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Single PMMA/PPy composite microtubes as chemical sensors

Single PMMA/PPy composite microtubes as chemical sensors
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Fabrication of polymeric micro/nanotubes is receiving increasing attention because of their potential applications in microfluidic, catalysis, tissue engineering, drug delivery, photonics, sensing, and so on. Main ingredients of polymeric micro/nanotube structures, however, have been mostly limited to single component polymers. Polymer composite that has broad applications as one-dimensional (1-D) fibers, wires, or cables by its enhanced mechanical property and thermochemical stability is largely unexplored as a form of hollow microtube structure. Integration of micro/nanotubes with various electrical, chemical or biological properties has been more and more important in bio- or chemical sensors. Here, one of the issues is to regulate the scales of tubes (diameters and wall thicknesses) as well as their positions. Conventional fabrication methods, such as self-assembly, templating, electrospinning, co-electrospinning, wetting, TUFT(tube by fiber template) process, self rolling, co-extrusion method, direct laser polymerization, and solvent evaporation induced phase separation, are not suitable for such regulation. For polymer composite microtubes, it is furthermore a challenge to well regulate their scales, even if successfully fabricated. In this study, single PMMA/PPy composite microtubes are realized by a meniscus-guided approach conjugated with the “coffee-ring effect” in a microscale meniscus of a colloidal solution. Furthermore, this thesis provides that individual addressable PMMA/PPy composite microtubes as stretchable chemical sensors. The first part of the thesis is the development of direct writing of single freestanding hollow microtubes of a polymer composite (poly(methyl methacrylate)/polypyrrole (PMMA/PPy)). In fact, single freestanding PMMA/PPy microtubes were fabricated by a meniscus-guided approach, based on the “coffee-ring effect” in an evaporating microscale meniscus of a colloidal solution. This method permitted to regulate the outer diameter and wall thickness of the PMMA/PPy microtubes simply by controlling the meniscus-guiding and the coffee-ring effect. Furthermore, an array of freestanding microtubes, individually controlled in scale and in position was realized. The second topic is to develop individually addressable PMMA/PPy composite microtubes as chemical sensors based on direct-writing of the composite microtubes. In fact, the composite microtubes showed a good potential as chemical sensors by demonstrating gas-concentration-driven modulation of electrical resistance for various gases such as NH3 and Volatile Organic Compounds (i.e. methanol, ethanol). The composite microtubes exhibited a higher sensitivity than those of PMMA/PPy composite microwires, much higher than those of PPy microwires at room temperature. Remarkably, strong robustness against humidity (20 to 80%) was demonstrated in the composite microtubes, due to the water-insensitivity of PMMA layer. The last topic of the thesis is to develop stretchable chemical sensor based on the PMMA/PPy composite microtubes. By writing the composite microtube into a stretchable 3-D architecture, specifically a 3-D arch shape, a very high stretchability up to ~ 110% was achieved with little change in the electrical conductance. Notably, the composite microtubes were robust against a stretching cycle up to 2 x 104. Furthermore, chemical sensitivity of the composite microtubes was not affected by stretching up to 80% for various NH3 concentrations (1 to 1000 ppm). From this work, one may challenge to research for a number of other composite materials and more complicated structures, based on the regulation of the meniscus geometry-dependent coffee-ring effect. Furthermore, individually addressable conductive polymer composite microtubes with a stretchable 3-D architecture could have possibility to application of a variety of sensors with high sensitivity and high stability to various environment conditions.
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