Construction Of Polymer-based Thermal/Electrical Conductive Composites Based On Pickering Emulsion Interfacial Assembly Strategy

The development of the 5G era is associated with a high degree of integration and miniaturisation of electronic products,which inevitably leads to overheating and electromagnetic radiation pollution,affecting the performance and service life of electronic products and even threatening people’s health.Therefore,it is essential to develop high performance thermally conductive,electrically conductive and electromagnetic shielding materials to enhance the thermal and electromagnetic pollution protection of electronic products.Polymer-based thermally/electrically conductive composites can be prepared by introducing functional filler particles into the polymer matrix,which are widely used in thermal management and electromagnetic shielding of electronic devices due to their low density,easy processing,chemical stability,low cost,excellent mechanical properties and easy control of thermal and electrical conductivity.There are still significant challenges in the preparation and performance modulation of polymer-based thermally/electrically conductive composites.Firstly,functional fillers are difficult to disperse uniformly in the polymer matrix due to their large specific surface area or high aspect ratio,making it difficult to take advantage of their inherent advantages in terms of structure and properties.The main idea in the preparation of traditional polymer-based thermally/electrically conductive composites is to add functional filler particles directly to the polymer matrix by melt blending or solution blending to increase the overall thermal and electrical conductivity of the composites.Because of the lack of effective interconnection between fillers,a large amount of fillers need to be incorporated to form a continuous filler network for better thermal and electrical conductivity,resulting in high production costs with this approach and affecting the overall processability and mechanical properties of the composites.Therefore,how to achieve a satisfactory distribution and orientation of the fillers in the polymer matrix at a low filling amount,so as to construct an efficient thermal and electrical conductive network pathway to enhance the thermal management and electromagnetic shielding performance of the composite material is an urgent challenge to be solved.Pickering emulsion interfacial assembly strategy is an effective way to assist in the formation of continuous thermally conductive and electrically conductive networks in polymer matrix.However,most of the functional filler particles are difficult to assemble individually as Pickering emulsion stabilized particles at the oil-water interface,making this method limited.Regenerated cellulose(RC)with networked nanofiber structure is widely used as stabilized particles in Pickering emulsions because of its good amphiphilic properties.In this project,we obtained stable Pickering emulsions based on non-covalent bonding interactions between RC and functional filler particles with the assistance of RC’s excellent emulsification ability to achieve oriented assembly of functional filler particles at the oil-water interface,and polymer-based composites with excellent thermally conductive,electrically conductive and electromagnetic shielding properties were developed by combining hot pressing and melt blending techniques,the specific study of this thesis is as follows:Thermally conductive poly(lactic acid)(PLA)/BN composites were prepared using PLA as matrix and boron nitride(BN)as thermally conductive particles,incorporating Pickering emulsion interfacial assembly strategy and hot pressing technique,expected to be applied in the field of thermal management of electronic devices.A stable RC suspension was prepared using the phosphoric acid dissolution regeneration method and the microscopic morphology,chemical structure and crystallographic changes of the cellulose before and after regeneration were investigated.The hydrophobic-hydrophobic interactions between RC and BN were analysed,the necessity of RC incorporation for improving the wettability of BN in aqueous media and the directed assembly at the emulsions oil-water interface was investigated by comparative experiments,and we explored the formation mechanism of PLA/BN Pickering emulsions thoroughly.The effect of filler loading on the establishment and improvement of the threedimensional thermal conductive network of BN and the thermal conductivity was clarified by characterising the microscopic morphology of PLA/BN composites with different BN loadings.The obtained PLA/BN composites feature a thermal conductivity of 1.06 W/(m·K)at 28.4 wt%BN loading,representing an enhancement of 430% comparing to neat PLA.This work effectively verified the feasibility of RC-assisted stabilized Pickering emulsion-based interfacial assembly strategy for the preparation of functional polymer-based composites.We proposed a two-step dispersion strategy based on the Pickering emulsion templating method and masterbatch blending method to improve the dispersion of CNT via selecting RC/carbon nanotubes(CNT)as new emulsion-stabilized particles,and prepared a series of PLA/CNT conductive composites with different