Preparation And Properties Of PEEK Thermally Conductive Composites Based On Segregated Network Structure

Poly（ether ether ketone）（PEEK）,as a semi-crystalline special engineering plastic,has been widely used in civilian fields and the national defense industry such as aerospace,electrical and electronic vehicles,precision machinery,and medical devices,because of its excellent high-temperature resistance,anti-aging,creep resistance,corrosion resistance,irradiation resistance,good biocompatibility and other excellent properties.However,affected by the phonon scattering from a large number of non-harmonic vibrations inside the polymer,the intrinsic thermal conductivity of PEEK is only 0.25 W·m^-1·K^-1,which is not conducive to the heat transfer of its components in high-temperature application conditions.And the resulting heat accumulation will cause a greater risk of failure to equipment friction,seals and other key components.Therefore,a lot of research has been carried out to improve the thermal conductivity of PEEK materials,and significant progress has been made in preparing filling-type PEEK thermally conductive composites,but at the same time,there are also some bottlenecks.Existing work usually involves the addition of thermally conductive fillers such as multi-walled carbon nanotubes（MWCNT）,graphite,boron nitride,etc.to the PEEK matrix by melt blending to construct thermally conductive pathways,while the composition of a through thermal pathway often requires the filling of a large amount of thermally conductive fillers.The introduction of a large number of thermally conductive fillers usually leads to a significant decrease in the mechanical strength and toughness of the composites,which affects the practical applicability of PEEK thermally conductive composites.At the same time,due to the limitations of high viscosity and solvent resistance of PEEK melt,the thermally conductive fillers introduced into the resin often suffer from the difficulties of pathway structure design and poor interfacial compatibility.Moreover,the high melt processing temperature restricts the use of interface modifiers,which makes it difficult to effectively improve the poor interface between the resin matrix and the thermally conductive filler,and the efficiency of the thermally conductive filler cannot be fully utilized.Considering the above research background,in this dissertation,a thermally conductive filler network consisting of nano-graphite micro flakes（GNS）was built inside PEEK by designing the segregated structure,and optimized in situ polymerized PEEK/MWCNT composite preparation processing.The molecular structure modification was adopted to introduce large conjugated groups into the main chain of PEEK to improve the interfacial strength between matrix and filler throughπ-πinteractions,and its effect on the properties of the composites was systematically investigated.Based on the prepared PEEK nanocomposites,combined with the micron-segregated structure,a double-segregated thermal conductivity network was jointly constructed,which can simultaneously improve its thermal and mechanical properties.On this basis,high-temperature soluble PEEK was synthesized and used as the interface modifier,and based on the high efficiency of nanofiller modification and the remarkable effect of carbon fiber reinforcement,the introduction of carbon nanotubes and carbon fibers in the PEEK matrix.It simultaneously realized the high thermal conductivity and high strength of PEEK composites.This methodology provides an effective design approach for solving the high-temperature thermal conductivity problem of special engineering plastics such as PEEK.Through the segregated network structure design,the PEEK powder was processed into 100–300μm bulks,and the graphite flakes were adhered to the surface of the PEEK bulks by ball milling to prepare the core-shell structure.And finally,the graphite heat-conducting network with a segregated network structure was obtained inside the PEEK resin by hot press molding process.The optimum ball milling time for the preparation of PEEK composites was chosen to be 2 h,and the hot pressing temperature was 340^oC near the melting point of PEEK.The size and thickness of graphite flakes were optimally selected,and the PEEK/graphite composite achieved a thermal conductivity of 1.18 W·m^-1·K^-1 with the addition of 20 wt%GNS,which was19%higher than the composite obtained by direct melt blending while maintaining a mechanical strength of 60 MPa.Though the thermal conductivity of the PEEK/GNS segregated network thermally conductive composites prepared by the above method was improved,the improvement is still not significant enough,and the single thermal conductivity network constructed by micron filler has the problem of long-distance between pathways and low efficiency of phonon transmission across the resin matrix.With the above result,to further improve the thermal conductivity of PEEK/GNS segregated network composites,based on the above-mentioned micron thermal conductivity pathways,it was proposed to further construct nanocapillary thermal conductivity pathways within the PEEK resin matrix to expand the transmission pathways between the micron pathways while enhancing the distribution of the pathways within the matrix.The maximum addition and uniform dispersion of MWCNT in PEEK can be improved through the regulation of ultrasonic process and sample treatment,and up to 10 wt%of MWCNT can be uniformly dispersed in PEEK.PEEK/MWCNT nanocomposites with good