Competing Influences of Cooling Rate and Diluent Concentration on Crystallization of Poly(3-hexylthiophene)

Crystallization of poly(3-hexylthiophene) (P3HT) was studied in mixtures with different concentrations of 3-hexylthiophene (3HT) as a diluent. Two coexisting crystal populations, so-called "thin" and "thick" crystals, with melting temperatures differing by more than 10 °C, were observed in these mixtures. Intriguingly, the "thick" crystals represented transient, unstable structures, with a mass fraction ϕH that was the highest for rapidly crystallized P3HT in the presence of a significant amount of a diluent. Reflecting its dependence on growth kinetics, ϕH decreased with increasing crystallization temperature, extended crystallization time, or lower cooling rate. The presence of a diluent led to a "depression" of the melting temperature, described by Raoult's law and the Flory equation, which was similar for P3HT samples crystallized in various ways. A kinetically controlled "enhancement" of the melting temperature caused by the presence of the diluent depended sensitively on the cooling rates.