Study On Structure, Conductivity And Rheological Behaviors Of Vgcf-filled Polymer Composites

As one of the most important novel fillers, vapour grown carbon nanofibers (VGCFs), have extraordinary mechanical, thermal and electrical properties similar to carbon nanotube (CNT) and have been used as one of the most potential reinforcing materials and conductive fillers. The final performances of polymer/VGCF composites are always related to the dispersion, agglomeration and network formation of VGCF in the matrix.In this thesis, VGCF-filled polystyrene (PS) or polypropylene (PP) have been selected as research models. Influences of raw materials, mixing conditions and external fields on filler-phase structures, conductivity and rheological behaviors of the composites have been investigated in order to probe mechanisms of linear or nonlinear rheologies for VGCF-filled polymer, and detect response of VGCF network to actions of external fields, and establish relationship among VGCF-phase structure, conductivity and rheological behavior.The linear rheological behavior of VGCF-filled PS can be well fitted using the two phase model thus the global complex modulus of VGCF-filled polymers can be divided into a certain strain amplification e(?)ect and a structural contribution of the filler phase. These composites with high level of VGCF entanglement and contact have high values of strain amplification factor (Af) as well as characteristic moduli of the filler phase.Effect of volume fraction (φ) and aspect ratio of VGCF and coupling agent styrene-maleic anhydride copolymer (SMA) on nonlinear behaviors of VGCF-filled PS was investigated. The results indicate that the strain-softening strength increases with increasing (p and aspect ratio of VGCF, and decreases in case of SMA adding. Using Af arising from hydrodynamic effect as vertical and horizontal shifting factors, curves of the dynamic storage modulus (G) and the dynamic loss modulus (G") as a function of y for the composites can be superposed on those of pure matrix in large y region, suggesting that the matrix provides the main contribution to strain softening in large y region. Significant deflection from the master curves can be observed in small y region and the deflection becomes more marked at higher (p. Electrical resistance (R) tested as a function of y and TEM micrographs provide direct evidences for breakdown of filler-filler contact by y perturbation. Moreover, fiber slippage and orientation maybe accompanied by debonding of PS chain from the fiber surface, result in strain softening in small y region.Nonlinear relaxation behaviors of PS/VGCF composites follow time-strain separation principle. Whenφ≤8 vol%, composites show one relaxation behavior mainly arising from the matrix. When (φ≥12 vol%, composites exhibit another long-time relaxation due to filler network. Addition of SMA has no effect on nonlinear relaxation of composites.The evolution of VGCF network under shearing, heating or solidification of matrix was studied by using simultaneous measurements of G'and R. The results show that shearing destroys the percolation network and the destruction degree depends on the shearing mode. The VGCF network collapses under 100% oscillation strain shear and while it is just partly destructed under 1 s-1 steady shear. Moreover, thermal treatment and solidification of PS affect VGCF-network reformation. In PP/VGCF composites, crystallization of PP destroys the VGCF network firstly. When crystallinity is up to a certain value, VGCFs gather in the amorphous region to form a percolation network, and the shrink in volume upon cooling causes VGCF to get closer together.The processing-microstructure-property relationship for PS/VGCF composites was investigated. A remarked decrease in fiber length from 35 to 15μm was observed after mixing at low-shear condition (190℃-30rpm-2.5min). Composites with different dispersion levels of VGCF can be formulated by changing the mixing conditions. At low rate and short time mixing conditions, the composites have high Af values, characteristic moduli of the filler phase as well as electrical conductivity due to high entanglement level of VGCF.The effect ofφand VGCF size on viscosity and die-well ratio of VGCF filled polymers was studied through steady rheology measurement. Composites, containing 2 vol% VGCF exhibit shear thinning even at low shear rate and the shear thinning became more significant with increasingφand aspect ratio of VGCF. Addition of VGCF into polymer may increase flow activation energy and decrease primary normal stress difference (N1) and die-well ratio.It is noted that these effects were more significant with increasing (p and aspect ratio of VGCF.