(521d) Dynamic Fluctuations in Poly(3-alkylthiophene)s and Their Relation to Structure and Bulk Properties

This work investigates the relationship between structure and dynamics for a class of “model” conjugated polymers, poly(3-alkylthiophene)s (P3ATs), and their effect on bulk physical properties. The dynamics of conjugated polymers are probed using quasi-elastic neutron scattering (QENS) techniques. QENS data probes characteristic timescales (1 ps - 10 ns) and activation energies for molecular motions corresponding to the backbone (i.e. thiophene rings) or the substitution moieties (i.e. alkyl side chains of varying length) at variable length scales (0.3-1.9 Å^-1). The effect of regioregularity is also considered by comparing QENS data from regioregular and regiorandom poly(3-hexylthiophene). Fits of the QENS spectra using Kolrausch-William-Watts stretched exponential functions show that simple diffusion dominates the observed dynamics at large distances (0.3-1.1 Å^-1), but nonlinear dynamics are evident at shorter length scales.  To better understand the nature of these dynamic processes, QENS data is being used to validate molecular dynamics (MD) simulations of these systems since time and length scales are commensurate. Initial simulated spectra of P3HT have revealed that the torsion potentials of the backbone and side-chain carbons play an integral role in the dynamics observed. The semi-crystalline structure of these materials is also investigated using wide-angle x-ray scattering and neutron diffraction, and key structural transitions are noted using differential scanning calorimetry (DSC). From this data, the crystalline fraction of the material is determined for use as a starting point for MD simulations. Validated simulations will help clarify the specific motions that contribute to the determination of electronic properties. These motions are correlated to molecular relaxations observed through impedance spectroscopy of P3AT thick films, establishing a link between the dynamics observed from QENS and the AC conductivity of the conjugated polymer. The results of this work will help guide polymer chemists to rationally identify the optimal composition and placement of substitution moieties when designing new conjugated polymers. It will also help elucidate the role of molecular dynamics in the solid state with regards to the modification of electronic properties.