Local dynamics of bulk polybutadienes of various microstructures: Comparison of theory with NMR measurements

Theoretical and experimental results are presented for the local dynamics of bulk polybutadienes (PB) of various microstructures, i.e., various fractions of cis and trans units. The cooperative kinematics (CK) method is used to analyze the specific mechanism of motion involved in the rotational isomerization of the bonds belonging to the cis and trans units. Correlation times measured by NMR experiments at room temperature are reported for various microstructures and interpreted using the CK model. The effective monomeric friction coefficient and its dependence on the microstructure of PB are seen to dominate the rate of local motions. On the other hand, the specific mechanism of relaxation shows little dependence on the frictional resistance or on the composition of the polymer, but is uniquely defined depending on the given conformational state, cis or trans, of the double bond of the repeat unit. The amplitude of the spatial reorientations of the methylene C-H bond vectors and the frequencies of isomeric jumps are the two contributions affecting the observed correlation times in NMR experiments. The frequencies of isomeric jumps are comparable in cis and trans units, in general. On the other hand, the C-H bonds belonging to cis units are shown to undergo significantly larger amplitude reorientations during the cooperative motions accompanying bond isomerization, compared to C-H bonds in trans units. This latter effect explains the differences in the NMR correlation times of cis and trans units in the chains of various microstructures.