific products from chain end scission (depolymerization), which would be indicated by separate low MW peaks, were not observed. The absence of specific products in the GPC chromatograph and the increase of polydispersity, approaching a value of 2 at long times,2,38 confirm the random scission of the polymer. In this study, all the three initiators (oxidizers) undergo homolytic cleavage to yield two identical radicals. This reaction occurs only negligibly at ambient temperature in the absence of UV/solar exposure. Under UV/solar exposure, the rate constant for this reaction is influenced by the intensity and frequency of radiation. This reaction can be represented as kp

C2 98 2C*

(A)

The hydrogen abstraction of the polymer chain, P(x), of molecular weight x through oxidizer radicals can be written as kd(x)

C* + P(x) 98 CH + R*(x)

(B)

The molar concentration of the oxidizer, cp, as a function of time, from reaction A is cp ) cp0e-kpt, where cp0 and kp are the initial molar concentration and the dissociation rate constant of oxidizer, respectively. This can be substituted into the population balance equation for the consumption of oxidizer radicals from (B)

dc(t)/dt ) 2kpcp(t) - c(t)

∫0∞kd(x′) p(x′,t) dx′

and solved to yield

c(t) )

2kpcp0 (1)

(kdp

(e-kpt - e-kdp t) (1)

- kp)

(1)

where c(t) is the molar concentration of oxidizer radicals at any time (t). The initiation and termination that occurs during polymer degradation can be written as ka

P(x) {\ } R*(x′) + R*(x-x′) k

(C)

b

This step is less frequent compared to the depropagation steps and can be neglected.39 The reversible hydrogen abstraction from the polymer chain is kh(x)

} R*(x) P(x) {\ k (x) H

(D)

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The depropagation step that occurs by the irreversible β-scission of the polymer chain is ks(x)

R*(x) 98 R*(x′) + P(x-x′)

(E)

The population balance equations for polymer and polymer radicals can be written as

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

∫x∞ks(x′) r(x′,t) Ω(x,x′) dx′

(2a)

∂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′

(2b)

For random chain scission, the stoichiometric kernel, Ω(x,x′), which determines the distribution of scission products is given by 1/x′.39,40 In the above expressions the rate coefficients, kd, kh, kH, and ks, are assumed to be linearly proportional to the molecular weight.3-6,41 Applying moment operation on eqs 2a and 2b

Figure 1. Variation of [(Mn0/Mn) - 1] of PEA without oxidizers (a) under UV exposure and (b) in solar radiation. Lines are model fit.

When no oxidizer is present, eq 6 reduces to

ks (j+1) dp ) -kdc(t) p(j+1)(t) - khp(j+1) + kHr(j+1) + r dt j+1 (3a) (j)

j dr(j) ) kdc(t) p(n+1)(t) + khp(j+1) - kHr(j+1) - ks r(j+1) dt j+1 (3b) Applying quasi-steady-state approximation to the radicals, eq 3b can be simplified and substituted into eq 3a to yield

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

(4)

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) ) k0p(1) dt

(5)

where the overall rate coefficient k0 is given by koxidc(t) + kt. The oxidative degradation coefficient, koxid, is kdks/kH, and the degradation coefficient in the absence of oxidizer, kt, is khks/ kH. Since the degradation in the presence of oxidizer is much faster than in its absence, the overall rate coefficient (k0) can be written as k0 ≈ koxidc(t). Thus eq 5 becomes

dp(0) ) koxidc(t)p(1) dt

(6)

The simultaneous solution of eq 6 along with eq 1, with the initial condition p(0)(t)0) ) p0(0), will give the number-average molecular weight for any time (t). The analytical solution is given by

2Mn0cp0koxid Mn0 (1) (kp(1 - e-kdp t) -1) (1) (1) Mn kdp [kp - kdp ] kdp(1)(1 - e-kpt)) (7) where Mn ()p(1)/p(0)) is the number-average molecular weight of polymer.

dp(0) ) ktp(1) dt

(8)

Solving eq 8 with the initial condition p(0)(t)0) ) p0(0)

(Mn0/Mn) - 1 ) kMn0t

(9)

