Local dynamics of cis-1,4-polybutadiene and cis-1,4-polyisoprene. A comparative study based on cooperative kinematics theory and NMR experiments

Cooperative motions of cis-1,4-polybutadiene (cis-PB) and cis-1,4-polyisoprene (cis-PI) in the bulk state are analyzed and compared using the cooperative kinematics (CK) theory. The mechanisms of relaxation on a local scale are closely similar for the two polymers in dense media, resulting from the same geometry of the backbone, which is the major determinant of the types of accessible cooperative conformational rearrangements. The correlation times for the reorientation of C-H bonds of cis-PI are found to be about 1 order of magnitude longer than those for cis-PB at the same effective friction coefficient. The results are in accordance with previous NMR experiments which indicated a ratio of about 10. The molecular characteristics responsible for this difference in the local mobilities of the two polymers are examined. Three sources are identified: (i) the excess volume swept by the moving units in cis-PI due to the CH3 units appended to the backbone, (ii) the higher short-range conformational potential energies in cis-PI compared to those in cis-PB, and (iii) the restriction in the number of accessible isomeric transitions in cis-PI compared to those of cis-PB, which is due to the excluded-volume effect.