Mechanism Of The Continuous Spatial Confining Network Assembly Method For Preparation Of High Performance Polymeric Electrically/thermally Conductive Composite

Electrically and thermally conductive composite polymer materials have a wide range of application requirements in the fields of flexible electronics,biomedicine,soft robotics,energy storage and electromagnetic shielding.Limited by the chemical structure of the material,most polymers exhibit electrical insulation and low thermal conductivity.Filling conductive and thermally conductive fillers into polymers to prepare composite materials is a common method to improve the electrical and thermal conductivity of polymers.However,in order to obtain good electrical and thermal conductivity,it is necessary to add a large number of fillers,which will cause a significant decline in the mechanical properties and processing properties of composite materials.Appropriate processing technologies are effective strategies to improve the electrical and thermal conductivity of composite materials at lower fillers loading by forming continuous filler conductive networks such as self-assembled networks,isolated networks,and oriented networks.It is worth noting that with the integration and enlargement of electronic devices,there is also a urgent demand for continuous large-area preparation of high-performance polymer electrically conductive and thermal conductive composite materials.Several years ago,our team proposed a processing method named Spatial Confining Forced Network Assembly(SCFNA)method to produce high-performance conductive/thermal composite materials.The method achieved compaction of the packing network through the positive superposition of the external force field and the packing self-assembly force.The conductive/thermal conductivity of the composite material is greatly improved,and the thermal conductivity of the composite material can be improved by several times and the electrical conductivity can be improved by2 to 4 orders of magnitude without increasing the content of the filler.However,the applicability of SCFNA method is limited because it can only prepare conductive/thermal composites in batches.Based on this,the method named Continuous Spatial Confining Force Network Assembly(CSNA)is proposed from the process flow and structure design.The continuous largearea preparation of polymer-based conductive and heat-conducting composites can be realized simply and efficiently.At the same time,due to the forced assembly effect of "network compaction" and "filler orientation" reinforcing effect,the electrical conductivity and thermal conductivity of the composites can reach or even exceed that of the composites prepared by SCFNA method.Using short carbon fiber(SCF),carbon black(CB),and carbon grease as conductive fillers,hexagonal boron nitride(h-BN),and multi-walled carbon nanotube(MWCNT)as thermally conductive fillers,a series of electrically conductive and thermally conductive polymer composite were prepared by CSNA method,and their electrical/thermal conductivity reached or exceeded properties of composites fabricated by SCFNA method.This thesis systematically explored the continuous strengthening mechanism of the CSNA method,and obtains the influence of process parameters on the electrical conductivity and thermal conductivity of composite materials.The electrically and thermally conductive polymer composites prepared by the CSNA method show good comprehensive properties and have been used in flexible sensors,soft robotics,and thermal interface materials.The main findings of this thesis were as follows:1.Based on the SCFNA method,the CSNA method with continuous preparation of forced conductive networks was proposed: the flexible roller was used to carry out single or multiple surface contact continuous rolling compression on the composite substrate to realize the continuous forced assembly filler networks.The essence of this method was the large deformation of the flexible roller forms a “similar plate compression” on the composite material substrate.The continuous rolling of the flexible roller makes the composite substrate subject to the plane squeezing effect similar to the SCFNA method at every moment,thus realizing continuous force assembly of filler networks.The mathematical model of the CSNA process was established,and the influence of the rolling pressure P,rolling compression area S,and holding time t on the force assembly effect of "network densification" and "filler orientation" reinforcing effect were further studied,and relevant regularity results were obtained.2.Using the polydimethylsiloxane(PDMS)as matrix and SCF as conductive filler,the PDMS/SCF electrically conductive composites were prepared by the CSNA method.The relationship between the rolling times process parameters in the CSNA method and the electrical conductivity of the composites was analyzed.When the rolling number was 20,the percolation threshold of PDMS/SCF conductive composites prepared by the CSNA method was about 0.1 wt%.When the content of SCF is increased to 4 wt%,the electrical conductivity of the composite can achieve 49.69 S/m.The electrical conductivity of the composites reached the electrical conductivity level of the same composites prepared by SCFNA,which was 2-4 orders of magnitude higher than that of the composite material prepared by the blending method.The “network compaction” force assembly effect and “filler orientation” reinforcing effect of CSNA on the composites were verified by scanning electron microscope and optical micrograph.A modified Weber and Kamal model successfully predicted the trend of electrical conductivity of the composites.Combining the characteristics of continuous preparation of the CSNA method,the PDMS/SCF electrically conductive composites with different sizes were prepared according to the application occasions and were successfully used in preparing and applying capacitive flexible pressure sensors.The feasibility and practicability of the CSNA method in the continuous preparation of high-performance polymeric electrically conductive composites were verified.3.Using silicon as the polymeric matrix,carbon black and carbon grease as conductive fillers,the flexible electrodes with good comprehensive properties and smooth surfaces were prepared by the CSNA method and were used to construct the dielectric elastomer actuators.The selected electrode has an elongation at break of 256.1%,the tensile stress of 0.8 MPa,electrical conductivity of 0.52 S/m,the average surface roughness Ra of 12.55 nm,and a water surface contact angle of 104.9°,which solve the balance between the electrical conductivity and flexibility of electrodes.On this basis,combined with the vacuum lamination process,multilayer dielectric elastomer actuators with a detachable and reconfigurable structure were prepared.Due to the flexibility of the electrodes,the actuation performances of the dielectric elastomer actuator prepared by the new process were similar to that of the actuator prepared by the traditional brush coating process,and the electrodes also exhibited recyclable and reconfigurable characteristics.The spring actuator prepared based on the detachable and reconfigurable multilayer dielectric elastomer actuator has been successfully used to prepare lower limb auxiliary devices with the function of knee flexion and extension.The utility and reliability of the CSNA method in fabricating polymeric electrically conductive composites have been further validated.4.Using PDMS as the polymer matrix and h-BN and MWCNT as thermally conductive fillers,the PDMS/h-BN/MWCNT thermally conductive composites were prepared by CSNA method.The “network compaction”effect of CSNA on the composites and the synergistic effect among fillers was verified by SEM micrograph.The effect of film thickness on the thermal conductivity of the composites was studied.It was found that the thermal conductivity of the composites increased with the increase of compressive ratio of the samples,and there was a characteristic thickness.The thermal conductivity of the composites will increase faster when their thicknesses were smaller than the characteristic thickness,which is related to the degree of fillers assembly.When the filler contents of h-BN and MWCNT were 30 wt%and 2 wt%,respectively,the in-plane thermal conductivity of PDMS/hBN/MWCNT can reach 4.28 W m-1 K-1,and the thermal conductivity enhancement factor was 1543.5% comparing with pure PDMS matrix.The thermal conductivity of these composites reached the thermal conductivity of the composites prepared by the SCFNA method,which is about ten times higher than that of the composite material prepared by the traditional blending method.The composite has been applied to the heat dissipation of LED chips and batteries.The feasibility and potential use of the CSNA method in preparing polymeric thermally conductive composite have been tested.This research provided new sight into the continuous preparation of large-scale,high-performance polymeric electrically and thermally conductive composites.