Electrochromic carbon nanotube polydiacetylene and zinc oxide hybrids

Abstract

Materials that have the inherent ability to respond to external stimuli with a visible color change have tremendous potential in the field of sensing. It is unlikely that a single material exhibits all the favorable properties required for use of such chromatic systems practically. For instance polydiacetylenes (PDA) have the inherent ability to change color in response to external stimuli, however, the physical attributes of these polymers are poor. By combining the useful stimuli response of PDAs with physically robust materials, composites with the positive attributes of both can be obtained. A literature study conducted towards this end has revealed that carbon nanotube (CNT) fibers are ideal for forming composites with PDAs not only because of the improved mechanical properties of the resulting composite, but also because it invokes an electrically induced chromatic transition of the same. Although the phenomenon of electrochromism in CNT-PDA composite fibers has been observed, an understanding of the influence of CNTs on the color transition is limited. This work explores the science behind electrochromism in CNT-PDA composites, by systematically studying composites prepared through different strategies. This research provides insight into the science of electrochromic CNT-PDA composites and facilitate tailoring the properties of these composites to provide some degree of control over the electrochromic transition. Towards achieving the objectives of this research, CNT-PDA composites with different extent of polymerization were prepared. To understand the effect of order in the PDA layers on the electrochromism, the composites were exposed to different types of heat treatments. Further, ZnO nanoparticles were used as an anchoring phase to prepare unique CNT-ZnO-PDA hybrids. The electrochromism in these composites was explored in detail using in-situ spectroscopy and surface characterization tools. Although both partially and highly polymerized CNT-PDA composites exhibited electrochromism, it was found that the electrically induced red phase in CNT-PDA composites need not have stressed PDA backbones, unlike other forms of PDA chromism induced by heat and solvents. It was also found that heat treatment in general, increased the critical electrochromic transition voltage in CNT-PDA composites. Specifically, heat treatment before polymerization increased the chromatic contrast and critical transition voltage significantly when compared with conventional CNT-PDA composites. In contrast, incorporation of ZnO to form CNT-ZnO-PDA hybrids lowered the critical transition voltage but significantly improved the recovery of blue phase when electric stimulus was removed. Detailed mechanistic interpretations of the structural changes responsible for the different electrochromic behaviors in these composites are provided in this thesis.