Construction Of Thermal/electrically Conductive Channels In Polyvinyl Alcohol-based Composites And The Study On Their Properties

With the development of electronic equipment towards high integration,miniaturization,high frequency,flexibility and wearability,the corresponding requirements such as thermal conductivity and electrical conductivity are increasing.In order to meet the requirements of the next generation of flexible/wearable electronic devices,the key is to endow the flexible substrate with good thermal and electrical conductivity.Based on the internal structure of the materials,it is necessary to construct thermal/electrically conductive channels in the flexible substrate with thermal/electrically conductive fillers.However,some problems still exist,such as there are fewer types of flexible polymeric substrates and it always needs high loading of inorganic thermal/electrically conductive fillers and complex construction process for effective transmission channels.In this work,polyvinyl alcohol（PVA）is selected as flexible substrate benefiting from its outstanding flexibility.According to the loading sequence of the fillers in the polymer matrix,a series of polyvinyl alcohol-based composites have been prepared to optimize the loading capacity of the fillers and their distribution in the matrix.Systematical work has been done to study the rules on the construction of effective thermal/electrically conductive channels.Their properties and further applications in the fields of thermal and electrical conductivity（electromagnetic shielding and strain sensing）are investigated.The main research results are as follows.（1）Based on pre-addition of thermal conductive fillers with constant content and taking covalent bonding between fillers and polymer matrix into consideration,the heat transfer performance of thermal conductive channel is regulated by the structure of covalent bonding coupling agent.Since the interfacial thermal resistance is an important factor to influence phonon transport,while the establishment of covalent coupling between filler and polymer matrix can promote effective phonon transport in the thermal conductive channel,but the coupling agent itself may also cause thermal barriers.In this work,different silane coupling agents（SCAs）has been investigated to PVAand functionalized boron nitride（fBN）,intrinsic relationship between molecular structure of silane crosslinkers and thermal conductivity（TC）of PVA-fBN composite is studied systematically.The results show that longer side chains of SCAs would inhibit the TC enhancement of the composite.More hydrolyzable Si-O-R in SCAs will increase the degree of self-condensation and reduce the TC of the composites.It shows that the SCAs molecules with short side chain,i.e.vinyl triethoxysilane（VTES）increases the TC of composite polymer,with maximum value of 1.636 W/m·K,which is 337.3%of that of PVA/fBN.In contrast,3-glycidoxypropyltrimethoxy silane（GPTMS）with long side chain decreases the TC to 54.4%of that of PVA-fBN.（2）Based on pre-addition of electrically conductive fillers with low metal loading,the structure of the fillers is explored during the process of construction of electrically conductive channel when there is no covalent bonding between fillers and polymer matrix.A spontaneous process to construct segregated laminar structure with three-dimensional electrically conductive network has been investigated.In this work,Ag nanoparticle-coated polystyrene（PS@Ag）microspheres with large PS core are selected as electrically conductive fillers,with the assistance of gravity-driven effect and the volume exclusion effect,segregated laminar structure with three-dimensional electrically conductive network could be constructed spontaneously with low silver loading.The flexible thin films with low amount of silver exhibits outstanding EMI SE per unit thickness（SE/d）.To further investigate the effect of structure on the EMI SE efficiency,it is found that the alternate interfaces play a critical role in enhancing shielding performance by stacking multiple films compared to the bulk films by casting.The EMI SE for only 0.3 mm triple-piece stacking film with 6.3 wt%Ag loading could reach 55.3 dB compared to 37.2 dB for triple-thickness casting film.By stacking process,it shows easy accessibility for tailored composites with desired EMI SE,which supplies a new strategy for the design of shielding materials in the next-generation electronics.（3）Based on without pre-addition of electrically conductive fillers,electronic conductive channels and free ion conductive paths are constructed in the matrix by self-growth method.In this work,the pre-prepared polyvinyl alcohol-tannic acid hydrogel was freeze-dried,and then immersed in AgNO₃ solution for rehydration without additional reducing agent or pre-addition of inorganic conductive filler.It results in densified polymer networks along with "fish scale-like" surface,consisting of inorganic materials originated from spontaneously formed Ag nanoparticles（AgNPs）which are tightly packed on the exterior surface of hydrogels.Benefiting from superior conductive pathways,the freely moving salt ions in the hydrogel matrix and the dense layers of AgNPs on the outer surface endow hydrogel with good electrical conductivity and sensing sensitivity.The densified polymer networks dramatically improve the mechanical properties.Consequently,the prepared hydrogels realize 20-times improvement on both strength（3 MPa）and toughness（2.12 MJ/m³）along with ideal electrical conductivity（3.98 mS/cm）and strain sensitivity（GF=4.12）.The obtained conductive hydrogels can behave as strain sensors to monitor human motions and also exhibit desirable antibacterial properties.（4）Based on without pre-addition of electrically conductive fillers,the process of salt ions constructing ionic conductive channels in the matrix with freeze-resistant complex components is investigated.Since the flexible conductive hydrogel in work（3）would be frozen at sub-zero to lose electrical conductivity and flexibility.In this work,PVA,tannic acid（TA）,[2-（methacryloyloxy）ethyl]dimethyl-（3-sulfopropyl）ammonium hydroxide（SBMA）were selected to construct semi-interpenetrating network structure.With assistance of calcium chloride in glycerol-water solution,the ionic conductive gel with excellent antifreeze-conductivity-mechanical properties was prepared.Herein,glycerol,calcium chloride together with SBMA would lower the freezing point of water and improve freeze-resistance.Amphoteric SBMA and the salt ions penetrating in the semi-interpenetrating network endow the gel with favourable ionic conductivity.The physical crystallization crosslinking point of PVA,the complexation of Ca²⁺ with TA,and the formation of multiple hydrogen bonds by PVA-TA-glycerol would construct effective energy dissipation network to improve mechanical properties.The desired product is PTS-6Ca-2G-3W with toughness of 3.08 MJ/m³,tensile strength of 2.44 MPa,elongation at break of 269%,electrical conductivity of 12.1 mS/cm at room temperature,freezing point of-41.3℃,and good flexibility and electrical conductivity after stored at-18℃ for 7 days.PTS-6Ca-2G-3 W can also be used for monitor human motions.