18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS DETERMINATION OF THERMODYNMIC SURFACE CHARACTERISTICS OF CARBON NANOTUBES VIA INVERSE GAS CHROMATOGRAPHY METHOD H. Lim, Y. Kim, C.R. Park* Carbon Nanomaterials Design Laboratory, Global Research Laboratory, Research Institute of Advanced Materials, and Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea * Corresponding author (crpark@snu.ac.kr) Keywords : Inverse Gas Chromatography, Carbon Nanotubes, Surface Characteristics, Solubility Parameter surface characterization method such as XPS, FT-IR Abstract Inverse gas chromatography method is widely used etc. for the characterization of polymers and surfactants In this research, we introduced an IGC method. IGC that are capable to offer useful information to predict is a powerful method which offers essential miscibility of polymer composites. However thermodynamic surface characteristic information of adaption of IGC to carbon nanotubes (CNTs) is still column packing materials, including surface tension, challenging due to many problems arising in interaction parameter and solubility parameter. This establishment and there are only limited reports method has advantages for investigating the about CNTs. Thus we researched the characteristics of solid surfaces in powder form and thermodynamic surface characteristics of CNTs has been widely used to the characterizing of the study. And we confirmed that the IGC method can surface properties of polymers. And recently it is be successfully applied for various CNTs to applied for determination of surface characteristics determine unknown thermodynamic surface of carbon nanomaterials. However adaption of IGC characteristics, including solubility parameter in on CNTs is still challenging and only a few studies particular. have been published by this time. This is because strong dispersive interaction and high specific surface area result in weak and distorted retention chromatograms. [3] 1. Introduction Herein we tried to find method to avoid such CNTs are attractive candidate for reinforcing filler in problems, and moreover thermal decomposition of CNT based composites.[1] However, due to the the surface functional groups in the case of surface strong Van der Waals interaction between CNTs, modified CNTs. Then we determine thermodynamic they tend to agglomerate and form bundles, which parameters representing the surface characteristics, hinder good dispersion in surrounding media. So including Hansen Solubility Parameters. their compatibility with common polymer matrix is very limited and phase separation, a cause of 2. Theory performance deterioration, occurs in composites.[2] To enhance the miscibility, modification of surface Surface characteristics determined by IGC are characteristics with various functional groups has calculated with retention volume obtained by been most widely adopted. However, finding symmetrical peak of chromatogram under ideal suitable pair of modified CNT/matrix for a given dilute condition of probe molecules with next eq. 1 type of CNTs still entirely relies on the experimental [3] trials and errors. To overcome this situation, it is ( ) jF t t T required to find appropriate guidelines or parameters (eq. 1) r m V g 273.2 enabling it to predict the dispersion behavior of m K CNTs which cannot be determined by general
(V g : specific retention volume, F: flow rate of carrier Δ E A is approximately identical with Δ H A , adsorption gas, t r : retention time of probe, t m : void retention enthalpy, calculated from slope of plot of Δ G A /T vs time measured by non-interacting marker, T: column 1/T. temperature, m: mass of stationary phase (packing By adjusting multi linear regression method to eq. 6 material in column), j: James-Martin correction with known solubility parameter of probes, Hansen factor [4]) solubility parameter of stationary phase can be Adsorption free energy Δ G A can be calculated from obtained. V g using the following relationship 3. Experimental V P 0 ln( g ) (eq. 2) G RT A SB 3.1 Materials 0 Multi-walled carbon nanotubes (<96% carbon (R: gas constant, S: specific surface area, P 0 : reference partial pressure of probes = 1.013×10 5 Pa, contents) are produced using chemical vapor deposition (CVD) method by JEIO Co., Ltd. B 0 : reference bidimensional spreading pressure of (denoted APMWCNT). the adsorbed probe=3.38×10 -4 Nm -1 ) Dispersive surface tension γ D is obtained by following relation equation [5] 3.2 Inverse Gas