Replication for TEM

Direct or single stage replicas have the best possible resolution, are the fastest and, unfortunately, are the most difficult to prepare. The 

The removal of direct carbon replicas is dependent upon the polymer. Boiling xylene vapor was used to remove drawn PE from replicas [296] in work on drawn polymer morphology. Hobbs and Pratt [297] described a direct carbon replica method for replication of a PBT impact fracture surface by evaporation of platinum at 20° and PBT removal in hexafluor-oisopropanol (HFIP). Latex film coalescence in poly(vinyl acrylate) homopolymer and vinyl acrylic copolymer latexes was studied using direct replicas [298]. As the latex films have a low glass transition temperature, they were cooled by liquid nitrogen to about -150°C in the vacuum evaporator and shadowed with Pt/ Pd at 45° followed by deposition of a carbon support film at 90° to the specimen surface. The latex films were dissolved in methyl acetate/ methanol. TEM micrographs of the latex films show the difference between films aged for various times (Section 5.5.2). 

Polymers are used to aid removal of thin replicas when the specimen cannot be dissolved. 

Two stage, or double, replicas provide positive impressions of the specimen surface, although they require more time to prepare. The steps involved in formation of a two stage replica are  

Examples of replicating plastics (along with their solvent) are cellulose acetate (acetone), gelatin (water or dilute sodium hydroxide), acrylic resin (acetone) and poly(vinyl alcohol) (water). Solutions of 2% formvar in chloroform or dioxane and 1-4% collodion in amyl acetate are appropriate for rough surfaces where tapes and films are not easily removed. Washing and extractions are similar to those described above for one stage replicas. A major problem can be the incomplete dissolution of the plastic replica which interferes with TEM imaging. 

Direct or single stage replicas have the best possible resolution, are the fastest and, unfortunately, are the most difficult to prepare. The method involves the deposition of the replicating media and its removal or dissolution of the polymer. Materials used for direct replication include polymers, evaporated carbon films or metal oxides. Carbon is widely used for replication, especially if the polymer specimen to be replicated can be readily removed or dissolved. 

Replicas of fibers and yarns are difficult to strip off due to their size and shape. Fibers can be prepared for replication by semiembedding them first in a resin or gelatin [250-252]. The fibers are replicated with metal and carbon and then stripped. Scott [41] used the peelback method to 

Carbon replicas [427] are formed by the evaporation of a thin layer of carbon in a vacuum evaporator. Metal shadowing, at an angle of 20 to 45° to the specimen surface is performed while the specimen is in the evaporator. The highest resolution direct replica material is carbon/platinum (C/Pt). After evap- 

A commonly employed method for the preparation of positive, two stage replicas, uses PAA as the first stage rephca  

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Fig. 1. Preparation of sections through an anodic fihn for replication for TEM examination carbon deposition direction A, shadowing direction B...

Scale-Up Fermenters ranging from about two to over 100 liters (0.07-45.5 fp) have been used for research and development, but the smaller sizes provide too little volume for sampling and are difficult to replicate, while large vessels are expensive and nse too much medium. Autoclavable small fermenters that are placed in a water bath for tem-... 

Specimen preparation for TEM generally in-voles the formation of a thin film of the material less than 100 nm thick. The methods used for this preparation depend upon the nature of the polymer and its physical form. In the case of thick or bulk specimens, microtomy is generally used. In the case of solutions, powders or particulates, simpler methods can provide a thin, dispersed form of the material. Three types of simple preparations will be described later in this section dispersion, disintegration and film casting. The more complex methods such as microtomy, replication, etching and staining will be described in other sections of this chapter. 

Peck and Kaye [240] immersed cellulose acetate specimens in acetone, at —50°C, and then flooded the surfaces with cold absolute alcohol, followed by replication which showed the skin, orientation, voids and pigment. Reding and Walter [21] etched PE with hot carbon tetrachloride (high density PE), benzene (low density PE) or toluene, which removed the amorphous material. Bailey [241] used a rapid xylene etch to reveal spher-ulites in PE and PP, while Li and Kargin [242] etched with benzene. A method was developed for obtaining PE for TEM. Melt crystallized PE, backed with carbon and gelatin, was exposed to xylene and dissolved to a thin layer and examined directly by TEM [243, 244]. 

Early TEM studies were by replica methods [18], as in a study of replicated and etched fiber surfaces [15]. Such studies are now conducted by SEM of external and bulk structures and by ultrathin sectioning for TEM. Microstructural studies generally require complementary optical and SEM study to understand the arrangement of the fine structural details within the macrostructure. 

The etching techniques described above in connection with SEM can also be used for TEM, in conjunction with so-called replication. In this, an accurate facsimile of the sample s surface topography is produced, frequently in the form of a thin carbon film, which is then coated at an oblique with an electron dense metal (shadowed). As a consequence, the distribution of metal oti the surface is non-uniform and directly related to the surface topography of the specimen and, hence, the morphological features present within it. Figure 2.20a shows an equivalent structure, as revealed by etching followed by replication, to the lamellar... 

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