| Nylon,successfully synthesized in 1935,has been the one kind of bestselling engineering plastics due to its corrosion resistance,low cost,excellent mechanical properties,abrasive resistance and so on.And the nylon composites,equipped with higher strength,toughness and antistatic property,are widely used to prepare parts of engine,heat sink and electronic equipment.However,with the development of miniaturization and power density integration of engine and electronic equipment,it is necessary for their structural components to have superior heat dissipation capability.The extremely low thermal conductivity of nylon(0.25W/(m·K))fails to meet the needs of fast heat dissipation for the equipment.So,the study of thermally conductive nylon composites is of crucial importance.At present,thermally conductive nylon composites are mainly prepared by melting and solution mixing,and the forming of thermally conductive paths mainly depend on the increase of filler content,which leads to the less improvement of thermal conductivity((27)10 W/(m·K)).Furthermore,the interfacial interaction between fillers and nylon matrix is weak owing to the surface chemical inertness of fillers,causing many problems including interfacial defects,filler aggregation inside polymer and so on.And these problems make it very difficult to prepare nylon composites with excellent comprehensive properties.Aiming at these problems,it is the key to achieve obvious enhancement of thermal conductivity of composites by adding less fillers,meanwhile the mechanical properties including strength and toughness should be improved through enhancing the interfacial interactions between fillers and nylon and adjusting filler dispersion morphology.All of these have been the key to prepare thermally conductive nylon composites with excellent comprehensive properties.In this paper,the aim is to prepare thermally conductive nylon composites with excellent comprehensive performance,by the way of adjusting the interfacial interaction,processing-induced self-assembly,surface-induced situ synthesis and so on.Specially,the adjustment mechanism of microstructure of nylon composites has been systematically studied,and the relationship between structure and performance has been deeply explored.Firstly,we explored the effect of elastomer on the interfacial interaction between fillers and nylon matrix and the influence on the toughness of nylon composites.Followingly,the nylon composite films with laminate structure were prepared through layer by layer self-assembly.The filler dispersion inside composites and the related performance have been studied,and the mechanism of thermal conductivity and electromagnetic shielding has been illustrated based on the ordered arrangement of fillers.Finally,inspired by mussel bionics and the super-hydrophobicity of lotus leaf,the idea of in situ synthesis of multi-functional nano-silver coating on the surface of nylon gauze was proposed,and the synthesis process conditions and the regulation mechanism of the second component on the surface physicochemical properties of nylon composite films were systematically studied.The main research contents and results are as follows:(1)Inspired by metal welding effect,a new strategy was proposed to toughen the composite by taking advantage of elastomer to enhance interfacial interaction.Nylon was used as the matrix,carbon nanofibers(CNFs)were chosen as thermal conductive fillers,and Anhydride-modified ethylene-based copolymer(AEC)was regarded as the elastomer toughening agent.Interfacial interaction calculations indicate AEC exhibits strong interactions with both nylon and CNFs,and when the three components coexist,AEC tends to be distributed between nylon and CNFs.The micromorphological analysis confirms the CNFs in the composite are surrounded by AECs,significantly enhancing the interfacial adhesion between the CNFs and the nylon matrix,which could effectively prevent the CNFs from pulling out from the matrix during the fracture process.That is to say,the elastomer exerts"interface welding"effect inside the composite.When the ternary composite contains 30 wt%CNFs and 40 wt%AEC,its thermal conductivity is 1.63 W/(m·K),and its impact toughness is still as high as 40 k J/m~2,even though the sample is not completely broken.Owing to the fitting results of effective medium theory(EMT),the existence of AEC indeed reduces the interfacial thermal resistance between CNFs and nylon matrix,which proves that the"interface welding"effect of AEC effectively enhances the interfacial interaction between CNFs and nylon from the perspective of theory.