利用電漿反應進行聚乳酸聚合與二氧化碳轉化行為之研究

Study on Polymerization of Lactic Acid and Conversion of Carbon Dioxide by Plasma Activation

張懿心(Yi-Hsin Chang)

指導教授 ： 楊長謀

國立清華大學/工學院/材料科學工程學系/博士(2010年)

https://doi.org/10.6843/NTHU.2010.00311

摘要

近來低溫電漿廣泛的的應用於材料合成或是化學反應製程上，電漿由高能帶電粒子組成，電子經由電場加速碰撞分子進行活化反應，生成離子也可經由電漿鞘區電場加速碰撞驅使化學反應，因此可有效提升分子反應速度。在本研究中我們利用電漿活化乳酸分子進行聚合反應，乳酸反應前驅物包含氣態單體與液態寡聚物，藉以討論前驅物型態對電漿聚合反應之影響。此外，我們也研究利用電漿技術轉化二氧化碳為可再利用之有機產物，電漿的高能電子可活化二氧化碳分子，進而與小分子進行化學反應以生成燃料。由於電漿製程具有高效率、低耗能、低污染等優點，非常適用於生醫及環保用途上。
本論文的第一部份研究，我們利用改良電漿製程，進行高分子密度電漿聚合反應，發現經活化後乳酸單體可快速沉積成薄膜，且薄膜具有分子級平坦的表面形貌。此外，所得高分子薄膜具有交聯結構且交聯密度隨電漿功率與反應時間增加而增加，此交聯結構歸因於電漿中分子碎片重組所造成。再者，此薄膜具有良好的機械強度與細胞相容性，可適用於生醫薄膜等用途上。如改採用電漿活化液態乳酸寡聚物聚合，則主要反應路徑為酯化聚合反應，分子於吸收電漿中電子震動能後可驅使該反應進行，而實驗結果顯示，寡聚物分子量對於生成高分子的化學組成與交聯密度有顯著影響。此研究驗證藉調控電漿反應條件及反應前驅物密度，可利用電漿技術快速製備高分子。
本研究第二部分實驗是利用電漿活化二氧化碳分子，活化後的二氧化碳分子分解為氧原子和一氧化碳自由基，可在無催化劑環境下，與碳氫小分子進行化學反應，藉由調整碳氫小分子結構可得不同化學組成之產物。研究結果顯示，產物的固體與液體含量與分子構造有關，當碳氫分子具有較多氫原子可生成較多液體產物，這是因為氫原子可飽和分子活化位置，減少長鏈分子生成機率。因此在實驗中，二氧化碳與飽和碳氫分子較與不飽和碳氫分子反應所得之液體產物含量高，固體產率隨不飽和碳氫分子中碳碳雙鍵數目增加而增加。此外，二氧化碳與雙鍵反應時具有較高的反應性，因為雙鍵的高活性可在不斷鍵下與氧原子和一氧化碳自由基反應。至於苯環分子，於開環後生成的分子碎片具有與二氧化碳良好的反應性且易形成固體產物，該高固體產率可再加入水分子進行共反應時下降。因此，此研究證明在合適的條件與共反應分子結構下，二氧化碳可在無觸媒環境，經由電漿反應轉化為有用產物。

關鍵字

電漿 ； 聚乳酸 ； 生物可分解 ； 二氧化碳 ； 轉化

並列摘要

Low-temperature plasmas are a well-known technique for material synthesis and chemical reactions. They offer a unique combination of energetic electrons, radicals, and ions to trigger plasma chemistry efficiently. Here, we performed a modified plasma process to induce polymerization from L-lactic acid (LLA) vapors and liquid LLA oligomers. We also aimed to convert carbon dioxide (CO2) into fuels by plasma activation without using catalysts. The high-efficient, low energy waste and environmental-friendly of plasma processes are beneficial for biomedical and environmental applications.
Firstly, novel biocompatible polymer films were derived from LLA with molecularly smooth surfaces by using a plasma deposition method significantly enhanced with higher monomer vapor densities. It was found that hydrocarbons and chain crosslinks increased relative to the oxygen-containing moieties as plasma power or reaction time increased, following a reaction scenario dominated by energy-mediated molecular scission pathways. With the hydroxyl groups being retained, the films of excellent mechanical strength were highly hydrophilic and manifested excellent biocompatibility applicable for a wide range of biomedical coatings. For the polymerization from liquid LLA oligomers, the polymerization was dominated by esterification processes activated by increased molecular vibrational energies imparted by electron bombardments from the plasma. The effects of oligomer molecular weight were studied and found useful for tailoring the properties of the final polymers. This work demonstrated the feasibility of converting common molecules to useful polymers under appropriate plasma conditions.
Secondly, selected reactions between carbon dioxide and small hydrocarbon molecules (CnHm) of various bond structures and sizes (6≤n≤12) were investigated under plasma activation without catalysts. CO2 broke up into CO and O to form oxygenated functionalities in liquid and solid products. The liquid/solid ratio depended on plasma energy and molecular structures of the hydrocarbons. A high yield of liquids was obtained when enough hydrogen atoms were provided to saturate the active sites on CO2 and hydrocarbon fragments. Hence, the liquid yield from CO2 conversion with saturated hydrocarbons is higher than unsaturated hydrocarbons. For unsaturated hydrocarbons, the yield of solid products increases with increasing the number of C=C bonds. The bi-functional radicals produced through pi-bond dissociation can propagate into polymers easily. Such high solid yields from unsaturated hydrocarbons are able to decrease by adding water into the plasma system. In addition, the C=C bonds were found to have high activities with CO2 due to an effective recombination process of CO and O radicals with C=C bonds. This study clarifies the reaction routes for CO2 and hydrocarbon molecules under plasma activation and affords proper selections of molecules for optimal syntheses with CO2 without catalysts.

並列關鍵字

plasma ； poly(L-lactic acid) ； biodegradable ； carbon dioxide ； conversion

參考文獻

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