TRANSLATIONAL AND ROTATIONAL DIFFUSION IN SUPERCOOLED ORTHOTERPHENYL CLOSE TO THE GLASS-TRANSITION

Self diffusion coefficients in supercooled orthoterphenyl (OTP) have been determined down to D(t)=3 . 10(-14) m2s-1 using a H-1-NMR technique applying static field gradients up to 53 T m-1. In a range of more than two decades the D(t) values agree with those of photochromic tracer molecules of the same size determined by forced Rayleigh scattering down to the glass transition temperature T(g). A change of mechanism is found for translational diffusion at T(c) almost-equal-to 1.2 T(g) where D(t) is proportional to the inverse shear viscosity eta-1 at T>T(c) but D(t) approximately eta(-xi) with xi = 0.75 at T<T(c). Rotational correlation times determined by H-2-NMR stimulated echo techniques in deuterated OTP remain proportinal to eta-1 down to T(g). Our results are discussed in relation with mode coupling theory and with models of cooperative motion at the glass transition.