Langevin Field-Theoretic Simulation of Conventional and Frank–Kasper Sphere Packings in Conformationally Asymmetric Diblock Copolymer Melts

Conformationally asymmetric diblock copolymer melts exhibit close competition between conventional body-centered-cubic (BCC) and face-centered-cubic (FCC) sphere packings and more complex Frank–Kasper phases near the order–disorder transition (ODT). How compositional fluctuations reshape this competition remains poorly understood. We use Langevin field-theoretic simulation (L-FTS), with discrete-chain self-consistent field theory (SCFT) as a mean-field reference, to obtain ODT estimates and examine nearby morphological transformations among BCC, FCC, and four Frank–Kasper phases (σ, A15, C14, and C15). In monodisperse melts at an invariant polymerization index of N¯=105, where fluctuations are relatively weak, the estimated transition thresholds remain closely spaced, consistent with small SCFT free-energy differences per chain. At N¯=104, stronger fluctuations increase the separation between BCC disordering and Frank–Kasper transformation thresholds. BCC remains ordered at lower segregation strengths, and several Frank–Kasper initial states transform into BCC-like packings before disordering. In disperse systems modeled by binary blends at N¯=104, increasing chain-length dispersity narrows the separations between BCC and Frank–Kasper thresholds. Nevertheless, BCC remains ordered at lower segregation strengths than other initial morphologies, even at higher dispersity where SCFT favors FCC. These results suggest that compositional fluctuations and chain-length dispersity affect spherical morphologies differently near the ODT.