Abstract
The order-to-disorder transition (ODT) has been studied in a poly(styrene-b-isoprene) diblock copolymer with conventional rheology and an ultrasonic shear wave reflection technique. Despite the operational frequency in the latter experiment (f = 5 MHz) being much higher than the critical frequency ωc, the storage modulus exhibits a discontinuous increase at the same temperature at which in conventional rheology a discontinuous decrease is obtained. This results from a change in the mixing law at the ODT. The method constitutes a non-destructive probe of the viscoelastic properties of "soft" materials in the vicinity of phase transitions.