What Is the Assembly Pathway of a Conjugated Polymer From Solution to Thin Films?

Abstract

The hierarchical assembly of conjugated polymers has gained much attention due to its critical role in determining optical/electrical/mechanical properties. The hierarchical morphology encompasses molecular-scale intramolecular conformation (torsion angle, chain folds) and intermolecular ordering (π–π stacking), mesoscale domain size, orientation and connectivity, and macroscale alignment and (para)crystallinity. Such complex morphology in the solid state is fully determined by the polymer assembly pathway in the solution state, which, in turn, is sensitively modulated by molecular structure and processing conditions. However, molecular pictures of polymer assembly pathways remain elusive due to the lack of detailed structural characterizations in the solution state and the lack of understanding on how various factors impact the assembly pathways. In this mini-review, we present possible assembly pathways of conjugated polymers and their characteristics across length scales. Recent advances in understanding and controlling of assembly pathways are highlighted. We also discuss the current gap in our knowledge of assembly pathways, with future perspectives on research needed on this topic.

Resonant soft X‐ray scattering in polymer science

“…Using the sensitivity to molecular orientation, polarized RSoXS could be used to investigate orientation evolution during solution self-assembly of molecular nanostructures that includes molecular nanocrystals as well as more exotic structures. 230,231 Even more advanced, would be incorporating orientation sensitivity of individual residues into current solution SAXS characterization of peptides and proteins. 232,233 Such work could significantly improve characterization of critical biological structures such as membrane proteins that govern permeation of cell walls but evade protein crystallography investigations.…”

“…Using the sensitivity to molecular orientation, polarized RSoXS could be used to investigate orientation evolution during solution self‐assembly of molecular nanostructures that includes molecular nanocrystals as well as more exotic structures 230,231 . Even more advanced, would be incorporating orientation sensitivity of individual residues into current solution SAXS characterization of peptides and proteins 232,233 .…”

Systematic Investigation of Solution-State Aggregation Effect on Electrical Conductivity in Doped Conjugated Polymers

“…1 Conjugated polymers usually form significant pre-aggregates in solutions due to the interchain interactions, including π-π interaction between the polymer backbones and the dispersion forces among the alkyl side chains. 2 Solution-state aggregation structures of conjugated polymers were demonstrated to be inherited to thin films and finally conspicuously reflected on electrical performances. 3,4 To improve the optoelectrical performances of conjugated polymers, some methodologies through modulating solution aggregation structures have been developed in the past years, [5][6][7][8][9] such as modulating solution preheating temperature, 10 tuning solvent quality, 1 and changing dissolution temperature.…”

Not all aggregates are made the same: Distinct structures of solution aggregates drastically modulate assembly pathways, morphology and electronic properties of conjugated polymers

“…The solidstate properties of conjugated polymers depend sensitively on their morphologies across all length scales, which encompasses molecular-scale intramolecular conformation and intermolecular ordering, mesoscale domain size, orientation and connectivity, and macroscale alignment and (para)crystallinity 5, 6 . This multiscale morphology in the solid-state is largely affected by solution-state aggregates and their assembly pathways from solution to thin films [7][8][9] . In-depth understanding of the solution aggregation and assembly pathways will lead to precise control of many electronic and optical properties, such as charge carrier generation and transport 10,11 , interfacial charge transfer 12 , signal transduction 13 , spin transport 14 , optical absorption 15 , etc -properties of close relevance to a wide range of device applications including transistors, thermoelectrics, solar cells, bioelectronics, photocatalysts, and spintronics.Recent literature has suggested there is a positive correlation between degree of solution-state aggregation and solid-state morphology and electronic property 16,17 , and solution aggregated polymers are often found to have significantly improved device performance compared with well-dissolved polymers 18, 19 .…”

“…We believe that resolving the controversy around the role of solution aggregation requires probing the actual aggregate structure, which has been largely neglected in literature in part due to the analytical challenge. Besides, the evolution of solution aggregate structure during assembly has been mostly overlooked, which is a critical missing link between solution aggregation and solid-state properties 8 . In sum, there is currently a gap in our knowledge regarding how to control the precise solution aggregate structures and assembly pathways of conjugated polymers, and how the solid-state morphology and electronic properties can be determined by different assembly pathways in a controllable fashion.…”