A Spectroscopic Analysis of the Role of Side Chains in Controlling Thermochromic Transitions in Polydiacetylenes

Abstract

A systematic study has been carried out to investigate the role of the side chains in lowering the reversible thermochromic transition temperature of polydiacetylenes. The type of interacting groups also proved to be crucial. Polymers with hydrogen-bonded side chains have consistently behaved differently as compared to polymers with less specific interactions associated with metallic ions. The most striking factor is the orderliness of the polymethylene portion of the side chains. The length of the side chains is critical in determining the transition temperature. In some cases, the packing order of the side chains is of sufficient order to crystallize. When high order exists, a reversible thermochromic transition is difficult to achieve. Conversely, if the side chains are disordered at any temperature, the transition temperatures are low and the transition is always reversible. This is the case for side chains stabilized by the presence of metallic ions. It is apparent that the presence of long-range nonspecific electrostatic interactions is important in determining reversible thermochromic transitions. These structural features are most easily observed using infrared spectroscopy and Raman scattering. Supporting evidence have been obtained using thermal analysis and X-ray diffraction.