Molecular fluctuations in polyethylene melts. Dependence of the longitudinal and transverse proton relaxation on the chain length

Abstract

Melts of diverse fractions of linear polyethylene have been investigated by longitudinal and transverse proton relaxation. A three-component model of molecular fluctuations allows to describe the longitudinal relaxation times in the frequency range of our experiments (10 4 to 10 8 Hz). The components are ‘anisotropic segment reorientation’, ‘reptation’ and the ‘conformational fluctuation of the surrounding tube’. The molecular weight dependences of the corresponding time parameters are τ s ∼ M 0, τ l ∼ M 1 and τ r ∼ M 3, respectively. These quantities also permit us to explain the molecular weight dependence of the transverse relaxation curves which have been described by the aid of the Anderson-Weiss theory. It is shown that the influence of the molecular weight distribution is not negligible. Rather it is the reason for non-exponential relaxation decays. The free induction decays have been found to decrease essentially faster than the relaxation decays even if the inhomogeneity of the magnet was negligible. As an explanation, internal inhomogeneities caused by microscopic voids have been assumed. An estimate of this effect is given. In addition to the pure fractions we have also studied a mixture of short chains in a matrix of long deuterated chains. It turns out that the fluctuation of the local network depends on both molecular weights and the ratio of mixture.