Study On Preparation,Microstructural Regulation And Properties Of Polystyrene Thermal Conductive Composites

With the continuous development of integrated circuit technology,electronic devices are developing toward the direction of ultra-light,miniaturization,multi-functionality,high integration and high performance.The low operating efficiency and short service life caused by heat dissipation become more and more serious,and there is an urgent need to develop materials with high thermal conductivity.Polymers have significant advantages for the development of light and thermally conductive materials due to their light weight,low cost,chemical corrosion resistance,electrical insulation and good processing performance,etc.However,most of the polymer materials are poor conductors of heat with intrinsic thermal conductivity lower than 0.5 W/mK,which greatly limits their application in the field of heat dissipation.At present,the main way to improve the thermal conductivity of polymer materials is incorporating the fillers with high thermal conductivity.Generally,a large number of fillers are usually required due to the presence of the extremely high thermal resistance between fillers and polymer matrix.However,the presence of the high content of fillers not only increases the viscosity of the matrix,leading to the deterioration of the processing flowability,but also results in the deterioration of the mechanical properties of the composite.Therefore,it is very significant to prepare the highly thermal conductivie polymer composites at relatively low filler content.To prepare the required thermally conductive composites,the key is to construct an efficient thermally conductive network structure in the polymer matrix.This thesis focuses on the construction of efficient heat conductive network structure and its influence on the thermal conductivity of polystyrene(PS).Firstly,the regulation of the preparation method on the structure of the filler network was studied.On this basis,the hybrid network structure of the filler was constructed by introducing nano-fillers with different dimensions,i.e.two-dimensional graphene nanoplatelets(GNPs)and one-dimensional carbon nanofibers(CNFs).The synergetic effects of the hybrid network structure on the thermal conductivities of the PS composites were studied.Finally,in order to solve the problem of poor impact resistance of PS-based composites,the ternary composites were prepared by introducing SEBS-g-MAH,and the influence law and mechanism of maleic anhydride grafted styrene-ethylene-butylene-styrene block copolymer(SEBS-g-MAH)on the thermal conductivity and mechanical properties of the composites were studied.The main results obtained in this work are listed in the following.(1)PS/GNP-S and PS/GNP-SM composites were prepared by one-step method(solution blending)and two-step method(solution blending followed by melt blending).It was found that the thermal conductivity,electrical conductivity and glass transition temperature(T_g)of PS/GNP-S composites were higher than those of PS/GNP-SM composites.The rheological property test and micromorphology characterization showed that the dispersion of GNPs in PS/GNP-S composites was poor compared with PS/GNP-SM composites,but GNPs formed a relatively complete network structure.The analysis suggested that solution blending was not conducive to the uniform dispersion of GNPs in the composite,but the contact of adjacent GNPs was beneficial to construct an efficient network structure in the composite.During the melt blending process,the existence of shear stress further promoted the uniform dispersion of GNPs,but damaged the network structure to a certain extent.The existence of dense GNPs network structure in PS/GNP-S composites was further verified by the effective medium model,and the interface thermal resistance between GNPs and PS matrix was relatively low,which was the fundamental reason for the high thermal conductivity of PS/GNP-S composites.(2)GNPs and CNFs fillers were introduced into PS matrix to construct hybrid network,and PS/GNP/CNF thermally conductive composites were successfully prepared by solution blending method.Thermal conductivity and electrical conductivity tests showed that PS composites had good electrical and thermal conductivity.On the one hand,the conductive percolation threshold of PS/GNP/CNF composites was greatly reduced.On the other hand,CNFs and GNPs had a good synergistic effect in improving the thermal conductivity of composite materials.In addition,the T_g and the vicat softening point(T_v)of the PS/GNP/CNF composites had also been significantly improved.Through microscopic morphological characterization and rheological behavior research,it was found that CNFs could attach to flaky GNPs and form a dense network structure with GNPs in ternary composites.In practical applications,PS/GNP/CNF composites were expected to be used as heat-dissipating materials for microelectronic devices,and have great application potential under complex conditions such as high temperature and high load.(3)Considering the poor impact resistance of PS,toughening and modification of PS/CNF thermally conductive composites were proposed.Based on this,the PS/SEBS-g-MAH/CNF ternary composite was prepared by melt blending.Thermal conductivity tests showed that the addition of SEBS-g-MAH could further improve the thermal conductivity of PS/CNF composites.The impact resistance tests showed that SEBS-g-MAH could effectively improve the unnotched impact strength of PS/CNF composites.Through rheological property test and micromorphology analysis,it was found that SEBS-g-MAH could induce the distribution of CNFs along the elastomer phase,which made the CNFs form a partial lap network,thus further improving the thermal conductivity of the composites.