Order-to-disorder transition in a diblock copolymer studied at ultrasonic frequencies with a shear wave reflection technique

The order-to-disorder transition (ODT) has been studied in a poly(styrene-b -isoprene) diblock copolymer with conventional rheology and an ultrasonic s hear wave reflection technique. Despite the operational frequency in the la tter experiment (f = 5 MHz) being much higher than the critical frequency o mega (c), 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 meth od constitutes a nondestructive probe of the viscoelastic properties of "so ft" materials in the vicinity of phase transitions.