Junction Point Fluctuations in Microphase Separated Polystyrene−Polyisoprene−Polystyrene Triblock Copolymer Melts. A Dielectric and Rheological Investigation

Dielectric spectroscopy is employed in two polystyrene−polyisoprene−polystyrene (SIS) triblock copolymers well below the order-to-disorder transition temperature and in the frequency range from 10 -2 to 10 6 Hz. Small angle X-ray scattering has shown the formation of lamellar structures with a long period of about 25 nm. Besides the polyisoprene and polystyrene segmental relaxations and a slower process associated with the reorientation of the interface, we provide evidence for a new type of chain dynamics associated with the mobility of the junction points at the interface. From the relaxation strength of this process which is very much reduced as compared to the chain relaxation in bulk polyisoprene we extract a characteristic length of the end-to-end vector fluctuations in the interface in the range 4−6 nm. This value compares well with an independent estimate of the interfacial thickness based on thermodynamics. Dielectric spectroscopy can therefore be used as a dynamic probe of the interface in ordered triblock copolymers. Over the same temperature range rheology is influenced by a broad spectrum of modes related to the dynamics of tethered polyisoprene chains.