Chromatography 2 IGC chromatograms were recorded on a Perkin G 1 CH D (eq. 3) 2 Elmer Clarus 600 Gas Chromatograph (Perkin S 2 N a Elmer, USA) with a thermal conductivity detector. CH A CH 2 2 CNTs were packed into a stainless steel tube of 1/4 ( Δ G CH2 : adsorption free energy per mole of CH 2 inch in diameter to prepare columns. Each end of the groups of aliphatic hydrocarbon, N A : Avogadro's columns was plugged with silane-treated glass wool number, a CH2 : cross sectional area of a methylene to fix the stationary phase. The columns were group (0.06 nm 2 ), γ CH2 : surface energy of infinite stabilized in the GC system under a helium flow to polyethylene chain, ) eliminate pre-adsorbed volatile impurities if any. Δ G CH2 and γ CH2 can be represented with following To find optimum working conditions of IGC, a equation.[6] relationship between adsorption thermodynamic parameters determined with operating variables V 1 ln (eq. 4) n G RT including the quantity of the injected probe CH 2 V molecules, and column temperature was investigated. n And the validity of conditions was confirmed by a 35.6 0.058(20 ) (eq. 5) T linear relationship of enthalpy or dispersive surface CH 2 tension. (V n , V n+1 : retention volume of aliphatic hydrocarbon of carbon number n, n+1) Using the model developed by Karger et al. [7], the 4. Results and Discussion Hansen solubility parameters of solids can be 4.1 Data correction calculated from IGC data with Δ E A as follow: As shown in Fig. 1, the adsorption free energy V ( P S P S P S ) (eq. 6) E increases exponentially with decreasing quantity of A P d d p p h h the injected probe molecules. It is well known that (V P : molar volume of probes, δ : solubility parameter, as the concentration of the probe decreases, the d, p, h: dispersion, polar, hydrogen-bonding retention chromatogram becomes more symmetrical. component of solubility parameter, P: solubility As denoted above theory section, determination of parameter of probes, S: solubility parameter of surface characteristics by IGC should be performed stationary phase) under ideal dilute condition. In this condition, Assumed that the change in volume on adsorption of chromatogram shows symmetrical peak and infinite dilute gas probe on stationary phase is zero, retention time doesn`t depend on quantity of probe.
PAPER TITLE 4.2. Dispersive surface tension Fig.1 . Injected quantity of chloroform vs adsorption free energy of chloroform on APMWCNTs. Fig.3 . Dispersive surface tension of APMWCNTs When the concentration is infinitely dilute, it is at (200~240 o C). the zero surface coverage (ZSC) condition, which is required to determine the surface characteristics Dispersive surface tension of CNTs were obtained precisely. Based on this definition of ZSC, an by calculation of above eq. 3~5. For APMWCNTs, extrapolation to y-axis in Fig. 1 can yield the it showed 90.7mJ/m 2 under 200 o C condition with adsorption free energy at ZSC, which make it linear relationship between temperatures. And it possible to determine the precise thermodynamic showed similar results with reported values about surface characteristics different type of as-received CNTs or graphite powders [5]. 4.3. Hansen solubility parameters Hansen solubility parameters of APMWCNTs were determined by multi linear regression with adsorption enthalpy of various probes. Obtained data δ d =19.7, (unit:MPa 1/2 ) were δ p =4.1, δ h =5.7 respectively. It is compared with reported value which is obtained by calculation from center of Hansen sphere radius with observation of sedimentation behavior of CNT dispersion which reported δ d =19.8, δ p =6.1, δ h =4.3 [8] Fig. 2 Reciprocal of temperature vs adsorption free From the results, Hansen solubility parameters energy divided by temperature (50 o C ~250 o C) obtained by IGC showed relatively similar with the value obtained by Hansen sphere radius. To predict Indeed, Fig. 2 clearly shows a linear relationship the dispersion property of CNTs in solvents by between the reciprocal of temperature and comparing the parameters of CNTs and solvents, adsorption free energy divided by temperature. The they showed almost similar tendency with common results of Figs. 1 and 2 indicate that it is possible to solvents. So we conclude that prediction of the determine the surface characteristics of CNTs even dispersion behavior of CNTs with IGC is acceptable at low temperature whereby the ZSC condition is met. 3
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