Effect of Alkyl Group Substituents, Temperature, and Solvents on the

The effect of alkyl group substituents on the degradation rate of poly(n-alkyl acrylate) was investigated by studying the ultrasonic degradation of po...
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Ind. Eng. Chem. Res. 2005, 44, 6572-6577

Effect of Alkyl Group Substituents, Temperature, and Solvents on the Ultrasonic Degradation of Poly(n-alkyl acrylates) J. P. Mahalik and Giridhar Madras* Department of Chemical Engineering, Indian Institute of Science, Bangalore 560 012, India

The effect of alkyl group substituents on the degradation rate of poly(n-alkyl acrylate) was investigated by studying the ultrasonic degradation of poly(methyl acrylate) (PMA), poly(ethyl acrylate) (PEA), and poly(n-butyl acrylate) (PBA) in o-dichlorobenzene at 30 °C. A continuousdistribution kinetic model with midpoint scission was used to determine the rate coefficients. The rate coefficient increased with a decrease in the number of carbon atoms in the alkyl group substituent; thus, the degradation rate followed the order PMA > PEA > PBA. This was explained based on the delocalization of charge on the carbon atoms joining two monomer units. The effect of temperature and solvents on the ultrasonic degradation of PBA was also investigated. The degradation rate coefficient decreased with an increase in the vapor pressure of the solvent and with an increase in the temperature. The effect of an initiator, 2,2′-azobis(isobutyronitrile) (AIBN), on the degradation of PBA was also investigated. The degradation rate decreased with an increase in the AIBN concentration, and the experimental results were simulated by the model. Introduction Polyacrylates are industrially important because of their wide applications in paint, paper, adhesive, textile, and leather industries.1 The knowledge of polymer degradation is useful in understanding the life cycle of the products or disposing of the product after end use and recycling of polymers.2,3 Among various methods of polymer degradation (thermal, light, enzymatic, and ultrasonic), ultrasonic degradation is an attractive option. Because of these attractive features and the wider availability of sonochemical apparatuses, ultrasonicassisted degradation of polymers has been widely studied.4-9 The effect of solvents6 and temperature7 on the degradation of polymers has also been studied. It has been hypothesized that during ultrasound exposure in a solution, cavitation occurs followed by the rapid growth and collapse of microbubbles.4 Near the collapsing bubble, polymer chains are caught in the highgradient shear field and move at a higher velocity than those farther away from the collapsing cavity. This relative motion of the polymer segments and solvent produces stresses near the polymer chain that cause scission4,10 due to mechanical shearing, with scission occurring near the midpoint of the chain. Ultrasound has also been used for suspension and emulsion polymerization,11 ring-opening polymerization,12 condensation polymerization,13 and Ziegler-Natta polymerization.10 It has been reported that the molecular weight (MW) rapidly rises, then declines, and reaches a limiting MW. The monomer concentration is reported to increase the initial rate of polymerization and limiting MW.14 Several investigations have been done on the degradation kinetics of acrylates. The pyrolytic thermal degradation of polyacrylates has been extensively studied by many investigators in different environments (nitrogen, oxygen, and vacuum).15-21 The degradation involves random homolytic scission followed by a series * To whom the correspondence should be addressed. Tel.: 091-80-22932321. Fax: 091-80-23600683. E-mail: Giridhar@ chemeng.iisc.ernet.in.

of intra- and intermolecular transfer reactions. The polymer undergoes different mechanisms of hydrogen abstraction at different temperatures22 in photodegradation. There have been investigations on the ultrasonic polymerization of polyacrylates,23 but no studies have been conducted on the ultrasonic-assisted degradation of polyacrylates. The objective of this study is to investigate the effect of alkyl substituents on the degradation kinetics of poly(n-alkyl acrylate). The effect of various solvents, temperature, and 2,2′-azobis(isobutyronitrile) (AIBN) on the degradation of poly(n-butyl acrylate) (PBA) was also investigated. A continuous-distribution kinetics model is developed to account for the effect of AIBN on the ultrasonic degradation of polymers. Experimental Section Materials. Methyl acrylate (Merck Chemicals), ethyl acrylate (Rolex Chemicals), and n-butyl acrylate (S.D. Fine Chemicals) were freed from inhibitors by washing with a 5% caustic solution followed by washing with distilled water and double distillation under reduced pressure. All solvents used in the study were distilled and filtered prior to use. The solvents used for dissolution of the polymer are inert and do not undergo chemical reaction during ultrasonication. Polymer Synthesis. Solution polymerization was used to polymerize the acrylates at 60 °C in benzene with benzoyl peroxide as the initiator. An initiator concentration of 1-2 g/L and a monomer concentration of 40-60 vol % in benzene were used to synthesize the polymers. The number-average MWs of the obtained PBA, poly(ethyl acrylate) (PEA), and poly(methyl acrylate) (PMA) were 490 000, 460 000, and 400 000, respectively, with polydispersities of 1.7, 1.7, and 1.5, respectively. Degradation Experiments. Solutions of 2 kg/m3 of PBA were made in different solvents [benzene, toluene, xylene, o-dichlorobenzene (DCB), chlorobenzene (MCB)], and 40 mL of each solution was ultrasonically degraded

