The Effects of Particle Size on the Morphology and Properties of Polyimide/nano-Al 2. Composite Films

Transcription

1 The Effects of Particle Size on the Morphology and Properties of Polyimide/nano- The Effects of Particle Size on the Morphology and Properties of Polyimide/nano- Lizhu Liu 1,2*, Hui Shi 1, Ling Weng 1, Jun Ding 1, and Weiwei Cui 1 1 College of Material Science and Engineering, Harbin University of Science and Technology, Harbin, , People s Republic of China 2 Key Laboratory of Engineering Dielectric and Its Application, Ministry of Education, Harbin University of Science and Technology, Harbin, , People s Republic of China Summary Polyimide/nano-sized films based on pyromellitic dianhydride(pmda) and 4,4 -oxydianiline(oda) were synthesized by incorporation with different particle sizes of nano- via in situ polymerization. The morphology and structure of films were studied by scanning electronic microscopy (SEM). The properties including the tensile strength, corona resistance, electric breakdown strength and dielectric properties were investigated. Results indicated that the size of nano-particles had a great effect on the microstructures of films. The smaller size of the particles, the more regularity of films were obtained. Properties testing results indicated that by increasing the particles dimension would decrease the mechanical properties and electric breakdown strength of films, while the dielectric constant remain nearly stable. Results also showed that after addition, the corona resistance of films was improved, compared with that of pure PI films. The longest corona resistance time was obtained when the smallest particles was used, which had a three times longer than PI film with largest addition. Keywords: Polyimide, Nano-, Morphology, Electrical property 1. Introduction Polyimides have been extensively used in microelectronics industry as a material for electrical insulating and electronic packaging due to their favorable mechanical and electrical insulating properties. Moreover, their excellent high thermal stability and easily processing characteristics make them play an important part in aerospace applications. With the rapid development of industry, polyimide materials with more outstanding functions are usually required 1-2. In recent years, polyimide materials have received much attention due to its dramatic improvements in mechanical property, thermal stability, corona-resistance property and other special characteristics by Smithers Rapra Technology, 2014 introducing small fraction inorganic compounds, such as clay, silica and aluminum nitride 3-5. Among these inorganic additives, nano-alumina ( ) is often chosen to improve the electrical property of polyimide materials 6-8. Many researchers have investigated the polyimide/ membranes and reported that the mechanical property, electric breakdown strength and thermal property of polyimide could be considerably improved by the dispersed alumina particles in polyimide matrix. However, the studies about the property and microstructure of this kind of membranes are still far from perfection. Especially, little information has been focused on the effects of different sizes of inorganic particles on properties of polyimide/ nano- membranes. In our present work, we prepare polyimide/nano- membranes via in situ polymerization with alumina particles as the reinforcing materials and polyimides as the matrix. A series of novel PI/ nano- films based on PMDA and ODA were obtained with different sizes of nano- particles. The microstructure and properties of these films were investigated. Especially, the effect of alumina particles dimension on the microstructure and property of membranes were studied. 2. Experimental 2.1 Materials N, N-dimethylacetamide (DMAc) was analytical grade and purchased from Tianjin Basifu Chemical Co. 4, 4 -Oxydianiline (ODA) and pyromellitic dianhydride (PMDA) were chemical grade and purchased Polymers & Polymer Composites, Vol. 22, No. 2,

