Aggregation, crystallization, and resistance properties of poly(3-hexylthiophene-2,5-diyl) solid films gel-cast from CHCl3/p-xylene mixed solvents

• The gelation time of P3HT can be adjusted by changing the ratio of its good/poor solvents.
• The P3HT film cast from 20 vol% CHCl3 exhibited the lowest resistance in samples.
• With more crystalline gels, a higher crystallinity of the film can be observed.
• The peak 75 °C plays a key role on interchain overlapping of P3HT backbone.

In this study we found that the gelation time and crystallinity of P3HT solid films are adjustable when aging and casting from CHCl3/p-xylene mixed solvents. After aging for 36 h in pure p-xylene, CHCl3, or various mixtures of the two as cosolvents, we found that the solid P3HT film gel-cast from 20 vol% CHCl3 had the highest degree of crystallinity of its main chain (ϕm = 0.54), highest melting point of its main chain (Tm = 232.7 °C), fastest gelation time (30 min), largest melting enthalpy of its main chain (ΔHm = 19.81 J g−1), and lowest resistance (RP = 0.76 MΩ); the latter value was three orders and one order of magnitude lower than those of the films cast from pure CHCl3 (ca. 110 MΩ) and pure p-xylene (ca. 4.4 MΩ), respectively. In differential scanning calorimetry scans, we attribute the presence of melting peaks near 75 °C to the solid-to-solid phase transition of the side chain crystallites of P3HT, thereby affecting the aggregation of the P3HT main chain and resulting in the changes in resistance, crystallinity, melting enthalpy, and melting point of the gel-cast P3HT solid films.

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