Mechanism Of A Novel Method Of Spatial Confining Forced Network Assembly For Preparation Of Highly Performance Polymer Composites

As one of the most important thermally functional materials,thermally conductive polymer composites are widely used in LED,battery and electronic packaging.Traditional melt blending is an important method to prepare polymer composites.Thermal conductivity(TC)of polymer composites increases slowly through increasing filler content,which always deteriorates the processing and mechanical properties of composites.Besides,there is no percolation threshold in TC,unlike in electrical conductivity.One reason of the slow increase of TC is that the conductive network was not compacted enough,resulting in a rather high thermal resistance during heat travelling.The other reason is that most heat which should travel along the network is absorbed and dissipated by polymer in and around the network.The main purpose of this dissertation is to reduce the thermal resistance and the heat dissipation when heat transfers in polymer composites to improve thermal properties of polymer composites.The concept of sand-like network and stone-like network in polymer composites and the Mechanism of Transforming between Sand-like network and Stone-like network(MTSS))were proposed firstly.In the network,a method called Spatial Confining Forced Network Assembly(SCFNA)was used to decrease the average gap among fillers to decrease the thermal resistance and to realize the transforming between stone-like network and sand-like network.The main mechanism of SCFNA method can be explained that a mechanical external load was applied on the composites with loose self-assembly network to compress the composites until to a thickness less than the characteristic thickness in a confining space to restrict the wiggling of self-assembled network.Fillers in self-assembly network get closer and extra polymers can be extruded easily due to the strong forced-assembly force,destroying the balance between aggregating force and hindering force,resulting in a densified network.In this way,the average gap among fillers in the network could be reduced and the TC could be enhanced by SCFNA method.Around the network,appropriate rigid particles were added along the thermally conductive network resulting in a stone-like network transformed from a sand-like network to increase the heat flow in network.On one hand,the heat leaking into the polymer partially returmed to the thermally conductive network through the scattering of rigid particles.On the other hand,rigid particles were used to replace some polymers around the thermally conductive network to reduce the heat dissipation in corresponding polymers.Therefore,the great improvement of thermal properties can be achieved by rigid particles.The main works related to the theories and experiments of SCFNA method and MTSS related to rigid particles are shown as below:1.The relationship between mechanics and dynamics in the distribution-dispersion,self-assembly and forced-assembly process of SCFNA method,as well as the mechanism of SCFNA method to realize the compacting network were analyzed.The three types of silicone elastomers with trade name of HY-E620,HY-E640 and SYLGARD 184,respectively,were used as polymer matrix,and short carbon fiber was used as filler to prepare polymer composites.The results showed that an excellent dispersion of filler could be obtained by appropriate dispersing force to get uniform distribution of heat conducting network.However,the TC of polymer composites could not be improved further by excessively dispersed due to the length of fiber broken by high dispersing force.The optimal process parameters used in SCFNA method was obtained based on the TC and the quality of composites.The silicone elastomers with trade name of SYLGARD 184 was used as polymer matrix,and short carbon fiber and whisker carbon nanotube were used as filler to prepare polymer composites.The results showed short carbon fiber was more effective in enhancing the TC of PDMS composites than whisker carbon nanotube.2.The mechanism model of TC enhanced by SCFNA method was built,indicating that the average gap among fillers influenced the TC of binary polymer composites largely.Polydimethylsiloxane(PDMS),with trade name of SYLGARD 184,was used as the polymer matrix,combined with fillers of various dimensions to prepare PDMS composites.The results showed that SCFNA method was more effectively to improve TC than traditional compounding method within a certain range of filler content,and the processing and mechanical performance of the polymer composites could also be guaranteed.Compared with the traditional compounding method,the SCFNA method has obvious advantages in TCs and flexibility.3.The mechanism model of TC enhanced by SCFNA method was built,indicating that the average gap among fillers influenced the synergistic effect of micro and nano fillers largely.PDMS was used as polymer matrix,and fillers with various size were used as filler to prepare PDMS composites.The results showed that only when the thickness of the composite samples was below a critical value,the micro and nano fillers were able to bridge and form a compact conducting network by SCFNA method.SCFNA method can further improve the TC and ensure the mechanical properties of composites without further adding filler content.4.The movement of carbon fiber was analyzed based on the mechanism of SCFNA method.The mathematical model predicting the TC of polymer composites was built based on the two fillers in series.Factors of properties of filler and polymer composite,compression ratio and interface thermal resistance were considered in this model.The results showed that this model was applied well to the polymer composites with carbon fiber.5.Brass network and 3Y shape forced-assembled network were used as the continuous network,respectively,and the hollow glass ball,i.e.iml6K(the TC is lower than that of ordinary polymers)and Al2O3(the TC is higher than that of ordinary polymers)were used as rigid particles to prepare composites with stone-like network.The results showed that the rigid particles can improve the thermal properties of polymer composites by ensuring the effective distribution of certain rigid particles along the continuous network,even though the TC of rigid particles is much lower than that of ordinary polymer.