10.1021/ie0504607 CCC: $30.25 © 2005 American Chemical Society Published on Web 07/16/2005

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in a horn type sonicator (Vibronics, 25 kHz) at 30 °C. The processor delivers the sound energy to the polymer solution through a horn tip with a flat radiating circular surface. The temperature of the water bath was maintained at the desired temperature ((1 °C) using a feedback proportional-integral-derivative controller, and the temperature of the polymer solution was monitored using a thermometer and was also within (1 °C. The intensity and frequency of the sound generated was 36 W cm-2 and 25 kHz, respectively. Samples of 200 µL were collected at various time intervals and analyzed in the gel permeation chromatograph (GPC). The limiting MW was obtained by conducting experiments for 8 h, when the MW reaches a constant value. Multiple experiments indicated that the error in the rate coefficient is approximately 3%. MW Determination. The MW distribution of the samples was determined by a GPC (Waters, Milford, MA) consisting of an isocratic pump (Waters 501) with an automated gradient controller, size exclusion columns (Styragel HR 5E, HR 3, and HR 0.5; 300 mm × 7.5 mm), a differential refractometer (Waters R401), and a data acquisition system. Samples were injected in a Rheodyne valve with a sample loop of 50 µL, and the refractive index was continuously monitored using a differential refractive index detector. The chromatograph was converted to the MW distribution using a universal calibration based on polystyrene standards (Polymer Laboratories, Shropshire, U.K.). AIBN Concentration Determination. To determine the dissociation of AIBN during ultrasonic degradation, experiments were conducted by adding AIBN in DCB and degraded in the presence of ultrasound at 30 °C. Samples were taken at regular intervals, and the concentration of AIBN was determined using a UV/ visible spectrophotometer. The intensity of the AIBN absorbance was determined using a UV/visible spectrophotometer (Shimadzu UV-2100) in the wavelength range of 200-800 nm. Calibration based on the BeerLambert law was used to quantify the concentration of AIBN. 13C NMR Spectrophotometry. The relative electron densities on the carbon atoms of PMA, PEA, and PBA were determined using a 400-MHz Bruker spectrophotometer at room temperature with CDCl3 as the solvent.

weight x undergoes binary fission to two radicals, R*(x′) and R*(x-x′). According to the long-chain approximation (LCA),24 although reaction A is essential for degradation to occur and for the completeness of the mechanism, it has a negligible effect on the overall reaction rate24 because reactions B and C occur much more frequently24 than reaction A. Thus, the rate coefficients of the initiation and termination reactions are assumed to be zero.24 Reaction B represents the intermolecular hydrogen abstraction, where the polymer undergoes hydrogen abstraction with polymer radicals. Reaction C represents the depropagation reaction. Reaction D represents the decomposition of AIBN, where D2 represents AIBN and D* represents the AIBN radical. Reaction E represents the interaction of AIBN with the polymer radical, where the polymer radical interacts with the AIBN molecule to form a stable polymer. The population balance equations for polymer p(x,t) and radical r(x,t) can be written as

∂p(x,t) ) -kh(x) p(x,t) + kH(x) r(x,t) + ∂t kD(x) r(x,t) D2(t) +

∫x∞ks(x′) Ω(x,x′) r(x′,t) dx′

(1)

∂r(x,t) )kh(x) p(x,t)-kH(x) r(x,t) - kD(x) r(x,t) D2(t) ∂t ks(x) r(x,t) +

∫x∞ks(x′) Ω(x,x′) r(x′,t) dx′

dD2(t) ) -kdD2(t) dt

∫0∞kD(x) D2(t) r(x,t) dx

(2) (3)

