Abstract
A simplified and novel kinetic model was firstly developed in this study by way of characterization of Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), high performance liquid chromatography (HPLC) with refractive index detector (RID) and the weight-loss method for PMMA decomposition under microwave assisted digestion system. By Newton’s approximation method, the experimental results of the remaining weight of PMMA were closely fitted by the model combined with zero-order and first-order kinetics, in which the former dominated the reaction at lower temperatures (423-443K) and the latter at higher temperatures (≥453K). Kinetic parameters of PMMA decomposition under 423-453K including rate constants and the mass fractions (α) via main-chain (453K) and side-chin (423K-443K) scission were determined by this empirical model. The average activation energies of PMMA 996,000 g/mol decomposition estimated by Arrhenius equation were 2.63 and 25.25 kcal/mol for the zero- and first-order reactions, respectively. For PMMA 350,000 g/mol, 0.76 and 35.97 kcal/mol were estimated for the zero- and first-order reaction, respectively. For PMMA 120,000 g/mol, 1.79 and 29.18 kcal/mol were obtained for the zero- and first-order reaction, respectively. In addition, the average pre-exponential factors of the respective zero- and first-order reactions for PMMA 996,000 g/mol were 2.4×10-2 g min-1 and 2.50×1012 min-1 respectively. For PMMA 350,000 g/mol, 2.33×10-3 g min-1 and 9.31×1017 min-1 were calculated for the respective zero- and first-order reactions. For PMMA 120,000 g/mol, 8.5×10-3 g min-1 and 3.72×1014 min-1 were obtained for the zero- and first-order reaction, respectively. Effect of HNO3 volume on PMMA decomposition was further investigated at 423-473K for three different molecular weights of PMMAs. At 473K, the digestion efficiency increased to 100% as HNO3 volume was ≥3 mL for PMMA 996,000 and 350,000 g/mol, respectively. This was due to the increase of oxidizing potential of HNO3. For PMMA 120,000 g/mol, the decomposition was also almost completely digested (100%) when the amount of HNO3 was >3 mL. The estimated α values for the decomposition of PMMA 350,000 g/mol with 2-7 mL of HNO3 were increasing with HNO3 volume at 423, 443K and 453K, yet varying insignificantly at 473K. The predicted α values for the decomposition of PMMA 996,000 and 120,000 g/mol were also increasing with HNO3 volume at 423 and 443K, but not apparently varied at 453 and 473K.