Spin contrast variation study of fuel-efficient tire rubber

The scattering length of a proton against a polarized neutron depends strongly on the polarization of proton spins (P_H). This dependence can be utilized for contrast variation in small angle neutron scattering (SANS). We applied this spin contrast variation technique to a silica-filled SBR rubber specimen, which is widely used for tread rubber of fuel-efficient tires. For realizing high P_H, we used dynamic nuclear polarization (DNP) technique, in which large polarization of electron spin at low temperature and high magnetic field is transferred to proton spin by microwave irradiation with a tuned frequency. As this electron spin source, we doped stable radical TEMPO (2, 2, 6, 6-tetramethyl piperidine 1-oxyl) into the rubber sample by use of a vapor sorption technique. For the TEMPO-doped rubber sample, SANS measurements were conducted at P_H = 20%, 0%, and +13%, with almost fully polarized neutron beam (98.5 %) with its wavelength of 6.5±0.6 Å. The SANS profile clearly changed as a function of P_H, which can be explained by the P_H dependence of the neutron scattering length densities of the main three components (SBR, silica and zinc oxide). By a linear transformation of the profiles obtained at the three different P_H values, we successfully determined the partial scattering function of silica, which reflects the aggregation of silica particles.