NOTE: This article appears in the Journal of Polymers Science,41, 2630 (2003).

Nanoscale Structure/Property Relationships in Conjugated Polymers:
Implications for Current and Future Device Applications

M.J. Winokur and W. Chunwachirasiri
Department of Physics, University of Wisconsin, Madison, WI 53706

        

Abstract: The most prevalent molecular level structures and structure/property relationships for three basic classes of conjugated polymers are summarized. The discussion encompasses linear unsubstituted conducting polymers and those containing linear side chain and branched side chain substituents. The impact of these structural attributes on charge transport and photophysics is emphasized.

Polymer based electronics are, at the molecular level, fundamentally different[1] from conventional silicon constructed devices because of the highly anisotropic nature of polymer chains and chemical bonding. In conventional semiconductors almost all properties can be derived from the perspective of a rigid tetrahedrally coordinated Si framework. On the other hand conjugated polymers are representative of a low dimensional solid with strong covalent bonds along the molecular backbone and very much weaker interchain interactions in the two orthogonal directions. Thus there is an intimate and often subtle relationship between the chemical and physical make-up of the polymer chain and the resulting electronic and optical properties. Controlling and manipulating structure of these polymers in both bulk and at surface interfaces has proven to be an extraordinarily challenging task. A centrally important step for developing and exploiting new device technologies is simply learning the key structure/property relationships that exist at molecular length scales.

The ``simplest'' electronic polymers (see Fig. 1) were synthesized and studied well over twenty years ago[2] with a strong focus on polyacetylene (PA) and its role as the quintessential degenerate ground state material. More complex homopolymers were soon developed and major parallel efforts were made in the study of $\pi$-conjugated polymers with non-degenerate ground states including polythiophene (PT), polyaniline (PANi), polypyrrole (PPy) and poly($p$-phenylene) (PPP). Analogous studies[3,4,5] of a $\sigma$-conjugated material, polysilane [(SiH$_2$)$_n$ or PSil], were also pursued. Copolymers, such as poly($p$-phenylene vinylene) (PPV), comprised of repeating AB type repeating phenylene/vinylene sequences were synthesized and studied as well. Many of these polymers have material specific electronic and optical properties although there is one nearly universal and overriding disadvantage with respect to technological applications. With the exception of polyaniline all of the these linear unsubstituted materials are intractable and infusible. In some instances synthesis of soluble precursor polymers has been achieved but, by and large, most of these polymers are not easily processible and therefore better suited for fundamental studies of optical and transport properties.

Figure 1: Sketches of prototypical unsubstituted $\pi$ and $\sigma$ conjugated polymers.

A key step in the continuing efforts to develop more processible materials has been the addition of solubilizing side chains[6]. This straightforward chemical modification has helped create a wealth of new polymers with a seemingly endless stream of interesting and, in many instances, counter-intuitive structure/property relationships. Even minor changes in the size and structure of the side chain substituents can produce dramatic differences in the structural phases, the phase behavior and, thus, specific device properties. Extending beyond the needs of processing many recent studies of electroactive polymers now emphasize molecular level engineering (i.e., creation of tailored molecular structures) in order to achieve specialized behavior. Specific functional groups may be introduced for a variety of purposes including: chemical affinity which results in molecular recognition in biological or chemical sensors; surface interaction with silicon for use in hybrid organic/inorganic technologies; optimization of charge transport or luminescence; enhanced solubility enabling either aqueous or organic solvent processing. Current research also emphasizes engineering and synthesizing systems with multiple architectural constituents so that these materials mimic the self-assembly process that occurs in more conventional polymers and biological systems.

This article reviews some of the most basic structural motifs[7] and paradigms that are encountered when simple alkyl side-chain substitution is used to confer solubility, enhance processibility and, in some cases, improve structural order. As a first step it is necessary to introduce key structural attributes of selected linear unsubstituted materials. Next we address a subset of electronic polymers having linear alkyl side chain substitutions with strong emphasis towards poly(3-alkylthiophenes) or P3ATs. The final section includes an extended discussion of polymers containing branched side chains; especially those containing di-$n$-alkyl type moieties. Even within this very restricted setting there is a surprisingly rich range of behavior and many important molecular level properties remain incompletely understood. The impact that these underlying structural relationships have on charge transport and photophysics are discussed as well. A comprehensive review of all important structural behavior and structure/property relationships in the electronic polymer field is clearly beyond the scope of this article.