Rheology of the Lower Critical Ordering Transition

Abstract

Linear viscoelastic dynamic rheological measurements were performed on symmetric diblock copolymers of deuterated polystyrene and poly(n-butyl methacrylate), denoted P(d-S-b-nBMA). Isothermal frequency sweep data weee time-temperature superposed. A transition from a disordered to ordered morphology was found with increasing temperature as a function of molecular weight. P(d-S-b-nBMA) with molecular weights less than 6.8 × 104 were always disordered, as evidenced by the scaling of the storage and loss moduli with frequency at low frequencies, G' ∝ ͩ2 and G' ∝ ͩ. Copolymers with molecular weights above 9.9 × 104 were always ordered, exhibiting very long relaxation times and appearing almost solidlike in the low-frequency limit. Mixtures of the 6.8 × 104 and 9.9 × 104 copolymers, however, showed a transition from fluid to solidlike behavior as the temperature was increased. This rheological data is consistent with the existence of a lower critical ordering temperature, LCOT, where a transition from a disordered to ordered state occuss with increasing temperature. © 1995, American Chemical Society. All rights reserved.