Chemical Kinetics of Polymethyl Methacrylate ... - ACS Publications


Mar 14, 2008 - By Newton's method, the experimental results of the remaining weight of PMMA are closely fitted by the model combined with zero-order a...
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Chemical Kinetics of Polymethyl Methacrylate (PMMA) Decomposition Assessed by a Microwave-Assisted Digestion System Tsuey-Lin Tsai,†,§ Chun-Chih Lin,‡ Gia-Luen Guo,§ and Tieh-Chi Chu*,†,| Department of Biomedical Engineering and EnVironmental Sciences, National Tsing Hua UniVersity, Hsinchu, 30013, TaiwanDepartment of Natural Biotechnology/General Education Center, Nanhua UniVersity, 62248, TaiwanChemical Analysis DiVision, Institute of Nuclear Energy Research, 32546, Taiwan Department of Radiological Technology, Yuanpei UniVersity, 30015, Taiwan

A simplified and novel kinetic model was first developed by way of characterization of Fourier transform infrared spectroscopy, scanning electron microscopy, and the weight-loss method for PMMA decomposition under a microwave assisted digestion system. By Newton’s method, the experimental results of the remaining weight of PMMA are closely fitted by the model combined with zero-order and first-order kinetics, in which the former predominates the reaction at lower temperatures (423-443 K) and the latter at higher temperatures (g453 K). Kinetic parameters of PMMA decomposition under 423-453 K including rate constants and the mass fractions (φ) via main-chain scission were determined by this empirical model. Activation energies of PMMA decomposition estimated by the Arrhenius equation are 0.74 and 36.07 kcal/mol respectively for the zero- and first-order reaction. The pre-exponential factors of the zero- and first-order reactions are 2.28 × 10-3 and 2.57 × 1017 g min-1 respectively. The effect of HNO3 volume on PMMA decomposition was further investigated at 423-473 K. At 473 K, the digestion efficiency has increased to 100% as the HNO3 volume is g3 mL. The estimated φ values of the decomposition are increasing with the HNO3 volume at 423, 443, and 453 K, yet varying insignificantly at 473 K. Introduction Polymethyl methacrylate (PMMA) is a major type of thermoplastics used throughout the world in such applications as photonic of nanotechnology because of the uniform optical index of its structure,1 prosthetic composites used in dentistry because of its excellent cell adhesion and biocompatibility,2 and also is widely used in consumer products, such as transparent all-weather sheets, electrical insulation, bathroom units, automotive parts, surface coating, ion exchange resins, and so forth. PMMA exhibits excellent mechanical properties and performs well under various processing conditions.3 Therefore, on the basis of environmental concern, the study of PMMA thermal degradation is imperative to understand its stability and the conditions for recycling waste polymer in the rubber industry.4 The properties of polymers, such as PMMA, are governed distinctively by molecular weight, temperature, and gas atmosphere during decomposition, all of which can change the material characteristics and have been intensively investigated.5 For the past several years, considerable effort has been made to study reactions occurring in polymer degradation and copolymerization incorporating with other polymers.6 The thermal degradation kinetics in polymers is more complicated than that in inorganic materials due to the nature of the polydispersity of polymer chains. Great effort has been devoted to the study of thermal degradation of pure PMMA and PMMA composites7,8 by high-temperature heating, pyrolysis, photochemical reaction, mechanical stressing, and oxidation.9 However, the decomposition of PMMA by an efficient microwave irradiation and its mechanism has not yet been addressed. * To whom correspondence should be addressed. E-mail: [email protected] mx.nthu.edu.tw. Tel.: +886-3-5711808. Fax: +886-3-5725461. † National Tsing Hua University. ‡ Nanhua University. § Institute of Nuclear Energy Research. | Yuanpei University.

