Translational and rotational diffusion in supercooled orthoterphenyl close to the glass transition

Self diffusion coefficients in supercooled orthoterphenyl (OTP) have been determined down toD_t=3·10^–14 m²s^–1 using a¹H-NMR technique applying static field gradients up to 53T m^–1 In a range of more than two decades theD_t values agree with those of photochromic tracer molecules of the same size determined by forced Rayleigh scattering down to the glass transition temperatureT_g. A change of mechanism is found for translational diffusion atT_c1.2T_g whereD_t is proportional to the inverse shear viscosity^–1 atT>T_c butD_t with =0.75 atT<T_c. Rotational correlation times determined by²H-NMR stimulated echo techniques in deuterated OTP remain proportinal to^–1 down toT_g. Our results are discussed in relation with mode coupling theory and with models of cooperative motion at the glass transition.