Fluctuations, Order, and Disorder in Short Diblock Copolymers

The phase behavior for a series of poly(1,4-isoprene-b-DL-lactide) diblock copolymers characterized by a relatively large Flory-Huggins segment-segment interaction parameter (χ) and low degrees of polymerization (N) over a range of compositions was reported. Ordered-state morphologies were deduced from small-angle x-ray scattering (SAXS) measurements, and χ(T) was determined from order-disorder transition temperatures (T_ODT's) associated with the compositionally symmetric specimens and assuming the mean-field theory, that is, (χN)_ODT=10.5. The ODT was determined by differential scanning calorimetry, SAXS, and dynamic mechanical spectroscopy, and shown to be weakly first-order, with latent heats of transition that vary strongly with composition. We interpret this behavior in terms of the mean and Gauss interfacial curvature of the ordered-state morphologies and with respect to composition fluctuations in the disordered state. These results offer a fresh strategy for investigating weakly first-order phase transitions within the Brazovskii universality class.