Preparation Of Multidimensional Stress Flexible Sensing Material By Hipe Interface Film Formation Method

The porous-structure monolith has the advantages of high porosity,light weight,air permeability and resistance to compression deformation,which is one of the ideal materials for preparing flexible stress sensors currently.As far as its preparation method is concerned,since the high internal phase emulsion polymerization method can control the pore structure of the monolith,thereby optimizing and improving the sensing performance of the device,it has priority over the foaming method and the commercial template method.However,during high internal phase emulsion polymerization,in order to maintain the stability of the polymerization reaction system,high-quality crosslinking monomers（>10%）were added to the formulation,which makes the monolith produced can only undergo compression deformation（lose the deformation of bending and torsion）,it is difficult to meet the requirements of flexible sensors for detecting multi-dimensional stress such as bending and torsion of monoliths.In this paper,hydrophilic PU particles are used as Pickering particles to stabilize the"cyclohexane/water（O/W）"high internal phase emulsion.During the freeze-drying process of the emulsion,since the glass transition temperature of PU is much lower than the freeze-drying temperature,PU can form a film to form a pore wall with a certain mechanical strength,which improves the stability of the emulsion during the freeze-drying process and avoids the dependence of emulsion stability on high crosslinking agent content in high internal phase emulsion polymerization,and a flexible sensor material that can detect multi-dimensional stress is obtained.（1）.The modification of cetyltrimethylammonium bromide（CTAB）significantly changes the amphiphilicity（hydrophilic and lipophilic）of graphene oxide（GO）,which enhances the interfacial activity of GO modified by CTAB（CGO）,and reduces the interfacial tension of"cyclohexane/water"greatly.Studies have shown that when CTAB/GO=0.8,the"cyclohexane/water"interfacial tension drops to 20 m N/m;at the"cyclohexane/water"interface,CGO with high interfacial activity can compete with PU particles for adsorption,and attached to the inner wall of the porous material cavity formed by freeze-drying,which improves the electrical conductivity of the material significantly.（2）.After freeze-drying,the cyclohexane in the internal phase and the water in the external phase are removed at the same time,the porous material with low density and high porosity was formed.By reducing the content of PU particles or increasing the volume fraction of the internal phase,the density can be further reduced,and increased the porosity and the pore size significantly.The density,porosity and pore size of the porous composite material can be adjusted between 14-50 mg/cm³,95.01-98.54%and 50-200μm,respectively（3）.The prepared porous composite material has the ability to withstand the multidimensional stress of compression,bending and torsion,and as the pore structure of the material changes,the density and porosity of the material change accordingly.The prepared porous composite materials can withstand up to 95%compression deformation,and when the density of the material decreases from 40.11 mg/cm³（P20I80）to 19.94 mg/cm³（P10I80）,its compressive strength decreases from680 kPa to 180 kPa.Meanwhile,as the density decreases,the stress response interval of porous composites shows a trend of shrinking but increasing sensitivity.Taking P20I80（0-680 kPa）and P10I80（0-180 kPa）as examples,the sensitivity of the material has increased from 0.013 kPa^-1（400-680 kPa）of P20I80 to 1.02 kPa^-1（<4 kPa）of P10I80.In addition,the prepared porous composite material（P10I80）also has the ability to detect multi-dimensional stress response with bending curvature between 0.008 mm^-1 and 0.3 mm^-1 and twisting between 0 and 270°.The experiment has good repeatability and reproducibility,and the prepared flexible sensing material has excellent response stability and durability.（4）.The sensor device assembled from P10I80 material has the ability to detect stress as low as 0.7 Pa,fast response and recovery characteristics of≤60 ms,and the ability to detect bending and torsional deformation.The sensor（P10I80）can distinguish human neck pulse,vocal cord/throat muscle movement differences,elbow bending during exercise,and fingertip stress changes when grasping heavy objects;and it can be used to record human standing and walking（P20I80）,guidance Squat（P20I80）and response spatial stress distribution（4X4 array）scene.