Microwave digestion has been proven to be suitable for sample preparation10-11 over the last decades due to advantages of low digestion time, high decomposition efficiency, low reagent consumption and precise monitoring of the digestion process. Applications of microwave digestion in analyzing environmental, biological,12-14 and material samples15-17 (e.g., polymer, metal alloys, and ceramics) were widely reported. A liquid sample is frequently required for subsequent analysis, which for polymers usually necessitates laborious digestion procedures. Deploymerization often takes place at high temperatures and pressures and requires long reaction time as well. Recently, microwave digestion for wet oxidation of polymers in sealed containers is available to reduce significant time required for sample preparation. Compared to conventional pyrolysis, degradation of polymer in solution is favorable because of good and uniform temperature control.18 Another application of microwave digestion was also reported for retrieving the monomers from polymer waste.19-20 By utilizing closed vessels, sample-loss and contamination can be minimized in the microwave digestion. Therefore, microwave-assisted digestion in closed vessels is regarded as a state-of-the-art sample digestion technique. To the best of our knowledge, the use of larger amounts of digestion reagents, longer digestion time, or higher digestion temperature can enhance digestion efficiency. Nevertheless, destruction of the matrix by a specific decomposition method has seldom been evaluated quantitatively. Conventionally in experiments, when clear or colorless solutions are visually obtained, the complete oxidation of the organic matter is thought to have occurred, especially in acid digestion for sample pretreatment, in the field of analytical chemistry. However, such assumptions are unreliable in all cases. The residual matter retained following acid digestion should be present and identified, because such matter may interfere with subsequent measurements. Therefore, a systematical database of digestion kinetics will make the selection of digestion recipes more efficient. The

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principal difficulty in studying kinetics is the characterization of the composition of the solution following microwave digestion, especially when the species concentration is one of the kinetic indicators. Hence, another appropriate digestion indicator is required to describe kinetics. In general, the products of polymer after microwave digestion can be classified into three groups: nonvolatile, semi-volatile, and volatile products. During digestion, most of the long-chain molecules can be fragmentized into small molecules, which usually have relatively low boiling points and are thus volatile. These products can be removed by further heating. Therefore, the weight-loss method is convenient to evaluate the efficiency of microwave digestion in a closed vessel. The mechanism of thermal degradation for polymers depends on molecular structure and experimental conditions. Thermal degradation of PMMA sequentially occurs as initial scission, depropagation reaction, and further degradation into volatile compounds.18 To date, at least two mechanisms have been validated for the initial scission of PMMA, including random scission (C-C bonds) of the main-chain and homolytic scission of the methoxycarbonyl side groups (-COOCH3). In the initial scission, the random C-C scission is the dominant mechanism, which decomposes PMMA into methyl methacrylate (MMA) as the major product.21,22 However, the products, such as CO2, CO, CH3OH, CH4, and char, other than the monomer, could also form along with the elimination of the methoxycarbonyl side group.23 The kinetics of PMMA decomposition was studied as a firstorder reaction for most previous studies.9,18,21,24 Recently, it was reported that the reaction orders were 1.0-1.3 for PMMA decomposition.7,8 The difference of activation energies and Arrhenius constants were also observed depending on decomposition approaches and investigated conditions.9,18,21,24 However, the kinetics for the acid-catalyzed digestion of PMMA in solution has not yet been examined. Parameters including reaction order, rate constant, and activation energy are important to predict the digestion kinetics for PMMA decomposition. Accordingly, this work aimed to explore the chemical kinetics and decomposition behavior of PMMA (MW ) 350 000) during microwave digestion at various holding temperatures, digestion time, and added volumes of HNO3. Experimental Section Reagents and Equipment. Reagent-grade polymethyl methacrylate (PMMA) with an average molecular weight of 350 000 g/mol was purchased from Aldrich Chemical Co. Inc., which was in the form of a solid powder and well stored in the dampproof cabinet. The concentrated HNO3 (∼69-70%) was supplied by J.T. Baker Inc. The electronic balance, Model-AT201, obtained from the Mettler-Toledo company (Switerland), was used to measure sample weights. The microwave device (Model MARS-5, CEM; Matthews, North Carolina) comprises a power system with a selectable output of 0-1200 W ((15%) at a frequency of 2.45 GHz, a Teflon-coated cavity with fan and tubing to vent fumes, a digital computer programmable with 100 programs with up to 5 stages each, an alternating turntable-rotating system for homogeneous heating, and a 14-position sample carousel. A pressure sensor (ESP-1500 Plus) was attached to the control vessel to monitor the pressure, and the pressure maximum was set at 350 psi. The oven power was automatically turned off as the pressure in the vessel exceeded the limit and restarted as the endogenous pressure decreased. An optical fiber was used to monitor and control the digestion temperature up to 210 °C by a feedback

