The investigation on the dispersion of ionic liquids-physisorbed MWCNT and the applications of MWCNT/epoxy nanocomposites
Date Issued
2014
Date
2014
Author(s)
Chang, Yu-Hsun
Abstract
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.
Subjects
奈米碳管
離子液體
流變
物理性吸附
環氧樹脂
Type
thesis
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