Conjugated poly(3-hexylthiophene) (P3HT) chains are known to exist at least
in two distinct conformations: a coiled phase and a better ordered aggregated phase.
Employing steady state absorption and fluorescence spectroscopy, we measure the course of
aggregation of P3HT in tetrahydrofuran (THF) solution within a temperature range of 300 K
to 170 K. We show that aggregation is a temperature controlled process, driven by a
thermodynamic order−disorder transition. The transition temperature increases with the
molecular weight of the chains and can be rationalized in the theory of Sanchez. This implies
a smearing out of the phase transition in samples with increasing polydispersity and erodes
the signature of a first order phase transition. The detection of a hysteresis when undergoing
cooling/heating cycles further substantiates this reasoning.