Phase Diagrams of n-Type Low Bandgap Naphthalenediimide-Bithiophene Copolymer Solutions and Blends

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.

Phase Behavior of Amorphous/Semicrystalline Conjugated Polymer Blends

“…Figure 1 shows the chemical structures of (a) P(NDI2OD-T2) and (b) PCPDTBT. In the previous works, we reported that the solubility parameter (δ) of P(NDI2OD-T2) is 7.99 [22], whereas that of PCPDTBT (M n = 3.2 kg/mol and PDI = 2.2) is 10.70 [21]. Note that in this work, PCPDTBT has M n = 4.5 kg/mol and PDI = 3.0, indicating a very minor difference between the two PCPDTBT samples.…”

“…Note that in this work, PCPDTBT has M n = 4.5 kg/mol and PDI = 3.0, indicating a very minor difference between the two PCPDTBT samples. Hence, based on the former δ data, the Flory–Huggins interaction parameter ( ) [ 22 , 39 , 40 ] for the PCPDTBT/P(NDI2OD-T2) polymer blends could be estimated using Equation (1), where (= lattice site volume) and were the molar volume of a solvent [ = (112.56 g/mol)/(1.11 g/cm 3 ) = 101.41 cm 3 /mol for CB] and the solubility parameter [subscript i or j = 1, 2; PCPDTBT = 1 and P(NDI2OD-T2) = 2]. R and T were the gas constant and temperature (K).…”

“…Note that in this work, PCPDTBT has M n = 4.5 kg/mol and PDI = 3.0, indicating a very minor difference between the two PCPDTBT samples. Hence, based on the former δ data, the Flory-Huggins interaction parameter (χ ij ) [22,39,40] for the PCPDTBT/P(NDI2OD-T2) polymer blends could be estimated using Equation (1),…”

“…Here, NG proceeds in the metastable region, whereas SD takes place in the unstable region without any energy barrier. However, if P D and/or P A are semicrystalline, the liquid-solid (L-S) phase transition (crystallization) also takes place with L-L phase transition [20][21][22]. Indeed, in many P A /P D blend systems, they contain a stereoregular (or regioregular) polymer, leading to both L-L and L-S phase transition, directly affecting the morphologies of P D /P A blend films.…”

“…Recently, we investigated the phase behavior of crystalline/crystalline conjugated polymer blends, composed of r-reg P3HT and 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)). Then, we also explained the upper critical solution temperature (UCST) behavior of each polymer solution as a function of solvent species, for which the Flory-Huggins lattice theory was employed [22].…”