Abstract
Phase diagrams of n-type low bandgap poly{(N,N′-bis(2-octyldodecyl)naphthalene -1,4,5,8-bis(dicarboximide)-2,6-diyl)-alt-5,5′,-(2,2′-bithiophene)} (P(NDI2OD-T2)) solutions and blends were constructed. To this end, we employed the Flory–Huggins (FH) lattice theory for qualitatively understanding the phase behavior of P(NDI2OD-T2) solutions as a function of solvent, chlorobenzene, chloroform, and p-xylene. Herein, the polymer–solvent interaction parameter (χ) was obtained from a water contact angle measurement, leading to the solubility parameter. The phase behavior of these P(NDI2OD-T2) solutions showed both liquid–liquid (L–L) and liquid–solid (L–S) phase transitions. However, depending on the solvent, the relative position of the liquid–liquid phase equilibria (LLE) and solid–liquid phase equilibria (SLE) (i.e., two-phase co-existence curves) could be changed drastically, i.e., LLE > SLE, LLE ≈ SLE, and SLE > LLE. Finally, we studied the phase behavior of the polymer–polymer mixture composed of P(NDI2OD-T2) and regioregular poly(3-hexylthiophene-2,5-dyil) (r-reg P3HT), in which the melting transition curve was compared with the theory of melting point depression combined with the FH model. The FH theory describes excellently the melting temperature of the r-reg P3HT/P(NDI2OD-T2) mixture when the entropic contribution to the polymer–polymer interaction parameter (χ = 116.8 K/T − 0.185, dimensionless) was properly accounted for, indicating an increase of entropy by forming a new contact between two different polymer segments. Understanding the phase behavior of the polymer solutions and blends affecting morphologies plays an integral role towards developing polymer optoelectronic devices.
Highlights
The phase behavior of conjugated polymer–solvent and polymer–polymer mixtures has been an interesting topic of research in which there are two important phase-separation mechanisms, i.e., liquid–liquid (L–L) and liquid–solid (L–S) phase transitions [1,2,3]
P(NDI2OD-T2) solutions as a function of three different solvents, chlorobenzene, chloroform, and p-xylene, for which the Flory–Huggins lattice model was employed with a χ interaction parameter estimated from the solubility parameter
The P(NDI2OD-T2) solutions showed the three different types of phase behavior depending on the solvent, Case 1: liquid–liquid equilibria > solid–liquid equilibria, Case 2: liquid–liquid equilibria ≈ solid–liquid equilibria, and Case 3: solid–liquid equilibria >
Summary
The phase behavior of conjugated polymer–solvent and polymer–polymer mixtures has been an interesting topic of research in which there are two important phase-separation mechanisms, i.e., liquid–liquid (L–L) and liquid–solid (L–S) phase transitions [1,2,3]. We report that how n-type low bandgap P(NDI2OD-T2) solution undergoes L–L and L–S phase transition depends on the solvent in comparison with the phase behavior of the r-reg P3HT solutions. These phase behaviors are qualitatively described by the Flory–Huggins (FH) lattice theory [1,40,41], for which the χ interaction parameter is estimated from contact angle measurements, leading to a solubility parameter [2,3]. Note that, when a polymer contains impurities (e.g., solvents or copolymerized units or other polymers), the melting point is shifted by re-establishing the condition of equilibrium between liquid and crystalline polymer, which could be described by combining the melting point depression theory with the FH model [1]
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