微波消化系統應用於高分子材料分解之動力學研究

Research on the Kinetics of Polymer Decomposition Using Microwave Digestion System

蔡翠玲(Tsuey-Lin Tsai)

指導教授 ： 朱鐵吉

國立清華大學/原子科學院/生醫工程與環境科學系/博士(2009年)

https://doi.org/10.6843/NTHU.2009.00028

摘要

本研究以紅外線光譜、掃描式電子顯微鏡、高效率液相層析示差折光檢測器之特徵化分析及重量損失法探討聚甲基丙烯酸甲酯(PMMA)之三種不同分子量之高分子材料在微波加速反應系統之熱分解行為，首創建立ㄧ簡化、新穎之消化動力學模式-零級與ㄧ級反應。利用牛頓逼近法，將PMMA由微波消化分解之殘餘重量的實驗結果相當逼近此動力學模式。對於較低的消化溫度(423-443K)下，主要進行零級反應；而ㄧ級反應則在較高的溫度下進行(≥453K)。聚甲基丙烯酸甲酯在423K-453K溫度間，其分解機制經由主鏈(453K)與側鏈(423K-443K)裂解反應，透過此經驗模式，計算出動力學參數，包含反應速率常數與質量分率(α)。根據阿瑞尼士方程式，對於PMMA 分子量為996,000 g/mol的零級與ㄧ級分解反應，其平均活化能分別為2.63與25.25 kcal/mol；對於PMMA分子量為350,000 g/mol的零級與ㄧ級分解反應，其平均活化能分別為0.76與35.97 kcal/mol；對於PMMA分子量為120,000 g/mol的零級與ㄧ級分解反應，其平均活化能分別為1.79與29.18 kcal/mol。此外，對於PMMA 分子量為996,000 g/mol的零級與ㄧ級分解反應，其平均碰撞頻率分別為2.4×10-2 g min-1與2.50×1012 min-1；對於PMMA分子量為350,000 g/mol的零級與ㄧ級分解反應，其平均碰撞頻率分別為2.33×10-3 g min-1 與 9.31×1017 min-1；對於PMMA分子量為120,000 g/mol的零級與ㄧ級分解反應，其平均碰撞頻率分別為8.5×10-3 g min-1與3.72×1014 min-1。
本實驗亦對硝酸體積在423K-473K溫度間。探討對聚甲基丙烯酸甲酯三種不同分子量之分解效應影響。在溫度為473K，硝酸體積大於3mL時，對PMMA 996,000 與 350,000 g/mol分子量而言，消化效率達到100%，其原因來自於硝酸之氧化能力。對PMMA 120,000 g/mol分子量而言，當硝酸體積大於3mL時，消化效率亦幾乎達到100%。對350,000 g/mol分子量而言，硝酸體積為2-7mL，溫度為423K、443K與453K時，估算之質量分率(α)隨硝酸體積增加而增加，但溫度到達473K，α值隨著酸體積變化卻不明顯。同理，對於PMMA 996,000 與 120,000 g/mol分子量而言，溫度為423K與443K時，估計之α值隨著酸體積增加而上升，但溫度在453K與473K時，α值的改變卻不顯著。

關鍵字

動力學 ； 數值模擬 ； 微波消化 ； 聚甲基丙烯酸甲酯 ； 高分子 ； 照射

並列摘要

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.

並列關鍵字

kinetics, ； mathematical modelling ； microwave digestion ； poly(methyl methacrylate) ； polymers ； irradiation

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