Thermochromism in polydiacetylene-ZnO nanocomposites

Room temperature attentuated total reflection (ATR)-Fourier transform infrared (FTIR), temperature-dependent Raman, colorimetric (using optical densitometry), and differential scanning calorimetry (DSC) are used to provide a qualitative mechanism of the thermochromic reversibility introduced by composite formation of poly-10,12-tricosadiynoic acid (poly-TCDA) with ZnO in the particle size range below 100 nm. The results indicate that in pure poly-TCDA, heating above the chromatic blue to red transition temperature leads to an irreversible stress on the polymer backbone due to break up of hydrogen bonding on the side chain head groups to form an irreversible red phase. By contrast, poly-TCDA composites with nanosize ZnO at concentrations above 5 wt % display rapid chromatic reversibility. Reversibility is only partial for composites with 5 wt % ZnO, and interestingly for composites only of this composition the Raman data indicate a broad scattering band which can be assigned to the irreversible formation of an amorphous poly-TCDA/5 wt % ZnO phase network. The appearance of a new line at 1540 cm^-1 in the ATR-FTIR spectra of poly-TCDA/ZnO composites is indicative of chelation between head groups on neighboring side chains and ZnO, which facilitates the release of stress on cooling from the red phase to rapidly form the blue phase. Moreover, the colorimetric data show an increase in chromatic transition temperature with increase of the amount of ZnO from 5 to 15 wt % in poly-TCDA. The potential of these nanocomposites as thermochromic temperature sensors is also considered.