Normal and segmental mode dynamics of end-functionalized poly(propylene oxide) by dielectric relaxation spectroscopy and dynamic mechanical spectroscopy

An investigation was carried out of the normal and segmental dynamics of poly(propylene oxide) (PPO) chains with (1) symmetrical dipole inversion and (2) three-arm star configuration. PPO chains exhibit, in addition to the transverse dipole moment component (μ^⊥) that gives rise to segmental (α) process, a persistent cumulative dipole moment along the chain contour (μ^∥) that can be relaxed via the normal mode (α_N) process. Data were generated by broad-band dielectric relaxation spectroscopy (DRS) and dynamic mechanical spectroscopy (DMS), and the findings were contrasted. In comparison with the previous studies of PPO dynamics, our DRS and DMS results were generated over a broader range of frequency and temperature and on samples of a wider range of molecular architecture (molecular weight, type, and functionality of end group). Segmental and normal mode spectra were thermodielectrically simple (with the KWW β parameter of ca. 0.51). The average relaxation times for the segmental mode (τ_S) from DRS and DMS were in excellent agreement. A direct comparison of the normal mode relaxation times (τ_N) obtained from DRS and DMS data had to account for the longest viscoelastic relaxation times (τ_N1DMS) of the Rouse chain being one-half the longest dielectric relaxation time (τ_N1DRS) and twice the second dielectric normal mode relaxation time (τ_N2DRS), which is experimentally measured. We observed excellent agreement between (1) our DRS and DMS data, (2) our data and the results of other groups, and (3) our data and the prediction of the Rouse model for MW less than 12000 g/mol. The molecular weight dependence of τ_N1DMS at 273 K is characterized by an exponent of 2, in perfect agreement with τ_N2DRS.