Bulk preparation techniques limitations

This process is highly suitable for rubbers with poor solubility. In this process, the rubber sheet is soaked in TEOS or quite often in TEOS-solvent mixture and the in situ sUica generation is conducted by either acid or base catalysis. The sol-gel reaction is normally carried out at room temperature. Kohjiya et al. [29-31] have reported various nonpolar mbber-silica hybrid nanocomposites based on this technique. The network density of the rubber influences the swelling behavior and hence controls the silica formation. It is very likely that there has been a graded silica concentration from surface to the bulk due to limited swelling of the rubber. This process has been predominantly used to prepare ionomer-inorganic hybrids by Siuzdak et al. [48-50]. 

Third, the bulk of the items in Table 1 address method performance. These requirements must be satisfied on a substrate-by-substrate basis to address substrate-specific interferences. As discussed above, interferences are best dealt with by application of conventional sample preparation techniques use of blank substrate to account for background interferences is not permitted. The analyst must establish a limit of detection (LOD), the lowest standard concentration that yields a signal that can be differentiated from background, and an LOQ (the reader is referred to Brady for a discussion of different techniques used to determine the LOD for immunoassays). For example, analysis of a variety of corn fractions requires the generation of LOD and LOQ data for each fraction. Procedural recoveries must accompany each analytical set and be based on fresh fortification of substrate prior to extraction. Recovery samples serve to confirm that the extraction and cleanup procedures were conducted correctly for all samples in each set of analyses. Carrying control substrate through the analytical procedure is good practice if practicable. 

Dimethylphenylphosphine-borane has been prepared in good yield by the displacement of trimethylamine by the less volatile dimethyl-phenylphosphine from trimethylamine-borane.1 This method could be extended to the syntheses of other phosphine-boranes, but it involves the use of expensive trimethylamine-borane. The direct combination2 of diborane with substituted phosphines could give good yields of pure phosphine-boranes, but the required use of vacuum-line techniques limits its utility to small-scale preparations. For the bulk synthesis of phosphine-boranes, the method reported by Nainan and Ryschkewitsch3 has been found to be most suitable, and we have now extended this method to the syntheses of dimethylphenylphosphine-borane and methyldiphenylphosphine-borane. 

Both solid and liquid samples can be analyzed by XRF as described earlier in the chapter. With the exception of micro-XRF, XRF is considered to be a bulk analysis technique. This means that the analysis represents the elemental composition of the entire sample, assuming the sample is homogeneous. The term bulk analysis is used to distinguish such techniques from surface analysis techniques (Chapter 14), which look at only a very thin layer at the sample surface. But there are conditions that must be considered in XRF in order to obtain accurate results. The limiting factor for direct XRF analysis or the analysis of prepared samples is that the signal of the characteristic radiation from the sample originates from different layers within the sample. 

As shown in the previous section. Sc elastomers can be well aligned in electric fields. These preparation techniques are limited to thin film geometries, while mechanical orientation offers the possibility to prepare uniformly oriented bulk samples. 

LEED requires a coherence length of ordered domains on the order of at least 100 A to be useful (36). SEM images are typically limited to resolution of 50 A, but are useful due to their depth of field and wide field of view (37). High resolution TEM can yield images of solids to a resolution of 1-2 A, but requires extensive sample preparation and also requires that the surface atoms be in registry with the remainder of the bulk sample under study (14,38). Clearly, each technique provides valuable information which complements the others, and ideally a combination of the probes would be used to obtain a complete characterization of the surface under study. 

Near-field scanning techniques are relative newcomers, and the basis for their interpretation is less well established. However, AFM has opened up new perspectives for morphological studies, particularly given that excessive surface damage in soft specimens can be avoided by use of non-contact or intermittent contact modes. Its sensitivity to surface topography nevertheless makes AFM prone to artefacts when used to observe surfaces prepared by microtoming, and its effective depth of field is limited compared with SEM. On the other hand, if lamellar surfaces can be prepared such that the surface relief (or hardness, friction variations) is representative of the bulk texture, very striking detail can be recorded at the nanometre scale in deformed polyolefins [11]. 

---