Guoqiang Zheng

Electrically conductive thermoplastic elastomer nanocomposites at ultralow graphene loading levels for strain sensor applications

An electrically conductive ultralow percolation threshold of 0.1 wt% graphene was observed in the thermoplastic polyurethane (TPU) nanocomposites. The homogeneously dispersed graphene effectively enhanced the mechanical properties of TPU significantly at a low graphene loading of 0.2 wt%. These nanocomposites were subjected to cyclic loading to investigate the influences of graphene loading, strain amplitude and strain rate on the strain sensing performances. The two dimensional graphene and the flexible TPU matrix were found to endow these nanocomposites with a wide range of strain sensitivity (gauge factor ranging from 0.78 for TPU with 0.6 wt% graphene at the strain rate of 0.1 min−1 to 17.7 for TPU with 0.2 wt% graphene at the strain rate of 0.3 min−1) and good sensing stability for different strain patterns. In addition, these nanocomposites demonstrated good recoverability and reproducibility after stabilization by cyclic loading. An analytical model based on tunneling theory was used to simulate the resistance response to strain under different strain rates. The change in the number of conductive pathways and tunneling distance under strain was responsible for the observed resistance-strain behaviors. This study provides guidelines for the fabrication of graphene based polymer strain sensors.

Lightweight conductive graphene/thermoplastic polyurethane foams with ultrahigh compressibility for piezoresistive sensing

Lightweight conductive porous graphene/thermoplastic polyurethane (TPU) foams with ultrahigh compressibility were successfully fabricated by using the thermal induced phase separation (TISP) technique. The density and porosity of the foams were calculated to be about 0.11 g cm−3 and 90% owing to the porous structure. Compared with pure TPU foams, the addition of graphene could effectively increase the thickness of the cell wall and hinder the formation of small holes, leading to a robust porous structure with excellent compression property. Meanwhile, the cell walls with small holes and a dendritic structure were observed due to the flexibility of graphene, endowing the foam with special positive piezoresistive behaviors and peculiar response patterns with a deflection point during the cyclic compression. This could effectively enhance the identifiability of external compression strain when used as piezoresistive sensors. In addition, larger compression sensitivity was achieved at a higher compression rate. Due to high porosity and good elasticity of TPU, the conductive foams demonstrated good compressibility and stable piezoresistive sensing signals at a strain of up to 90%. During the cyclic piezoresistive sensing test under different compression strains, the conductive foam exhibited good recoverability and reproducibility after the stabilization of cyclic loading. All these suggest that the fabricated conductive foam possesses great potential to be used as lightweight, flexible, highly sensitive, and stable piezoresistive sensors.

Continuously prepared highly conductive and stretchable SWNT/MWNT synergistically composited electrospun thermoplastic polyurethane yarns for wearable sensing

Highly conductive and stretchable yarns have attracted increasing attention due to their potential applications in wearable electronics. The integration of conductive yarns with large stretching capability renders the composite yarns with new intriguing functions, such as monitoring human body motion and health. However, simultaneously endowing the yarns with high conductivity and stretchability using an easily scalable approach is still a challenge. Here, highly conductive and stretchable yarns based on electrospun thermoplastic polyurethane (TPU) fiber yarns successively decorated with multi-walled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs) were prepared by a combined electrospinning, ultrasonication adsorbing, and bobbin winder technique. The improved thermal stability of the SWNT/MWNT/TPU yarn (SMTY) indicated strong interfacial interactions between the CNTs and electrospun TPU fibers. The synergism between the successively decorated SWNTs and MWNTs significantly enhanced the conductivity of the TPU yarns (up to 13 S cm−1). The as-fabricated yarns can be easily integrated into strain sensors and exhibit high stretchability with large workable strain range (100%) and good cyclic stability (2000 cycles). Moreover, such yarn can be attached to the human body or knitted into textiles to monitor joint motion, showing promising potential for wearable electronics, such as wearable strain sensors.

