Intradomain Phase Transitions in Block Copolymers with Intradomain Ordering

Block copolymers (BCPs) form a variety of microphase separated structures depending on e.g. monomer fractions, monomer immiscibilities, and chain topology. Here we turn our attention to the effect of orientational interactions among chain segments in the limit of flexible chains. Even in the absence of orientational interactions, flexible BCPs exhibit orientational order of chain segments which is coupled to their mesophase domain structure. Due to the intrinsic alignment generated by micro-domain formation alone, even a weak enthalpic preference for local segment alignment has strong non-linear effects on the pattern of segment alignment at the sub-domain scale. We show here that by increasing the enthalpy of alignment, lamellar BCP phases undergo a new type of SmA-SmC transition in which lamellar sub-domains exhibit a spontaneous tilt relative to the lamellar layers. This transition from normal to in-plane tilt patterns takes place within a single lamellar domain due to the intrinsic spatial variations of segment alignment favored by chain connectivity. We also explore the influence of Frank elastic penalties for sub-domain orientational gradients and their tendency to suppress in-plane ordering associated with the SmC phase.