We examine the thermodynamic behavior of diblock copolymers representing the binary pairs of the ternary system: poly(dimethylsiloxane)/poly(ethylene-stat-propylene)/poly(styrene-ran-styrenesulfonic acid), D/EP/SS. We employ small-angle X-ray scattering, electron microscopy, and rheology to characterize the order-to-disorder transition temperature TODTand lamellar period d of 28 materials with varying molecular weights and sulfonation extents. These data are then interpreted in the context of self-consistent mean-field theory employing the continuous Gaussian chain model to deduce the interaction parameters as a function of temperature and sulfonation extent. We find that while EPD and SSEP are amenable to such treatment, the SS/D interaction forces SSD chains to stretch beyond the realm of applicability of the Gaussian chain model.