Abstract
Photon correlation spectroscopy in the depolarized mode and dielectric
relaxation spectroscopy were employed to investigate local and global
chain motions in poly(styrene-b-1,4-isoprene) diblock copolymer melts
near the order-to-disorder transition (ODT). The presence of two
distinctily different segmental orientation processes and the
temperature dependence of the width of the slow relaxation function in
the disordered state imply strong composition fluctuations in
calorimetrically and rheologically homogeneous diblocks. The estimated
amplitudes of composition fluctuations increase with the proximity to
the ODT. The polyisoprene subchain orientation is characterized by a
single broad distribution of relaxation times above the ODT, that
apparently senses an average friction. This is due to the fact that the
end-to-end vector orientations sample fluctuations at a different length
scale than segmental orientations do.