ORDERING KINETICS IN A SYMMETRICAL DIBLOCK COPOLYMER

Small angle X-ray scattering (SAXS) and rheology are employed to study the microphase separation transition in a symmetric poly(styrene-b-isoprene) diblock copolymer. The order-disorder transition temperature T(ODT) has been identified with a good precision (T(ODT)=359 K) and the interaction parameter is extracted using mean field theory at temperatures well above T(ODT). The kinetics of microphase separation have been studied by means of rheology in a small temperature range below the order-disorder transition, following a quench from the disordered state. The time evolution of the storage and loss moduli exhibited shapes which are reminiscent of the crystallization isotherms of semicrystalline materials (of the Avrami type). For shallow quenches, we find a temperature range of 3 K below T(ODT) where the ordering proceeds by heterogeneous nucleation and growth of three-dimensional objects with lamellar microstructure. At higher undercooling, the kinetic curves change in form,indicating a different mechanism of structure formation. Our results are compared with theoretical predictions for the ordering kinetics of symmetric diblocks.