On the origin of the non-exponential behaviour of the alpha-relaxation in glass-forming polymers: incoherent neutron scattering and dielectric relaxation results

The question of whether the primary or-relaxation in glass-forming systems is homogeneous or heterogeneous in nature is investigated by incoherent neu tron scattering techniques for glass-forming polymers. The results obtained for more than ten different polymers show that the incoherent intermediate -scattering function is well described by a Kohlrausch-Williams-Watts funct ion with a constant shape parameter Moreover, the momentum-transfer depende nce of this function approximately follows the Gaussian approximation. Thes e results do not agree with those expected from the heterogeneous picture u sually invoked as the origin of the nonexponential behaviour of the alpha-p rocess. It seems that the apparent stretching of the alpha-relaxation funct ion in glass-forming polymers relates dominantly to sub-linear diffusion an d is not a result of a superposition of single-exponential processes, at le ast in the mesoscopic timescale covered by neutron scattering techniques. D eviations from the Gaussian behaviour are evaluated for the different polym ers investigated and interpreted in terms of distributions of stretched rel axation functions. The validity of the extension of these results to the lo w-temperature range close to the glass transition temperature, where neutro n scattering measurements are not feasible, is discussed in the light of ne w dielectric results for this range.