2 Lizhu Liu, Hui Shi, Ling Weng, Jun Ding, and Weiwei Cui from Shandong Wanda Chemical Co. Three kinds of α-phase powder, of which the particle size 30 nm, 50 nm and 100 nm, were purchased from Hangzhou Wanjing New Material Co. 2.2 Preparation of Nano- / PI Composite Membranes In this work, the PI/nano- films were prepared by in-situ polymerization. The polyamic acid (PAA) solution with 20 wt.% solid concentrations was prepared by dissolving ODA powder in DMAc mixture solution placed in a 250 ml three-necked round-bottom flask at room temperature. Then the calculated amount of nano- particles with three particle sizes (30 nm, 50 nm, and 100 nm, respectively) were added into pre-solution with the aid of ultrasonic wave treatment for 0.5 h. Then an equimolar amount of PMDA was gradually added into the mixture solution to ensure dissolution completely 9, and stirred for 10 h to obtain the uniform PAA/nano- precursor solution. Then the solution was casted on a glass substrate and dried at 80 C for 1 h, 100 C for 1 h, 140 C for 1 h, 220 C for 2 h, 300 C for 1 h, 350 C for 2 h, separately. The PI/nano- films were obtained after the films were stripped off from the substrate. The films were 30±2 μm in thickness. 2.3 Measurement Scanning electron microscopy (SEM) was conducted by using a HITACHI S-4300 electron microscope to observe the dispersion of alumina particles in polyimide matrix. Mechanical properties (tensile strength and elongation at break) were tested on XLD Electronic Tensile Apparatus at a tension rate of 50 mm/min. At least five samples were tested for each membrane. The dimensions of samples were 10 mm 150 mm. The electric breakdown strength was measured with the HT-5/20A equipment. Dielectric properties, such as dielectric constant and dielectric loss, were measured on a capacitance meter (Precision Impedance An-alyzer-4294A) in 50 Hz at 30 C. The corona resistance of films was measured by using HP-3kV High-frequency pulse voltage tester under a voltage of 2 kv. 3. Results and Discussions 3.1 Morphology of Composite Membranes The fracture surfaces microstructures of PI/nano- membranes with different sizes of nano- particles were observed by SEM and results were shown in Figure 1. As shown in Figure 1a, it could be clearly seen that the nano- particles with 30 nm size were distributed uniformly in PI membranes. There was no obvious phase separation between the inorganic particles and the polyimide matrix, which indicated a good compatibility between the matrix and fillers. The interfacial bonding between the particles and the PI matrix were considerably good because any gaps or porosities in the membranes were barely observed. However, with the particles dimension increase to 50 nm and 100 nm, as showed in Figure 1b and c, both the SEM micrographs revealed that the nano- particles were packed and obvious phase separation between the inorganic particles and the polyimide matrix could be found. This should be attributed to the strong interaction force and aggregation tendency between large particles and the rather weak dispersion capability of equipment for large particles. 3.2 Mechanical Properties of The mechanical properties of the PI/ nano- films were examined and the results were listed in Table 1. It could be found that the tensile strength for the PI/nano- (30 nm) films was MPa, while the tensile strength of PI/nano- (50 nm) films was decreased to MPa. With the particle size further increased to 100 nm, the tensile strength Figure 1. The fracture surfaces SEM of PI/nano- films with different sizes of particles: (a) 30 nm; (b) 50 nm; (c) 100 nm (a) (c) (b) 118 Polymers & Polymer Composites, Vol. 22, No. 2, 2014

3 The Effects of Particle Size on the Morphology and Properties of Polyimide/nano- decreased markedly to MPa, which was 16% lower than the PI/ nano- films mixed with 30 nm diameter. The decreased in the tensile strength reflected that the dispersion of large size nano- particle into polyimide films was poor. Table 1. Mechanical properties of PI/ nano- membranes with different sizes of Particle size (nm) Tensile strength (MPa) Elongation (%) Table 1 also showed that the elongation at break for PI/nano- (30 nm) films was 23.28%, while the elongation at break increased to 24.07% as the membrane added with 50 nm. This increase in toughness may be attributed to the strong interfacial interaction between the polyimide matrix and the particles. Further increase in the dimension led to a great decrease in the elongation at break, which could be attributed to the addition of large nano- particles that interrupted the integrity of PI molecule and formed some defects such as aggregation and small hole 10,11. particles showed the electrical breakdown strength of MV/m, which was just a slightly higher than that of un-doped PI film (201 MV/m). However, with the nano- dimension increased to 50 nm and 100 nm, the electrical breakdown strength of the films reduced to MV/m and MV/m, which were lower than the un-doped ones. Combined with the SEM microstructural analysis, this phenomenon may be owing to the aggregation, which mainly found at the films incorporated with a relative large particle, might act as an impurity that cause some defects to deteriorate the electrical breakdown strength. 3.4 Dielectric Properties of Figure 3 showed the effect of particle size on the dielectric properties value of the PI/nano- membranes at the sweep frequency of 50 khz measured at room temperature. It could be seen that the inorganic particle size had little effect on both the dielectric constant and dielectric loss. However, many other researchers had reported that the particles size had a great effect on the dielectric properties of s, which was not similar with us We thought that this difference should be attributed to the generation of a large number of defects in the films. These defects commonly included the reunion of nano- particles, the bad combination between nano-particles and PI matrix and the formation of the tiny pores in the films. 3.5 Corona Resistance of The phenomenon generally known as corona that could cause ionization in the insulating layer is recognized as the major reason for electric breakdown of an insulation material when the voltage stress reached a critical level. Many researchers had reported that the polyimide films with addition of some ultrafine inorganic additives showed good corona resistance property 17. Figure 4 illustrated the time to failure for nano- /PI films in electrical aging test, which was determined by the breakdown time of the membranes under a voltage of Figure 2. Electrical breakdown strength of the PI/nano- films 3.3 Electric Breakdown Strength of Being a key parameter, the electrical breakdown strength has been widely used to measure the insulating capability of the dielectrics, because breakdown would cause short circuit which could be a fatal malfunction for the power equipment 12,13. Figure 2 was the electric breakdown strength of PI/nano- films with different sizes. It was seen that the electrical breakdown strength were gradually decreased while the particle size increased. The films with 30 nm Polymers & Polymer Composites, Vol. 22, No. 2,

