Segmental and normal mode dynamics during network formation

Broad-band dielectric relaxation spectroscopy was used to investigate the molecular dynamics of reactive systems where one of the components exhibits, in addition to the transverse dipole moment component (μ_⊥ or type B) that gives rise to the segmental α process, a persistent cumulative dipole moment along the chain contour (μ_∥ or type A). The systems studied were composed of an amine-terminated poly(propylene oxide) (PPO), that contains type A and B dipoles, and a bifunctional epoxy prepolymer. The segmental and normal mode processes were clearly discernible in the neat PPO and the PPO/epoxy blends with PPO MW > 2000 g/mol. The α process broadens upon mixing. The normal mode has a narrower spectrum than the segmental mode and is not affected by mixing. Cross-linking causes the segmental and normal modes to gradually overlap and we observe a deviation of the normal mode from the Rouse dynamics and a considerable broadening of the α process. The molecular architecture of the growing network plays an important role in determining the dynamics, as the compositional and spatial heterogeneities arise during cross-linking. Interestingly, while the increased heterogeneity results in the broadening of the relaxation spectra, the temperature dependence of the α process remains unaffected by cross-linking. This is a curious finding; the apparent lack of a direct correlation between non-exponentiality and fragility in cross-linking PPO networks may reflect the length scale of relaxation and warrants further investigation.