Equation 9 indicates that the variation of (Mn0/Mn) - 1 with time is linear for the degradation in the absence of oxidizer with the slope of kMn0, where k is the degradation rate coefficient that is independent of initial molecular weight. Results and Discussion Several kinds of experiments were conducted for the degradation of polymers. The polymers were degraded (a) under UV (Figure 1a) and solar exposure (Figure 1b) without oxidizers or catalyst, (b) under UV exposure without oxidizers but with catalyst (Figure 2), (c) under UV exposure with oxidizers but without catalyst (Figures 3a, 4 ,and 5), (d) under UV exposure with oxidizers and catalyst (Figure 3b), (e) under solar exposure with oxidizers but without catalyst (Figure 6), and (f) without UV/solar exposure at various temperatures with oxidizers and without catalyst (Figure 7). It is important to recognize the differences in various systems. In the presence of oxidizer alone without any catalyst, the system is homogeneous. However, in the presence of the catalyst, the reaction occurs both homogeneously and heterogeneously. However, the rate coefficients of oxidizer dissociation (kp) and the hydrogen abstraction (kd) are unaffected by the presence of the catalyst. Thus, the catalyst only enhances the rate coefficient for depropagation (koxid), as shown in Table 1. In the case of reactions conducted in the absence of oxidizers (cases a and b), eq 9 applies while eq 7 applies to all other cases. No observable degradation of polymer was observed in the absence of both TiO2 and UV. However, the poly(alkyl acrylates) degraded in the presence of either UV or UV + TiO2. Initially the degradation of poly(ethyl acrylate) was carried out without catalyst. Experiments were then carried out in the presence of Degussa P-25 and with CS-TiO2. The random mode of chain scission exhibited by the polymer in solution is evident from the linear variation of (Mn0/Mn) - 1

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Table 1. Rate Parameters Obtained under Various Experimental Conditions kp × 104 (s-1)

kdp(1) (s-1)

koxidcpo × 105 (mol g-1 s-1)

AIBN BPO DCP

(a) Degradation of PEA with Various Oxidizers in UV without Catalyst 7.90 11.10 4.50 same as above 1.95 same as above

6.32 same as above same as above

AIBN BPO DCP

(b) Degradation of PEA with varIous Oxidizers in UV in the Presence of 1 g/L of CS-TiO2 7.90 11.10 4.50 same as above 1.95 same as above

11.25 same as above same as above

PMA PEA PBA

(c) Degradation of PMA, PEA, and PBA in UV in the Presence of BPO without Catalyst 4.50 12.31 4.50 11.10 4.50 0.19

13.01 6.32 0.04

BPO

(d) Degradation of PEA under Solar Exposure in the Presence of BPO without Catalyst 0.33 0.08

0.07

60 °C 70 °C 80 °C

(e) Thermal Oxidative Degradation of PEA in the Presence of BPO at Various Temperatures 0.42 3.89 1.55 5.70 5.32 39.51

0.22 0.31 1.26

with time with and without catalyst (Figure 1a, eq 9). This suggests that the presence of catalysts increases the degradation rate without altering the degradation mechanism. The rate coefficients kt (× 10-5 mol g-1 s-1) obtained from the slopes of the regressed lines are 3.37, 5.93, and 12.27 for the degradation of polymer without catalyst, with Degussa P-25, and with CS-TiO2 respectively. This indicates that the presence of Degussa P-25 nearly doubles the noncatalytic reaction rate while the presence of CS-TiO2 increases the noncatalytic reaction rate by nearly 4 times. The photocatalytic degradation of poly(alkyl acrylates) under solar exposure was also investigated to examine whether the polymers degrade under solar radiation. However, the rate coefficients obtained for degradation under UV and solar exposure cannot be compared because the emission spectra and intensities are different. The solar degradation of poly(ethyl acrylate) with a concentration of 3.0 g/L was conducted under the conditions mentioned earlier. Figure 1b shows the variation of molecular weight in the presence of combustion-synthesized, Degussa P-25 catalysts and in the absence of catalyst. The rate coefficients kt (× 10-5 mol g-1 s-1) obtained from the slopes of the regressed lines are 1.22, 2.62, and 4.17 for the degradation of polymer without catalyst, with Degussa P-25 TiO2, and with CS-TiO2 respectively. This indicates that the presence of Degussa P-25 nearly doubles the noncatalytic reaction rate while the presence of CS-TiO2 increases the noncatalytic reaction rate by nearly 3.5 times. Thus the enhancement in the photodegradation of polymer by these catalysts is of the same order of magnitude as compared in solar and UV degradation. This indicates the feasibility of the combustion-synthesized catalyst as a photocatalyst for the photodegradation of poly(alkyl acrylates). The higher photocatalytic activity of CS-TiO2 compared to that of Degussa P-25 TiO2 can be due to the higher number of hydroxyl groups and reduced band gap in addition to the higher surface area, nanocrystalline size, and pure anatase form of TiO2.36 The effect of alkyl group substituents on the photocatalytic degradation of poly(alkyl acrylates) was investigated by studying the degradation of PMA, PEA, and PBA in the presence of 1 g/L CS-TiO2. The rate coefficients kt (× 10-5 mol g-1 s-1) obtained from the slopes of the regressed lines (Figure 2) are 21.34, 12.27, and 6.36 for the degradation of PMA, PEA, and PBA, respectively, and the degradation rate follows the order PMA > PEA > PBA. A similar trend has been reported42,43

