Characterization by IGC

Increased interest in IGC has resulted in a dramatic increase in the number of papers on the subject. In the decade following the first mention of IGC in 1967, approximately 30 papers were published about IGC. In the ensuing decade, more than 300 IGC papers were published. This book, the first to focus exclusively on IGC, contains 19 of the 20 papers presented at the Symposium on Polymer Characterization by IGC. Three chapters were added to broaden the scope of this volume. 

Lately the most frequently used technique for the determination of thermodynamic and acid/base characteristics is inverse gas chromatography [30,73-75]. In IGC the unknown filler or fiber surface is characterized by compounds, usually solvents, of known properties. IGC measurements can be carried out in two different ways. In the most often applied linear, or ideal, IGC infinite concentrations of n-alkane are injected into the column containing the filler to be characterized. The net retention volume (V ) can be calculated by ... 

Carbon fiber reinforced composites are at the forefront of current developments in polymer composites, and there is additional evidence for the important role being played by IGC in characterizing the interface in such systems. The Gutmann theory is used by Bolvari and Ward, who report add/base interactions for surface-treated carbon fibers and a series of thermoplastic polymer hosts, including polysulfone, polycarbonate, and... 

A further illustration of IGC as a source of data for acid/base characterization of polymers and of solid constituents of complex polymer systems, is given by Osmont and Schreiber (49), who rate the inherent acid/base interaction potentials of glass fiber surfaces and of polymers by a comparative index, based on the Drago acid/base concepts (SO). The interaction index is conveniently measured by IGC and is shown to differentiate clearly among untreated and variously silane-modified glass fiber surfaces. Conventional methods are used to determine adsorption isotherms for fiber-polymer pairs, and the IGC data ate used to demonstrate the relationship between acid/base interactions and the quantity of polymer retained at fiber surfaces. 

IGC has been used at zero surface coverage to characterize the surfaces of cellulose (5), cellophane (6), and poly(ethylene terephthalate) film (7 ). Surface properties of Intact textile fibers were also studied by IGC (8). Domlngo-Garcla et al. (9 ) have recently characterized graphite and graphltlzed carbon black surfaces with this method, and some zero coverage results on carbon fibers have appeared (10). 

Kamdem et al. [31] have used the AN values of polar probes as suggested by Riddle and Fowkes in characterization of birch wood meal by IGC method. The reported values of Kd and Ka indicated that white birch wood surface has an amphoteric character and predominantly acidic sites are involved in the adsorption process. 

Abstract A CaCOs filler was coated with various mono- and dicarboxylic acids in a dry-blending process. The coated fillers were characterized by various techniques, including dissolution experiments, thermal analysis (differential scanning calorimetry) and inverse gas chromatography (IGC) to determine the amount of surfactant needed to achieve mono-layer coverage IGC proved to be the most convenient, reliable and universal method for this purpose. The dispersion component of the surface tension and the specific interaction potential of the coated filler can be derived from the results, but indirect conclusions can be also drawn from them about the orientation of the molecules on the filler surface and the structure of the layer formed. The coverage of the filler with an organic compound leads to a... 

Polar surfaces enter into acid-base interactions with other materials. Such interactions can be characterized by the free-enthalpy change of adsorption, AG , of polar probes, which can also be determined by IGC from the retention volume of an appropriate solvent. CaCOs is basic in character and its basicity can be determined by using an acidic solvent CHCI3 was used in this study. The AG values obtained on behenic acid coated fillers are plotted in Fig. 4 as a function of the surfactant used for the treatment. The same tendency is observed as in Fig. 3 and the minimum appear practically at the same surfactant content. The free-enthalpy change of adsorption was determined for fillers coated with all the surfactants studied and the results are listed in Table 2. 

As mentioned before, austenitic stainless steels are susceptible to IGC due to sensitization caused by exposure to high temperatures (450-850 C). The IGC of austenitic stainless steel can also be characterized by normalized classical tests ASTM G28, ASTM A262-86, SEP 1877, AFNOR A05-159 and AFNOR A05-160, currently known as the Strauss, Huey and Streicher tests [54-57]. These methods however are destructive, difficult to perform on site and require sampling that can be harmful to the integrity of materials during service. For this reason, the electrochemical, non-destructive tests commonly known as EPR (electrochemical potentiokinetic reactivation) and DL-EPR (double loop electrochemical potentiokinetic reactivation) were developed to measure the sensitivity of austenitic stainless steels to IGC [58-66]. However, EPR and DL-EPR are based on measurements of characteristic potentials and currents of passive/active zones on potentiody-namic curves in an aqueous solution (linear voltammetry curve from oxygen to hydrogen evolution in the... 