Organic vapor sensing behaviors of conductive thermoplastic polyurethane–graphene nanocomposites

Conductive thermoplastic polyurethane (TPU) nanocomposites filled with graphene were fabricated and tested for organic vapor sensing. The observed finely dispersed graphene in the TPU matrix benefited from the formation of efficient conductive paths and the generation of stable electrical signals. Organic vapor sensing behaviors of the conductive polymer composites (CPCs) were evaluated using four kinds of organic vapors possessing different polarities (p), including cyclohexane (p = 0.1), tetrachloromethane (CCl4, p = 1.6), ethylacetate (p = 4.3) and acetone (p = 5.4). Unlike conventional CPCs that only respond to certain specific groups of organic vapors, the current CPCs showed a novel negative vapor coefficient (NVC) effect for all tested vapors. This observed NVC was due to both the inherent microphase segregation structure of TPU containing soft and hard segments and the wrinkled structure of graphene. In successive immersion-drying runs (IDRs) at 30 °C, fast response, good reversibility and reproducibility were observed for the non- and low- polar vapors (cyclohexane and CCl4), but residual resistance was observed for polar organic vapors (ethylacetate and acetone) after their desorption. The temperature dependent vapor sensing behaviors indicated that the vapor sensing responsivity increased with increasing the temperature due to higher absorption activation energy at higher temperature. This study provides guidelines for the fabrication of organic vapor sensors using CPCs possessing fast response, good discrimination ability and reproducibility.

Comparative assessment of the strain-sensing behaviors of polylactic acid nanocomposites: reduced graphene oxide or carbon nanotubes

Reduced graphene oxide (RGO)/polylactic acid (PLA) and carbon nanotubes (CNTs)/PLA nanocomposites were prepared by ultrasound-assisted dispersion and a hot-pressing method for comparative studies. The RGO and CNT nanofillers, which had a hydrogen-bonding interaction with the PLA matrix, were uniformly dispersed in the PLA matrix, and low percolation thresholds (0.11 wt% for RGO/PLA and 0.80 wt% for the CNTs/PLA nanocomposites) were achieved in the PLA nanocomposites. The addition of RGO resulted in weak crystallization ability and significant enhancement in thermal stability of the PLA matrix owing to the two-dimensional characteristic of RGO, while opposite results were obtained for the CNTs nanocomposites (the degree of crystallinity Xc is 49.82% for 0.8 wt% CNTs and 9.21% for 0.8 wt% RGO, respectively). During ten extension–retraction cycles, the values of the max and min ΔR/R0 for the RGO/PLA nanocomposites shift upwards gradually with the increase of the cycle number, resulting from the breakdown of conductive networks caused by the slippage of overlapped RGO layers; while the values of the max and min ΔR/R0 for CNTs/PLA nanocomposites decrease gradually owing to the formation of a better conductive network caused by the rearrangement of CNTs. This study is meaningful for the application of conductive polymer composite based strain sensors in many fields, such as structural health monitoring, wearable electronic devices, soft robotics, etc.

Crystallization of poly(lactic acid) accelerated by cyclodextrin complex as nucleating agent

Poly(lactic acid) (PLA) is a well-known biodegradable and biocompatible polyester with intrinsically slow crystallization rate. To extend its applications to the field where heat resistance is required, increasing the crystallization rate of the material becomes critical. In this note, the nucleation effect of supramolecular inclusion complex (IC), organized by non-covalent interactions through threading α-cyclodextrin molecules onto PLA chains, on the crystallization of PLA was investigated by differential scanning calorimetry (DSC) and polarized optical microscopy. The formation of IC was confirmed by wide-angle X-ray diffraction and DSC measurements. It was found that the presence of PLA-IC significantly promoted the crystallization of PLA from both the non-isothermal and isothermal crystallization experiments. The nucleation mechanism was also discussed to some extent.