system (EST-300 Plus). The sample and HNO3 were mixed and digested at various temperatures in a double-walled vessel (HP500 Plus, volume ) ∼100 mL), which consists of a chemically resistant inner shell and a cover made of Teflon PFA and an Ultem polyetherimide outer shell. The cap (Autovent Plus) on the vessel was used to release excessive pressure and prevent the loss of sample and volatile analytes. Determination of the Kinetics of PMMA Digestion. The effects of digestion temperature, digestion reagent (HNO3) volume, and digestion time on the kinetics of PMMA decomposition by microwave digestion were examined by the weightloss method. In gravimetric method, the digestion efficiency (%), DE, as described in eq 1 was evaluated by weights of the totally dry solid sample before and after digestion, where Wi denotes the initial sample weight and Wf represents the final remaining weight after digestion for a period of time t (min).

DE (% ) )

Wi - Wf × 100 Wi

(1)

An aliquot of 0.1 g PMMA (molecular weight ) 350 000 g/mol) and 3 mL of concentrated HNO3 were added into the reaction vessels to examine the effect of temperature on digestion efficiency. To investigate the effect of the volume of the digestion reagent (HNO3) on digestion efficiency, the above procedures were followed except that 2-7 mL of HNO3 was added. The digestion efficiency under five target temperatures (423, 433, 443, 453, and 473 K) at a constant heating rate with various digestion times was studied. Before digestion, the PMMA powder was attached to the wall of the vessel or suspended on the surface of the HNO3 solution. The PMMA material was totally dissolved when it was fully digested; otherwise, brown solid was observed to aggregate on the sidewall or the bottom of the container. During microwave digestion, the color of PMMA powder changed from white to brown and finally became transparent. To evaluate the amount of solid residual, the digested product was cooled and transferred to a 20 mL beaker, evaporated to complete dryness on the hot plate, and was measured by direct weighting. Sample Preparation for Fourier Transform Infrared (FTIR) Spectroscopy. The digested solution was dried completely and formed as pellets with KBr. Pure PMMA was directly dissolved in CH2Cl2, cast onto a KBr disk, and then baked at 60 °C in the oven as a control group. The residues of PMMA after digestion at 453 K were also examined. Pure and digested PMMA samples were then analyzed using a Nicolet Avatar 320 (Madison, Wisconsin) FTIR spectrometer with a resolution of 1 cm-1. The films used in this work were thin enough to obey the Beer-Lambert law. Sample Preparation for Scanning Electron Microscopy (SEM). The digested solution was diluted by a factor of 500 by dissolving 20 µL of the solution in 10 mL of HNO3. An aliquot of 2 µL diluted solution was pipetted and coated on the silicon wafer (0.8 cm × 0.8 cm) with a spin coater (model KW4A, SPI Supplies Inc.). After it was dried at 60 °C, the sample was coated with gold in the coating chamber for 110 s and then analyzed using a scanning electron microscope (JOEL-JSM6500F). The size distribution of the PMMA was determined by image analysis software (IRCON). Results and Discussion FTIR Characterization. To identify any change of chemical structure of the PMMA before and after digestion, results on the FTIR spectra are presented in Figure 1. The absorbance

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Ind. Eng. Chem. Res., Vol. 47, No. 8, 2008 Table 1. Average Particle Size Distribution of PMMA 350 000 after Microwave Digestion at Various Temperatures temperature control 443 K 453 K 463 K 473 K

groupb

mean ( SD (nm)a 45 ( 6.37 33.07 ( 11.84 16.94 ( 8.88 12.41 ( 2.37 6.75 ( 1.21

a Standard deviation of the particle size distribution. b Acid must be removed prior to baking for sample pretreatment.

