TEM Studies on Thin Films of Natural Rubber and Polychloroprene Crystallized under Molecular Orientation

Abstract

ABSTRACT:Thin films of natural rubber (NR) or polychloroprene (CR) were made by casting a 2.0 wt¾ 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°C for NR or at -S°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 "·cl-filaments" in NR reported by Andrews [Proc. R. Soc. A, 277, 562 (1964)] and the filaments in CR (corresponding to the cl-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.KEY WORDS Natural Rubber/ Polychloroprene / Orientation-Induced Crystallization/ Morphology/ Transmission Electron Microscopy / Dark-Field Image / Defocus Contrast / Crystalline morphologies of natural rubber (NR) were discussed for the first time by Andrews 1 -3 on the basis of the results observed by transmission electron microscopy of its thin films, which had been crystallized isothermally at -26°C ( or -28°C) and most of which had been fixed/stained with osmic acid (OsO 4 ): spherulitic structure in unstrained NR and sheaf-like structure in N R lightly prestretched before crystallization, in both of which "a-filaments", named by Andrews,2 were arranged in the radial direction. In addition, row ( or shish-kebab) structure was observed in uniaxially prestretched NR, in which the a-filaments grown perpendicularly to the prestretching direction were stacked in that direction. 2 Figures l(a)-(e) show the schematic crystalline morphologies in NR thin films proposed by Andrews,4 which were prestretched up to respective given amounts of strain, s (%). In a series of his experiments, 1 -3 • 5 NR thin films cast onto the water surface from a benzene solution were prestretched by a desired s at room temperature, crystallized isothermally and then most of them were fixed/stained with OsO 4 • Based on transmission electron microscope (TEM) investigations of the specimens thus prepared, the resulting morphologies were classified into five groups as shown in Figure 1: An almost similar classification into five groups was given later by Reed, 6 in which slightly different correlation was, however, proposed between e and morphologies.The morphology observed for s = 0% (no strain) is characterized by the spherulitic structure in which the a-filaments are arranged radially (see Figure l(a)). The morphology for c =: 50% is well expressed by the sheaflike structure (Figure l(b)). Figure l(c) shows the morphology for c =: 100% corresponding to the structure in which a-filaments grown perpendicularly to the prestretching direction are sta...

Strain‐induced crystallization behavior of polychloroprene rubber

“…13,[16][17][18] And this is verified by the XRD and DSC experiment results of CR gum in our work i.e., the sharp XRD reflection peak at 2y ¼ 20 and large endothermic peak during DSC first heating. Hence the addition of vulcanizing agents will not change the crystalline behavior of CR until vulcanization occurs.…”

Biaxial Orientation Mechanism of Drawn Natural Rubber. I. Appearance of the Biaxial Orientation

“…[7,8] Andrews examined thin NR films drawn up to a desired strain followed by isothermal crystallization and reported several kinds of morphologies based on transmission electron microscopy (TEM). In the same way, Shimizu et al [9,10] and Tsuji et al [11,12] studied thin NR films crystallized under strain. Mitchell [13] studied uniaxially stretched NR sheet and calculated crystallinity and orientation based on WAXD.…”

Mechanism of strain-induced crystallization in filled and unfilled natural rubber vulcanizates

“…36 Subsequently, the oriented crystallites will be formed onto the stretched chains. The molecular chains that are consumed for the crystal growth ͑in the same way as the shishi-kebab structure [37][38][39][40][41] ͒ would be supplied by the surrounding coiled chains.…”