DIRECT OBSERVATION OF STRUCTURE FORMATION AT THE TEMPERATURE-DRIVEN ORDER-TO-DISORDER TRANSITION IN DIBLOCK COPOLYMERS

The order-to-disorder transition in a nearly symmetric diblock copolymer of polystyrene/poly(cis-1,4)isoprene (M(w) = 15 700) was reproducibly observed in heating and cooling runs using small-angle scattering and rheological measurements. Small-angle X-ray scattering characterizes the ordered structure as consisting of stacks of about 20 lamellae. The transition temperature is T(MST) = 362 K and about 30 K below the spinodal temperature T(s) which is extrapolated from the high-temperature phase. Measurements of the dynamic shear modulus reveal a drastic change of the spectrum of relaxation times at T(MST). The transition is shown to be discontinuous and accompanied by strong concentration fluctuations in accordance with theoretical expectations.