On the source of X‐ray small angle scattering in rubbers vulcanized with methacrylic acid salts

Abstract

AbstractStyrene‐butadiene rubber (SBR) is mixed with zinc methacrylate (ZnM) powder SBR: ZnM = 5:1 by weight. Wide angle X‐ray diffraction proves that the ZnM crystallites are larger than 240Å. The pure SBR and the SBR‐ZnM mixture exhibit the same small angle X‐ray pattern, indicating particle sizes for SBR and ZnM beyond the resolving power. After annealing the SBR‐ZnM mixture for 30min at 155°C a weak small angle X‐ray scattering (SAXS) pattern appears which indicates particles of 160Å diameter with a polydispersity of gy = 0,2. The same particle size is found from wide angle X‐ray scattering (WAXS) which proves that these particles are ZnM microcrystals and split from the larger crystals. Heating the mixture SBR‐ZnM (100 parts by weight) with 0,8 parts by weight dicumylperoxide, (bis(1‐methyl‐1‐phenylethyl)peroxide) for 30min at 155°C gives rise to a seven times stronger WAXS pattern with the same particle dimension 160Å and polydispersity gy = 0,2. The particle scattering is superimposed by a paracrystalline interference which indicates that the ZnM particles have a mean distance of 250Å and hence are homogeneously spread over the SBR matrix. The ZnM particles have lost their innercrystalline structure, since the WAXS reflection has become very diffuse. Striking similarities to the graft copolymerization of styrene on polyethylene are discussed.