Structure Regulation And Sensing Performance Research Of Flexible Conductive Polymer Composites

Conductive polymer composites（CPC）,as a new type of functional material,have been widely studied for their light weight,ease of molding,low cost,easily adjustable performance,and flexibility,etc.The electrical properties of CPC can produce rich response behaviors under strain.It can be widely used as a strain sensor in medical monitoring,sports science,human-computer interaction,electronic skin and other fields,showing broad application prospects in wearable flexible electronic devices.However,the current CPC-based flexible strain sensor（FSS）generally suffers from low sensitivity,small sensing range,lagging response,poor stability and durability,etc.In addition,these sensing properties are mutually constrained and difficult to balance,and the sensing function is relatively single,which seriously hinders its popular application in the field of wearable flexible electronics.Therefore,the development of CPC-based flexible sensors with excellent comprehensive performance remains a great challenge.In this paper,through the structural design of CPC and the construction of conductive network,the comprehensive performance of CPC-based flexible strain sensor is enhanced and regulated.The internal relationship between CPC and strain sensing response behavior and sensing performance is systematically discussed.The sensing mechanism is revealed,and the preparation of high-performance CPC-based FSS is realized,and the practical application of the prepared flexible sensor in human motion monitoring,physiological health detection,expression and speech recognition is explored.Firstly,the embedded conductive network structure is designed to solve the problems of insufficient flexibility,low sensitivity and poor stability of the sensor,but the sensor cannot identify the strain direction.Based on this,an asymmetric multi-oriented conductive network structure is proposed to realize the simultaneous detection and recognition of the corresponding variables and strain directions of the sensor.Then,to meet the requirements of green development in the future,a self-driven flexible sensing microsystem is developed,which lays a foundation for the preparation of sustainable intelligent sensor devices.Finally,a composite structure combining gradient distribution conductive network and medium-sized dome array is designed,which improves the accurate detection ability of the sensor to compressive strain and expands the sensor’s application field.The specific research results are as follows.（1）Based on the anchoring principle,the design of embedded conductive network structure is proposed to achieve the strong combination of conductive filler and flexible matrix,which solves the problems of instability of conductive functional layer and lack of flexibility and elasticity of FSS.The flexible polyurethane/silver nanowire（PU/Ag NW）composite fiber with embedded conductive network structure was prepared by using capillary glass tube molding method.Low content of Ag NW conductive filler is evenly embedded in the surface of the flexible PU matrix and formed a perfect conductive network.The unique embedded conductive network structure do not destroy the original flexibility and stretchability of PU matrix,giving the composite fiber both good electrical conductivity（3.1 S/cm）,high elasticity（266%elongation at break）and excellent cyclic stability and durability（2500 strain cycles）.The simultaneous strain of Ag NW conductive functional layer and flexible PU matrix significantly improves the sensitivity（Gauge factor（GF）of 3051 at 39%strain）and fast response time（49 ms）of PU/Ag NW composite fiber.The destruction and reconstruction of Ag NW conductive network during strain results in the regular resistance change of PU/Ag NW composite fiber,presenting a rich sensing response behavior.As a flexible wearable strain sensor,PU/Ag NW composite fiber is applied to full-range human motion monitoring,showing great application potential in flexible electronics,intelligent wearable devices,etc.（2）The design strategy of constructing asymmetric multi-oriented conductive network structure is proposed to realize the independent response of CPC-based FSS to multi-dimensional strain stimuli,which solves the problem that FSS cannot identify the strain direction.A flexible thermoplastic polyurethane/Carbon-Silver nanowire（TPU/C-Ag NW）strain sensor with an asymmetric orthogonal conductive network was prepared by screen printing process.The detection of anisotropic response of strain stimulation in different dimensions is achieved by taking advantage of the significant change in the electrical response of the sensor with the strain direction.The ability of the sensor to accurately detect and identify the corresponding variable and direction of strain is improved by using two sets of electrodes to collect the sensing signal simultaneously.The crack structure generated by the conductive network under strain endows the sensor with extremely high sensitivity（GF value of 9032.3）.As a dynamic conductive bridge connecting cracks,Ag NW gives the sensing stability（5000 strain cycles）and a wide response range（728.3%）.Integration of the prepared flexible strain sensor with a wireless data transmitter module shows promising applications in personal health monitoring and telemedicine as a wireless wearable device for comprehensive detection of multi-dimensional human joint activities（wrist bending and twisting）,expression changes,swallowing,pulse and respiration,etc.（3）To meet the requirements of future self-driven sensing development,a wearable flexible sensing micro-system integrating power supply unit,energy storage unit and sensing unit is designed,which simultaneously realizes the functions of capacity,energy storage and multi-dimensional strain sensing,and solves the strong dependence of sensors on external power supply,showing great convenience and practical application potential in sustainable intelligent sensor devices and wearable electronic devices.Triboelectric nanogenerator（TENG）,flexible solid-state supercapacitors（FSSC）and multifunctional flexible strain sensors were prepared by using multifunctional flexible C-Ag NW/TPU conductive films as substrates,and they were integrated into a self-powered,multi-modal,flexible sensing micro-system.In this system,the TENG can generate an open-circuit voltage of 12.5 V and a short-circuit current of 18.4μA（at 4 Hz）,and can be used as a green energy supply device to provide stable power output to external circuits.FSSC has stable electrochemical performance in the voltage window of 0～0.8 V.Its area specific capacitance is 3.44 m F/cm² at a current density of 50μA/cm² and its volume energy density reaches 0.31 m Wh/cm³ when the power density is 20 m W/cm³.Multiple FSSC devices can change the output capacitance and voltage in series to achieve dual capacitance enhancement effect.FSSC can store the electric energy generated by TENG and supply uninterrupted power to external loads.The C-Ag NW/TPU strain sensor with excellent comprehensive performance can be used as a wireless wearable device for various human motion detection,speech recognition and physiological signal monitoring.This flexible sensing micro-system provides an important technological reserve for manufacturing green and sustainable flexible electronic devices.（4）Based on the synergistic effect of multiple sensor mechanism,a design strategy combining gradient-distributed conductive network and medium-sized dome array structure is proposed to enhance the response signal intensity of CPC-based FSS to compressive strain stimulation,which solves the contradiction of difficult balance between the sensitivity and the sensing range of flexible sensor.The PU/Ag NW pressure sensor constructed by a mesoscale PU dome array embedded with a gradient-distributed Ag NW network on the surface was prepared by glass template method and surface coating method.The evolution of the dome array and the conductive network in the strain stage of the sensor is described.The sensing response mechanism between the resistance-strain response behavior and the’crack effect’and’contact effect’of the conductive network is revealed.The gradient-distributed Ag NW conductive network ensures the high sensitivity(6.258 k Pa^-1)in the low pressure range（0-40Pa）and effective electrical response output in the high pressure range.The mesoscale flexible PU dome structure gives the sensor a wider response range（0-204.7 k Pa）.The novel composite structure design enables the prepared flexible sensor to achieve ultra-low detection limit（as low as 10 Pa）,an ultra-fast sensing response（20 ms）,excellent reliability and durability（2300 strain cycles）.Based on the above excellent comprehensive performance,the PU/Ag NW pressure sensor can be applied to multiple types of human motion monitoring,such as slight expression movements,fist clenching and relatively intense joint bending,walking and other human activities.