Xiao-Su Yi

Time dependence of piezoresistance for the conductor-filled polymer composites

The piezoresistance and its time dependence of conductor-filled polymer composites have been investigated. To reveal the origin of the time dependence of piezoresistance, the creep of the polymer matrix is also studied. Based on the interparticle separation change under the applied stress, a model has been developed to predict the piezoresistance and its time dependence. By analyzing this model, the influences of applied stress, filler particle diameter, filler volume fraction, matrix compressive modulus, potential barrier height, and the matrix creep behavior on the piezoresistance and its time dependence are interpreted quantitatively. These predicted results are compared with the experimental data obtained on the polymer composites filled with conductor fillers, and good agreements were obtained.

Morphology and electrical properties of short carbon fiber-filled polymer blends : High-density polyethylene poly(methyl methacrylate)

Morphology and electrical properties of short carbon fiber-filled high-density polyethylene (HDPE)/poly(methyl methacrylate)(PMMA) polymer blends have been studied. The percolation threshold of HDPE50/PMMA50 blends filled with vapor-grown carbon fiber (VGCF), 1.25 phr VGCF content, is much lower than those of the individual polymers. The SEM micrographs verified that the enhancement of conductivity could be attributed to the selective location of VGCF in the HDPE phase. A double percolation is the basic requirement for the conductivity of the composites, i.e., the percolation of carbon fibers in the HDPE phase and the continuity of this phase in the blends, which hereby are defined as the first percolation and the second percolation, respectively. The SEM micrographs also showed that the short carbon fibers could affect the morphology of the blends. With the increase of VGCF content, the HDPE domains are elongated from spherical into strip shape, finally develop to a continuous structure. As a result, the second percolation threshold of the blends filled with 2.5 phr VGCF, 20 wt % HDPE, is lower than that of the blends filled with 1.5 phr VGCF, 30 wt % HDPE. The influence of molding temperature and time on the second percolation threshold has also been investigated. For the composites molded at a lower temperature, the second percolation threshold is shifted to a higher VGCF content, but there is little influence of molding time on the second percolation threshold.

Selective location and double percolation of short carbon fiber filled polymer blends: high-density polyethylene/isotactic polypropylene

The percolation threshold of high-density polyethylene (HDPE)/isotactic polypropylene (iPP) blends filled with vapor-grown carbon fiber (VGCF), 1.25 parts per hundred parts resin (phr) VGCF content, is much lower than those of the individual polymers. SEM micrographs verified that the improvement of electrical conductivity can be attributed to the selective location of VGCF in the HDPE phase. A double percolation is the basic requirement for the conductivity of the composites, i.e., the percolation of carbon fibers in the HDPE phase, and the continuity of this phase in the polymer blends. Short carbon fibers may affect the morphology of the polymer blends.

Property balancing for polyethylene‐based carbon black‐filled conductive composites

Carbon black (CB) filled polyethylene (PE) and/or ethylene vinyl acetate (EVA) composites were prepared by conventional blending. Their electrical properties, including resistance-temperature relation, resistivity-CB content relation, and positive or negative temperature coefficient (PTC, NTC) intensity; mechanical properties such as break strength and the elongation at break; and processing conditions involving mixing torque and equilibrium temperature were studied to balance them from the fabrication and application points of view. A balanced property profile was achieved by mixing EVA with the primary CB-PE system to form a trinary blend. To enhance the thermal stability, the prepared samples were then filled with ZnO and talc and subsequently undertook either chemical or radiation crosslinking. This treatment mostly eliminated the NTC effect, the PTC transition was significantly intensified, and the room temperature resistance was slightly increased.

Thermoplastic starch/PVAl compounds: Preparation, processing, and properties

As a renewable and native polymeric material, thermoplastic starch is gaining increased acceptance because of resource and environment concerns. This study reports the formulation design of native starch/polyvinyl alcohol (PVAl) compounds; the preparation technology to make them available for thermoplastic processing; and their rheological, mechanical, and degradation properties. Among various plasticizers investigated, a mixture of glycerin and water (50/50 wt %) is found to be the best one for the compounds in terms of rheological behavior, mechanical properties, and environmental stability. The mechanical properties do not increase with the PVAl content to an expected extent, mainly due to the poor interface adhesion between the fibrous PVAl structure and the starch matrix.

