Piezoresistive Fibers with Large Working Factors for Strain Sensing Applications.

Piezoresistive fibers with large working factors remain of great interest for strain sensing applications involving large strains, yet difficult to achieve. Here, we produced strain-sensitive fibers with large working factors by dip-coating nanocomposite piezoresistive inks on surface-modified polyether block amide (PEBA) fibers. Surface modification of neat PEBA fibers was carried out with polydopamine (PDA) while nanocomposite conductive inks consisted of styrene-ethylene-butylene-styrene (SEBS) elastomer and carbon black (CB). As such, the deposition of piezoresistive coatings was enabled through nonconventional hydrogen-bonding interactions. The resultant fibers demonstrated well-defined piezoresistive linear relationships, which increased with CB filler loading in SEBS. In addition, gauge factors decreased with increasing CB mass fractions from ∼15 to ∼7. Furthermore, we used the fatigue theory to predict the endurance limit (Ce) of our fibers toward resistance signal stability. Such a piezoresistive performance allowed us to explore the application of our fibers as strain sensors for monitoring the movement of finger joints.

Lignin-based carbon fibers: Insight into structural evolution from lignin pretreatment, fiber forming, to pre-oxidation and carbonization.

Lignin remains the second abundant source of renewable carbon with an aromatic structure. However, most of the lignin is burnt directly for power generation, with an effective utilization rate of <2 %, making value addition on lignin an urgent requirement. From this perspective, preparation of lignin-based carbon fibers has been widely studied as an effective way to increase value addition on lignin. However, lignin species are diverse and complex in structure, and the pathway that enables changes in lignin structure during pretreatment, fiber formation, stabilization, and carbonization is still uncertain. In this review, we condense the common structural evolution route from the previous studies, which can serve as a guide towards engineered lignin carbon fibers with high performance properties.

Electromechanical Performance of Strain Sensors Based on Viscoelastic Conductive Composite Polymer Fibers.

Flexible conductive polymer composite (CPC) fibers that show large changes in resistance with deformation have recently gained much attention as strain-sensing components for future wearable electronics. However, the electrical resistance of these materials decays with time during dynamic cyclic loading, a deformation performed to simulate their real application as strain sensors. Despite the extensive research on CPC fibers, the mechanism leading to this decay in the electromechanical response under repetitive cycles remains unreported. Herein, this behavior is investigated using fiber-based strain sensors wet spun from thermoplastic polyurethane (TPU) consisting of a carbonaceous hybrid conductive filler system of carbon black (CB) and carbon nanotubes (CNTs). We found electrical viscosity to predict the observed electromechanical resistance decay. This implies that cycling these materials enables the relaxation of both the polymer chains and the conductive network. In addition, the resulting piezoresistive fibers are sensitive to deformation in the region of low strain (gauge factor of 6.0 within 3.0% strain), remain conductive under 280.5% deformation, and are stable for more than 2000 cycles. Finally, we demonstrate the potential of TPU/CB-CNT fibers as strain sensors for monitoring human motion.

Effect of pre-oxidation temperature and heating rate on the microstructure of lignin carbon fibers.

Lignin is a biopolymer with high carbon content, making lignin-based carbon fiber an important research direction. In the process of carbonization to prepare carbon fibers, lignin fibers are easily softened and fused, which destroys the microstructure of fibers, thereby reducing the quality of lignin-based carbon fibers. Therefore, it is non-negligible to pre-oxidize lignin fibers before carbonization to prevent fiber fusion and maintain fiber structure. Therefore, the effects of pre-oxidation temperature and heating rate on the structure of pre-oxidation lignin fibers with controllable diameter and thickness prepared by melt-blowing were studied in detail. During pre-oxidation, crosslinking and aromatization of lignin fibers occurred, and alkyl and benzene rings were mainly oxidized to form carbonyl groups. The aromatization degree of the pre-oxidized product was recorded at 280 °C and 0.25 °C/min, and the oxygen content reached 15 %-20 %, making it suitable for the preparation of bio-based carbon fibers. On this basis, carbon fibers with porous morphology can be prepared with a graphitization of 0.54 and a resistivity of 0.02 Ω cm-1. These materials are expected to be applicable in sensors, catalytic materials and other fields.

Kraft lignin-based piezoresistive sensors: Effect of chemical structure on the microstructure of ultrathin carbon fibers.

In an attempt to diversify lignin applications, in this study, electrospun mats composed of ultrathin carbon fibers from Eucalyptus globulus Lignin (EKL) and sugarcane bagasse Kraft lignin (BKL) capable of detecting certain human body motions were fabricated and their chemical structures fully investigated accordingly. Results suggested that the main chemical structure of EKL was slightly linear with sinapyl alcohol and coniferyl alcohol as the basic units and pinoresinol, arylglycerol-ß-aryl ether, phenylcoumaran, with diphenylethane as the main linkages. The slightly linear EKL and cross-linked BKL tended to form amorphous carbon and graphite crystallites under the same heat treatment process respectively, thereby resulting in a large difference in the resistance of fabricated sensors. The amplitude of signals due to changes in the relative resistance (△R/R0) for EKL ultrathin carbon fiber (CF)-based sensor was approximately 9 × 104 during the finger bending process, while large (△R/R0 ~380) and small (△R/R0 < 10) relative resistance variations due to BKL-CF-based sensors were detected during the arm bending and finger pressing motions respectively.

Tuning the microstructure and electrochemical behavior of lignin-based ultrafine carbon fibers via hydrogen-bonding interaction.

Hardwood Kraft lignin (HKL)-based ultrafine carbon fibers with different pore structures and properties were prepared by controlling the intermolecular interaction between HKL and incorporated poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG) triblock copolymer. The thermal properties of HKL-based ultrafine fibers together with the morphology and pore structures of HKL-based ultrafine carbon fibers were extensively investigated with DSC, TG, SEM, BET, DLS and HRTEM to provide comprehensive knowledge on the effect of added PEG-PPG-PEG on the properties of obtained fibers. Results suggested that addition of PEG-PPG-PEG increased the thermal stability of HKL and promoted the formation of graphite crystallites in HKL-based ultrafine carbon fibers via enhanced intermolecular hydrogen bonding interactions. The electrochemical behavior of HKL-based ultrafine carbon fibers with different PEG-PPG-PEG contents were also characterized to expand their potential application in electrochemical capacitors. All the HKL ultrafine carbon fibers-based electrodes showed good capacitive behavior and stability. Besides, the specific capacitance of HKL-based ultrafine carbon fibers can be significantly tuned by the addition of PEG-PPG-PEG.