Phase separation transmission electron

Whereas examples in Fig. 19.6A,B pertain to immiscible polymer melts. Fig. 19.6C pertains to a miscible polymer pair [31]. Submicron layers were formed in a blend of 30% by volume EVOH and polyamide (PA). To improve phase contrast in the scanning electron image, samples were etched in a solvent so that some or portions of EVOH layers were removed. Thicknesses of layers were determined by separate transmission electron examinations to be less than 0.5 pm. The potential amount of layer refinement obtained with a smart blender is demonstrated for a blend consisting of 15% by volume LDPE and high density polyethylene (HDPE) [5]. Because interfacial tension was very low due to... 

Local composition is very useful supplementary information that can be obtained in many of the transmission electron microscopes (TEM). The two main methods to measure local composition are electron energy loss spectrometry (EELS), which is a topic of a separate paper in this volume (Mayer 2004) and x-ray emission spectrometry, which is named EDS or EDX after the energy dispersive spectrometer, because this type of x-ray detection became ubiquitous in the TEM. Present paper introduces this latter method, which measures the X-rays produced by the fast electrons of the TEM, bombarding the sample, to determine the local composition. As an independent topic, information content and usage of the popular X-ray powder dififaction database is also introduced here. Combination of information from these two sources results in an efficient phase identification. Identification of known phases is contrasted to solving unknown stmctures, the latter being the topic of the largest fiaction of this school. 

The extent of phase separation can be measured directly by the scanning electron microscope (SEM), transmission electron microscope (TEM), optical microscope, and light or X-ray scattering technique. It is also investigated indirectly by measuring certain physical properties, such as glass transition temperature. 

Hill and Barham [133] showed by transmission electron microscopy that blends of high and low molar mass polyethylene melts were homogeneous with no detectable phase separation. The blends were prepared by solution mixing to obtain an initially homogeneous blend before the thermal treatment in the melt. It should be realised that the mechanical mixing of high and low molar mass linear polyethylenes to obtain a homogeneous melt may require considerable work and time. 

Phase separation has been studied in great detail, e.g. in the GalnAsP material system, where a large miscibility gap was found both experimentally and theoretically [3], On a microscopic scale, large compositional variations were observed by dark-field transmission electron microscopy, with length scales of the order 10-20 nm. 

Transmission Electron Microscopy. Films of all samples designated R were obtained by evaporation of toluene from solutions of the block copolymers and were observed without staining using a Hitachi Hu-125 or a JEOL JEM 100 S electron microscope. Methods of preparing the films have been described previously (24). So far, we have obtained evidence for microphase separation in only the four highest molecular-weight samples by TEM. We have not obtained continuous films of the lower molecular-weight samples we plan to examine sections of these samples later. Because of the very small compatibility of styrene and polydimethylsiloxane, however, we expect phase separation in all of these samples. 

The method used to provide contrast in transmission electron microscopy was successful in demonstrating the presence of a two-phase structure in homopolymer blends of BR and IR (Figure 2a). The opposite situation, i.e., a clear absence of any phase separation in the block copolymers, also is demonstrated, but much less convincingly by the comparison of Figures 2b and 2c. It is necessary to consider the evidence from all of the mechanical and thermal analysis experiments, along with the evidence from microscopy. 

A transmission electron micrograph of a composite whose composition is given in Table 7 is presented in Fig. 33. This figure shows that the dispersed phase is composed of polyhedral cells of polyacrylamide separated by films of polystyrene. The obtained material is white, soft, and exhibits some elasticity. 

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