for the degradation of poly(alkyl acrylates) by thermal, enzymatic, and ultrasonic degradation. To study the effect of oxidizer on the degradation rate, the degradation of poly(ethyl acrylate) (PEA) 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 to eq 7. The dissociation rate constants of the oxidizers were determined as discussed in the experimental section. The kinetic parameters for hydrogen abstraction and oxidative random chain scission were used as model fitting parameters. The model is first used to predict the number-average molecular weight of PEA for the degradation in the presence of AIBN. The dissociation rate constant of AIBN was substituted in the governing equation (7) and solved using Polymath 5.1. The nonlinearly regressed values of kdp(1) and koxidcp0 obtained by Levenberg-Marquardt (LM) algorithm are given in Table 1. The same values of kdp(1) and koxidcp0 obtained for the degradation in the presence of AIBN are used for predicting the number-average molecular weight for the degradation in the presence of DCP and BPO by substituting the corresponding dissociation rate constant values of the oxidizers in the governing equation (7). The model prediction is in good agreement with the experimental values (Figure 3a). Thus it is apparent that the hydrogen abstraction and oxidative random

Figure 2. Variation of [(Mn0/Mn) - 1] of PMA, PEA, and PBA with time under UV exposure in the presence of CS-TiO2 of concentration 1 g/L. Lines are model fit.

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Figure 4. Variation of number-average molecular weight of PMA, PEA, and PBA with time at constant initial BPO concentration of 5 g/L. Lines are model fit.

correspondence with the model prediction where the numberaverage molecular weight from eq 7 at infinite time reduces to

Mn lim ) 1+

Figure 3. (a) Variation of number-average molecular weight of PEA with time in the presence of various initiators without catalyst. Lines are model fit. (b) Variation of number-average molecular weight of PEA with time in the presence of various initiators and CS-TiO2. Lines are model fit.

chain scission rate coefficients are independent of the oxidizer and depend only on the nature of polymer. It is clear that, by calculating the values of kdp(1) and koxidcp0 (which are dependent only on the nature of the polymer) from the polymer degradation in the presence of any one of the oxidizers, the above model can be used to predict the degradation rates in the presence of any other oxidizers by only knowing the rate constant values (kp) of the oxidizers. Since the rate coefficient for eq 3, kd, 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. Similar results were assumed by Ikeda et al.33 for the oxidative degradation of poly(methylstyrene) initiated by photodecomposition of AIBN. Thus 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. Figure 3a shows the variation of the number-average molecular weight of poly(ethyl acrylate) with time at a constant initial oxidizer concentration of 5 g/L. From the figure, it can be seen that, for the same oxidizer concentration of 5 g/L, the degradation of poly(ethyl acrylate) is faster in the presence of AIBN than in the presence of benzoyl peroxide and dicumyl peroxide. This is due to the higher rate of consumption of oxidizer and availability of more oxidizer free radical for AIBN. This is due to the faster photodissociation rate compared to the other two initiators for the same initial oxidizer concentration. From the figure, it is also evident that the number-average molecular weight reaches a limiting value at long times. This is in

Mn0 2Mn0koxidcp0 kdp(1)