Theoretical Model

The initial conditions for the rate equations are p(x,t)0) ) p0(x) and r(x,t)0) ) 0. p(x,t) dx represents the molar concentration of the polymer in the MW interval (x, x + dx), and D2(t) represents the molar concentration of AIBN. Ultrasonic degradation occurs by midpoint chain scission with a stoichiometric kernel5,9 of Ω(x,x′) ) δ(xx′/2). Because the hydrogen abstraction does not depend on the chain length of the polymer, the rate coefficients kh(x) and kH(x) are assumed to be independent of the MW,9 while the rate coefficient kD(x) is assumed to be a linear function of the MW [kD(x) ) kDx]. To ensure that the degradation rate is zero when the limiting MW (xl) is reached, the depropagation rate coefficient is assumed to be linearly dependent7,9 on x such that ks(x) ) ks(xxl). When moment ∫∞0 f(x,t) xj dx is applied,

The model developed here for the effect of AIBN on the ultrasonic degradation of the polymer is new and builds upon the existing models that discuss ultrasonic degradation in the absence of7,25 and in the presence of peroxides.9 The radical mechanism for the degradation of the polymer can be represented as follows:

dp(j)(t) ) -khp(j)(t) + kHr(j)(t) + dt ks kDr(j+1)D2(t) + j [r(j+1)(t) - xlr(j)(t)] (4) 2

ki

P(x) y\ z R*(x′) + R*(x-x′) k t

kh

z R*(x) P(x) y\ k H

ks

R*(x) 98 P(x′) + R*(x-x′) kd

D2 98 2D* kD

R*(x) + D2 98 P(x) + D* + DH

(A) (B) (C) (D) (E)

Reaction A represents initiation and termination steps, where the polymer molecule, P(x), of molecular

dr(j)(t) ) khp(j)(t) - kHr(j)(t) - kDr(j+1)(t) D2(t) dt ks ks[r(j+1)(t) - r(j)(t) xl] + j [r(j+1)(t) - r(j)(t) xl] (5) 2 dD2(t) ) -D2(t) [kd + kDr(1)(t)] (6) dt The initial conditions for the moments are p(j)(t)0) ) (j) p(j) 0 and r (t)0) ) 0. Using a quasi-steady-state assumption (QSSA),24 the rate of change of radicals can be neglected. Therefore, ∂r(j)(x,t)/∂t ) 0; thus, dr(j)(t)/dt ) 0. For j ) 1, dp(1)(t)/dt ) 0 and p(1)(t) ) p(1) 0 , indicating

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mass conservation. With j ) 0 and 1 and with application of QSSA, the zeroth and first moments of the radical concentration are

r(0)(t) ) r(1)(t) )

kh (0) kDD(t) (1) p (t) r (t) kH kH

(7)

kh kDD2(t) + ks/2 (2) p(1)(t) r (t) kH - xlks/2 kH - xlks/2

(8)

Because r(2) is assumed to be small, the second term is neglected in the expression for r(1). With j ) 0 in eq 4, the time variation of the molar concentration of the polymer is

dp(0)(t) ) -khp(0)(t) + kHr(0)(t) + kDD2(t) r(1)(t) + dt ks[r(1)(t) - xlr(0)(t)] (9) Equation 6 can be simplified to

dD2(t) ) -kdAD2(t) dt

(10)

where kdA ) kd + kDkhp(1) 0 /(kH - xlks/2), and solved to obtain

D2(t) ) D20 exp(-kdAt)

(11)

where D20 represents the initial concentration of AIBN. Substituting eqs 7, 8, and 11 into eq 9 gives

dp(0)(t) (1) ) k1(-xlp(0) + p(1) 0 ) - k2(xlD20p0 ) exp(-kdAt) dt (12) where k1 ) kskh/kH and k2 ) k1kD/kH(1 - xlks/2kH). As t f ∞, Mn f xl; therefore, k2 ) k1kD/kH, implying that xlks/2kH , 1. When eq 12 is solved with the initial condition p(0)(t)0) ) p(0) 0 ,

ln Y ) k1xlt -

{ [ ]

ln 1 - D20

k2xl kdA - k1xl [1 - exp[-(kdA - k1xl)t]] 1 1 Mn0 xl

}

(13)

where Y ) (1/Mn0 - 1/xl)/(1/Mn - 1/xl) in which Mn ) (0) and M p(1) n0 represent the number-average MW at 0 /p time t and time zero, respectively. In the absence of AIBN, D20 ) 0, and eq 13 reduces to

ln Y ) k1xlt

(14)