flowability obtained by controlled polymerization reaction conditions.With the addition of 10 wt%MWCNT,the thermal conductivity of the nanocomposite reached 2.20 W·m^-1·K^-1,and the tensile strength was maintained at 80 MPa.Replacing pure PEEK as the matrix resin with this nanocomposite and combining graphite micron network to prepare a thermally conductive network with a double network segregated structure,the thermal conductivity of the composite was increased to 3.78 W·m^-1·K^-1 with the addition of 10wt%MWCNT and 20 wt%graphite,and reached 4.30 W·m^-1·K^-1 at 150^oC of the glass transition temperature.However,with the introduction of a large number of fillers,undesirable resin-filler interfaces also emerged,and the interfacial thermal resistance limited the thermal conductive fillers to play their maximum lifting efficiency,while the interfacial defects also caused the tensile drop to 46 MPa.To improve the interfacial quality between resin and filler,and to further enhance the thermal and mechanical properties of the above PEEK nanocomposites,interfacial modifications were first applied to the PEEK nanocomposites by introducing fluorene groups with large conjugated structures into the main chain structure of PEEK and improving the interfacial bonding between the resin matrix and the thermally conductive filler throughπ-πinteractions.The introduction of the fluorene group resulted in a 17%reduction in interfacial thermal resistance and a 10%increase in tensile strength at 2 mol%bisphenol fluorene addition,compared with that without fluorene-functional.Combining the above-prepared nanocomposites with GNS-segregated networks,the fluorene groups enhanced the interfacial quality inside the nanocomposites while allowing for a higher-quality interfacial bond between the resin particles and the external GNS.The thermal conductivity of the prepared composites was improved to 3.91 W·m^-1·K^-1,which was 17%higher compared to that without the addition of fluorene groups,and further improved to 4.45 W·m^-1·K^-1 at 150^oC.Meanwhile,the tensile strength of the composites was also improved to 58 MPa.The above results indicated that the introduction of fluorene groups can effectively improve the interfacial bonding strength between the PEEK matrix and carbon material.As the graphite network was only hot-pressed into a network by the physical force of ball milling,lacking necessary interface modification,resulting in its interface defects and stress concentration phenomenon,limited the thermal conductivity modification effect and significantly reduced the mechanical strength.Therefore,in this dissertation,a soluble high-temperature resistant fluorene-containing PEEK（FD-APEEK）was prepared as an interfacial modifier to improve the interface of the graphite network within the composites.With the non-covalent bonding modification of graphite by solution dispersion coating method,the tensile strength of the composite was increased to 62 MPa by the same compression molding process as described above,but the improvement in the interface did not significantly improve the thermal conductivity of the material,which was only increased by 4%.During processing,the molten FD-APEEK filled the voids between graphite layers and improves the interface,thus enhancing the performance of the composite.However,introducing polymeric interfacial modifiers will affect the thermal conductivity by cutting off part of the thermal conductivity pathway.Therefore,inspired by the encouraging improvements in micro-nano conducting dual networks,MWCNT was further introduced between graphite layers to construct multi-scale composite filler.The MWCNT acted as a"bridge"for heat and stress transfer between the graphite layers,significantly improving the overall performance of the composite.When 2 wt%MWCNT was added,the thermal conductivity of the composites increased to 4.15 W·m^-1·K^-1,and the tensile strength increased to 76 MPa.Combined with carbon fiber reinforced PEEK composites commonly used in practical applications,the porous carbon fiber mat was used as the skeleton,and the prepared PEEK-modified powder was filled into the pores of the carbon fiber mat to prepare sandwich structure reinforced composites,and a three-dimensional thermal conductivity network was constructed in the composite.When 20 wt%carbon fiber was introduced,the tensile strength of the composite reached 261 MPa,and the thermal conductivity remained at 3.75 W·m^-1·K^-1 which was a 698%improvement compared to the PEEK/CF composite without the addition of the powder prepared in this dissertation.The above work shows that by designing and preparing a dual segregated thermal conductivity network,the thermal conductivity of PEEK composites can be significantly enhanced while maintaining excellent mechanical properties,and a high-strength thermal conductivity PEEK composite can be prepared.In summary,by segregated network structure design,optimized processing and preparation process,combined with polymer main chain structure design approach,the interfacial bonding between PEEK matrix and filler can be significantly improved with efficient construction of thermal conductivity pathway,thus achieving significant enhancement of thermal conductivity and mechanical properties of the composites.This work broadens the application prospects of PEEK-based high thermal conductivity composites and provides a general design and preparation method for resin-based composites with high-temperature resistance,high strength,and high thermal conductivity.