On Electrically Conductive In-Situ Microfibrillar Polymer Composites And Their Morphology, Structure And Properties

Functionalization of general-purpose plastics (mainly polyolefin (PO)) is one of the most important research subjects in the field of polymer materials science and engineering at the present and in the future. Electrical conductivity is one of the most important functions of the general-purpose plastics. Filling conductive particles is the main route to fabricate this function. Controlling the distribution and arrayment of the conductive fillers in the polymer matrices is an important method to obtain the conductive polymer composites with high performance and low price. It is also the development tendency of the conductive polymer composites. Based on the technologies of controlling morphology during the processing, this dissertation has put forward a simple and effective method to obtain low price and high performance conductive polymer composites.The fibrillation during the processing, morphology, microstructure, percolation behavior, and temperature and chemical liquid response properties of the PO based carbon black (CB)/ poly (ethylene terephthalate) (PET)/ polyethylene (PE) composite were investigated in this thesis. A large quantity of valuable data and results were obtained, which is of importance to develop the conductive theory and double percolation theory of the conductive polymer composites. These results also provide new ideas and methods to prepare conductive polymer composites with low price and excellent balanceable property. The main results are:(â… ) Preparation, microstructure and percolation behavior for the eleetrieaUy conductive in-situ microfibrillar composites The melt mixing -extrusion at high temperature-quenching-cold moulding process was used to prepare the conductive microfibrillar CB/PET/PE composites in this thesis. The formation of microfibrils, the distribution of conductive fillers and electrical properties were systemically studied.(1) CB aggregates are selectively located in the PET microfibfils during the whole melt mixing -extrusion at high temperature-quenching-cold moulding process. This phenomenon can be explained by thermodynamics and dynamics. By the view of thermodynamics, based on Young's equation and the minimization of the dissipative energy, CB aggregates prefer to locate in PET owing to the two points: (1) the interfacial tension between CB and PET is lower than that between CB and PE, and (2) the apparent viscosity of PET is lower than that of PE at the extrusion temperature. On the other hand, CB was first compounded with PET prior to extrusion and hot stretching. Hence there was sufficient time for CB particles to mix with PET by the view of dynamics.(2) The droplet-fiber transition in the electrically conductive in-situ microfibrillar CB/PET/PE composite strongly depends on the viscosity ratio of CB/PET dispersed phase and PE matrix. When the viscosity ratio is lower than 1, well defined in-situ micro fibrils with high length/diameter ratio can be formed in the system. As the viscosity ratio is slightly higher than 1, micro fibrils can also be formed, but the length/diameter ratio is relatively low. With the further increase of length/diameter ratio, the microfibrils can not be formed when the length/diameter ratio is far higher than 1. The viscosity ratio depends on the CB/PET dispersed phase as the matrix is fixed at PE. High structure CB and high loading of CB go against the formation of in-situ micro fibrils.(3) A special microstructure is formed in the microfibrillar composite, in which a 3D network is formed by in-situ CB/PET microfibrils, and CB is selectively located in the PET dispersed phase. As a result, the percolation threshold of electrically conductive in-situ microfibrillar CB/PET/PE composite is obviously lower than that of common CB/PET/PE and common CB/PE composite.(4) Owing to the reinforcement of the CB/PET microfibrils, the in-situ microfibrillar composites can keep their mechanical strength while reducing the percolation threshold and the cost.(â…¡) The role of the surface microstructure of the microfibrils on the percolation behavior of the in-situ microfibrillar composite(1) The relationship between the microstructure and the percolation behavior of the in-situ microfibrillar CB/PET/PE composite was studied in this thesis. It was found that the percolation behavior of the composite can not be explained by the classical double percolation theory.(2) The influences of the surface microstructure of the fibrils on the conductive properties were studied. The surface microstructure of the microfibrils was found to be the key factor affecting the percolation behavior of the ternary composite. When the CB loading is lower than the maximum packing fraction, there are no CB particles on the surface of the microfibrils, resulting in the high contact resistance among the microfibrils, and thus, the volume resistivity of the ternary composite remains high. As the content of CB is beyond the maximum packing fraction, the number of the CB particles dispersed on the fibrils' surface increases quickly, and the conductive contacts among the microfibrils increase accordingly. When the concentration of CB particles on the CB/PET microfibrils is higher than a critical value, the microfibrils network connected by electrically conductive contact points is able to sustain the electron transmission in the whole system and as a result, the volume resistivity of in-situ microfibrillar CB/PET/PE composite drops sharply and the percolation happens.