Gregory M. Newbloom

Structure and property development of poly(3-hexylthiophene) organogels probed with combined rheology, conductivity and small angle neutron scattering

The structural, mechanical and electrical properties of poly(3-hexylthiophene) (P3HT) organogels have been probed during the sol–gel transition through combined rheology, AC dielectric spectroscopy and small angle neutron scattering (SANS). SANS shows that structural features of P3HT gels, which are crucial for the optimization of organic photovoltaic devices, evolve throughout the gelation process. In situ structure–property analyses also demonstrate that there are very different mechanisms for the formation and dissolution of fibers and networks prepared from these polymeric semiconductors. It is determined that P3HT gels form conductive pathways that are maintained even after up to 50% of the fibers re-dissolve upon heating. P3HT organogels formed in different aromatic solvents also show differences of more than two orders of magnitude in conductivity despite having similar nanoscale fiber structures. These results demonstrate the importance of controlling the self-assembled morphology of fiber networks for maintaining optimal electronic properties. This work also highlights the potential for using organogels as flexible platforms for designing efficient organic photovoltaic devices.

Solvatochromism and Conformational Changes in Fully Dissolved Poly(3-alkylthiophene)s

Absorption spectroscopy is commonly utilized to probe optical properties that can be related, among other things, to the conformation of single, isolated conjugated polymer chains in solution. It is frequently suggested that changes in peak positions of optical spectra result from variations in the stiffness of polymer chains in solution because this modifies the conjugation length. In this work we utilize ultraviolet-visible (UV-vis) spectroscopy, small angle neutron scattering (SANS), and all atom molecular dynamic (AA-MD) simulations to closely probe the relationship between the conformation of single-chains of poly(3-alkylthiophene)s (P3ATs) and their optical properties. SANS results show variations in the radius of gyration and Kuhn length as a function of alkyl chain length, and structure, as well as the solvent environment. Furthermore, both SANS and MD simulations show that dissolved P3HT chains are more rigid in solvents where self-assembly and crystallization are possible. Shifts in P3AT optical properties were also observed for different solvent environments. However, these changes were not correlated to the changes in polymer conformation. Furthermore, changes in optical properties could not be perfectly described by generalized solvent-solute interactions. AA-MD simulations provide new insights into specific polymer-solvent interactions not accounted for in generalized solvatochromic theory. This work highlights the need for experiments and molecular simulations that further inform the specific role of solvent molecules on local polymer conformation and on optical properties.

Understanding Interfacial Alignment in Solution Coated Conjugated Polymer Thin Films

Domain alignment in conjugated polymer thin films can significantly enhance charge carrier mobility. However, the alignment mechanism during meniscus-guided solution coating remains unclear. Furthermore, interfacial alignment has been rarely studied despite its direct relevance and critical importance to charge transport. In this study, we uncover a significantly higher degree of alignment at the top interface of solution coated thin films, using a donor-acceptor conjugated polymer, poly(diketopyrrolopyrrole-co-thiophene-co-thieno[3,2-b]thiophene-co-thiophene) (DPP2T-TT), as the model system. At the molecular level, we observe in-plane π-π stacking anisotropy of up to 4.8 near the top interface with the polymer backbone aligned parallel to the coating direction. The bulk of the film is only weakly aligned with the backbone oriented transverse to coating. At the mesoscale, we observe a well-defined fibril-like morphology at the top interface with the fibril long axis pointing toward the coating direction. Significantly smaller fibrils with poor orientational order are found on the bottom interface, weakly aligned orthogonal to the fibrils on the top interface. The high degree of alignment at the top interface leads to a charge transport anisotropy of up to 5.4 compared to an anisotropy close to 1 on the bottom interface. We attribute the formation of distinct interfacial morphology to the skin-layer formation associated with high Peclet number, which promotes crystallization on the top interface while suppressing it in the bulk. We further infer that the interfacial fibril alignment is driven by the extensional flow on the top interface arisen from increasing solvent evaporation rate closer to the meniscus front.

Section II: Microstructure of Semiconducting Polymers Chapter 5:Characterization of Polymer Semiconductors by Neutron Scattering Techniques

Neutron scattering is a powerful tool for the analysis of structure and dynamics in organic semiconductor systems, including conjugated polymers. This chapter covers the major techniques that have been applied to study phase behaviour, structure and dynamics in these systems. Elastic scattering techniques, including small-angle neutron scattering, neutron reflectometry and grazing incidence small-angle neutron scattering serve as advanced and complementary tools to non-destructively probe structure over nanometre length scales. Quasi-elastic neutron scattering is also briefly discussed to demonstrate its value in the analysis of dynamic motion in conductive polymers. An introduction to the basic principles, theories and data analysis are provided for each technique, in addition to example applications from the literature.

A Structuring Repeat for Peptide Design: Long Beta Ribbons.

Beta sheets are inherently length‐limited; adding residues to the ends of model β‐sheets does not necessarily grow the β‐sheet. Here, we present a method for extending β‐sheets to any length with a stabilizing repeat unit containing cross‐strand Trp residues. Beta ribbons as long as 35 residues (approaching 100 Å in length) are reported and characterized.