If the redox state of the fuel salt is characterized by an uranium ratio [U(IV)]/[U(III)] < 1, the alloy specimens get a more negative stationary electrode potential than equilibrium electrode potentials of some uranium intermetaUic compounds and alloys with nickel and molybdenum. This leads to a spontaneous behavior of alloy formation processes on the specimen siur-face and further diffusion of uranium deep into the metaUic phase. As a consequence, films of intermetaUic compounds and alloys of nickel, molybdenum, and tungsten with uranium are formed on the aUoy specimen surfaces, and IGC does not take place. 

In order to calculate polymer/filler interaction, or more exactly the reversible work of adhesion characterizing it, the surface tension of the polymer must also be known. This quantity is usually determined by contact angle measurements or occasionally the pendant drop method is used. The former method is based on the Young, Dupre and Eowkes equations (Eqs. 21,8, and 10), but the result is influenced by the surface quality of the substrate. Moreover, the surface (structure, orientation, density) of polymers usually differs from the bulk, which might bias the results. Accuracy of the technique maybe increased by using two or more liquids for the measurements. The use of the pendant drop method is limited due to technical problems (long time to reach equilibrium, stability of the polymer, evaluation problems etc.). Occasionally IGC is also used for the characterization of polymers [30]. 

Inverse gas chromatography (IGC) refers to the characterization of the chromatographic stationary phase (polymer) using a known amount of mobile phase (solvent). The stationary phase is prepared by coating an inert support with polymer and packing the coated particles into a conventional gas chromatography column. The activity coefficient of a given solvent can be related to its retention time on the column. The equipment itself is commercially available, easily automated, and extremely versatile. 

By analogy to the above technique, gas chromatography is considered a useful tool to obtain data for gas and vapor adsorption on polymeric surfaces. In contrast to liquid chromatography, the general principle of the IGC technique is well established for the characterization of polymeric materials this technique called inverse gas chromatography (IGC), enables the study of various polymeric properties, including interfacial properties (15-18). 

IGC appears to be a useful and powerful method for the characterization of divided or fibrous solid surfaces. Because of its extreme sensitivity to small variations in the surface properties of the solid, IGC reveals interesting phenomena to be eventually confirmed by independent analytical methods. 

IGC is a gas phase technique for characterizing surface and bulk properties of solid materials. The principles of IGC are very simple, being the reverse of a conventional gas chromatographic (GC) experiment. A cylindrical column is uniformly packed with the solid material of interest, typically a powder, fiber, or film. A pulse or constant concentration of gas is then injected down the column at a fixed carrier gas flow rate, and the time taken for the pulse or concentration front to elute down the column is measured by a detector. A series of IGC measurements with different gas phase probe molecules then allows access to a wide range of physicochemical properties of the solid sample. The flow and retention of gas is shown in Figure 3. 

IGC has also been employed by Donnet et al. [135] to characterize silica xerogels. From chromatographic data the authors estimated the BET equation C parameter which is a measure of the gas—sohd interaction energy. They found that increasing the silylation of the surface decreases the value of C in agreement with the results obtained by Cascarini de Torre et al. [41] with gas adsorption and computer simulations. 

Evidenced has been obtained previously that the reinforcing effect of different grades of fumed silicas on silicone elastomers is influenced by the surface fractality [1] and that the surface roughness increases with the specific surface energy. The aim of the present work is to demonstrate variations by calling on NMR and infrared spectroscopic methods, which are applied to fumed silica samples that have been carefully characterized through adsorption methods including IGC analysis. 

IGC is a variation of coventional gas chromatography. Figure 1 shows a typical arrangement for IGC. In IGC a finely divided non-volatile material of interest (polymer, fiber, plasticizer, etc) is placed within a chromatographic column. It may be packed directly into the column, or coated onto a suitable support, or onto the walls of the column. A volatile "probe" of known characteristics is swept through the column by an inert mobile phase (eg. helium), and the output is monitored. The residence time of the probe and the shape of the output signal characterize the stationary phase and its interaction with the volatile phase. 

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