Unveiling re-entrant phase behavior and crystalline–amorphous interactions in semi-conducting polymer:small molecule blends

It has been reported recently that conjugated polymer:small molecule systems might exhibit complex, re-entrant phase behavior with hourglass or closed-loop miscibility gaps due to an 'apparent' lower critical solution temperature branch. However, the study did not firmly establish if the observations were reflecting equilibrium or not. To assure that the observed shapes of the binodals via a mixing experiment represent local near-equilibrium conditions that capture complex molecular interactions or equation-of-state effects, we present here the liquidus and the binodal for the exact same systems, i.e., PTB7-Th:PC₆₁BM, PffBT4T-C9C13:PC₇₁BM and PTB7-Th:EH-IDTBR, with the liquidus measured via a demixing experiment with long annealing time of days to weeks. We observe that the binodal displayed consistent trends with the liquidus, revealing an underlying thermodynamic and not microstructural or kinetic cause behind the complex phase behavior. Our results highlight the need for a novel, sufficiently complex physical model for understanding these non-trivial phase diagrams of semi-conducting materials. We also discover that the composition difference (Δϕ) between liquidus and binodal reflects the crystalline-amorphous interaction, exhibiting a linear relationship with the binodal composition (ϕ_b,polymer), i.e., Δϕ increases as χ_aa decreases. This possibly provides a new approach for obtaining the crystalline-amorphous interaction parameter χ_ca(T) beyond the commonly used melting point depression method, which estimates χ_ca near the melting temperature T_m of the crystalline component. The capability of obtaining χ_ca(T) over a more extended temperature range may encourage more extensive studies and facilitate the understanding of χ_ca in general, but particularly for all the novel non-fullerene acceptors that are able to crystallize.