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 (mu(perpendicular to) or type B) that gives rise to the segmental alpha process, a persistent cumulative dipole moment along the chain contour (mu(parallel to) 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 alpha 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 alpha 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 alpha 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. (C) 2002 Elsevier Science B.V. All rights reserved.