物理性吸附離子液體之奈米碳管分散性探討及其在奈米碳管/環氧樹脂奈米複合材料之應用

The investigation on the dispersion of ionic liquids-physisorbed MWCNT and the applications of MWCNT/epoxy nanocomposites

張育勳(Yu-Hsun Chang)

指導教授 ： 林金福

國立臺灣大學/工學院/高分子科學與工程學研究所/博士(2014年)

https://doi.org/10.6342/NTU.2014.01794

摘要

本研究目的主要是探討奈米碳管的分散行為，利用離子液體物理吸附於奈米碳管表面上，探討奈米碳管表面與離子液體之陽離子結構之間的陽離子-π作用力。經過表面吸附過後的奈米碳管與未改質奈米碳管各別添加到diglycidyl ether of bisphenol-A (DGEBA)環氧樹脂中，研究未交聯環氧樹脂奈米複材之加工特性及流變行為。隨後將未交聯環氧樹脂奈米複材進行交聯並製備成複材薄膜與塊材，探討薄膜複材之電性性質，並以可控制剪切平面角度之特殊量測裝置量測奈米複材本身的剪切強度。
本研究分為四部份，第一部份主要是合成三種離子液體1-(2-acryloyloxy-ethyl)-3-methyl benzoimidazol-1-ium iodide (AMBImI)、10-Methyl-acridinium iodide(MAcI)及1,3-dimethyl-3H-benzotriazolium iodide(DMBTAzI)與奈米碳管進行物理性吸附，即可得到MWCNT-MAcI、MWCNT-DMBTAzI及MWCNT-AMBImI，隨後再以相似結構非離子型化合物與奈米碳管吸附進行比較，探討陽離子-π作用力對吸附性之影響。從熱重分析儀(TGA)、高解析電子顯微鏡(HRTEM)和X光光電子光譜(XPS)可觀察出陽離子-π作用力在奈米碳管吸附之差異。物理吸附過後的奈米碳管則在有機溶劑中有較好的分散性，從電子顯微鏡(TEM)也可觀察到改質過奈米碳管在薄膜中仍有較佳之分散。從DMA之結果可發現改質過後之奈米碳管與環氧樹脂有較好的介面作用力，其複材之玻璃轉移溫度最高可提升到129℃左右。
本研究之第二部份，為了瞭解奈米碳管複材之加工性及流變特性，我們將探討奈米碳管濃度變化以及物理吸附改質過後的奈米碳管對未交聯奈環氧樹脂之流變性質影響。流變之相位移角度(δ)主要關係式為tan-1(G”/G’)， 當損失模數G”大於儲存模數G’且相位移角度大於45度時，未交聯環氧樹脂複材之流變行為為流動態行為。儲存模數G’大於損失模數G”且相位移delta小於45度時，此時流變行為為近固態行為。因此，我們視相位移角度45度為相變化轉折點之物理性膠化點。當添加表面物理改質的奈米碳管，奈米碳管則是因為分散性已改善，因此有較多的單一根奈米碳管彼此間進行作用，未交聯環氧樹脂奈米複材之物理性膠化則會提早發生。值得注意的是，在固定1wt%的含量下，添加MWCNT-MAcI的儲存模數G’已是未改質奈米碳管的170倍，而且可觀察到儲存模數並不隨著頻率而變化。穩定剪切黏度則會受奈米碳管濃度增加而上升，並可利用Thomas訂正過的Einstein viscosity equation可計算出環氧樹脂被包覆住在奈米碳管網狀結構的體積含量的變化，小角度X光散射則可推測改質過奈米碳管有較大的網狀結構。除此之外，我們也改變溫度狀態觀察溫度效應對環氧樹脂/奈米碳管複材之流變影響。在固定含量的未改質奈米碳管狀態下，複材之相位移角度則是隨著量測溫度的上升而下降，逐漸呈現出近固態之流變行為，加熱似乎有助於物理性膠化的發生。而奈米碳管含量的增加及分散性的改善，皆會改變未交聯環氧樹脂/奈米碳管複材之膠化溫度。溫度效應也造成環氧樹脂被包覆住在奈米碳管網狀結構中體積含量的改變。
