Dielectric normal mode spectroscopy in the ordered and disordered states of diblock copolymers

The microphase separation transition (MST) in a series of diblock copolymers of polystyrene/cis-1,4-isoprene of low molecular weight (10500-20800) and various compositions is studied using small-angle X-ray scattering (SAXS) and dielectric spectroscopy. The full regime from the ordered to the disordered state is investigated. The combination of both methods makes it possible to relate dynamical to structural properties. SAXS identifies a broad transition regime which is characterized by strong concentration fluctuations in the absence of long-range order. The polymers are shown to go through their MST at experimentally accessible temperatures. Dielectric spectroscopy of the polyisoprene normal mode in the diblock copolymers is compared to the homopolymer case. It is used to study the temperature dependence of the effective composition in the volume of the polyisoprene block. The ordered state consists of well-separated polyisoprene regimes. However, the relaxation time spectrum is considerably broadened with respect to the homopolymer case due to the coupling to polystyrene. The order-to-disorder transition does not lead to a homogeneous mixing of both blocks. The full temperature dependence of the normal mode relaxation time is described in a free volume model