Figure 1. FTIR spectra of PMMA 350 000 (dissolved in CH2Cl2) (a) before microwave digestion and (b) after digestion at 453 K.

intensities, such as aliphatic C-H (SP3), CdO, and -CH3 groups are pronounced before microwave digestion as shown in part a of Figure 1. The absorption band that occurs in the region of 3000-2840 cm-1 is characteristic of methyl ester stretching vibrations. Several saturated hydrocarbons that contain methyl groups correspond to two distinct peaks at 2997 and 2946 cm-1, resulting from asymmetrical and symmetrical stretching modes of C-H bonding in the methyl group, respectively. Symmetrical bending vibration (δsCH3) occurs near 1383 cm-1, whereas the asymmetrical bending vibration (δasCH3) is identified near 1450 cm-1. The absorption frequency in the region 1750-1735 cm-1 is characteristic of carbonyl stretching vibration in the saturated aliphatic esters.4 Following microwave digestion (453 K) with nitric acid, the broad absorption band of -OH in the carboxylic acid group specifically appears in the region 3500-3200 cm-1 (νOH) as shown in part b of Figure 1, suggesting that the PMMA polymer is hydrophilic after hydrolysis in digestion. Additionally, the absorption peak of the carbonyl group (ν CdO, stretching of carboxylic acid group) at around 1700 cm-1 becomes substantially broadened due to the hydroxyl group formed by acid hydrolysis. Numerous peaks arise in the 1300-1500 cm-1 region after digestion, representing the broadening peaks of PMMA branches, which suggests that the dominant products are volatile monomers accompanied with a number of lowmolecular-weight stable compounds and small gaseous molecules. Decomposition products with a carbonyl group (e.g., formaldehyde, acetone and methyl pyruvate) at high temperature from the oxidation of MMA (methyl methacrylate) monomers were also reported.25,26 The hydrophilic products with carboxylic acid groups may be derived from transition compounds by acid hydrolysis in microwave-assisted digestion. According to the absorption of CdO and OH groups in the FTIR spectra, the formic acid (HCOOH) suggested occurred as investigated in our previous study using solid-phase micro-extraction followed by GC-MS.27 However, it is difficult to identify whether the bond breaking of functional groups during decomposition originates from PMMA or MMA monomers. SEM Characterization. Table 1 exhibits the variation of the particle size of the PMMA with digestion temperature, which gradually decreases as the temperature increases. The PMMA particle size reveals a Gaussian distribution, and the particle density shifts toward smaller size as the digestion temperature increases, which enlarges the surface area of the materials. Besides, the range of size distribution becomes narrower when

Figure 2. Variation of the digestion efficiency with temperature and endogenous pressure.

the temperature increases, as revealed by standard deviations. Furthermore, the uncertainty in the mean particle size decreases with increasing digestion temperature. The decomposition is generally agreed to proceed by depolymerization, in which lowmolecular-weight products such as monomers and radicals are generated.25 Effects of Temperature on PMMA Digestion. Figure 2 shows the effect of the digestion temperature on the digestion efficiency for PMMA polymers. When the temperature increases, the pressure under the maximum setting also increases dependently during microwave digestion. The endogenous pressure inside the close digestion vessel results mainly from the evaporation of HNO3 and partly from volatile decomposition products (e.g., a large amount of MMA, CO, CO2, H2O, and a trace of some organic compounds) of PMMA. The average translational kinetic energy of HNO3 molecules as well as the pressure increases with temperature, according to the kinetic molecular theory of gases28 and the van der Waals equation,29 respectively. This is due to the increase in the oxidizing potential of HNO3 and in the vibrational energy of PMMA with temperature, which enhances the efficiency of PMMA digestion. No apparent effect was observed on PMMA digestion at less than 423 K. Figure 3 shows the variation of digestion efficiency with the digestion time at various heating temperatures. The digestion efficiency of PMMA material increases with digestion time under the same digestion temperature. In addition, the digestion efficiency also increases with digestion temperature at the same digestion time. These tendencies demonstrate that the decomposition of PMMA material with nitric acid is a time-dependent reaction. The weight-loss rate of PMMA is similar during the digestion period at relatively lower digestion temperatures, 423 and 433K, which appears as nearly but not a simple zero-order reaction under these temperatures. As the digestion temperature increases to 443 K, most weight loss of PMMA intensively occurred within 10 min and then slowly decreased for the following time. By the fractional-life (τ) method,30 the order of the reaction under this temperature

Ind. Eng. Chem. Res., Vol. 47, No. 8, 2008 2557 Table 2. Kinetic and Statistical Parameters of PMMA Digestion with 3 mL of HNO3 digestion temperature, T (K)

mass fraction, φ (g/g)

rate constant, ks × 103 (g min-1)

rate constant, km (min-1)