Equation 14 is exactly the same equation obtained in the previous models7,25 for ultrasonic degradation in the absence of AIBN. Results and Discussion The effect of alkyl group substituents on the ultrasonic degradation of poly(n-alkyl acrylates) was inves-

Figure 1. Variation of ln Y with the sonication time in DCB at 30 °C for different polymers. The solid lines represent linear regressed lines, and the slope corresponds to the rate coefficient, k1. Legend: 9, PBA; b, PEA; 2, PMA.

tigated by conducting ultrasonic degradation of PMA, PEA, and PBA. The rate coefficient of degradation was determined by plotting the variation of ln Y with the sonication time. The plot is nearly linear, as shown in Figure 1, indicating that the model (eq 14) is satisfactory. It should be noted that the degradation rate coefficient, k1, is independent of the initial MW or the limiting MW. The degradation rate coefficients (×10-7 mol g-1 min-1), k1, are 4.08, 3.40, and 2.95 for PMA, PEA, and PBA, respectively. The decrease in the rate coefficient with an increase in the alkyl group chain length can be explained in terms of the delocalization of electrons.26 The longer the alkyl group, the higher will be the delocalization of the charge at the carbon atom linking two monomer units, and thus it is more stable. When the alkyl group is small, the delocalization of the electrons will be less, so the tendency to form radicals is more.26 The 13C NMR spectra of PMA (Figure 2a), PEA (Figure 2b), and PBA (Figure 2c) support this hypothesis. The signals of 13C NMR for PMA, PEA, and PBA are listed below. 13C NMR (CDCl , ppm): PMA, δ 174.9 (CdO), 51.8 3 (-CH3), 41.3 (-CH-), 35 (-CH2); PEA, δ 174.55 (Cd O), 60.69 (-CH2O-), 41.36 (-CH-), 36.13, 35.3 (-CH2), 14.18 (-CH3); PBA, δ 174.55 (CdO), 64.58 (-CH2O-), 41.43 (-CH-), 36.47, 35.39 (-CH2), 30.61 (-CH2CH2), 19.11 (-CH2CH3), 13.76 (-CH3). The δ for -CH- increases with the alkyl substituent, indicating that the electron density on the carbon atoms joining two monomers follows the order PMA > PEA > PBA and hence the probability of radical formation.26 Because the rate of radical formation is directly related to the depropagation, the degradation rate follows the order PMA > PEA > PBA. Further, PBA degrades slower than polybutadiene,25 poly(methyl methacrylate),8 poly(vinyl acetate),6 and polystyrene.27,28 To investigate whether the degradation rate of PBA further decreases, the degradation of PBA was investigated in the presence of other solvents and at higher temperatures. The effect of solvents on the ultrasonic degradation of PBA at 30 °C is shown in Figure 3. The rate coefficient of degradation was determined by linear

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Figure 2.

13C

NMR spectra of (a) PMA, (b) PEA, and (c) PBA, recorded at 30 °C, in CDCl3.

regression of the data points of ln Y with the sonication time. The degradation rate coefficients (×10-7 mol g-1 min-1) decreased from 2.95 to 1.96 as the vapor pressure of the solvent increased from 0.26 kPa for DCB to 15.80 kPa for benzene. As the vapor pressure of the solvent increases, a large quantity of the solvent vapor enters the cavitation bubbles during their expansion and exerts a cushioning effect4 during the collapse, leading to a diminishing of the intensity of the shock wave, reducing the jet velocity, and leading to reduced degradation. Because the highest degradation was observed when DCB was used as the solvent, experiments were conducted in DCB to study the effect of the temperature (30-70 °C) on the degradation kinetics of PBA (Figure 4). The degradation rate coefficient (×10-7 mol g-1 min-1) decreased from 2.95 to 1.38 as the temperature increased from 30 °C (vapor pressure ) 0.26 kPa) to 70