(â…¢) The temperature response properties of the electrically conductive in-situ microfibrillar composite(1) The resistance-temperature effect of the electrically conductive CB/PET/PE composite was studied in this thesis. The composite exhibits a strong PTC effect and a weak NTC effect during the first few heating-cooling cycles. Compared with common CB/PE composite, in-situ microfibrillar composite has excellent PTC/NTC property. The large size effect of CB/PET microfibrils is the origin of weak NTC effect during the early heating-cooling recycles.(2) After ten heating-cooling cycles or after high-temperature thermal treatment for a long time, the PTC effect of the composite exhibits an anomalous strong attenuation. The resistance becomes insensitive to the temperature. That is, the resistant can keep stabilization as the surrounding temperature change. The unique microstructure and the relatively large size of the microfibrils is the key factor of this anomalous phenomenon. Based on the inhomogeneous microstructure of the surface of microfibrils consisting of conductive and insulative areas, during the heating-cooling recycles or the thermal treatment for a long time, the electrically conductive network becomes more stable and more perfect owing to the interacting among the CB aggregates. During the cooling process, the large size of CB/PET microfibrils can effectively protect the conductive network from being destroyed by crystallization. The more stable and perfect conductive microfibrillar network generated in PE melt can, thus, at least partially survive during crystallization, and consequently, the PTC effect attenuates.(3) This anomalous phenomenon is of great importance to develop an effective way is developed to fabricate recyclable semicrystalline thermoplastic (SCTP) based conductive composite with stable conductive properties.(â…£) Chemical liquid response properties of the electrically conductive in-situ microfibrillar composite(1) The chemical liquid response properties of the electrically conductive in-situ microfibrillar CB/PET/PE composite were studied in this thesis, compared with the common CB/PE composite. The intensity of the chemical liquid response for the microfibrillar composite was found to be much higher than that for the common CB/PE composite. A new idea to develop high sensitive chemical response polymer materials can be provided according to this phenomenon.(2) The influence of the sample's thickness of the composites on the rate of response was studied. Compared with that for common CB/PE, the rate of chemical liquid response for in-situ microfibrillar composite is more sensitive to the sample's thickness. Preferentially occupying the interface of PET/PE for the chemical liquid was found to be the origin of this phenomenon.(3) The rate of chemical liquid response for in-situ microfibrillar composite is more sensitive to the liquid temperature compared with that for common CB/PE composite. The restriction effect of the microfibrillar network to the PE chains is regarded as the main reason of this phenomenon.(4) The chemical liquid responsible PTC effect of in-situ microfibrillar CB/PET/PE composite was studied. It was found that both the in-situ microfibrillar composite and the common CB/PE composite exhibit strong chemical liquid responsible PTC effect. The surface of these two composites was destroyed during the heating-cooling cycle. Owing to the microfibrils network, the damage of the surface of microfibrillar composite was weaker than that of common CB/PE, resulting in the better reversible properties of the resistivity for the microfibrillar composite.(5) A unique voltage induced saltation of resistivty in the liquid response measurement was found in the in-situ microfibrillar CB/PET/PE composite. It provides a chance to thoroughly study the conductive mechanism of in-situ microfibrillar composite with the chemical liquid surrounding according to this unusual phenomenon.Base on the content of this thesis, three techniques of the preparation of the following blend materials were obtained: (a) conductive polymer composites with electrically conductive in-situ microfibril network; (b) recyclable SCTP based conductive composites with stable conductive properties; (c) high sensitive chemical response polymer composites. The main raw materials in this thesis including conductive CB, PET and PE can be exchanged by common CB, common PO and common general engineering plastics (GEP). For these raw materials, many grades can be chosen, the resource is wide and the price is low. In addition, the processing operation of these materials can be controlled easily, and it does not have excessive requirements for the processing apparatus. Therefore, the industrial manufacturing of these three materials can be successfully carried out.