R2

423 433 443 453

0.09 0.17 0.33 0.60

0.94 0.96 0.97 1.00

0.05 0.18 0.55 0.80

0.9740 0.9900 0.9948 0.9978

chain and side-chain scission during digestion may be described respectively by eqs 4 and 5. Figure 3. Variation of the digestion efficiency as a function of digestion time with different heating temperatures in 3 mL of nitric acid.

can be determined from the slope of a linear regression line diagrammed for log τ versus initial weight (log[PMMA]0). The fractional life reaction order is 1.19 with R2 ) 0.9832 (correlation coefficient) as estimated by defining the fractional life as the time interval of decreasing the initial weight of PMMA to 75%, which indicates that the decomposition may involve two simultaneous reactions with different orders.31 At a higher temperature of 453 K, the weight loss of PMMA decreases drastically within 5 min. According to the fractional-life method by defining the fractional life as decreasing the initial weight of PMMA to 50% (i.e., the half-life (τ) method30), the reaction order can be determined as 1.55 with R2 ) 0.9739. This indicates that the reaction probably proceeds by two simultaneously occurring mechanisms with different reaction orders, and the estimated order (1.55) may be a pseudo order.31 Moreover, according to the digested products reported in literatures24,25 and the FTIR and SEM characterization of the products of PMMA after digestion as described above, the decomposition of PMMA may result from mainchain and side-chain scission. Accordingly, the decomposition of PMMA may be proposed as a multi-order combined reaction, and the rate of the reaction (r) as eq 2 to obtain a better fit,

d[PMMA] ) r m + rs ) dt km[PMMA]R + ks[PMMA]β (2)

weight decomposition rate ) -

where rm and rs denote the rates of PMMA decomposition by main-chain and side-chain scissions, and km and ks denote the rate constants, respectively. The superscript letters R and β are integers for the order of main-chain and side-chain scission reactions. In the main chain scission reaction, the MMA monomer may generally be the major products, and the kinetics of this type of reaction is generally expected as a first-order reaction.9,18,24 Besides, the reaction corresponding to side-chain scission, such as homolytic scission of methoxycarbonyl side groups, forms low-molecular-weight compounds and is usually considered as a zero-order reaction.21,32-33 Hence, the decomposition of PMMA under a fixed HNO3 volume (3 mL) is postulated as a combination of first- and zero-order reactions, and the orders, R and β in eq 2, are replaced by 1and 0 respectively as shown in eq 3.

weight decomposition rate ) -

d[PMMA] ) rm + r s ) dt ks + km[PMMA]1 (3)

In addition, the remaining weight ([PMMA]m, [PMMA]s) and the initial weight ([PMMA]m0, [PMMA]s0) of PMMA via main-

[PMMA]s ) [PMMA]s0 - kst

(4)

[PMMA]m ) [PMMA]m0 exp(-kmt)

(5)

Because the total mass of the initial PMMA ([PMMA]0) is fixed as 0.1 g in a close digestion system, it is supposed that the φ fraction of original PMMA is decomposed by main-chain scission (i.e., [PMMA]m0 ) φ[PMMA]0) and the remaining (1 - φ) fraction is decomposed via side-chain scission {i.e., (1 φ)[PMMA]0}. Therefore, the relationship between the weight of PMMA and the digestion time can be described by eqs 6 and 7.

[PMMA] ) {(1 - φ)[PMMA]0 - kst} + φ[PMMA]0 exp(-kmt) (6)

[

] ]

PMMA ) PMMA0 kst 100 (1 - φ) + φ exp(-kmt) [PMMA0]

remaining weight (%) )100

{[

}

(7)

