Breadth of the alpha-relaxation function in 1,4-polybutadiene

Mechanical and dielectric spectroscopy studies of local segmental motion (a lpha-relaxation) in polybutadienes are reviewed and new results presented. Previous mechanical spectra for 1,4-polybutadiene relied on time-temperatur e superpositioning, leading to discrepancies regarding the breadth of the r elaxation function. By measuring over a wide range of actual frequencies, w e show that the mechanical dispersion is narrow, relative to 1,2-polybutadi ene. The disagreement among published dielectric alpha-relaxation results i s shown to be a consequence of the intrusion of the secondary relaxation fo r temperatures near the glass transition. The actual breadth of the relaxat ion function can be gleaned from analysis of the temperature dependence of the alpha-relaxation time and is consistent with the determination from mec hanical spectroscopy (i.e., a narrow dispersion). Thus, decreasing vinyl co ntent is associated with a narrower segmental relaxation function, whereby the correlation of chemical structure with both the breadth of the relaxati on function and its temperature dependence holds for polybutadiene. This is in accord with an interpretation of the segmental dynamics based on steric interactions involving the pendant vinyl group enhancing intermolecular co operativity. The T-g-normalized temperature dependences for 1,2- and 1,4-po lybutadiene are also consistent with the respective energy barriers between conformational states of their backbones.