TEM studies on thin films of natural rubber and polychloroprene crystallized under molecular orientation

Thin films of natural rubber (NR) or polychloroprene (CR) were made by casting a 2.0 wt% solution in benzene onto the water surface, and some of them were prestretched up to a desired amount of strain (mostly, about 200%) at ambient temperature. The specimens thus prepared were then crystallized at -25 degrees C for NR or at -5 degrees C for CR in a transmission electron microscope (TEM) column with a cryo-transfer specimen-holder and examined by TEM observation without any electron staining. Morphological observations in bright- and dark-field imaging modes and selected-area electron diffraction analysis revealed directly the following: The "alpha-filaments"' in NR reported by Andrews [Proc. R. Soc. A, 277, 562 (1964)] and the filaments in CR (corresponding to the alpha-filaments in NR) reported by Andrews and Reeve [J. Mater. Sci., 6, 547 (1971)], both of which are observed in the TEM images to orient perpendicularly to the prestretching direction, are undoubtedly identified as edge-on lamellar crystals.