Brillouin scattering and dielectric relaxation in PPG of different chain lengths and end groups

We have investigated the structural relaxational behavior of poly(propylene glycol), PPG, for polymer chains of increasing number of monomer units, n (= 3,7,70), and for different end groups (OH and CH₃) using Brillouin scattering and dielectric relaxation. In the case of the OH capped systems the relaxation dynamics are rather insensitive to chain length variations, whereas large effects are demonstrated when changing to CH₃ end groups. There is a radical decrease in the sound velocity and the average relaxation time, when the hydroxyl terminations are replaced by methyl terminations. By using the non-polar CH₃ end-capping the linking of adjacent chains, as is the case in the hydrogen bonded OH capped systems, is avoided.