RHEOLOGY OF LAMELLAR POLYSTYRENE-BLOCK-POLYISOPRENE DIBLOCK COPOLYMERS

The linear viscoelastic properties of symmetric polystyrene-block-poly isoprene diblock copolymers (PS-b-PI) were studied over a range of mol ecular weights between 20 000 and 200 000 using dynamic mechanical spe ctroscopy. For all samples the low frequency rheological response can be divided into two frequency regimes in agreement with previous repor ts on this subject, indicating the onset of a third relaxation process at very low frequencies related to the microphase separated microstru cture. Various methods to determine two characteristic frequencies, om ega(c) and omega(d), Separating different orientation regimes of the l amellar microstructure observed under large amplitude oscillatory shea r flow in the vicinity of T-ODT (T < T-ODT) are compared. It is furthe r shown that for PS-b-PI diblock copolymers an apparent entanglement e ffect sets in at molecular weights which are much lower than those exp ected from the corresponding homopolymer data. A microscopic picture i s proposed to account for the experimental results.