The predicted limiting value of the number-average molecular weight is independent of oxidizer and the value for the abovementioned experimental condition is 59 000 g/mol, which is confirmed in Figure 3a. To investigate the effect of catalyst on the photoinitiated oxidative degradation, the degradation of poly(ethyl acrylate) was carried out in the presence of the above-mentioned three initiators with CS-TiO2 of concentration 1 g/L. The experiments were also conducted for the photodissociation of initiators in the presence of 1 g/L CS-TiO2. There are no significant changes observed in the values of dissociation rate constants (kp) of the initiators. The optimized values of kdp(1) and koxidcp0 obtained for the degradation in the presence of AIBN and CS-TiO2 are given in Table 1b. The same values of kdp(1) and koxidcp0 obtained for the degradation in the presence of AIBN are used for predicting the number-average molecular weight for the degradation in the presence of DCP and BPO as it was used for the oxidizer effect. For this case also, the model predicts the experimental values (Figure 3b) and confirms the observation that the overall degradation rate can be predicted by knowing the dissociation rate constant of the initiators (oxidizers). From Table 1a,b, it is clear that the presence of catalyst enhances the random chain scission of the polymer radical (reaction E) and it does not affect the hydrogen abstraction by oxidizer radicals from the polymer (step B), which is much faster than chain scission of the polymer radical (reaction E). The predicted limiting value of number-average molecular weight for the effect of catalyst is 38 000 g/mol, which is less than the value (59 000 g/mol) in the absence of catalyst. In the abovementioned Figure 3, the polymer degradation follows the order AIBN > benzoyl peroxide > dicumyl peroxide. The effect of alkyl group substituents on the photoinitiated oxidative polymer degradation was investigated by studying the degradation of PMA, PEA, and PBA in the presence of benzoyl peroxide of 5 g/L initial concentration under UV exposure. Figure 4 shows the variation of number-average molecular weights of PMA, PEA, and PBA with time. From the figure, it is clear that PMA degrades faster than PEA and PBA. Table 1c

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Figure 5. Variation of number-average molecular weight of PEA with time at different initial BPO concentrations (g/L). Lines are model fit.

shows that the rate coefficients for the hydrogen abstraction and the oxidative random chain scission decrease with increase in alkyl group and the degradation rate follows the order PMA > PEA > PBA. For the same dissociation rate 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 variation of oxidizer radical concentration with time shows that the oxidizer radical availability is constant for the degradation of poly(alkyl acrylates) for the same experimental conditions. This is expected because the rate of peroxide radical consumption in reaction B to produce polymer radical is faster than the rate of peroxide radical formation from reaction A and the difference is 3-5 orders of magnitude (Table 1). The difference in the overall degradation rate of polymer is due to the random chain scission of the polymer radical for the alkyl group effect, and since poly(butyl acrylate) is more stable than other two polymers, its degradation rate is slower. The predicted limiting number-average molecular weights for PMA, PEA, and PBA are 36 000, 59 000, and 110 000 g/mol, respectively, which are in good agreement with the experimental values (Figure 4). To study the effect of concentration of oxidizer on the degradation rate, the degradation of poly(ethyl acrylate) was investigated at different initial concentrations of benzoyl peroxide. Figure 5 shows the variation of number-average molecular weight of poly(ethyl acrylate) with time at different benzoyl peroxide initial concentrations. The model predictions are in good agreement with the experimental data. It also shows the significant decrease in number-average molecular weight of the polymer with increase in oxidizer concentration. This is due to the larger amounts of oxidizer consumption and radical availability with increase in initial peroxide concentration, which leads to the corresponding increase in polymer molecular weight reduction. The limiting number-average molecular weights predicted by the model for the effect of oxidizers are 130 000, 59 000, and 35 000 for 1, 5, and 10 g/L initial BPO concentration, respectively, in reasonable agreement with experimental data (Figure 5). The degradation of poly(ethyl acrylate) was investigated in the presence of benzoyl peroxide of initial concentration 5 g/L under solar exposure to confirm the feasibility of photoinitiated oxidative degradation of polymer under solar radiation. The dissociation rate constant of the BPO was determined using HPLC. The optimized values for hydrogen abstraction (kdp(1)) and oxidative random chain scission (koxidcp0) are given in Table 1d. Figure 6 shows the variation of number-average molecular

Figure 6. Variation of number-average molecular weight of PEA with time under solar radiation at constant initial BPO concentration of 5 g/L. Lines are model fit.

Figure 7. Variation of number-average molecular weight of PEA with time at various temperatures with a constant initial BPO concentration of 5 g/L. Lines are model fit.

