Designing Amphiphilic Conjugated Polyelectrolytes for Self-Assembly into Straight-Chain Rod-like Micelles

Semiconducting polymers are a versatile class of materials that are used in many (opto)electronic applications, including organic photovoltaics. However, they are inherently disordered and suffer from poor conductivities due to bends and kinks in the polymer chains along the conjugated backbone, as well as disorder at grain boundaries. In an effort to reduce polymer disorder, we developed a method to straighten polymer chains by creating amphiphilic conjugated polyelectrolytes (CPEs) that self-assemble in water into worm-like micelles. The present work refines our design rules for self-assembly of CPEs. We present the synthesis and characterization of a straight, micelle-forming polymer, a derivative of poly(cyclopentadithiophene-alt-thiophene) (PCT) bearing two ammonium-charged groups per cyclopentadithiophene unit. Solution-phase self-assembly of PCT into micelles is observed by both small-angle X-ray scattering (SAXS) and cryo-electron microscopy (cryo-EM), while detailed SAXS fitting allows for characterization of intra-micellar interactions and inter-micelle aggregation. We find that PCT displays significant chain straightening thanks to the lack of steric hindrance between its alternating cyclopentadithiophene and thiophene subunits, which increases the propensity for the polymer to self-assemble into straight rod-like micelles. This work extends the availability of micelle-forming semiconducting polymers and points to further enhancements that can be made to obtain homogeneous nanostructured polymer assemblies based on cylindrical micelles.

Automated summary

Semiconducting polymers are important materials for (opto)electronic applications, but their disorder and poor conductivities limit their performance. This study presents a method to straighten polymer chains using amphiphilic conjugated polyelectrolytes (CPEs) that self-assemble into worm-like micelles. By synthesizing and characterizing a micelle-forming polymer called PCT, it is found that the lack of steric hindrance between its subunits leads to significant chain straightening and the formation of rod-like micelles. This work expands the availability of micelle-forming semiconducting polymers and suggests possibilities for obtaining homogeneous nanostructured polymer assemblies.