60th Annual Report on Research 2015

The structure and morphology of the conjugated polymers play an important role in controlling the device performance of the polymer-based organic electronics. We have developed a facile route to control the structures of various assemblies of conjugated polymers at different length scales. Conjugated polymers bearing ionic moieties in their side chains react with oppositely charged surfactants via electrostatic interactions to generate the supra-molecular complexes. In this project, we focused on carboxylated derivatives of poly(3-alkyl thiophene)s coupled with counter-ionic surfactants bearing single/double hydrocarbon tails (Figure 1). 

As-prepared complexes existed as hydrogels. We diluted the hydrogel to study structural transformation of individual polymer chains in the very diluted solution. Then, we employed various characterization methods including absorption and emission spectroscopies  and small angle X-ray scattering (SAXS) to investigate the dynamic process of structural transformation of the conjugated complexes.  We discovered the time-dependent chromism, which is indicative of a coil-to-rod transition of the polymer chains (Figure 2).

We used absorption spectroscopy and initial rate methods to study the kinetics and thermodynamics of the coil-to-rod transition. We found that this structural transition followed an intra-molecular mechanism, which was evidenced by an inverse first order of the kinetic law, Rate ~ C^-1, where C is the concentration of the conjugated complex. This intra-molecular mechanism was also supported by emission spectroscopy, evidenced by the red-shift emission without significant photo-quenching. It is believed that the coil-to-rod transition of individual polymer chains is determined by minimization of the total Gibbs free energy and therefore it is intrinsic to the polymer molecules.

To further understand the physical origin of the intra-molecular mechanism of the coil-to-rod transition, we studied the effect of the surfactant architecture on the kinetics of the coil-to-rod transition, including the length of hydrocarbon chains and single/double tail ratio of mixed surfactants. While increasing the length of hydrocarbon chains accelerated the transformation process, the introduction of the double tail surfactant decreased the coil-to-rod transition. We attributed this effect of the surfactant architecture on kinetics of the coil-to-rod transition to the steric interactions between hydrocarbon side chains within the individual polymer chains. We also studied the effect of the polymer structure on the coil-to-rod transition. The general trend was that increasing the side-chain length speeded up the transition process.

In summary, we studied the structures and properties of diluted solutions of conjugated complexes during the first year of this award. Future research work includes the phase behavior of the concentrated solutions (second year) and the structures of the solid thin films (third year).

To date, three undergraduate students and two graduate students have been involved in research from the support of this award. One student received a Master's degree and one student received an American Chemical Society (ACS) Meeting Travel Award. Cal Poly students presented oral talks in both ACS national meeting and Cal Poly summer research symposium.