Piezoresistive behavior of porous carbon nanotube-thermoplastic polyurethane conductive nanocomposites with ultrahigh compressibility

Ultrahigh compressibility has been observed in the lightweight porous carbon nanotube (CNT)-thermoplastic polyurethane (TPU) nanocomposites prepared by the thermally induced phase separation (TIPS) technique. The porous structure has significantly reduced the density to approximately 0.1 g·cm−3. The nanocomposites prepared with a sonication time of 16 min and a filler content of 0.51 vol. % possess uniform CNT distribution and show the highest saturated electrical conductivity. Furthermore, the observed CNT-dependent cell structure changes indicate that the added CNTs favor the formation of thicker and stronger cell structure to enhance its reproductivity as a piezoresistive sensor. Piezoresistive behaviors were then conducted under stepwise and cyclic compression. The porous nanocomposites possess fast sensing capacity over a wide strain range (up to 90%). In addition, good piezoresistive recoverability and reproducibility were observed in the nanocomposites after stabilization by cyclic compression. This study provides a guideline for fabricating porous electrically conductivenanocomposites as promising candidates for the flexible, high sensitive, and stable piezoresistance sensors.

Wide distribution of shish-kebab structure and tensile property of micro-injection-molded isotactic polypropylene microparts: a comparative study with injection-molded macroparts

In this paper, injection-molded isotactic polypropylene (iPP) microparts and macroparts were respectively fabricated by the molds with different thickness under the same processing conditions. Comparative study on microstructure and mechanical property was carried out by means of scanning electron microscopy (SEM), two-dimensional wide-angle X-ray diffraction (2D-WAXD), two-dimensional small-angle X-ray scattering (2D-SAXS), differential scanning calorimetry (DSC) and tensile test. SEM images reveal that the two parts show distinctly different hierarchical structure. An obvious ‘skin–core’ structure is present for the macroparts, while a wide distribution of shish-kebab structure develops in both shear and core layer for microparts, exhibiting a specific ‘core-free’ morphology. 2D-WAXD, 2D-SAXS and DSC results show that microparts have higher orientation degree and crystallinity as compared to macroparts, which are responsible for the remarkably high tensile strength and modulus. Our work provides a good example for better understanding processing structure–property relationship of iPP through tuning their internal microstructure.

A tunable strain sensor based on a carbon nanotubes/electrospun polyamide 6 conductive nanofibrous network embedded into poly(vinyl alcohol) with self-diagnosis capabilities

A new carbon nanotubes (CNTs)/polyamide 6 (PA6)/poly(vinyl alcohol) (PVA) conductive composite was prepared by embedding a CNT wrapped electrospun PA6 nanofibrous network into a PVA matrix. For this composite, CNTs were employed to pre-construct the conductive network by decorating the electrospun PA6 network. The effects of CNT content and the number of CNTs-PA6 layers on the tensile properties of the composite were investigated. The composite consisting of two layers of CNTs-PA6 conductive nanofibrous networks possessed a prominent integrated performance and was applied to evaluate its strain sensing capability. The resistance change of the composite under quasi-static tensile loading was classified into four stages, involving different damage modes. The performance of cyclic tensile tests revealed that the composite exhibited distinguishing strain sensing characteristics towards different deformation levels, which contributed to identifying the damage status of the composites containing conductive nanofibrous networks. A better repeatability has been achieved after several elongation/contraction cycles or pre-stretching treatment. This study opens up new opportunities to develop a nanofibrous network based tunable strain sensor with self-diagnosis capabilities for damage detection.

Nonisothermal crystallization kinetics of biodegradable poly(lactic acid)/zinc phenylphosphonate composites

The effect of zinc phenylphosphonate on the crystallization of poly(lactic acid) using differential scanning calorimetry operating in dynamical mode at various cooling rates is reported. Experimental data were analyzed using the Avrami, Tobin, and Ozawa models. It is concluded that the addition of zinc phenylphosphonate modifies the crystallization process of poly(lactic acid) (changing the value of the Avrami exponent). Various parameters such as the crystallization half-time and crystallization rate constant reflect that zinc phenylphosphonate significantly accelerates the crystallization process. The activation energy value of the crystallization of poly(lactic acid), determined by the Kissinger method, increases with the addition of zinc phenylphosphonate.