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

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Abstract

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 Ctriple bondC and Cdouble bondC 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.

Introduction

Polydiacetylenes (PDAs) are known to exhibit a color transition when exposed to various external stimuli such as heat, chemicals, biomolecules, UV light and electricity [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]]. The color transition, normally from blue to red, occurs rapidly and is easily perceived by naked eyes, rendering PDA-based materials potential candidates for various applications such as 2D and 3D temperature sensors [2], sensors for volatile organic compounds [3,4], sensors for biomolecules [7,10] and a sensor for hydrogen peroxide [15]. The as-prepared PDAs usually present blue phase and are not fluorescent [18]. The blue-to-red color transition of PDAs generally involves segmental rearrangement within PDA assemblies causing the decrease of conjugation length [[19], [20], [21]]. The red-phase PDAs become fluorescent with quantum yield of about 0.02.

Microscopic mechanism of the color transition of PDA has been investigated by utilizing various techniques. Early works on urethane-substituted PDAs illustrated that the color transition was dominated by the change of backbone conformation [22,23]. When the systems were perturbed, the inter- and intrachain interactions were weaken. This allowed segmental rearrangement within the PDA assemblies, affecting the conjugation length of systems. Later works on the PDAs constituting carboxylic head group observed the change of molecular packing during the color-transition process [[19], [20], [21],24]. Atkinson et al. reported that the color transition of PDA prepared from 10,12-pentacosadiynoic acid (PCDA) was related to the change from the orthorhombic to triclinic structure [24]. Lifshitz et al. observed the decrease of spacing between PDA backbones and the rearrangement of side chains during the color transition [20]. Fujimori et al. also detected the shrinkage of unit cell [21]. Our group utilized nuclear magnetic resonance spectroscopy to follow the molecular dynamics of each segment within PDA chain during the color transition [16]. These previous studies indicate that the segmental rearrangement plays an important role on the mechanism of color transition.

The color-transition properties of PDAs prepared from commercially available monomers such as PCDA are generally irreversible, limiting their utilization in various applications [14,16,25,26]. Many research groups have demonstrated that reversible color transition can be achieved by enhancing the interactions within the PDA assemblies via structural modification [17,[27], [28], [29], [30], [31], [32], [33]] or incorporating foreign materials [1,2,5,14,19,26,[34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47]]. For example, the PDA functionalized with hydrazide head group exhibits reversible color transition under acid-base treatments [28]. The azobenzene-substituted PDA exhibits reversible thermochromism due to the enhanced intermolecular π-π interaction [29]. The nanocomposites of PDA/polymers [5,14,26], PDA/cations [1,2,9,[34], [35], [36], [37]] and PDA/metal oxides [38,[35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47]] can provide reversible thermochromism as well.

Our group has achieved reversible thermochromism of PDAs by incorporating zinc oxide (ZnO) nanoparticles [[38], [39], [40], [41], [42], [43], [44], [45]]. Our latest study revealed the presence of Zn2+ ions, releasing from ZnO nanoparticles during the preparation process [44]. These Zn2+ ions intercalated between PDA layers and interacted with the carboxylate head groups while the ZnO nanoparticles provided anchoring sites. The PDA/Zn2+/ZnO nanocomposites possesses rather strong inter- and intramolecular interactions, making the system thermochromic reversible [38,39] and highly stable in various organic solvents [42]. The presence of ZnO nanoparticles also allows the colorimetric response to both acids and bases, which extends the utilization as a chemical sensor [40,43,45]. Recently, we have found a simple route for controlling the color-transition temperature of PDA/Zn2+/ZnO nanocomposites. The increase of photopolymerization time caused systematic variation of the color-transition temperature [41]. In this contribution, we present our continuation work, investigating into the molecular level of the color-transition behaviors of PDA/Zn2+/ZnO nanocomposites obtained by varying photopolymerization time.

Section snippets

Materials and methods

The diacetylene (DA) monomers used in this study, 5,7-hexadecadiynoic acid (HDDA), 10,12-tricosadiyoic acid (TCDA) and 10,12-pentacosadiynoic acid (PCDA) were commercially available at Fluka. The ZnO nanoparticles were purchased from Nano Materials Technology (Thailand). The diameter of ZnO nanoparticles revealed by transmission electron microscopy (TEM, Tecnai 12, D291) is ranged from 20 to 160 nm (Fig.1a) with the averaged diameter of 65 nm. The PDA/Zn2+/ZnO nanocomposites were prepared using

Thermochromism of PDA/Zn2+/ZnO nanocomposite films

In our previous studies, we explored thermochromic properties of poly(PCDA)/Zn2+/ZnO nanocomposite dispersed in aqueous suspension and polymeric matrices [41,42]. The poly(PCDA)/Zn2+/ZnO nanocomposite exhibited a two-step color transition upon increasing temperature, involving reversible blue-to-purple and then irreversible purple-to-red. The color-transition temperature of these two processes can be tuned by varying photopolymerization time [41]. In this contribution, we take a step forward to

Conclusion

This study demonstrates that the color/phase transition behaviors of PDA/Zn2+/ZnO nanocomposites can be systematically controlled by utilizing molecular engineering approach. The increase of PDA backbone length via photopolymerization process induces partial segmental relaxation within the nanocomposites. Raman spectroscopy detects the formation of new state of PDA conjugated backbone. The magnitude of backbone relaxation, depending on photopolymerization time, dictates the reversible

Acknowledgements

This research has been supported by the Thailand Research Fund [RSA 5980020]. This work has been partially supported by the Nanotechnology Center (NANOTEC), Ministry of Science and Technology, Thailand, through its program of Center of Excellence Network.

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