Polyurethane Based Flexible Conductive Fibers And Their Vapor Sensing Properties

As the growing of electronic industry, conductive fibers which combine the flexibility and weavability of traditional fibers and the sensing properties of functional materials, have been applied in many fields such as antictatic, electromagnetic shielding, smart clothing, sensors and super capacitor. The methods for preparing organic conductive fibers include direct spinning, blending spinning and post-processing. Among them, the post-processing method has been received especially attention as its easy operation and high conductivity of the resulted fibers. Unfortunately, most fibers prepared by post-processing method exhibit poor mechanical performances, which would limit their practical application. In this work, a series of polyurethane (PU) based conductive fibers covered with the polyaniline (PANI), polypyrrole (PPy) and grapheme sheets (GS) were fabricated, and more attentions were paid to their sensing properties. The results and conclusion are listed as follows:(1) Vapor sensing properties of PANI/PU composite fibers:Conductive PANI/PU fibers were fabricated by nanocoating PANI layers on the surface of traditional PU fibers through in-situ chemical polymerization. The thickness of the conductive layer can be controlled by adjusting the concentration of aniline monomer, and the maximum value of the conductivity was about 0.023 (Ω·cm)-1. The composite fibers exhibited high sensitivity (4×10 ppm-1), large saturation response (1.35), fast response time (less then 30 s) and low detection limit (150 ppm) for chloroform gas detection. It is further found that compared with that for larger-diameter composite fibers, higher responsiveness can be achieved by using smaller diameter fibers. Moreover, the stability would be improved with a little decrease in responsiveness by increasing the thickness of conductive layer. The remarkable sensing properties for the composite fibers to chloroform vapor can be benefited from both the conformational change of the PANI chains and the solvent-swelling behavior of PU matrix.(2) Effects of preparation methods on the sensing properties for PPy/PU composite fibers:PPy/PU fibers were generally prepared through one-step in-situ chemical polymerization of pyrrole monomers on the surface of traditional PU fibers. A modified approach is applied by carrying out the adsorption and polymerization for pyrrole monomers in different systems. More smooth and stable conductive layers can be achieved on the surface of PU fibers. More importantly, high elasticity and enhanced sensing properties can be obtained. As a strain sensor, the sensitive coefficient value is about 20 under the strain of 50%, which is 10 times larger than that for the composite fibers prepared in one system. As a sensor for chloroform vapor detection, saturation response of 0.36, fast response time of 36 s and low detection limit of 150 ppm can be obtained.(3) Preparation and sensing properties of GS/PU and GS/PANI/PU composite fibers:The adsorption of GS on PU fiber surface can be achieved through the strong interaction between the GS and aniline monomers absorbed on the surface of PU fibers. Two kinds of GS/PU and GS/PANI/PU fibers were fabricated through the removal of aniline monomers on the surface of the composite fibers and in-situ chemical polymerization, respectively. Compared with those for GS/PU fiber, the conductivity for GS/PANI/PU increased from 7.8×10-3 to 1.2×10=2 (Ω·cm)-1, but the sensitive coefficient decreased from 10.8 to 6.2 under the strain of 50%. All these composite fibers exhibited high response and good recovery for chloroform vapor detection. Compared with those for GS/PU fiber, the saturation response for GS/PANI/PU decreased from 0.89 to 0.3, whereas the response time increased from 50 to 54 s. The high response for GS/PU fibers can be resulted from large damage on the conductive layer during the swelling of the PU matrix at similar chloroform vapor concentration, but with unstable baseline drift.