Direct Observation of Structure Formation at the Temperature-Driven Order-To-Disorder Transition in Diblock Copolymers

Abstract

The order-to-disorder transition in a nearly symmetric diblock copolymer of polystyrene/poly(cis-1,4)isoprene (Mw = 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 TMST = 362 K and about 30 K below the spinodal temperature Ts 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 TMST. The transition is shown to be discontinuous and accompanied by strong concentration fluctuations in accordance with theoretical expectations.