本研究之第三部份主要探討交聯過後環氧樹脂/奈米碳管之電性質特性。交聯過後的環氧樹脂為標準絕緣材料，AC體積導電度會隨著頻率增加而增加。當添加未改質奈米碳管0.4wt%時，可觀察到在低頻率下AC體積導電度不受頻率影響，直到一臨界頻率(ωc)下AC體積導電度隨著頻率增加而上升，並且ωc會隨著奈米碳管添加量增加而往較高頻率位移。利用universal dynamic response(UDR)之ωs所fitting得到之s指數約為0.7-1。而分散較好之改質奈米碳管則會促使ωc往較高頻率偏移且也會影響UDR的s指數。在頻率趨近於0下之AC體積導電度可視為複材的DC體積導電度，在含有2wt%未改質奈米碳管時，體積導電度從原本10-11(S/cm)提升到1.25×10-5(S/cm)。當奈米碳管經過物理表面處理後，添加2wt%的MWCNT-MAcI其體積導電度可提升至4.56×10-5(S/cm)，2wt%的MWCNT-DMBTAzI之體積導電度來到3.36×10-5(S/cm)，是未改質奈米碳管的3.6倍。這是因為未改質奈米碳管在樹脂中分散性較差，經過吸附處理過後的奈米碳管則可在環氧樹脂中形成3D網狀結構，幫助電子傳遞進而提升體積導電度。DC導電度則也遵守percolation理論σDC＝σ0 (p-pc)t，未改質奈米碳管之t指數為1.3，pc為0.3wt%。而經過改質之奈米碳管則有較高的t指數與σ0常數，但pc已降至0.3wt%以下，最低可達0.18wt%。在SEM型態部分則可清楚觀察到未改質與改質奈米碳管其分散性的差別。
本研究最後一部分則是利用可控制試片剪切平面角度之新穎裝置量測環氧樹脂/奈米碳管複材塊材之剪切強度，由此實驗推算環氧樹脂之剪切強度可約為83.5MPa，當添加1wt%表面處理過之奈米碳管MWCNT-MAcI與MWCNT-DMBTAzI，複材之剪切強度則提升到98.69MPa和95.49MPa。其主要是改質過奈米碳管可呈現其卓越的機械特性，在剪切過程中，奈米碳管可輔助支撐住較高的剪切強度。在SEM型態部分，環氧樹脂的破壞截面型態會隨著角度增加及受力面積增加而有明顯變化，會從平坦的條紋(vein-like)結構轉變成粗糙的碎片結構。加入分散較好的MWCNT-MAcI與MWCNT-DMBTAzI，其條紋分枝結構更為明顯。傾斜角度增加到60°後，截面形貌則是因受到環氧樹脂中存在的改質奈米碳管的影響，因為改質後的奈米碳管與環氧樹脂有良好的介面作用力會使截面型態變得更加粗糙，分散性越好的奈米碳管，環氧樹脂結構破壞越嚴重。
總結，離子液體確實可藉由陽離子-π作用力吸附於奈米碳管，並可藉此改善奈米碳管的分散情況。此研究對於奈米碳管未來之應用有極大的幫助，奈米碳管不需要經由多步繁雜的處裡即可達到有效地分散，並且所製備之奈米複材也可達到良好的性質，其對於奈米碳管在未來工業科技上之應用的可行性則大大的提升。

