In Situ Observation of Oriented-Crystallization Process of Rubbers by Electron Microscopy

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Oriented-crystallization mechanism for rubbers has been investigated by an "indirect" method, viz. by transmission electron microscope (TEM) observation of their thin films fixed/stained at given stages of crystallization. The purpose of this research is to elucidate the mechanism by "direct" methodssuch as selected-area electron diffraction, bright-field defocus-contrast imaging and dark-field imaging of the films prestretched outside and then crystallized inside a TEM, and by time-resolved in situ TEM observation of the prestretched films during crystallization. The results are as follows :1. Prestretched films were isothermally crystallized one by one inside a TEM; -25℃ for natural rubber (NR) and -5℃ for polychloroprene (CR). In NR, the uniplanar axial orientation was confirmed. In NR and CR, the "α-filament" was identified as an edge-on lamellar crystal. In CR prestretched "quickly" up to a strain of ε=500%, paracrystalline nature was recognized.2. By using our homemade "film-stre … More tching device", highly prestretched NR films (ε=500〜800%) were made. In the case of, e.g. ε=800%, stacked-lamellar structure was recognized after isothermal crystallization at -25℃. On the other hand, thin fibrils of ca. 25nm wide were recognized before isotherml crystallization, but no lamellae were observed. Each fibril was judged to be a "shish" crystal (viz., γ-filament)" or a bundle of shishes.3. Time-resolved in situ observation of 200%-prestretched NR was tried, by repeating TEM observation at -80℃ and crystallization at -25℃. By using our homemade "temperature controller" for a specimen heating holder, in situ observation of growing edge-on lamellae in oriented amorphous films of isotactic polystyrene during crystallization at 150℃ inside a TEM was tried with a TV system. These trials suggested that the time-resolved in situ TEM observation is useful to follow oriented crystallization of polymers.4. On the basis of the above-mentioned results, an oriented-crystallization mechanism for rubbers was proposed. Less