Thermoreversible gelation of regioregular poly(3-hexylthiophene) in xylene

Abstract

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 (tgel-1) measured from the test tube tilting method is analyzed using the equation tgel-1 = f(C) f(T), where f(c) = φn, 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 (ΔF) 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 (ΔGdil*) is found to be 37.5 kcal/mol. The conductivity of the dried P3HT gel becomes enhanced by approximately 10 times that of the cast film in the undoped state, but in the doped state there is an approximately 50-fold increase.