加硫シス-1, 4-ポリブタジエンゴムのミクロ不均一構造と粘弾性

抄録

The theory of elasticity of vulcanized rubber treats its network as statistically homogeneous. It should be noticed that the vulcanizates with the same crosslink density show different mechanical strength when different vulcanizing agents and conditions are used. This fact is often interpreted as due to the difference in the chemical structure of the crosslink or that in the distribution of the crosslink density. In this paper, we have taken up the latter problem. The observation of the texture was made with electron microscope, and the measurement of viscoelasticity in its primary absorption was made to discuss the effect of the microheterogeneity on the absorption shape. The samples are natural rubber (NR) and cis-1, 4-Polybutadiene (BR).
Electron micrographs were taken on the ultrathin sections of the rubber vulcanizates embedded in PMMA and stained with osmic acid. Electron micrographs of sulfur vulcanizates of NR showed homogeneous texture independent of the cure time of 40-120min. In the case of BR, 40min sulfur vulcanizate showed comparatively homogeneous texture, but 120min vulcanizate showed remarkably heterogeneous texture. About 50Å diameter particles of highly crosslinked regions appeared as an aggregate with a few hundred Å composed of. With increasing cure time, the contrast of the image increased in BR. With increasing sulfur content, highly crosslinked regions of about 200Å diameter were dispersed like filler. In contrast to the sulfur vulcanizates, the γ-ray irradiated BR showed remarkably homogeneous texture in its electron microscope image.
The viscoelasticity in the primary absorption regions was measured by Vibron DDV-II. The curve of tanδ vs. temperature was analyzed under the assumptions that the lower temperature side of tanδ peak was expressed by single relaxation model and the temperature dependence of relaxation time was represented by Arrhenius type equation. According to these assumptions, tan δ vs. 1/T curves of sulfur vulcanized rubber were divided into absorption I and II from the lower temperature side. The tanδ vs. 1/T curve of BR showed an unsymmetrical shape broadened more than that of NR. The shape of tanδ curve of absorption I was analyzed to agree with that predicted by the single relaxation time model. The magnitude of absorption II of BR increased with increasing cure time and sulfur content, but that of NR did not increase with cure time. The γ-ray irradiated BR showed only absorption I and its shape was perfectly symmetrical.
Absorption II was concluded as characteristic of sulfur vulcanizate and its appearance was closely related to the microheterogeneity of the vulcanized texture revealed by the electron microsope.