Preparation Of Flexible Strain Sensors Based On Interfacial Interaction Induced Multilayer Structure And Study On Their Mechanism

In recent years,with the rapid development of flexible electronic technology and materials,a variety of flexible sensors emerge one after another.Among them,flexible strain sensors（FSS）,which can convert mechanical deformation into electrical signals,have attracted wide attention and been used in wearable devices,human-machine interaction,health monitoring and other fields.So far,FSS still faces the following challenges:1)the sensitivity of FSS has reached the theoretical upper limit according to the reported mechanism;2)high sensitivity and high stretchability are difficult to achieve together.Based on this,this thesis takes inspiration from the crack structure of spider and the strong interfacial structure of mussel,and proposes a novel sensing mechanism:interfacial interaction induced multilayer structure（IIMS）.And we study its sensing mechanism and sensing performance in detail.Further,the theoretical calculation formula of IIMS-based FSS gauge factor（GF）are established.According to the theoretical calculation formula,IIMS-based FSS with better performance are prepared.IIMS-based FSS provides a new direction and method for FSS with high sensitivity and high stretchability.Flexible substrate with high-toughness or functional is constructed and applied to IIMS-based FSS.The specific research contents and main results are as follows:（1）IIMS-based FSS with high sensitivity and good stretchability was prepared,and its structure,sensing mechanism,sensing performance and application were studied in detail.The strong hydrogen bond interfacial interaction between the cross-linked epoxide natural rubber（ENR）base layer and the brittle conductive layer of carboxymethyl chitosan/single-walled carbon nanotubes（CMCS/SWCNT）results in the separation of the CMCS/SWCNT layer into the flexible transition layer near the interface and the brittle bulk layer far away from the interface.The cracks only exist in the brittle bulk layer while the flexible transition layer is always continuous.Under the strain,the propagation of micron-scale cracks in brittle bulk layer endows FSS with extremely high strain sensitivity（GF=1.5×10⁶）.The continuous flexible transition layer provides a stable conductive path for FSS,and avoids irreversible failure of the cracked structure when overstretched,which makes the stretchability of FSS exceed 60%,this novel sensing model is called IIMS-based FSS.IIMS-based FSS has great potential in the field of self-powered FSS because of its excellent sensing performance at low voltage（0.1V）.（2）The theoretical models of different stages of crack growth are established,and the calculation formula of IIMS-based FSS GF is derived.The crack propagation process of IIMS-based FSS is divided into three stages and simplified as circuit diagrams formed by series-parallel resistances.The calculation formula of GF value of IIMS-based FSS is derived according to the change of the resistance value and the series-parallel mode.This formula is not only applicable to IIMS-based FSS,but also to all cracked-FSS.The parameters of IIMS-based FSS,including the thickness of the CMCS transition layer,the thickness and the resistivity of the CMCS bulk layer,are adjusted to verify the theoretical calculation formula.Two key parameters（larger crack size and lower resistivity of the cracked layer）to prepare FSS with extremely high GF value are calculated by the theoretical formula.The theoretical formula is further modified to make it more practical.（3）The universality of IIMS-based FSS is verified.By changing the flexible base layer（carboxyl nitrile butadiene rubber with more functional groups）and the conductive filler（silver nanowires with larger length to diameter ratio and lower resistivity）,IIMS-based FSS is successfully prepared,which verifies the universal suitability of IIMS mechanism.In addition,IIMS-based FSS based on XNBR and Ag NWs shows higher strain sensitivity（7.11×10⁸）and better stretchability（>100%）,indicating that the theoretical calculation formula of IIMS-based FSS GF has a wonderful guiding role in the preparation of FSS with extremely high GF.（4）Polymer substrate with high toughness was prepared and applied to IIMS-based sensor.Polylactic acid/natural rubber thermoplastic vulcanizate（PLA/NR TPV）is used as the flexible base layer in IIMS-based FSS.By adjusting the dispersion of Si O₂ in the TPV through processing technology,some Si O₂ is evenly dispersed in the NR phase,which plays a reinforcing effect on the NR phase,some Si O₂ is dispersed at the interface of the two phases,which effectively improves the interface compatibility of the two phases.Both of them led to a good toughness（4.42×10⁴ N/m）of PLA/NR/Si O₂ TPV.FSS with high sensitivity and high toughness was obtained by using the TPV as the flexible base layer.（5）Polymer substrate with both self-healing and electromagnetic shielding properties is prepared and applied to IIMS-based sensor.PLA/ENR TPV is used as the flexible base layer in IIMS-based FSS,and the modified multiwall carbon nanotubes is introduced and anchored in the PLA phase to form a conductive network,which endows the PLA-based TPV with good electrical conductivity.The disulfide bond in the ENR phase endows TPVs with the self-healing ability with the aid of shape memory behavior.The TPV is foamed with the assist of supercritical CO₂,which shows good electromagnetic shielding properties.The foamed TPV with both self-healing and electromagnetic shielding properties is applied in IIMS-based FSS,effectively expanding the application range of IIMS-based FSS.