Thermotropic Behavior

A number of interesting effects are observed during the thermal cycling of the various side chain substituted conducting polymer hosts. The most pronounced response is the distinctive thermochromic transition which is seen upon heating[,]. In the P3AT's, this attribute is correlated with a structural transformation to an isotropic melt state and a loss of the $\pi$-orbital conjugation length. In this state all of the relatively sharp scattering peaks are lost and only broad, liquid-like features remain[,126]. Cooling of the samples to temperatures well-below the respective P3AT ``melt'' temperature usually returns the polymer to its original state although the actual dynamics of this process appear to be rather sluggish.

There are many more subtle intermediate features which have also been observed. In addition to the endothermic behavior of the isotropic melt, calorimetry measurements[] detect a second endothermic peak at somewhat lower temperatures. This new feature corresponds to (on heating) a transition to a liquid crystalline (LC) phase wherein the individual stacked layers become decoupled from one another and are essentially free-floating. The main chain planarity is only somewhat affected by this transition while the alkyl containing side groups show a significant increase in the relative proportion of gauche conformers as the transition is approached from lower temperature.

There are also detailed structural changes which can be resolved in scattering experiments[]. Equatorial scattering peaks arising from the interlayer repeat are found to sharpen significantly in the LC phase of the longer alkyl P3AT'S (octyl and larger) indicating either an increase in the crystallite size or a reduction in the strain. On cooling the (h00) reflections are found to broaden once more. This implies, indirectly, that strain is the predominant effect. In samples containing (hk0) reflections, this LC phase is marked by the loss of these (hk0), $k\ne 0$, peaks. Significant variations also occur in the non-equatorial scattering. These effects are indicative of either changes in the side chain and/or main chain structure and/or conformation. In particular the meridional scattering near the (002) reflection is seen to shift its $2\theta$ position and reduce its intensity. While this may indicate a change in the thiophene repeat distance and in the skeletal main conformation, modest changes in the side chain structure can also strongly impact the closely spaced (hk2) scattering intensities near the meridional position.