weight of PEA under solar radiation in the presence of benzoyl peroxide with time. From the figure, it is clear that the trend in number-average molecular weight reduction is similar to the degradation under UV radiation. The figure also confirms the photooxidative degradation of PEA under solar radiation. The predicted limiting number-average molecular weight for the oxidative degradation of PEA under solar radiation (41 000 g/mol) is also in good agreement with the experimental values (Figure 6). The thermal oxidative degradation of poly(ethyl acrylate) was investigated at three different temperatures (60, 70, 80 °C) in the presence of BPO of initial concentration 5 g/L but in the absence of catalyst or UV. This was to compare the oxidative degradation under photo and thermal conditions. The rate coefficient for the dissociation of benzoyl peroxide under thermal conditions, kp, is given by ln kp ) 32.37 - 14900/T,44 where, kp is in s-1 and T is in K. The rate coefficients for hydrogen abstraction and oxidative random chain scission are optimized, and the values are given in Table 1e for all temperatures. Figure 7 shows the variation of the numberaverage molecular weight of PEA with time, and it is clear that the model fits the data well. The predicted limiting numberaverage molecular weights of PEA are 60 000, 62 000, and 84 000 g/mol for 60, 70, and 80 °C, respectively. The Arrhenius dependence with temperature is assumed for the rate coefficients, and it can be expressed from the Arrhenius plot as ln(kdp(1)) )