CNT loadings and applied them in the field of electromagnetic shielding.Molecular dynamics simulations were used to investigate the mechanism by which RC improves the surface wettability of CNT in aqueous media and the mechanism of pre-dispersion of CNT in PLA substrates assisted by the Pickering emulsion template method was analysed.The effects of different CNT dispersion states and loadings on the percolation threshold,conductivity,electromagnetic shielding and mechanical properties of the composites were systematically evaluated.The results showed that the hydrophobic-hydrophobic interaction between RC/CNT effectively improved the dispersion of CNT in water as well as assisted the CNT to assemble at the oil-water interface to further immobilize on the surface of PLA microspheres,achieving a pre-dispersion effect.Compared to the direct melt blending method,the masterbatch blending method enables the secondary dispersion of CNT in the PLA matrix,which is more favourable for the construction of a continuous three-dimensional CNT conductive network in the PLA matrix.The resulting PLA/CNT composite exhibited a percolation threshold of 0.46 vol% at 5.6 wt% CNT loading,with electrical conductivity and electromagnetic interference shielding effectiveness(EMI SE)of 72.2 S/m and 31.1 d B respectively.The two-step dispersion strategy provides an effective and versatile route to solve the dispersion problem of CNT and fabricate high performance conductive polymer composites.With the increase in assembly density and integration of electronic devices and the application environment becoming more complex,we selected acrylonitrile-butadiene-styrene copolymer(ABS)with excellent mechanical and processing properties as the matrix and continued to use RC-assisted stabilized Pickering emulsion as the design platform to prepare ABSbased composites with good thermal and electrical conductivity for thermal management and electromagnetic shielding applications.We investigated the effects of different graphene nano platelets(GNP)and CNT loadings on the morphology and particle size of the emulsions,characterised the dispersion of RC,GNP and CNT in Pickering emulsions to elucidate the synergistic assembly mechanism of RC,GNP and CNT at the oil/water interface.The microscopic morphology,thermal conductivity,electrical conductivity and electromagnetic shielding properties of ABS-based composites were characterised,and the progressive relationship between“filler loading-GNP/CNT synergistic network-thermally and electrically conductive properties"in the composites was investigated systematically,revealing the synergistic effect of GNP/CNT in improving the thermally and electrically conductive properties of the polymer.The results showed that the thermal conductivity of the composites was positively correlated with the filler content,achieving a thermal conductivity of 4.2 W/(m·K)when GNP and CNT were loaded at 27.8 wt%and 3 wt% respectively.With the formation of the “CNT-GNP” synergistic network,the addition of a small amount of CNT to the continuous GNP network greatly increases the electron transfer sites of the conductive network,resulting in a high electrical conductivity and EMI SE of 261.1S/m and 73.5 d B respectively under the above filler loadings.In addition,functional composite microspheres with different matrixes were fabricated by means of the Pickering emulsion interfacial assembly strategy,demonstrating the versatility of the method in the construction of functional composites.Compared with the rigid polymer-based conductive composites prepared above,natural rubber(NR)-based conductive composites show great potential for application in the field of electromagnetic shielding due to their good elasticity and mechanical properties.NR/CNT composites with excellent flexibility and electrical conductivity were fabricated using Pickering emulsions stabilized by a “ dual driver ” of hydrophobic-hydrophobic interactions and electrostatic interactions as a template.We compared the effects of different emulsification systems on the interfacial dispersion of CNT,emulsion morphology and stability,and evaluated the construction of CNT conductive network,electrical conductivity and mechanical properties within the composites.The results showed that the introduction of cetyltrimethylammonium bromide(CTAB)in the oil phase greatly enhanced the interfacial stability of the emulsion by electrostatic interaction with RC/CNT in the aqueous phase,and achieved directional assembly of RC/CNT at the oil-water interface,which was used as a template for the preparation of CNTswrapped NR-based conductive microspheres.After hot-compressing,the resultant NR/CNT composite film with three-dimensional CNT interconnection network exhibited a high electrical conductivity of 11.5 S/m with 4.59 vol% CNT loading.On the other hand,conductive inks with promising printing properties have been prepared based on the composite microspheres,which can be used in strain sensing applications when composite with nitrile rubber.The above results indicate that the RC/CTAB emulsification system used in this study has potential application to the preparation of functional filler particle-stabilized Pickering emulsions and the construction of flexible NR-based functional composites.