Controllable thermochromic and phase transition behaviors of polydiacetylene/zinc(II) ion/zinc oxide nanocomposites via photopolymerization: An insight into the molecular level

Reversible thermochromic polydiacetylene/zinc(II) ion/zinc oxide (PDA/Zn²⁺/ZnO) nanocomposites with a wide range of color-transition temperature have been prepared by varying photopolymerization time. This contribution presents our continuation study investigating into the molecular origins of this behavior. Infrared spectroscopy is utilized to investigate interfacial interactions of the systems while the conformation of PDA conjugated backbone is probed by Raman spectroscopy. X-ray diffraction explores molecular packing within the nanocomposites. We have found that the increase of photopolymerization time induces the relaxation of PDA backbone into a newly observed state indicated by systematic growth of new vibrational modes of C[tbnd]C and C[dbnd]C bonds. This relaxation process results in the decrease of reversible blue-to-purple color-transition temperature. In contrast, the increase of backbone length with photopolymerization time causes an opposite trend of the irreversible purple-to-red color transition observed at relatively high temperature region. Differential scanning calorimetry detects two distinct phase transitions corresponding to the melting of alkyl side chains and rigid backbone. These melting temperatures vary with photopolymerization time consistent with the variation of color-transition temperature.