RHEOLOGY AND THE MICROPHASE SEPARATION TRANSITION IN STYRENE-ISOPRENEBLOCK-COPOLYMERS

We have used dynamic oscillatory measurements to locate the temperatur e of the microphase separation transition (T-MST) for a series of near ly monodisperse styrene-isoprene diblock (SI) and triblock (SIS) copol ymers, all containing 13 wt % styrene and ranging in molecular weight from 48K to 149K. The microphase-separated morphology in all cases is a disordered (liquidlike) arrangement of polystyrene spheres in a poly isoprene matrix, as determined by small-angle X-ray scattering. Values of T-MST were obtained from isochronal plots of the storage modulus v ersus temperature. The marked change in slope at the MST demonstrates that even materials with a disordered microdomain structure can experi ence significant property changes when the MST is traversed. The diblo ck and triblock T-MST values are consistent with a single chi(T) relat ion, indicating that targe-scale polymer architecture has little effec t on chi.