Connection-improved Conductive Network Of Carbon Nanotubes In The PDMS Crosslink Network

Traditionally,the crosslink network in conductive rubbers limits the uniform dispersion of conductive fillers in them,even leading to the phase separation.The key to solve this problem is to establish a conductive network structure whose conductive fillers can uniformly disperse in crosslink networks.Based on the calculation of the connecting possibility of one-dimension(1D)nanofillers in crosslink networks,this study established a crosslink/conductive double-network model that can improve the connectivity of conductive fillers.In this model,the conductive network is constructed inside the crosslink network.The crosslink network can fix the connecting sites of 1D nanofillers in the grid with specific size to improve their connection.The critical percolation index of the conductive network can also increase.This model also suggest that the percolation value can exceed 100%when the nanofiller size and crosslink density are in a specific range.This predicts that the conductive network cannot be constructed inside the crosslink network in this range.The prediction is consistent with the fact that the conventional crosslink network can limit the construction of conductive network.Meanwhile,when the average distance between adjacent cross-linking points is larger than that of the rubber segment,the system conductivity should be similar to that of linear polymer composites.The double-network model was tested with poly dimethylsiloxane(PDMS)as crosslink matrix and carbon nanotube(CNT)as conductive filler.In specific range of crosslink density and CNT size,the relationship between the percolation threshold of PDMS/CNT nanocomposites and the crosslink density of PDMS was highly consistent with that of the predicted result from the model.The percolation threshold reached the lowest(0.03 vol%)at a crosslink density of 182 mol/m~3,which was lower than the reported percolation threshold value of the PDMS/CNT nanocomposite in literatures.Meanwhile,when the crosslink density was low enough,the percolation value varied little with the crosslink density,which was consistent with the result of double-network model.Through the characterization of the thermal stability and mechanical stability of PDMS/CNT nanocomposites,the nanocomposites were found to have high thermal stability.Their mechanical properties were stable during 20 cycles of reciprocating stretching.On the other hand,the size of carbon nanotube and the chemical structure on the surface were found to affect obviously the double-network of the conductive network.I compared the conductivity of PDMS/CNT composites prepared by several kinds of carbon nanotubes with different sizes and surface defects.It was found that the TNSM3 carbon nanotube,whose diameter is 15 nm,does not apply to this dual network theoretical model.Only when the diameter of one-dimensional nanofillers is in the range of several nanometers,the double-network model of the crosslinked network and the conductive network be constructed.This result was consistent with the prediction result of double-network model that the percolation threshold of the system with large-diameter 1D fillers can easily exceed 100%.Through the comparison of the surface defect levels,it was found that the large number of defects on TNSM2 CNT also limited the construction of the double-network.The cyclic voltammetry test also confirmed that the defects on TNSM2 can electrochemically reacted with PDMS.Such a reaction affected the chemical network of PDMS crosslink network and thus the formation of CNT conductive networks.In summary,the crosslink/conductive double-network model was consistent with the experimental results.By establishing such a double-network the percolation threshold of composite can be effectively reduced.Although this study proved that the PDMS crosslink networks are able to conduce the establishment of conductive CNT networks,the establishment condition is strict.CNTs need to have a diameter of several nanometers and have fewer surface defects.This study thus provides new ideas for constructing conductive networks in crosslink networks and provides new methods for reducing the percolation threshold of materials in crosslink polymer matrices.