Analytical Method for Rubber Materials Using Sulfur K-edge NEXAFS and Its Application

Abstract

Here, we develop a technique for curve-fitting of the sulfur K-edge near-edge X-ray absorption fine structure (NEXAFS) to analyze the crosslinked structure of vulcanized rubber based on the chemical state of sulfur. The proposed analytical method provides the sulfur chain length (S_x), crosslink density, and sulfur oxide content simultaneously, and it is highly sensitive for the analysis of the depth direction. Furthermore, we verify the aging mechanism of the crosslinked structure using this analytical method. It is found that the sulfur chains, shortened and oxidized by heat aging at 120°C, can cause scission of the S—C bond, thereby decreasing the crosslink density. Long sulfur chains are more susceptible to oxidation, which rapidly decreases the sulfur chain length (S_x) and, in turn, the crosslink density compared with short sulfur chains. By comparing the heat-aging dependence between the S—C bond and S100, it is found that the styrene-butadiene rubber polymer surface can form the C—C crosslink. We present the aging mechanism of the crosslinked structure and establish NEXAFS as a useful analytical method for rubber materials.