It is common to carry out optimization runs for each set of three kinds of parameters in the above nonlinear eq 7, which requires the use of Newton’s method with the commercial program Microsoft Excel by iterative trial-and-error computation. In eq 7, the parameters, φ () 0-1), ks, and km were confined as positive to be physically meaningful34 and were introduced with different values to closely fit the experimental data. The close fittings were carried out by minimizing the deviations between every experimental datum and calculated result as well as by maximizing the correlation coefficient (R2) of the linear regression curve between the experimental data and the modeled values. In the trial-anderror computation, the φ value was found governing the fitting principally. By introducing the values of φ, km, and ks under different temperatures listed in Table 2 into the model equation (eq 7), the relationship between the experimental and the modeled data was obtained as illustrated in Figure 4. The correlation coefficients (R2 ) 0.9740-0.9978) demonstrate that the theoretical model fits the experimental data well. Moreover, the φ values listed in Table 2 also indicate that the first-order reaction (φ ) 0.60) dominates at higher temperature (g453 K), whereas the zero-order reaction (φ e 0.33) dominates at lower temperatures (423-443 K). This agrees with the results illustrated in Figure 3. Other orders with the exception of zero and the first were also introduced into eq 2 to replace m and n for performing similar fittings; however, the derived models were abandoned due to poor coefficients of determination. To further obtain the activation energy (the minimal energy required for a collision to induce decomposition) of the decomposition reaction, the Arrhenius equation (eq 8)35 can be

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Figure 4. Relationships between the experimental value and model prediction of the remaining weight of PMMA during microwave digestion with 3 mL of nitric acid under various temperatures.

applied, where ki are the rate constants (i ) 0 or 1), A is a pre-exponential factor (corresponding to the frequency of collisions or the entropy,36 Ea is the activation energy, R is the universal gas constant (1.987 cal mol-1 K-1), and T is the absolute digestion temperature in Kelvin.

ki(T) ) A exp

( ) -Ea RT

(8)

This equation can be manipulated into linear form by taking natural logarithms of both sides as shown in eq 9.

lnki(T) ) lnA -

Ea 1 R T

()

(9)

A diagram of ln ki against 1/T should be a straight line, of which the slope is expressed as -Ea/R and the intercept with the vertical axis, at 1/T ) 0, is ln A. Therefore, the rate constants of the zero- and first-order reactions can be used to determine the activation energy using eq 9. The obtained slopes (-Ea/R) of the two regression lines are -374.63 and -18 152, and the regression coefficients of determination are 0.9605 and 0.9604, respectively. Thus, the activation energies of the zero- and firstorder reactions can be obtained as 0.74 and 36.07 kcal mol-1, and the A factors are 2.28 × 10-3 and 2.57 × 1017 g min-1, respectively. Figure 5 reveals that the rate constant of the weight degradation for the first-order reaction apparently depends on digestion temperature, whereas the zero-order reaction exhibits

Figure 5. Arrhenius plot of the rate constants of PMMA decomposition with 3 mL of nitric acid, according to the simplified zero- and first-order rate models.

less dependence on temperature; the former with the higher activation energy with an increase in digestion rate more rapidly than the latter with lower activation energy as the temperature increases. At low temperature, the zero-order reaction with the lower activation energy will predominate; at intermediate temperature, both mechanisms involving zero- and first-order reactions will contribute; at high temperature, the first-order reaction with higher activation energy will predominate due to higher values of the A factor. In other words, the increase of reaction order with temperature may be ascribed to the increase of effective collisions (i.e., collisions whose energy exceeds the

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results (R2 ) 0.9909-0.9998). As represented in part b of Figure 6, the φ values are increased with HNO3 volume at 423, 443, and 453 K, yet they vary insignificantly at 473 K. Conclusions

Figure 6. (a) Variation of the remaining weight of PMMA and (b) predicted φ values for PMMA decomposition under 5 min of digestion time with HNO3 volume at different digestion temperatures. Table 3. Predicted φ Value at Various Acid Volumes and Digestion Temperatures with 5 min of Digestion Time HNO3 volume (mL) φ value digestion temperature, T (K)

2

3

5

7

423 443 453 473

0.0001 0.230 0.570 0.680

0.090 0.330 0.600 0.750

0.350 0.585 0.742 0.750

0.630 0.630 0.820a 0.750

a

Result of the best fitting by Newton’s method.

