Type-A dipole moment and segmental dynamics of poly(styrene oxide)

We report the dielectric properties of poly(styrene oxide)s (PSO) in bulk and concentrated solution states. Since the structure of PSO is asymmetric along the backbone, the repeat unit of PSO is expected to possess the non-zero component of the dipole moment p_A parallel to the chain contour as well as the perpendicular component p_B. The former and latter cause the dielectric normal mode and segmental mode relaxations, respectively. Contrary to the above mentioned expectation the temperature dependence of ε″ exhibits only the primary (α) relaxation and weak secondary relaxation (β) in the glassy state. No loss peak due to the normal mode relaxation was observed in the frequency region expected from the viscoelastic terminal relaxation in bulk and toluene solutions. The dielectric behaviours of the α relaxation in the bulk state were analyzed in detail and the parameters of the Vogel-Fulcher and the Havriliak-Negami equations were determined. The Kirkwood correlation factor was determined to be 0.36. The ¹³C NMR spectra indicate that the present PSO samples contain about 2% head-to-head linkages. This cannot be the origin of the disappearance of the normal mode. We conclude that p_A of PSO is too small to be detected. The p_A calculated with molecular orbital methods supports this conclusion.