Evolution of the crosslink structure in the elastomers NR and SBR

An experimental positron annihilation lifetime spectroscopy (PALS), differential scanning calorimetry (DSC) and small angle X-ray scattering (SAXS) study of the effect of the advance of the crosslinking reaction on the free volume in a copolymer of styrene–butadiene and natural rubbers was carried out. The crosslink density developed in SBR specimens with different sulfur contents and cure temperatures was studied. SAXS technique was applied to study the process of crosslinking in NR as a function of the cure temperature. Finally, a study of different SBR/NR blends is presented using PALS and DSC. The effects of the sulfur content on the mechanical, thermal and electrical properties of styrene butadiene rubber (SBR) and natural rubber (NR) compounds are well documented; see Payne (1965), Marzocca and Goyanes (2004), Abd-El-Messieh and Abd-El-Nour (2003). By chemical or physical processes, crosslinking forms a three-dimensional network structure which has a strong influence on the physical properties of polymers. Therefore, an understanding of the mechanisms involved in the crosslinking formation is essential. In this connection, the free volume theory (Doolittle, 1951; Ferry, 1980) is a very useful tool to describe different mechanical and viscoelastic properties of the macromolecules involved in the vulcanization process. In this work, the crosslinking process in SBR, NR and SBR/NR blends is studied. Positron annihilation lifetime spectroscopy (PALS), small angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) experimental techniques were used. For previous works, see Marzocca et al. (2002) and Salgueiro et al. (2004).