Magnetorheological behaviour of magnetic carrageenan gels against shear and compression strains

Transcription

1 e-polymers 2011, no ISSN Magnetorheological behaviour of magnetic carrageenan gels against shear and compression strains Keisuke Negami, Tetsu Mitsumata * * Department of Polymer Science and Engineering, Graduate School of Engineering, Yamagata University, Yonezawa , Japan; fax: +81(0) ; tetsu@yz.yamagata-u.ac.jp. (Received:15 June, 2010; published: 17 April, 2011) Introduction Abstract: Magnetic field effect on the elasticity was investigated for magnetic carrageenan gel when a shear and compressional deformation was applied to the gel. The magnetic carrageenan gel consists of carrageenan of a polysaccharide, and carbonyl iron particles. The dynamic viscoelastic measurement with shear strain revealed that the storage shear modulus of the gel increased from to Pa by applying a magnetic field of 320 mt. On the other hand, the compression measurement showed that the Young s modulus increased from to Pa. The relative changes in the modulus with respect to the original modulus were 230 for shear strain and 9.5 for compressional strain, respectively. This strongly indicates that the magnetic field effect on viscoelasticity strongly depends on the geometry of directions of magnetic field and strain. The effect of vibration suppression of the present gel tuned by magnetic field is also presented. Dispersions of magnetic particles alter its viscoelastic properties in response to magnetic fields. This phenomenon is called the magnetorheological (MR) effect. Similarly, polymer gels containing magnetic fluids or magnetic particles exhibit the MR effect. Many attempts to fabricate MR gels using synthetic polymer [1], silicone elastomers [2-5], and rubbers [6] have been tried by many researchers. These papers reported that the relative change in storage modulus with respect to the original modulus was less than three fold. So far, we have investigated the MR behaviour of various polysaccharide hydrogels [7-10], and found a new class of magnetic gel that responds to giant and reversible changes in dynamic shear modulus by a relatively weak magnetic field (500 mt) [11]. The relative change in storage shear modulus reached 500 fold due to the magnetic field, which is far higher than the past studies. A chain structure bridging between two parallel plates is considered to bring the gel the dramatic changes in the shear modulus. This magnetic gel now attracts considerable attention of engineers to fabricate variable elastic devices which are aimed for active damper, tactile sense communicator, or smart furniture. In many cases for these applications, the magnetic gel is employed under compression deformation, therefore, the MR properties under compression deformation are important rather than shear deformation. Additionally, the geometry of directions of magnetic field and deformation should also affect the MR effect. Indeed, it has been reported that the longitudinal modulus measured by ultrasound varies by the geometry of magnetic field and deformation [12]. In this paper, we describe the MR effects of magnetic carrageenan gel measured by shear 1

2 G'/Pa and compression strains, and briefly discuss the geometry effect on the MR effect. As an application of the present gel, the attenuation effect using the elasticity changes is also presented. Results and discussion The strain dependence of the storage shear modulus, G, under various magnetic fields is shown in Figure mt mt mt 50 mt mt Fig. 1. Strain dependence of storage shear modulus of magnetic carrageenan gels under various magnetic fields. The storage modulus of the magnetic carrageenan gel without magnetic field was ~ Pa and was nearly independent of the strain. When magnetic fields were applied, the storage modulus increased significantly at whole strains. It needs to be mentioned that the storage modulus under magnetic field exhibited nonlinear viscoelasticity at high strains (γ> ). This phenomenon has been called the Payne effect that was observed for composite rubbers and elastomers with high volume fraction of fillers [13]. It has been interpreted that the Payne effect originates from the temporary destruction of a particle network within the materials. The nonlinear viscoelasticity observed under magnetic field strongly indicates that the magnetic particles form a chain structure within the gel, although the particles were embedded and fixed in the crosslinked network. Figure 2(a) shows the storage shear modulus at linear viscoelastic regime (γ=10-4 ) when a stepwise magnetic field between 0 and 320 mt was applied. The gel demonstrated huge, exceeding two orders of magnitude, and reversible changes in G synchronized with the magnetic field. The absolute change in G observed here was 2.3 MPa, which corresponds to the relative change in the modulus with 230, that is the ratio of the storage modulus at B=320 mt to the modulus at 0 mt. It has been reported for silicone gels with carbonyl iron particles that the increase in the storage modulus was 30 kpa and the relative change in the modulus was 2.5 [2], which is far smaller than the observed value. The observed value is also higher than the calculated value estimated by the finite element analysis based on magnetic dipoledipole interaction (=0.65 MPa) [14]. The magnetic particles embedded in gels might move easily compared to elastomers, since the gel is swollen by water (water content 2

