Direct Calorimetric Observation of the Rigid Amorphous Fraction in a Semiconducting Polymer

The performance of polymeric semiconductors is profoundly affected by the thermodynamic state of its crystalline and amorphous fractions and how they affect the optoelectronic properties. While intense research has been conducted on the crystalline features, fundamental understanding of the amorphous fraction(s) is still lacking. Here, we employ fast scanning calorimetry to provide insights on the glass transition of the archetypal conjugated polymer poly(3-hexylthiophene) (P3HT). According to the conceptual definition of the glass transition temperature (T_g), that is, the temperature marking the crossover from the melt in metastable equilibrium to the nonequilibrium glass, an enthalpy relaxation should be observed by calorimetry when the glass is aged below T_g. Thus, we are able to identify the enthalpy relaxations of mobile and rigid amorphous fractions (MAF and RAF, respectively) of P3HT and to determine their respective T_g. Our work moreover highlights that the RAF should be included in structural models when establishing structure/property interrelationships of polymer semiconductors.