| Polymer blending is one of the most promising and attractive methods to develop new materials, it can create new materials with excellent comprehensive properties by mixing two or more polymers together. Compared with the synthesis of new polymers, polymer blending has significant advantages such as shorter developing cycle, more diversified products, lower developing cost, more outstanding comprehensive properties, so it seems that polymer blending is attracting much more attention in the materials research & development industry. Poly(ether ether ketone)(PEEK) and thermoplastic polyimide(TPI) are two kinds of high performance polymers with high added value, which have wide applications in the aerospace, automotive, electronic, electrical, petroleum, chemical, bio-medical and other high-tech fields. Blending PEEK with TPI has been proved to be an effective method to obtain a kind of polymer alloy with excellent comprehensive properties, it is expected to enhance the high-temperature modulus of PEEK, and simultaneously improve the chemical resistance and processability of TPI. However, in the current market, the toughness of PEEK/TPI products is relatively poor and the variety is single, which limits their application fields greatly. Therefore, how to improve the toughness of PEEK/TPI blends and develop PEEK/TPI-based functional composites is becoming a super priority to further broaden the application areas of PEEK/TPI products.In order to improve the compatibility of PEEK and TPI, novel poly(ether ether ketone)-b-polyimide block copolymers(PEEK-b-PI) were designed and prepared by the polycondensation of amino-terminated PEEK oligomer and anhydride-terminated polyamic acid oligomer. By controlling the feed ratio of each polymization step, PEEK-b-PI copolymers with block molecular weights of 1500, 3000, 6000 and 12000 were successfully obtained. Then, PEEK-b-PI with different block molecular weights were added into the PEEK/TPI matrix(PEEK:TPI=50:50), with the same addition amount of 5 wt%. The results indicated that PEEK-b-PI with block molecular weight of 6000 showed the best compatibilizing effect, which can be explained as follows: the short block molecular weights of 1500 or 3000 were too short to adequately anchor into the interfacial areas of PEEK/TPI matrix, while the copolymer chains scission or breaking occurred when increasing the block molecular weight to 12000. So PEEK-b-PI with block molecular weight of 6000 was optimized to further discuss how the addition amount affects the properties of PEEK/TPI blends. The results showed that 5 wt% addition of PEEK-b-PI had the optimum compatibilizing effects to PEEK/TPI matrix. This is because when increase the PEEK-b-PI content in a certain range, the interfacial adhesion PEEK/TPI blend can be gradually enhanced, but when the PEEK-b-PI content is excessive, the formation of PEEK-b-PI micelles will reduce the performance of the whole system. In addition, the compatibilizing effects of PEEK-b-PI to the PEEK/TPI system were systematically investigated. With the introduction of PEEK-b-PI, the interfacial domain size of PEEK/TPI matrix was significantly reduced and the compatibility of PEEK and TPI was enhanced. Specially, 5 wt% content of PEEK-b-PI can improve the elongation at break of PEEK/TPI blends by 200%, and the 5% weigh loss temperature of the blends was all above 560 oC. The storage modulus and dimensional stability of the system were also improved effectively. Besides, for PEEK/TPI blends with various PEEK:TPI ratios(75:25 or 25:75), the addition of PEEK-b-PI can also effectively reduce the interfacial tension and improve the compatibility between PEEK and TPI. In a word, PEEK-b-PI is evidenced to be an ideal compatibilizer for PEEK/TPI blends.In order to broaden the application areas of PEEK/TPI blends, multi-walled carbon nanotubes(MWCNTs) were introduced into the PEEK/TPI matrix as the functional filler, and high performance conductive composites were prepared. According to the theoretical calculation, SEM and TEM results, MWCNTs were evidenced to be selectively distributed in the TPI phase, which is due to the different affinity of MWCNTs to PEEK and TPI. Then, the dependence of the electrical conductivity of PEEK/TPI/MWCNTs composites on the phase morphology was studied by tuning the PEEK/TPI ratios, and the maximum conductivity was obtained when PEEK:TPI=50:50, which can be attributed to the selective distribution of MWCNTs and the co-continuous phase structure of the composites. Therefore, PEEK/TPI blend with PEEK/TPI ratio of 50/50 was selected to further discuss how the MWCNTs contents affect the properties of PEEK/TPI blends. Due to the selective location of MWCNTs, the phase morphology of PEEK/TPI/MWCNTs composites was changed from sea-island to co-continuous structure with increasing contents of MWCNTs. Impressively, the electrical percolation threshold of the composites was reduced to as low as 0.6 wt% owing to the double percolation phenomenon, this is the lowest value reported so far for PEEK-based MWCNTs composites. Attributed to the excellent properties of high strength, high elastic modulus and outstanding thermal stability of MWCNTs, the introduction of MWCNTs could significantly improve the comprehensive properties of the blends. The storage modulus in the temperature range of 200-240 oC were increased by 31% with the addition of merely 0.4 wt% MWCNTs, and the 5% weigh loss temperature of PEEK/TPI/MWCNTs composites was all above 568 oC under air atmosphere. In summary, a high performance material with outstanding electrical properties and excellent comprehensive properties were obtained by controlling the selective location of MWCNTs in PEEK/TPI matrix.Furthermore, conductive carbon black(CB) was introduced into the PEEK/TPI matrix(PEEK:TPI=50:50) in order to develop a conductive material with lower price. According to the phase structure and galss transition temperature(Tg), CB was proved to be selectively distributed in TPI phase in the PEEK/TPI/CB composites, because of the lower interfacial tension between TPI and CB compared with that between PEEK and CB. With increasing CB contents, the phase morphology was changed from typical sea-island to co-continuous structure, which was attributed to the selective location of CB. Notably, the electrical percolation threshold of PEEK/TPI/CB ternary composites was reduced to as low as 5 wt%, which was significantly lower than that of corresponding binary TPI/CB(10 wt%) and PEEK/CB(9 wt%) composites. At identical 7.5 wt% CB content, the conductivity of PEEK/TPI/CB composites was 104 to 106 times higher than that of corresponding binary composites, and when increase the CB content to 12.5wt%, the electrical conductivity of PEEK/TPI/CB composites reached 5.33×10-1 S m-1. Moreover, the incorporation of CB could improve the comprehensive properties of PEEK/TPI blend effectively, including mechanical properties, storage modulus, dimensional stability, thermal stability. In summary, PEEK/TPI/CB composites were evidenced to be a high performance conductive material with relatively lower price and outstanding comprehensive properties. This structurally and functionally integrated material has wide applications in antistatic, conductive and electromagnetic interference shielding fields. |
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