Abstract
AbstractThe spin‐lattice relaxation time and the nuclear Overhauser enhancement (NOE) of 19F nuclei in dilute solutions of poly(p‐fluorostyrene) and poly(m‐fluorostyrene) were measured as a function of molecular weight, concentration, temperature, solvent, and field strength. Models for local motion of the chain backbone based on independent internal rotations or on one‐dimensional defect diffusion failed to provide a quantitative description of both T1 and the NOE. A model based on three‐bond crankshaft motions and a cutoff of coupling along the backbone gives a consistent account of these results as well as 13C nuclear magnetic resonance (NMR) relaxation data on polystyrene from the literature. The model provides a finite set of exponential correlation times to describe local motion, the center of the distribution lying in the nanosecond region for dilute solutions of polystyrenes in normal solvents. The apparent activation energy for the backbone motion obtained from the temperature dependence of the average correlation time based on NMR data is about 20 kJ mole−1. The local correlation times based on NMR are appreciably shorter than those determined by dielectric relaxation. This discrepancy points to the existence of several different local motions which make different contributions depending upon the experimental technique employed.
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