Rheology of lamellar polystyrene-block-polyisoprene diblock copolymers

The linear viscoelastic properties of symmetric polystyrene-block-polyisoprene diblock copolymers (PS-b-PI) were studied over a range of molecular weights between 20 000 and 200 000 using dynamic mechanical spectroscopy. For all samples the low frequency rheological response can be divided into two frequency regimes in agreement with previous reports on this subject, indicating the onset of a third relaxation process at very low frequencies related to the microphase separated microstructure. Various methods to determine two characteristic frequencies, ω_c and ω_d, separating different orientation regimes of the lamellar microstructure observed under large amplitude oscillatory shear flow in the vicinity of T_ODT (T < T_ODT) are compared. It is further shown that for PS-b-PI diblock copolymers an apparent entanglement effect sets in at molecular weights which are much lower than those expected from the corresponding homopolymer data. A microscopic picture is proposed to account for the experimental results.