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.