°C (vapor pressure ) 2.34 kPa). The dependence of the degradation rate coefficient on the temperature can be explained similarly to the effect of the vapor pressure on degradation. The dependence of the degradation rate coefficient on the vapor pressure of various solvents and of DCB at various temperatures is shown in Figure 5. It is observed that the degradation rate coefficient decreases with an increase in the vapor pressure of different solvents and also with an increase in the vapor pressure of DCB at different temperatures. However, the profiles are different. For example, although the vapor pressure of MCB at 30 °C is nearly the same as the vapor pressure of DCB at 70 °C, the degradation rate coefficient of PBA in DCB is significantly higher than that observed with MCB. However, the viscosity29 of MCB at 30 °C ()0.745 cP) is lower than that of DCB at 70 °C

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Figure 5. Variation of the rate coefficient of ultrasonic degradation of PBA with the vapor pressure of the solvent. Legend: 9, benzene; b, toluene; 2, xylene; 1, MCB; [, DCB at 30 °C; 0, DCB at 40 °C; O, DCB at 50 °C; ], DCB at 60 °C; 3, DCB at 70 °C.

Figure 3. Variation of ln Y with the sonication time for PBA in different solvents at 30 °C. The solid lines represent linear regressed lines, and the slope corresponds to the rate coefficient, k1. Legend: 9, benzene; b, toluene; 2, xylene; 1, MCB; [, DCB.

Figure 6. Variation of ln Y with the sonication time for PBA in DCB at 30 °C for different concentrations of AIBN: (a) 1 g/L; (b) 2 g/L; (c) 3 g/L; (d) 5 g/L; (e) 10 g/L. The solid lines are obtained from regression of the experimental data with eq 13 to obtain rate coefficient k2.

Figure 4. Variation of ln Y with the sonication time for PBA in DCB at different temperatures. The solid lines represent linear regressed lines, and the slope corresponds to the rate coefficient, k1. Legend: 9, 30 °C; b, 40 °C; 2, 50 °C; 1, 60 °C; [, 70 °C.

()0.8 cP). Because the degradation rate coefficient increases with an increase in the viscosity,25 the degradation of PBA is higher in DCB. Further, other factors such as differences in the surface tension and polymersolvent interactions4 for each solvent may also play a role in determining the ultrasonic degradation rate coefficients. The effect of the presence of AIBN on the ultrasonic degradation of PBA was investigated in DCB at 30 °C. The rate coefficient of decomposition (kdA) of AIBN at 30 °C in DCB was determined to be 0.0376 min-1. The rate coefficient, k1, obtained earlier for the degradation of PBA in the absence of AIBN is used. The other parameter, k2, is determined by a parametric fit of the model (eq 13) with the experimental data for degradation of PBA at various concentrations of AIBN. Figure 6 shows that the model fits are satisfactory with k2 ) 3 × 10-9 mol g-2 min-1. Although the model fit for lower concentration is a straight line, the contribution due to the second term in eq 13 (the AIBN effect) is almost

25% of the total degradation initially, and eventually the contribution goes down to almost 10% at the end of 3 h. The exponential effect is observed at the initial stage of degradation for a higher concentration of AIBN. For degradation using 10 g/L AIBN, the exponential effect is observed for up to 60 min (Figure 6e). The relative orders of magnitudes of the individual rate coefficients can be compared as follows. Because k2 ) k1kD/kH, kD/kH ) k2/k1 ∼ 10-4g-1, indicating that hydrogen abstraction by AIBN (kD) given by reaction E is slower than hydrogen abstraction (kH) as given by the reverse reaction in reaction B. Therefore, the capping of polymer radicals is enhanced by the presence of AIBN, leading to a decrease in the polymer degradation. Conclusions The effect of alkyl substituents on the degradation rate of poly(n-alkyl acrylate) has been investigated. The degradation rate of poly(n-alkyl acrylate) decreased with an increase in the number of carbon atoms of the alkyl substituents, with the degradation rate following the order PMA > PEA > PBA. The ultrasonic degradation of PBA was also investigated in different solvents and at different temperatures. The rate of degradation decreased with an increase in the vapor pressure of the solvent. The presence of AIBN decreased the degradation rate of PBA and was successfully simulated by the model.

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Acknowledgment We thank K. Nagesh of the Inorganic and Physical Chemistry department, IISc Bangalore, for help and interpretation of NMR. The authors thank the Department of Science and Technology for financial assistance.

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Received for review April 18, 2005 Revised manuscript received June 11, 2005 Accepted June 18, 2005 IE0504607