Thin films of various rubbers have been examined in a Siemens Elmiskop I. The materials used were all originally unvulcanized. They were dissolved in benzene and specimens were prepared in the following way: a drop of solution was allowed to fall on a clean water surface where it spread and — after evaporation of the solvent — left a floating rubber film. This was then picked up on a specimen grid brought through the water surface from below. It was found that the thickness of the rubber film was critical. If the film was too thick the resolution suffered, but if too thin it broke into a web of fine filaments. There was an intermediate thickness for which the film was coherent but had thin patches which were suitable for electron microscope examination. Such films could be obtained by using a solution of strength about 10 g/l, but it was found best to judge the film thickness by the interference colour and dilute the solution accordingly Films which showed low order white with veining of yellow colour were suitable: the average thickness was then about 1000 Å but there were thin patches which were probably less than 100 Å thick.
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