POLYMERS CONTAINING FLEXIBLE SIDE CHAINS

Addition of flexible side chains to the stiff $\pi$-conjugated polymer backbone of the host polymer has proven to be an extremely effective procedure for obtaining tractable and fusible materials. This approach is not limited to conducting polymers alone but has also attracted considerable attention in more conventional rigid rod-like polymers[94,,,] including various polyimides and polyamides and in other novel polymers such as the polysilanes[]. Not only does this modification enable the utilization of conventional polymer processing methods it can, in some cases, create new materials that exhibit enhanced electronic properties as compared to the linearly unsubstituted parent polymer. These side chain substituted conducting polymers also exhibit properties which do not exist in the unsubstituted hosts of the last section. Some of these attributes include a pronounced thermo- and solvato-chromism, thermotropic and lyotropic liquid crystallinity and structural self-assembly.

Most of the side chain containing conducting polymers synthesized to date employ alkyl, alkoxy or phenylalkyl side chains of varying lengths which are chemically substituted at various hydrogen atom sites along the polymer backbone. Alternatively, one can incorporate these side chains using unusual dopant molecules that have specific functionalities. A few example systems, employing both of these approaches, are shown schematically in Fig. 13.

 

Figure 13: Two examples of flexible side chain containing conducting polymers with the chains chemically anchored, a) monosubstituted poly(3-hexylthiophene), b) doubly substituted poly(2-methoxy-5-(2'-ethylhexoxy)-phenylene vinylene) and one example of a linear unsubstituted host having a functionalized dopant, c) polyaniline-dodecylbenzenesulfonate.

There is, relatively speaking, considerably less definitive structural information available from the side-chain substituted materials. In addition to the scattering from the structural components forming the main chain, there are necessarily contributions by the side chain constituents. Depending on the specifics of the polymer sample and its processing history a combination of crystalline, semi-crystalline and amorphous scattering signatures can be superimposed in the experimental data. As was the case for the linear unsubstituted conducting polymer hosts, a considerable effort has been made and significant details concerning the detailed structure and overall phase behavior can be discerned.