مطالعه ای بر روی اثر میدان های مغناطیسی بر روی مواد پلیمری و کاربرد آن Study on the Effect of Magnetic Fields on Polymeric Materials and Its Application

Magnetic effects on diamagnetic materials have been known since the age of Faraday, but it is very recent that the attention has been paid to the use of these effects to the processing of diamagnetic materials including inorganic, organic, and polymeric materials. This trend is partially due to the development of superconducting technology1 that enables us to use high magnetic fields (10 T or more) in the study of materials science at individual laboratory level. Cryogen free (liquid-helium free) superconducting magnets, mostly manufactured by Japanese companies, are now on the market, with various types including a large bore type (45 cm at 3.5 T), a rotate bore type, a split type, a low fringe field type, a high field type (15 T, 52 mm in diameter), etc. These magnets are used in academia as well as industries for the processing purposes. High magnetic fields provided by these superconducting magnets have made it possible to visualize the magnetic effects on “non-magnetic” materials such as diamagnetic materials. Because the diamagnetism is very small compared to the ferromagnetism, we hardly experience in daily life the effects of magnetic field on plastics, water, and living bodies, etc. Some means must be devised to visualize these effects. A straightforward way is to use high magnetic fields. If we use a superconducting magnet of 10 T instead of an electromagnet generating 1 T, a small change of 1 mm is magnified to 10 cm and a phenomenon that takes a day to occur is completed in 15 min, because the magnetic effect is proportional to the square of the magnetic flux density. Diamagnetic levitation2–۴ and Moses effect5 (water surface splits in a high magnetic field) are good examples among many others.6–۹ CONCLUSIONS Magnetic effects on “non-magnetic” materials are small but existing in any materials including polymeric materials. They are too small to be detected easily under the field strengths as low as generated by permanent magnets and electromagnets. Advent of liquid-helium free superconducting magnets has facilitated the use of high magnetic fields to result a number of new findings that would have been impossible under low fields. The use of high fields has been paid attention in various fields in academia and industries. Crystalline polymers were considered to be unable to undergo magnetic alignment because they lack ordered structures required for the alignment to occur. However, we have found that a number of crystalline polymers including poly(ethylene terephthalate), poly(ethylene-2,6-naphthalate), etc. do undergo magnetic alignment during crystallization from melts. The origin of the alignment is attributed to anisotropic structures (mesophase) transiently forming during crystallization and/or existing in the melt. Current understanding of the alignment mechanism is that these anisotropic structures rotate under magnetic torque, resulting alignment. Another possibility of preferential formation and preferential growth of anisotropic structures in a specific direction with respect to the field, resulting in alignment, is not ruled out. Since the polymeric systems are rich in mesophase especially during phase transition, many magnetic effects are expected if polymeric systems are exposed to the external magnetic field during they are undergoing a phase transition. The use of moderate field strengths may be preferred in the actual application of the magnetic alignment in industries. This is possible in suspensions. Fibers and fine crystalline particles of sub-micron sizes suspended in a low viscosity liquid can highly and quickly align in moderate fields provided by electromagnets or even permanent magnets. The key issue here is to disperse these particles in a stable manner and to fix the attained alignment. Precision alignment, which enables uniaxial alignment of negative anisotropic (χa < 0) fibers, for example polyethylene fiber, would be useful to attain desired alignment of crystallites and etc. Levitation is another field of interest. Pseudo-zerogravitation circumstances are provided by the magnetic levitation. They are different from the zero-gravity realized in the space in that the effect of trapping and alignment also comes in. In addition, if the medium surrounding the levitating particle is relevant, the hydrodynamic and magnetic buoyancies become dominant, which deviates the levitation from the pure zero-gravity circumstances. The magnetic force causing levitation would be also useful for micro- to nano-patterning of particles, if the microscopic inhomogeneity of magnetic field is available. Superconducting technology is continuously progressing. In future, 30-tesla class magnets may become widely accessible to individual laboratory level like 10- tesla ones at present time. Further progress is expected in future in academia as well as in industry