4 Lizhu Liu, Hui Shi, Ling Weng, Jun Ding, and Weiwei Cui Figure 3. Dielectric properties of films incorporation with in different particle size Figure 4. Corona resistance time of the PI/nano- films improved electrical aging performance of PI films should be attributed to the addition and highly dispersion of nano- particles in the PI matrix, which gave rise to the structural change in s and exhibited excellent effect to prevent the material from corona damaging. However, as the inorganic particles size became large and exceeded a certain value, particles would disperse heterogeneously and they would lead to a decrease in the corona-resistance property. Figure 5 and Figure 6 showed the surface SEM microstructures of nano- /PI films before and after corona tests. From it one could clearly see that after high-frequency corona resistant, the surface of PI/nano- films became uneven and many crackles on the surface were found. The corona resistant ability of PI/nano- films should be attributed to the existence of inorganic particles, which could dissipate the heat generated by corona and reduce the risk of thermal breakdown. Moreover, the inorganic particles themselves also have a better corona resistance and corrosion resistance, which makes the corona resistance time of films longer than pure film. constant electric field. Results indicated that the films doped with nano- powders showed improvement in electrical aging performance as compared with pure PI film. The electrical aging performance for with pure PI film was 18 min. When the PI film added with 30 nm particles, the corona resistance exhibited a significant enhancement to about 99 min, which is 5.5 times longer than that of pure PI film. The 4. Conclusions A series of PI/nano- films incorporation with different fillers diameters were prepared via in situ polymerization. The effects of the dimension of inorganic nano- particle on the properties and microstructure of films were investigated. Results indicated that the particle size had a great effect on the PI/nano- films. With the increase of nano- particle size, the dispersion of particles in PI matrix became poor. Moreover, the molecular order of PI matrix was destroyed by the addition of large particles. The addition of small nano- particles could 120 Polymers & Polymer Composites, Vol. 22, No. 2, 2014

5 The Effects of Particle Size on the Morphology and Properties of Polyimide/nano- Figure 5. Surface morphology of the PI/nano- film before highfrequency corona aging Figure 6. Surface morphology of the PI/nano- film after highfrequency coron aging improve the mechanical properties and electrical properties of PI films obviously. Acknowledgements This research was supported by the Nation Science Foundation Grant ( , ), the Natural Science Major Foundation of Heilongjiang Province of China (ZD201004) and the National Basic Research Program of China (2012CB723308). References 1. Ghosh M.K. and Mittal K.L., Polyimides; fundamentals and applications. New York, Marcel Dekker, Sroog C.E., Polyimides, Prog. Polym. Sci., 16(4) (1991) Rubia L., Vasconcelos F.S., and Wander L., Mater. Res., 5 (2002) Al-Kandary S., Ali A.A.M., and Ahmad Z., J. Appl. Polym. Sci. 98 (2005) Tong Y.J., Li Y.S., and Ding M.X., Polym. Inter., 49 (2000) Li H., Liu G., Liu B., Chen W., and Chen S., Mater. Lett., 61 (2007) Lizhu L., Bing L., Wei W., and Qingquani L., J. Comp. Mater., 40 (2006) Cai H., Yan F., Xue Q., and Liu W., Polym. Test., 22 (2003) Licari J.J., Coating Materials for Electronic Application: Polymers, Processing, Reliability, Testing, Noyes Publication/William Andrew Inc., 2003, pp Li H.Y., Liu G., Liu B., Chen W., and Chen S.T., Mater. Lett., 61 (2007) Magaraphan R., Lilayuthalert W., Sirivat A., Schwank J.W., Comp. Sci. Tech., 61 (2001) Li H., Liu Y., Liu G., Chen B., W. Chen S.T., Dielectric properties of polyimide/ hybrids synthesized by in-situ polymerization, Materials Letters, 61 (2007) R. Magaraphan, W. Lilayuthalert, A. Sirivat. Preparation, structure, properties and thermal behavior of rigid-rod polyimide/montmorillonite nanos, Composite Science and Technology, 61 (2001) Choi S.H., Kim T.D., Hong J.M., Park K.H., Oh S.G., Effect of the dispersibility of BaTi nanoparticles in BaTi /polyimide s on the dielectric properties, Materials Letters, 61 (2007) Wang S.F., Wang Y.R., Cheng K.C. and Hsaio Y.P., Characteristics of polyimide/barium titanate films, Ceramics International, 35 (2009) Wang F.J., Li W., Xue M.S., Yao J.P., Lu J.S., BaTi polyethersulfone nanos with high dielectric constant and excellent thermal stability, Composites Part B: Engineering, 42(1) (2011) Draper R.E., Jones G.P., Rehder R.H., and Stutt M., Sandwich insulation for increased corona resistance. US Pat 5,989,702. Polymers & Polymer Composites, Vol. 22, No. 2,

6 Lizhu Liu, Hui Shi, Ling Weng, Jun Ding, and Weiwei Cui 122 Polymers & Polymer Composites, Vol. 22, No. 2, 2014