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27.65 - 13400/T, ln(k0cp0) ) 6.27 - 10300/T, where, kdp(1) is in s-1, k0cp0 is in mol g-1 s-1, and T is in K. The hydrogen abstraction step under thermal conditions is also faster than random chain scission of the polymer radical as observed in the photodegradation. There are conflicting reports3,4 on the temperature dependence of the hydrogen abstraction rate coefficient. While it has been shown that the hydrogen abstraction step is temperature dependent with negative activation energy,3 Sterling et al.4 observed significant temperature dependence with positive activation energy. The latter observation is similar to the results obtained in this work and can be explained as follows. The hydrogen abstraction step can occur by two intermediate steps. In the first step, the initiator radical combines with the polymer to form the intermediate complex that dissociates into polymer radical and alcohol in the second step. If the formation of intermediate complex is the rate-controlling step, the activation energy is expected to be small and the hydrogen abstraction step will be temperature independent. However, if the dissociation of the intermediate complex is the rate-controlling step, then the temperature dependence of the hydrogen abstraction is expected.3 The value of activation energy for the hydrogen abstraction step in this study (26.8 kcal/mol) is similar to that found (25.8 kcal/mol) by Sterling et al.4 for oxidative thermal degradation of poly(R-methylstyrene). By comparing the values of hydrogen abstraction rate constants at 80 °C, it is clear that the rate of hydrogen abstraction (1.1 × 10-2 cm3 s-1 g-1) from poly(ethyl acrylate) is much faster than that (9.2 × 10-5 cm3 s-1 g-1) from poly(R-methylstyrene).4 Similarly, the value of the activation energy for the random chain scission of the polymer radical in this work (20.6 kcal/mol) is similar to that obtained3 for oxidative thermal degradation of polystyrene (26.7 kcal/mol). From Table 1, it is clear that the rate coefficients for hydrogen abstraction and random scission for photoassisted oxidative degradation are 5-6 orders of magnitude higher than the thermal-assisted oxidative degradation at the same temperature. This confirms the enhancement in the degradation rate of polymer under UV/solar exposure. Conclusions The photocatalytic degradation of poly(alkyl acrylates) was studied in the presence of CS-TiO2 and commercial Degussa P-25 catalysts. CS-TiO2 showed better photocatalytic activity than Degussa P-25 catalyst, and the mechanism of photocatalytic degradation of these polymers is not different from the noncatalytic photodegradation. The effect of oxidizer on the UVassisted oxidative degradation kinetics of poly(alkyl acrylates), namely poly(methyl acrylate), poly(ethyl acrylate), and poly(butyl acrylate), was studied. The presence of the catalyst in the UV-assisted oxidative degradation of poly(alkyl acrylates) enhances the random chain scission of the polymer radical, and it does not affect the hydrogen abstraction rate coefficient. The effect of alkyl group substituents on the UV-assisted oxidative degradation of poly(alkyl acrylates) was also investigated. The degradation rate of poly(alkyl acrylates) decreased with increase in the number of carbon atoms of the alkyl substituents and thus followed the order PMA > PEA > PBA. The effect of oxidizer concentration on the degradation rate was investigated, and the degradation rate increased significantly with oxidizer concentration. The oxidative-thermal degradation of poly(ethyl acrylate) was investigated, and its comparison with photoinitiated oxidative degradation confirms the enhancement in degradation rate under UV/solar exposure. The hydrogen abstraction and oxidative random chain scission rate coefficients 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. The model developed here can thus be used to predict the degradation rate of the polymer in the presence of any initiators by only knowing its dissociation rate constant values. Acknowledgment G.M. thanks the Department of Science and Technology, India, for financial support and a Swarnajayanthi Fellowship. Literature Cited (1) Miller, A. Industry invests in reusing plastics. EnViron. Sci. Technol. 1994, 28, 16A. (2) Madras, G.; McCoy, B. J. Oxidative degradation kinetics of polystyrene in solution. Chem. Eng. Sci. 1997, 52, 2707-2713. (3) Kim, Y. C.; McCoy, B. J. Degradation kinetics enhancement of polystyrene by peroxide addition. Ind. Eng. Chem. Res. 2000, 39, 28112816. (4) Sterling, W.; Kim, Y. C.; McCoy, B. J. Peroxide enhancement of poly(R-methylstyrene) thermal degradation. Ind. Eng. Chem. Res. 2001, 40, 1811-1821. (5) Madras, G.; Karmore, V. Continuous distribution kinetics for oxidative degradation of PMMA in solution. Polym. Deg. Stab. 2001, 72, 537-541. (6) Madras, G.; Chattopadhyay, S. Optimum temperature for oxidative degradation of poly(vinyl acetate) in solution. Chem. Eng. Sci. 2001, 56, 5085-5089. (7) Scott, T. F.; Schneider, A. D.; Cook, W. D.; Browman, C. N. Photoinduced plasticity in cross-linked polymers. Science 2005, 308, 16151617. (8) Fan, S.; Kyu, T. Reaction kinetics of thermooxidative degradation in a styrene-b-butadiene diblock copolymer. Macromolecules 2001, 34, 645-649. (9) Verdu, S.; Verdu, J. A new kinetic model for polypropylene thermal oxidation at moderate temperatures. Macromolecules 1997, 30, 2262-2267. (10) Reich, L.; Stivala, S. S. Elements of Polymer Degradation; McGraw-Hill: New York, 1971; p 164. (11) Kamiya, Y.; Niki, E. Oxidative degradation. In Aspects of degradation and stabilization of polymers; Jellinek, H. H. G., Ed.; Elsevier: New York, 1978; p 327. (12) Billmeyer, F. W. Textbook of Polymer Science; Wiley-Interscience: New York, 2003; p 391. (13) Masuda, M.; Jonkheijm, P.; Sijbesma, R. P.; Meijer, E. W. Photoinitiated polymerization of columnar stacks of self-assembled trialkyl1,3,5-benzenetricarboxamide derivatives. J. Am. Chem. Soc. 2003, 125, 15935-15940. (14) Kaczmarek, H.; Kaminska, A.; Swiatek, M.; Sanyal, S. Photoinitiated degradation of polystyrene in the presence of low-molecular organic compounds. Eur. Polym. J. 2000, 36, 1167-1173. (15) Takashi, K.; Robert, D.; Miller, J.; Ratnasabapathy, S.; Josef, M. Mechanism of the photochemical degradation of poly(di-alkylsilanes) in solution. J. Am. Chem. Soc. 1989, 111, 1140-1141. (16) Bengt, R. Photodegradation and photo-oxidation of synthetic polymers. J. Anal. Appl. Pyrolysis 1989, 15, 237-247. (17) Cohen, C. T.; Chu, T.; Coates, G. W. Cobalt catalysts for the alternating copolymerization of propylene oxide and carbon dioxide: Combining high activity and selectivity. J. Am. Chem. Soc. 2005, 127, 10869-10878. (18) Ajaya, N.; Kishore, K. Catalytic oxidative polymerization of 1,1diphenylethylene at ambient temperature and potential application of peroxide macroinitiator. Macromolecules 2002, 35, 6505-6510. (19) Gupte, S. L.; Agarwal, N.; Madras, G.; Nagaveni, K.; Hegde, M. S. Effect of aluminium chloride and Pt/TiO2 on the thermal degradation of poly(vinyl chloride) in solution. J. Appl. Polym. Sci. 2003, 90, 3532-3535. (20) Priya, M. H.; Madras, G. Kinetics of photocatalytic degradation of chlorophenol, nitrophenol and their mixtures. Ind. Eng. Chem. Res. 2006, 45, 482-486. (21) Priya, M. H.; Madras, G. Photocatalytic degradation of nitrobenzenes with combustion synthesized nano-TiO2. J. Photochem. Photobiol. A: Chem. 2006, 178, 1-7. (22) Sung, H. K.; Kwak, S.-Y.; Suzuki, T. Photocatalytic degradation of flexible PVC/TiO2 nanohybrid as an eco-friendly alternative to the current

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ReceiVed for reView September 26, 2007 ReVised manuscript receiVed December 31, 2007 Accepted January 26, 2008 IE0712939