關鍵字

奈米碳管 ； 離子液體 ； 流變 ； 物理性吸附 ； 環氧樹脂

並列摘要

The aim of this thesis was to discuss the dispersion of multi-walled carbon nanotube (MWCNT) modified by physical adsorption with ionic liquids. We investigated the cation-πinteraction between MWCNT and cation structure of ionic liquid. Pristine MWCNT and modified MWCNT would be incorporated individually into diglycidyl ether of bispheol-A (DGEBA) resin to fabricate epoxy/MWCNT suspension. We studied the processing and rheological behavior of epoxy/MWCNT suspensions, then the suspension was cured to form thin films or bulk nanocomposite. We also studied the electrical properties of nanocomposite films and the shear strength of bulk composite with the controlled applied normal stress.
The thesis divided into four parts. In the first part, we synthesized three types of ionic liquids, 1-(2-acryloyloxy-ethyl)-3-methyl benzoimidazol-1-ium iodide (AMBImI), 10-Methyl-acridinium iodide(MAcI) and 1,3-dimethyl-3H-benzotriazolium iodide(DMBTAzI), to physisorb onto MWCNT and obtained MWCNT-AMBImI, MWCNT-MAcI and MWCNT-DMBTAzI. We also used pre-ionized compounds physisorbed onto MWCNT to compare with ionic liquid-physisorbed MWCNT. Then, we investigated the effect of physisorption by cation-π interaction. We observed the excess adsorption with ionic liquid by thermo gravimetric analysis (TGA), high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). Ionic liquid-physisorbed MWCNT also showed well dispersion in organic solvent and nanocomposite observed by TEM. Consequently, the glass transition temperature of nanocomposites contained modified MWCNT was enhanced to 129℃ due to the well interaction between MWCNT and epoxy resin.
In the second part, we investigated the rheological behavior of epoxy/MWCNT suspension. MWCNT concentration effect on rheology was investigated in order to understand the processing and rheological properties of MWCNT composites. It was well known that storage modulus G’ and loss modulus G” of rheology were related to the degree of phase shift, δ, such that δ = tan-1(G” / G’). As the phase shift was higher than 45 degree, the G” was higher than G’ belonged to fluid behavior. On the contrary, as the phase shift was lower than 45 degree, it belonged to solid-like behavior. The suspension containing modified MWCNT gelled earlier than that containing pristine MWCNT because modified MWCNT had better dispersion to form network structure. We observed the plateau in storage modulus at lower angular frequency. The viscosity increased as the content of MWCNT was increased. We estimated volume fraction of particle domain aggregated by MWCNT through Thomas-modified Einstein viscosity equation and utilized small angle x-ray spectrum (SAXS) to measure the change of aggregated network structure. Besides, we also investigated rheological behavior of the epoxy/MWCNT suspension at various temperatures. At certain content of MWCNT, the suspensions showed more solid-like behavior as the temperature was increased. Increasing temperature facilitated the gelation of the suspension. The volume fraction of particle domain aggregated by MWCNT was changed by temperature effect.
In the third part, we investigated electrical properties of epoxy/MWCNT nanocomposite for modified MWCNT. We observed AC volume conductivity displayed a plateau at low frequency region until the critical frequency (ωc) for the nanocomposite containing 0.4 wt% pristine MWCNT. Above critical frequency (ωc), AC volume conductivity also was increased with frequency. ωc shifted to higher frequency with the increasing of MWCNT contents. We utilized universal dynamic response (UDR) to fit AC volume conductivity of nanocomposite and got s value was about 0.7-1. The value also shifted to higher frequency due to nanocomposites containing well dispersion modified MWCNT. By incorporation of 2 wt% pristine MWCNT, the volume conductivity of nanocomposite enhanced from 10-11(S/cm) of insulator to 1.25×10-5(S/cm). Moreover, the highest volume conductivity was improved to 4.56×10-5(S/cm) as the nanocomposite containing 2 wt% MWCNT-MAcI or MWCNT-DMBTAzI. DC volume conductivity also obeyed percolation theory σDC＝σ0 (p-pc)t. For modified MWCNT, it showed higher index of t and σ0 constant, and pc was below 0.3 wt%. The lowest pc of modified MWCNT was 0.18wt%.
In the last part of thesis, we measured the shear strength of bulk nanocomposite with controlled applied normal stress. The shear strength of cured DGEBA epoxy resin was 83.5 MPa. The shear strength of nanocomposite with 1 wt% MWCNT-MAcI or MWCNT-DMBTAzI was improved to 98.69MPa and 95.49MPa due to the excellently mechanical properties of MWCNT. In surface morphology, cross section of shear fracture of DGEBA epoxy resin dramatically changed from vein-like structure to rough chip-like structure as the inclined angle and compressive area was increased. With the incorporation of modified MWCNT-MAcI or MWCNT-DMBTAzI, the morphology of fracture clearly showed more branches in vein-like structure because of the improvement of MWCNT dispersion. As the inclined angle was increased to 60°, we could observe the fracture of nanocomposite containing modified MWCNT was damaged obviously due to the better dispersion.
Finally, cation-π interaction between ionic liquid and MWCNT was investigated that could improve the dispersion of MWCNT. It was not necessary to modify MWCNT through many complex processes. The nanocomposites we prepared also had excellent properties and had graet potential for application in the future.

並列關鍵字

carbon nanotube ； ionic liquid ； rehology ； physisorption ； epoxy

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