Proton spin—Lattice relaxation in polybutene—1 oxides: A comparison with related polymers

Abstract

T 1 measurements have been made for polybutene-1 oxides (PBO) of molecular weights 500, 1 000, 2 000 and 2 × 10 6 over wide temperature ranges. PBO 500 and 2 000 have also been studied in solution in the non-magnetic solvent carbon disulphide. Two T 1 minima were observed in all cases corresponding to chain backbone and sidechain motions. The dilution measurements enable the relaxation times at infinite dilution, ( T 1) ∞, to be determined and also indicate, on the basis of BPP theory, that the most important contributions to T 1 are from intramolecular interactions, as is usually assumed when analysing polymer T 1 measurements. Activation energies, Q, calculated from the slopes of T 1 curves for the solutions show a marked molecular weight dependence, and extrapolation to infinite dilution enables quantities Q ∞ to be obtained, which are in the order PBO 500 > PBO 2 000. Comparisons have been made between the activation parameters and temperatures of T 1 minima obtained for high molecular weight PBO and a number of other oxide polymers of the types ▪. Other polymers of the types ▪ and ▪ are also compared. The activation energies, which are assumed not to vary with temperature were calculated from slopes ( Q ±), and from areas under curves of 1 T1 against 1 T × 10 3 (〈 Q〉). Also included for comparison are barrier heights, E, for methyl group rotation derived from Stejskal and Gutowsky's quantum mechanical tunnelling calculations. A number of interesting trends are shown by the results obtained such as a rise in the temperatures of T 1 minima for polymers of the type ▪ as R becomes more bulky, the temperatures being in the order ø > t- Bu > Et > Me > H. In some respects. the simplest member of this series, polyethylene oxide, and polyethylene, the simplest member of the series ▪, appear to be anomalous. The effect of introducing an extra oxygen atom into the chain monomer unit seems to be the expected one of increasing chain mobility.