Thermoreversible Gelation of Regioregular Poly(3-hexylthiophene) in Xylene

Regioregular poly(3-hexylthiophene) (P3HT) produces thermoreversible gel in xylene. The gel is brownish-red in color. SEM and TEM studies indicate the presence of fibrillar network. WAXS and electron diffraction pattern indicate the presence of P3HT crystallites in the gel. The gels exhibit a first-order phase transition when heated in DSC. A time-dependent UV−vis study indicates that gelation in this system is probably accompanied by two different processes, e.g., (1) coil-to-rod transformation and (2) aggregation of rods to form the crystallites producing the gel. The gelation rate ( t gel -1 ) measured from the test tube tilting method is analyzed using the equation t gel -1 = f ( C ) f ( T ), where f ( c ) = φ n, φ being the reduced overlapping concentration and n is an exponent. The average “ n ” value determined is 0.52, which indicates that three-dimensional percolation is a suitable model for this gelation. The gelation rate is analyzed according to the Flory−Weaver theory of coil-to-helix transition, and the free energy of activation (Δ F ) for the coil-to-rod transformation is found to be 23.7 kcal/mol. It is also analyzed using the theory of fibrillar crystallization in solution, and the free energy of formation of critical size crystalline nucleus (Δ G * dil ) is found to be 37.5 kcal/mol. The conductivity of the dried P3HT gel becomes enhanced by ∼10 times that of the cast film in the undoped state, but in the doped state there is an ∼50-fold increase.