Properties and applications of filled conductive polymer composites

The electrical properties of polymers filled with different types of conducting particles are reviewed. Following a theoretical description of a general effective media (GEM) equation, the experimental conductivity–volume fraction data for thermoplastic filled with vanadium oxide particles as well as thermosetting polymer composites, are fitted to the equation. The calculated property-related parameters in the equation are discussed. The electrical conductivity of the composites is combined with an extremely large positive temperature coefficient (PTC) effect, depending on the filler type (V2O3 or carbon black), as well as on its distribution and volume fraction. Both melting and recrystallization behaviour are responsible for the PTC effect. Due to a conductive filamentary network across the medium, a localized thermal effect comes into existence, leading to self-heating of the body. This gives the composites potential application, for example, in plastic welding. Preliminary experimental results are reported. ©1997 SCI

Thermal volume expansion in polymeric PTC composites: a theoretical approach

Polymeric PTC (positive temperature coefficient) composites have been prepared by incorporating carbon black into polyethylene matrices. The dependence of conductivity upon filler loading level obeys a mixtures rule, which was determined by fitting the experimental data into the general effective medium (GEM) equation. A phenomenological model is proposed which is based on the GEM equation and the dilution effect of filler volume fraction due to thermal volume expansion. By using this model, the contribution of the thermal expansion of the matrix to the jump-like PTC switch transition of the composite is quantitatively estimated, and a mechanical explanation is given. The thermal-expansion-dependent percolation curves can also be predicted. Comparisons between predicted and measured PTC effects reveal that thermal expansion is one of the leading factors responsible for the thermal switch effect. Other factors influencing the composite resistivity are discussed.

Effects of glycerin and glycerol monostearate on performance of thermoplastic starch

The effects of glycerin and glycerol monostearate (GMS) on a variety of properties of anhydrous wheat starch are explored by means of DSC, TGA, as well as mechanical, rheological and moisture sorption assessment. It is found that, as glycerin content (GC) increases, both melting point and degradation temperature decrease, and the range of the processing window extends. Melt viscosity of the glycerin-plasticized starch depends on shear rate following power law, exponentially on GC, and on temperature in Arrhenius type. Increasing GC leads to a linear increment of the equilibrium moisture sorption, which, however, can be depressed significantly by incorporating a small amount of GMS. Moreover, the presence of 2% GMS in the formulations can efficiently reduce the melt viscosity and enhance its temperature sensitivity while has insignificant effect on the non-Newtonian behaviour and mechanical properties.

Co-occurrence of photochemical and thermal effects during laser polymer ablation via a 248-nm excimer laser

Abstract Polypropylene (PP) was ablated using a 248-nm KrF excimer laser with a fluence in the range of 0.4–2 J/cm 2 and at a repetition rate of 10 Hz. It has been found that both photochemical and thermal effects are co-operative during laser ablation process, evidenced by the existence of thermal affected zone (TAZ), together with ablative zone (AZ) and unaffected zone (UAZ) in the ablated crater. Increment of both fluence and the number of pulses leads to a more striking thermal effect, demonstrated by the spatial expansion of rim for PP. Relative importance of photochemical and thermal effects was theoretically discussed.

Piezoresistance of conductor filled insulator composites

Several series of electrically conducting composites composed of a conducting filler randomly dispersed into an insulating polymer matrix were prepared. The fillers were the tin–lead alloy powder, copper powder, aluminium powder and carbon black, and the matrices were polyethylene, polystyrene and epoxy resin. The piezoresistance effects of these composites have been investigated under uniaxial presses. It was observed that the piezoresistance depends on the applied stress, filler particle diameter, filler volume fraction, matrix compressive modulus and potential barrier height. Piezoresistance increases with increase of applied stress, filler particle diameter and potential barrier height, but decreases with increases of filler volume fraction and matrix compressive modulus. A model based on the change in interparticle separation under applied stress, is developed. By analysing this model, the piezoresistance of composites is studied and the effects of influencing factors are theoretically predicted quantitatively, showing good agreement with the experimental data. © 2001 Society of Chemical Industry