activation energy) and the multiplication of reacting surface area when PMMA is fragmentized. Effect of the Acid Volume of the Digestion Reagent. The effect of the digestion reagent (HNO3) volume on PMMA (MW ) 350 000) decomposition with 5 min of digestion time is shown in part a of Figure 6. The remaining weight of PMMA apparently decreases with increasing HNO3 volume at the tested temperatures of 423, 443, and 453 K because the increase of digestion efficiency is attributed to increasing the oxidizing potential of HNO3. At 473 K, the digestion efficiency for PMMA has increased to 100% as the volume of HNO3 is greater than 3 mL, which indicates that sufficient energy is available at high endogenous pressure (>250 psi) to complete PMMA digestion regardless of HNO3 volume. Because the rate constants in this study depend only on temperature, the effect of HNO3 volume should be corresponding to the mass fraction (φ) of PMMA decomposed via mainchain scission according to eq 7. By introducing digestion time (i.e., t ) 5) and the rate constants (km and ks) at various temperatures into eq 7, the φ values can be obtained by the approximation of Newton’s method, as listed in Table 3. The model prediction was very consistent with the experimental

In comparison with the conventional technique, the utilization of microwave digestion with acid-catalyzed hydrolysis provides another potential way for polymer degradation. Several parameters of the microwave-assisted heating system under different sets of conditions were presented for digestion efficiencies via kinetic methodology in this study, which facilitates the investigation of monomer recovery from polymer waste. From FTIR spectra observation, the PMMA polymer becomes hydrophilic due to acid-catalyzed hydrolysis of the chemical environment after microwave digestion. The dominant products are suggested to be volatile monomers, for example, MMA and hydrophilic products with -COOH groups. According to the particle size distribution of SEM, PMMA decreases, whereas the size distribution narrows with increasing digestion temperature and enlarges the surface area of the materials. A new kinetic model was proposed to describe the decomposition behavior of PMMA during digestion, which accounted for the effects of multi-order combined reactions via the first-order of main-chain scission reactions and zero-order of side-chain scissions. The mathematical model is in agreement with the experimental results (R2 ) 0.9740-0.9978). Moreover, the firstorder reaction dominates at higher temperature (g453 K), whereas the zero-order reaction is favored at lower temperatures (423-443 K). The average activation energies are 36.07 and 0.74 kcal/mol for the former and latter reactions, respectively. In addition, HNO3 obviously enhances the digestion efficiency of PMMA because the HNO3 volume is greater than 3 mL at 423-473 K, ascribed to increasing the oxidizing potential of HNO3. The digestion efficiencies and the φ values increase with HNO3 volume at 423-453 and at 473 K, and the digestion efficiency increases to 100% when the volume of HNO3 is g3 mL. Acknowledgment The authors would like to thank the National Nano Device Laboratory of the Republic of China, Taiwan, for financially supporting this research under contract no. NDL-92S-C014. Literature Cited (1) Nakajima, M.; Yoshikawa, T.; Sogo, K.; Hirai, Y. Fabrication of Muilti-Layered Nano-Channels by Reversal Imprint Lithography. Microelectron. Eng. 2006, 83, 876. (2) Jacobsen, N. L.; Mitchell, D .L.; Johnson, D. L.; Holt, R. A. Lased and Sandblasted Denture Base Surface Preparations Affecting Resilient Liner Bonding. J. Prosthet. Dent. 1997, 78, 153. (3) Denq, B. L.; Hu, Y. S.; Chiu, W. Y.; Chen, L. W.; Chiu, Y. S. Thermal Degradation Behavior and Physical Properties for Poly(Methyl Methacrylate) Blended with Propyl Ester Phosphazene. Poly. Degrad. Stab. 1997, 57, 269. (4) Marimuthu, A.; Madras, G. Effect of Alkyl-Group Substituents on the Degradation of Poly(Alkyl Methacrylates) in Supercritical Fluids. Ind. Eng. Chem. Res. 2007, 46, 15. (5) Wondraczek, K.; Adams, J.; Fuhrmann, J. Change of Tacticity during Thermal Degradation of PMMA. Macromol. Chem. Phys. 2002, 203, 2624. (6) Kuo, S. W.; Kao, H. C.; Chang, F. C. Thermal Behavior and Specific Interaction in High Glass Transition Temperature PMMA Copolymer. Polymer 2003, 44, 6873. (7) Holland, B. J.; Hay, J. N. The Effect of Polymerization Conditions on the Kinetics and Mechanisms of Thermal Degradation of PMMA. Poly. Degrad. Stab. 2002, 77, 435.

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ReceiVed for reView October 20, 2007 ReVised manuscript receiVed February 8, 2008 Accepted February 8, 2008 IE0714246