Study On Structure,Conductivity And Rheological Behaviors Of MWCHTs And Hydrophobic Nanosilica Filled Composites Based On Single Or Binary Polymer Matrix

Carbon nanotubes (CNTs), with the unique mechanical, electrical properties and high length/radius ratio (up to100-1000), can be introduced into polymer to obtain particle filled conductive polymer composite materials (CPC). It is observed that increasing CNTs content (φCNT) can improve the conductivity of CPC, but usually result in bad processability and conductivity stability. Hydrophobic nanosilica, SiO2as a reinforcing filler, can significantly enhance the mechanical properties and wear resistance. As an effective method of reducing critical conducton content (φC) and improving the conductivity, multi-walled carbon nanotubes (MWCNTs) and SiO2are used as conductive and nonconductive particles, respectively, to fill into the immiscible polymer blends matrix to obtain CPC.In this thesis, immiscible poly(methylmethacrylate)/polystyrene (PMMA/PS)(1/1in volume) blend have been selected as the matrix, MWCNTs and SiO2respectively as conductive and nonconductive fillers, to prepare binary, ternary and quaternary CPCs. Influences of fillers on phase structures, conductivity and rheological behaviors of the CPCs have been investigated. By simultaneous measurement of electrical conductivity and rheological behavior, relationship among phase structure, conductivity and rheological behavior has been established, and structural evolution during melt annealing has been investigated.Morphological observation by TEM revealed that MWCNTs are dispersed in PS phase and SiO2in PMMA phase, respectively. Degree of phase coarsening decreases with increasing volume fraction of MWCNTs in ternary CPC during melt annealing. The presence of SiO2could greatly restrain the coarsening in quaternary CPC. In a word, nanoparticles can reduce the phase size. Conductivity test shows that MWCNTs and SiO2can cause narrower conductive percolation threshold (φC). Under the dynamic rheological measurement, the viscosity percolation threshold (φη) and modulus percolation threshold (φG) are similar to φC, both veing related to the internal network formation for binary CPC; when temperature increases,φη and φG of binary and ternary CPC both decrease because MWCNTs aggregates faster and conductive percolation network forms at low φCNT. Quaternary CPC do not show obvious viscosity percolation phenomenon. When φCNT increases, binary, ternary and quaternary CPC exhibit strain softening effect (Payne effect) at higher strain (γ), and SiO2can promote Payne effect, it is related to failure of filler network of MWCNTs and SiO2. The relaxation of percolation network and phase structure of ternary CPC are accelerated as φCNT increases; SiO2can slower the relaxation of percolation network and phase structure of quaternary CPC.Improving CPC conductivity at low φCNT and conduction stability at external field stimulation is a research focus of CPCs filled with nanopaticles in recent years. Three dimension of particle network throughout the matrix is considered as the main reason of conduction behavior. The excess interfacial free energy between polymer chains and conductive particles results in unstable percolation network structure, which therefore tends to evolute in order to reduce interfacial energy during annealing. The conductive network formation is highly dependent on temperature and time during processing. By using simultaneous measurement, electrical conductivity and rheological behavior of ternary and quaternary CPC can be simultaneously tracked.During simultaneous measurement of ternary CPC, storage modulus (G’) first keeps constant and then decreases. The nonlinearity of G’ and R becomes more obvious as φCNT increases. The critical strain for modulus (γCG) decreases and critical strain for resistance (γCR) increases, which are related to the reconstruction and destruction of MWCNTs network.During melt annealing with large strain (γ≥50%), G’ of ternary CPC almost does not vary with time. Shear effect seriously damages MWCNTs aggregation structure and can cause the matrix phase caorsening effect. With the increase of y, R first increases and then decreases, reflecting the damage of the MWCNTs percolation network during annealing under the condition of high γ. SiO2can improve melt complex modulus but influence slightly the time-dependent dynamic conductive percolation (t-DCP) during melt annealing. MWCNT and SiO2have different effects on the time-dependent dynamic modulus percolation (t-DMP) during melt annealing. It is believed that MWCNTs and SiO2accelerates and decelerates the onset time for phase coarsening, respectively, while both significantly extends the time interval of phase coarsening.