(2)Commercialized nylon mesh was firstly used as the matrix and the composite with laminate structure was prepared by layer-by-layer self-assembly,achieving the purpose of order arrangement of fillers and directional thermal conductivity.It is found that graphene oxide(GO)can effectively promote the dispersion of graphene nanoplatelets(GNPs)in deionized water and GNPs tend to orient and align closely on nylon mesh under pressure,eventually the nylon composite films with lamellar structure has been prepared successfully.Due to the forming of thermally conductive network ordinating from GNPs orientation along the in-plane direction,the in-plane thermal conductivity of the composite is significantly improved.When the GNPs content is 11.8 wt%,the in-plane thermal conductivity is as high as 15.8 W/(m·K).Due to the barrier of the dense nylon film layer,the heat transfer efficiency of the composite film decreases significantly along out-plane direction,and the out-plane thermal conductivity is only 0.63 W/(m K)when the GNPs content is 11.8 wt%.In a word,the composite film exhibits anisotropic thermal conductivity.The COMSOL multiphysics simulation software was used to successfully simulate the heat transfer process of chip cooling,the simulation results confirm the excellent heat dissipation effect of this composite,which can achieve the effect of ideal contact between the chip and the heat sink.In addition,the nylon composite exhibits superior electromagnetic interference(EMI)shielding performance(58.1 d B)in the X-band(8.2–12.4 GHz),which is mainly attributed to the absorption loss inside the composite.It is worth noting that the hydrophobicity and flame retardancy are also improved obviously,which provides new opportunities for the design of multifunctional nylon composites and their functionalized applications.(3)On the basis of the research in the previous chapter,the regulation mechanism of the microstructure on the properties of the composite films was further discussed.In this chapter,it is found that cellulose nanofibers(Ce NFs)can effectively achieve the uniform dispersion of GNPs in DI water and improve the quality of the composite films,due to the characteristic of high aspect ratio and the existence of a large number of polar groups of Ce NFs.Based on the micro-morphological characterization,the composite shows the typical lamellar structure,and the GNPs are highly oriented along the in-plane direction and form the dense thermally conductive paths.The performance test results show that the thermal conductivity of the composite is highly anisotropic.When the GNPs content is 14.6 wt%,the in-plane thermal conductivity of the composite is as high as 16.0 W/(m·K),while the out-plane thermal conductivity is only 0.29 W/(m·K),which is close to the thermal conductivity of pure nylon(0.23 W/(m·K)).This phenomenon is attributed to the complete barrier of the nylon layer for the overlap of GNPs along the out-plane direction,which proves that there is no embedded GNPs inside nylon layer and further reflect the structural integrity of nylon layer.Based on the structure integrity of nylon layer inside the composite,the nylon composite exhibits superior tensile strength(48.3 MPa)and excellent elongation at break(34.2%)and toughness(13.15 MJ/m~3),endowing the thermally conductive nylon composite with the integration of strength and toughness.According to the COMSOL multiphysics simulation results,the composite can be used as the heat spreader of the device and exerts excellent heat dissipation effect.This composite film with excellent comprehensive properties such as high strength,high toughness,and highly anisotropic thermal conductivity exerts broad application prospects in the fields of personal wearable electronic devices and electric heating.(4)Due to the existence of a large number of polar groups,nylon usually shows strong hygroscopicity.Nylon’s hygroscopicity is always the bottleneck for its application.On the one hand,the hygroscopicity leads to the reduction of the dimensional stability of nylon products,and on the other hand,it also seriously affects the macroscopic properties of the products,such as mechanical properties,insulation and thermal conductivity.Inspired by the adhesion properties of mussels and the super-hydrophobic properties of the lotus leaf surface,a new method was proposed to modify the fiber surface with nano-silver coating in order to improve the thermal conductivity and hydrophobicity of the composite film.The study finds that the modification of polydopamine(PDA)on the nylon fiber surface is beneficial for the dense deposition of Ag and form the nano-silver coating,resulting in the increase of roughness.Moreover,the introduction of polydimethylsiloxane(PDMS),as a kind of low surface energy material,did not change the microscopic morphology of the silver layer and contributes to the further improvement of the hydrophobicity.The performance test results show that the thermal conductivity of the composite is positively correlated with the Ag loading.When the Ag content increases to 10.8 vol%,the out-plane thermal conductivity of the composite increases by 259%,while the in-plane thermal conductivity increases by 10226%.Due to the increase of the surface roughness and the introduction of PDMS,the water contact angle of the composite material increases steadily to 140°.Consequently,it is important for the composite film with excellent thermal conductivity and hydrophobicity to expand its application under extreme environment. |
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