Nonconductive samples polymers

Another popular approach to glow discharge spectroscopy is to use rf power instead of traditional dc power sources. The main advantage of rf-GD is its ability to sputter nonconductive samples, hence elemental analysis for polymers and ceramics becomes a matter of simple solids analysis. 

If the rf source is applied to the analysis of conducting bulk samples its figures of merit are very similar to those of the dc source [4.208]. This is also shown by comparative depth-profile analyses of commercial coatings an steel [4.209, 4.210]. The capability of the rf source is, however, unsurpassed in the analysis of poorly or nonconducting materials, e.g. anodic alumina films [4.211], chemical vapor deposition (CVD)-coated tool steels [4.212], composite materials such as ceramic coated steel [4.213], coated glass surfaces [4.214], and polymer coatings [4.209, 4.215, 4.216]. These coatings are used for automotive body parts and consist of a number of distinct polymer layers on a metallic substrate. The total thickness of the paint layers is typically more than 100 pm. An example of a quantitative depth profile on prepainted metal-coated steel is shown as in Fig. 4.39. 

The related fully sulfonated, self-doped polymer poly(2-methoxyaniline-5-sulfonic acid) (PMAS 9) may be prepared under normal atmospheric pressure by the oxidation of 2-methoxyaniline-5-sulfonic acid (MAS) monomer with aqueous (NH4)2S208 in the presence of ammonia or pyridine (to permit dissolution of the MAS monomer).141 The polymerization pH was therefore >3.5. Subsequent studies showed that the product consisted of two fractions a major fraction with Mw of ca. 10,000 Da whose electrical conductivity and spectroscopic and redox switching properties were consistent with a PAn emeraldine salt, as well as a nonconducting, electroinactive oligomer (Mw ca. 2,000 Da).143 144 Pure samples of each of these materials can be obtained using cross-flow dialysis.145... 

Examining nonconducting materials, particularly polymers and biological materials, may present other difficulties, such as thermal degradation, radiation damage, and sample volatility in the high vacuum. 

Applications now include the analysis of nonconductive materials such as polymers, ceramics, and glasses using the RF Marcus-type source. The advantages are similar to those just discussed, with a major improvement in detection limits and a decrease in sources of error. With a DC GD source, a nonconductive material had to be diluted with a conductive powder this decreased the amount of analyte that could be detected in the sample. Use of an excess of conductive powder and the process of blending and pressing always introduced the possibility of contamination of the sample. This source of error has been eliminated by direct analysis of nonconductive materials. 

Plastic composites are more or less inhomogeneous particularly due to the anisotropy of filler concentration. Plastic processing methods involving shear flow can lead to separation of the fillers and polymer matrix. For instance, a skin-core structure can be formed where the outer surface has a lower filler concentration than the inner part of the specimen. This phenomenon generates samples with a nonconductive surface (although the conductivity of the core may be high). Typically, the conductivity... 

Results for the dielectric PTF, which contained more solvent and was therefore the more difficult to cure, were reported in this study. Residual solvent and percent coating were measured on specimens of approximately 25 and 100 pm thicknesses coated onto stainless-steel mesh of siiMar thicknesses. Samples were heated rapidly in the TGA to 220 °C in nitrogen and held for 60 min to evaporate all residual solvent and other volatiles, and then ramped at 10 °C/min to 630 °C to pyrolyze the polymer binder, leaving the nonconducting filler and the mesh. In this way the amount of solvent-free coating, residual solvent, and filler content could be measured. 

Polymers are typically nonconducting specimens that collect rather than dissipate the electrons from the electron beam resulting in a range of spurious image details referred to as charging effects. Such samples are generally metal coated to provide an electrically conductive layer, to suppress surface charges, to... 

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