Preparation And Properties Of Polymeric Composites Filled With Low-Melting-Point Alloy

Due to the relatively low melting point, the low-melting-point alloy filled in the polymer matrix will melt during the processing and application, and the state transition between rigid particles and deformable droplets can thus be achieved. Therefore, the low-melting-point alloy filled polymer composites (LA-PMCs) have many special properties, which are not held by usual materials. In this Ph.D. dissertation, the researches have been focused on the preparation-structure-property relations of LA-PMCs, especially the electrical properties and Theological behaviors.The influences of preparation conditions on the electrical properties of LA-PMCs have been studied first. It is found that the processing temperature is the determining factor of the electrical conductibility of LA-PMCs. In order to achieve excellent electrical conductivity, the composites should be processed at temperatures below the melting point (Tm) of the alloy. This is because that the original structure and segregated dispersion of fillers are kept at these temperatures, and the conductive networks are thus easily formed.It is found that the resistivity-temperature characteristics of LA-PMCs vary with the selection of polymer matrices. When the amorphous polymer, such as the polystyrene (PS), is used, the LA-PMCs exhibit distinctive positive temperature coefficient (PTC) effects near the Tm of the alloy fillers instead of the Tm of polymer matrix. This is believed to be due to the surface energy difference between the alloy and polymer. However, when the semi-crystalline polymer, such as high-density polyethylene (HDPE), is used as matrix, double PTC effects appears. The first PTC transition emerges at Tm of the polymer matrix as usual, while the second one appears at the Tm of alloy. The two PTC transitions obey two different mechanisms, and possess different characteristics.The improvement of reproducibility for the resistivity-temperature characteristic of LA-PMCs has been performed by the surface treatment of alloy fillers with titanate. The enhancement of interfacial interaction decreases the movability of alloy fillers, and thus improves the reproducibility of resistivity-temperature characteristics. At the same time, the surface treatment of alloy fillers increases the PTC transition temperature, but decreases the PTC intensity.Besides the temperature, the applied pressure can also alter the electrical conductibility by changing the interparticle separation, which leads to the piezoresistance of conductive composites. By investigating eleven composite systems, it is found that the piezoresistance increases with increase of applied pressure, filler particle diameter and potential barrier height, but decreases with increase of filler content and matrix compressive modulus. Furthermore, the piezoresistance alsochanges with time at a fixed pressure. The increases of applied pressure, filler particle diameter and polymer matrix creep enhance the time dependence of piezoresistance, while the increase in filler content weakens it.A pressure-time-resistance model has been developed to predict the piezoresistance and its time dependence for the conductive polymer composites. In this model, the resistance-pressure relationship is established based on the investigation of interparticJe separation change under the applied pressure, while the resistance-time relationship based on the analysis of polymer creep. By analyzing this model, it is found that the actions of applied pressure, filler content, filler particle diameter and matrix compressive modulus (or polymer creep behaviors) on the piezoresistance and its time dependence are realized by influencing the change process of interparticle separation. However, the potential barrier height alters the piezoresistance by directly changing the transmission capability of electrons.After the study of electrical properties for LA-PMCs, attention has been paid to the rheological behaviors. In the static rheological measurement below Tra of the alloy, the rigid alloy particles retard t...