Fluctuation Effects on the Order–Disorder Transition of Symmetric Block Copolymers Mixed with Random Copolymers or Homopolymers

Abstract

Blending block copolymers (BCPs) with polymeric additives provides a direct route to controlling microphase separation, yet how such additives alter composition fluctuations remains incompletely understood. Here, we examine the order–disorder transition (ODT) in symmetric AB diblock copolymers mixed with either neutral random copolymers (RCPs) or symmetric pairs of A and B homopolymers (HPs). Fluctuation effects are quantified using Langevin field-theoretic simulations combined with well-tempered metadynamics and compared with fluctuation-free self-consistent field theory calculations. In BCP–RCP mixtures, RCP addition shifts the transition toward stronger incompatibility and amplifies the fluctuation-induced correction as the additive concentration increases. In mixtures containing A and B HPs, the response differs qualitatively, and the HP chain length controls both the direction of the transition shift and the magnitude of the fluctuation correction. Short HPs enhance the destabilization of lamellar order by fluctuations, whereas longer HPs suppress this effect. When fluctuations are included, the crossover in the overall ODT response shifts somewhat below its mean-field value. Together, these results show that polymeric additives alter the ODT through coupled mean-field and fluctuation effects, so their impact cannot be inferred from mean-field phase behavior alone.