Restricted Molecular Dynamics of Polymer Chains by Means of NMR Field Cycling Relaxometry

Abstract

The molecular dynamics of polymer melts with molecular weights above their critical values have been described by a power law dependence of the spin-lattice relaxation time on the Larmor frequency. Three different regimes of power laws describing the relaxation dispersion of polymers for the chain mode dynamics are distinguishable and have been observed experimentally for several types of polymers. In this study, the influence of chemical and physical constraints on the relaxation dispersion of polymers was investigated. This included the introduction of chemical cross-links and filler particles to produce elastomers, for which further constraints were imposed by mechanical deformation. As an alternative means of affecting molecular mobility, investigations were also carried out for poly(styrene-block-butadiene) diblock and poly(styrene-block-butadiene-block-styrene) triblock copolymers. Reducing the molecular degree of freedom of the chain, i.e., by introducing cross-links between different segments, was shown to affect the time scale of motion but left the spectrum of the reorientational dynamics virtually unchanged. Only a small effect was found on the addition of filler particles into the elastomer network. On the other hand, applying macroscopic deformation to natural rubber and butadiene rubber samples is shown to alter also the spectrum of reorientations. Generating chemical fixations to immobile microdomains by copolymerizing a soft butadiene phase with a hard polystyrene phase to produce a biphasic lamellar structure is demonstrated to have a comparable effect. The change in slow molecular motion in the kHz to MHz range is discussed using the corresponding frequency dependence of the NMR spin-lattice relaxation time, and a common description combining measurements at different temperatures is presented.