3 B/ mt G'/Pa G'/Pa ~60 vol%). Figure 2(b) shows the storage shear modulus at nonlinear viscoelastic regime (γ=10-2 ) when a stepwise magnetic field between 0 and 320 mt was applied. As shown in Figure 1, the magnetic field effect on the shear modulus was diminished at high strains. However, the magnetic gels demonstrate still huge change in shear modulus with 0.74 MPa, which corresponds to the relative change in the modulus with (a) (b) time/s Fig. 2. Pulsatile response of storage shear modulus of magnetic carrageenan gels at strains of (a) 10-4 and (b) Figure 3 shows the stress-stain curves of magnetic carrageenan gels with and without magnetic field at compression speeds of mm/min. At 0 mt, the stress was low and it was nearly proportional to the strain at whole strains. On the other hand, at 320 mt, the stress at low strain (γ<0.01) was insensitive to the magnetic field, however at high strain, it took higher values than those at 0 mt. The Young s modulus of the gel was evaluated at the region in which the stress was proportional to the strain. At 0 mt, the Young s moduli at 1 and 100 mm/min were determined to be and Pa, respectively. At 320 mt, the Young s moduli at 1 and 100 mm/min were determined to be and Pa, respectively. It was found 3

4 Stress/ Pa that the Young s modulus increased by magnetic field. Similar field effect was observed in the compression test for silicone rubber containing carbonyl iron powder (20% increase in the stress at a given deformation) [15]. The absolute changes in the Young s modulus at 1 and 100 mm/min were and Pa, respectively, which is one order of magnitude smaller than the modulus change measured by shear strain (= ). The relative changes in the Young s modulus with respect to the original modulus was 9.5 and 3.9 at 1 and 100 mm/min, respectively, which is also extremely smaller than that measured by shear strain (= ). This suggests that the chain structure formed by magnetic field is fragile against compressional strain than shear one, although there might be the influence of nonlinear viscoelasticity by large deformation mm/min 1mm/min 50mm/min 50mm/min 100mm/min 100mm/min Strain Fig. 3. Stress-strain curves of magnetic carrageenan gels with various compression speeds at 0 mt (open symbols) and 320 mt (closed symbols). Figure 4 displays the photographs of magnetic carrageenan gels with and without magnetic field. An opaque thin film was spread on the surface of the magnetic gel in order to see the deformation of the gel surface. The magnetic field was produced by a permanent magnet with a filed strength of 320 mt. At 0 mt, the magnetic gel did not support a weight of 200 g corresponding to a stress of Pa, and the weight immediately sank in the magnetic gel, accompanied with the collapse of the gel. On the other hand, in the presence of magnetic field, the weight stood on the magnetic gel without any deformation. Figure 5(a) shows the schematic illustration of vibration experiment for magnetic carrageenan gel. It is well known that the materials with tunable elasticity are able to control the mode of vibration. As shown in the figure, the magnetic gel was placed on a vibrating stage with a resonance frequency of approximately 20 Hz. The displacement of the water surface colored white on the gel was measured by a laser displacement sensor. A permanent magnet was inserted between the gel and the vibrating stage. The mass and height effects of the magnet were calibrated in the data. Figure 5(b) shows the time profile of the displacement of the water surface with and without magnetic fields. At B=0, the amplitude of the displacement was approximately 0.27 mm, on the other hand, it was suppressed to 0.07 mm at 320 mt. It is considered that the elasticity change due to magnetic field shifted the resonance frequency of the system. 4

