(617a) Tuning the Thin Film Self-Assembly of Radical-Containing Diblock Copolymers
Optoelectronically-active polymers have been studied extensively for their use in a range of energy conversion and energy storage devices. In many instances, block polymers containing at least one electronically-active moiety have been synthesized in order to control the nanoscale structure of the resultant material. In the majority of these efforts, however, the relatively rigid (i.e., rod-like) nature of the conjugated moiety and/or tendency of the conjugated moiety to crystallize has prevented the observation of self-assembled nanoscale structures observed frequently in block polymers containing only moieties with relatively flexible (i.e., coil-like) macromolecular backbones. Here, we address this crucial challenge through the introduction of an emerging class of electronically-active macromolecules, radical polymers. These radical polymers are composed of a non-conjugated macromolecular backbone with pendant groups that bear stable radical species. These stable radical sites are able to pass charge in the solid state through a simple oxidation-reduction (redox) mechanism. In this way, the promising electronic transport properties of radical polymers are combined synergistically with the impressive nanoscale self-assembly typically associated with electrically-insulating block polymers.
Specifically, we demonstrate the design, synthesis, and thin film self-assembly of a variety of A-B diblock copolymers where the A moiety contains either a poly(methyl methacrylate)-like (PMMA-like) or poly(norbornene)-like (PNB-like) backbone. The side chains of this A block also contain either one or two (2,2,6,6-tetramethylpiperidin-1-yl)oxyl-like (TEMPO-like) groups. In either instance, the B moiety of the diblock copolymer is an electrically-insulating polymer with a relatively low glass transition temperature. These types of B moieties were implemented in order to increase chain mobility and to allow for the thermodynamic ordering of the A-B diblock copolymers to occur on a convenient timescale. In this manner, we are able to establish how the glass transition temperatures and radical densities of the radical-bearing moieties affect the self-assembly properties of the radical-containing diblock copolymers when they are cast into thin films. Furthermore, we demonstrate that the relatively flexible macromolecular backbones of the radical polymers allow for the ready observation of classic diblock copolymer morphologies (e.g., lamellar, hexagonally-packed cylinders). Finally, we quantify the specific compositional phase boundaries associated with order-to-order transitions in these thin film diblock copolymer systems. Therefore, this effort presents a guide for important design considerations in generating well-ordered nanostructured materials for potential application in flexible energy conversion and energy storage devices.