5 z/mm Magnet field Permanent magnet Magnetic gel Fig. 4. Photographs of magnetic carrageenan gels with (left) and without (right) magnetic field. Weight: 200 g, Magnetic field: 320 mt. The field direction is indicated by an arrow. According to a theory of resonance frequency related to the materials elasticity, the resonance frequency of vibration shifts in proportional to the square root of the elasticity change [16]. The frequency shift with respect to the original frequency was estimated to be 2.2 when the relative change in Young s modulus was 9.5. This value is sufficiently high as a damping material in practical use. (a) (b) mT mT time/s Fig. 5. (a) Schematic illustration of the vibration experiment used in this study. (b) Displacement of the water surface on magnetic gel with and without magnetic fields. Conclusions We investigated the magnetorheological effect of magnetic carrageenan gels when the magnetic field was parallel to the compressional strain. It was presented that the ability to support a weight with 200 g is controllable by magnetic field. The relative change in Young s modulus was, at maximum, 9.5 fold, which was extremely smaller than that measured by shear strain. This indicates that the MR effect is strongly affected by the geometry of directions of strain and magnetic field. These features would be useful for designing a devise which alters its mechanical properties by 5

6 magnetic field. Recently, we have reported magnetic poly(vinyl alcohol) gels with both giant magnetorheology and high mechanical toughness [17]. Although, the magnetic gel has a disadvantage of water evaporation, we believe that the variable elastic materials using magnetic gels will be in a practical use soon. Experimental part Preparation of magnetic gels The magnetic gel consisted of carbonyl iron particles (BASF Japan) and carrageenan (San-Ei Gen F.F.I.). A pre-gel solution of the magnetic gel was prepared by mixing a 1.0 wt% aqueous carrageenan solution and carbonyl iron at 95 ºC using a mechanical stirrer for 30 min. The concentration of carbonyl iron was 70 wt%, which corresponds to a volume fraction of 34 vol%. The diameter of the particle was 2.7 µm, and the saturation magnetization and remanent magnetization were 207 and 1.3 emug -1, respectively. The particle has no magnetization at synthesis, it was magnetized only under magnetic fields. Dynamic shear modulus measurement The dynamic shear modulus at 1 Hz, 20 C was measured using a rheometer (MCR301, Anton Paar) under various magnetic fields up to 320 mt. The field direction was perpendicular to the strain. The samples were disks with dimensions of 20 mm diameter and 1.4 mm thickness. Stress-strain measurements The stress-strain experiment of magnetic gels was measured at room temperature using a compression apparatus (STA-1150, Orientec) with compression speeds of mm/min. The sample was synthesized within a laboratory dish (10 mm in thick, 35 mm in diameter). The measurement was carried out using the sample with the laboratory dish not to deform the magnetic gel by a gradient magnetic field produced by a permanent magnet. The magnetic field strength of the permanent magnet was 320 mt, and the field direction was parallel to the compressional strain. Acknowledgements We are grateful to San-Ei Gen F.F.I., Inc. and BASF Japan Ltd. for the offer of samples. This research was partially supported by Panasonic Electric Works, Co. Ltd. References [1] Mitsumata, T.; Ikeda, K.; Gong, J. P.; Osada, Y.; Szabo, D.; Zrinyi, M. J. App. Phys. 1999, 85, [2] Shiga, T.; Okada, A.; Kurauchi, T. J. Appl. Polym. Sci. 1995, 58, 787. [3] Jolly, M. R.; Carlson, J. D.; Munoz, B. C.; Bullions, T. A. J. Int. Mat. Sys. Struct., 1996, 7, 613. [4] Ginder, J. M.; Clark, S. M.; Schlotter, W. F.; Nichols, M. E. Int. J. Modern Phys. B 2002, 16, [5] Bossis, G.; Bellan, C. Int. J. Modern Phys. B 2002, 16, [6] Lokander, M.; Stenberg, B. Polym. Test. 2003, 22, 677. [7] Mitsumata, T.; Nagata, A.; Sakai, K.; Takimoto, J. Macromol. Rapid Commun. 6

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