New polyurethane-based magnetostrictive composites: dynamical mechanical properties.
Magnetostrictive effect, a general effect observed in any magnetic and deformable substance whereby a change in dimension accompanies an applied magnetic field, was first observed by Joule more than 150 years ago (1). It arises from "the change of the internal energy and from the symmetry lowering experienced by any sample when it is magnetized" (2).
An excellent magnetostrictive material discovered by Clark in 1970s was called Terfenol-D, a specially formulated compound of terbium, dysprosium, and iron. Monolithic Terfenol-D was first developed by the Naval Surface Warfare Center in the early 1970s (3), (4) and due to the compensation of intrinsically large anisotropies of Tb and Dy in [RFe.sub.2] compounds (R: rare earth element) possess nearly zero magnetocrystalline anisotropy and hence exhibit giant magnetostriction at low magnetic fields (5). Therefore, it has been a commercially available magnetostrictive material for application in many fields, as in sonar and actuator devices (5). However, Terfenol-D alloys exhibit enormous anisotropy of magnetostriction (5), (6) which together with the frequency limitation due to eddy current losses, the brittleness of the material., and the difficulty in manufacturing have limited its useful frequency range (7), (8).
To overcome this problem, alternative production routes such as sintering (9), (10) and polymer-bonding (11-13) were considered. Among the existing production routes, the method of combining Terfenol-D particles with polymers has been proved to be an effective way to overcome those shortcomings. The insulating layer created by the polymer matrix between the particles eliminates eddy current losses at high frequencies (14). Moreover, the polymer-bonding composite is considered to present high advantages respect to the monolithic magnetic material: the polymer matrix produces a relatively tough material that can better accommodate tensile and shear loading states (12), (15), (16) and it is easier to manufacture than the monolithic Terfenol-D (11), (17); although magnetostriction itself is reduced due to the dilution effect with the presence of a nonmagnetic polymer binder. Polymer-bonding Terfenol-D composites were first reported several years ago by Sandlund et al. (11) and later by Ruiz de Angulo et al. (12).
Foi those reasons, in recent years, the interest in developing and characterizing magnetostrictive composites has grown considerably. Moreover, polymer composites have historically demonstrated dramatic enhancement of neat polymer matrix properties. In some works of other authors, thermostable resins as rigid epoxy (12), (16-21), unsaturated polyester (22) and phenol-type resins (6), (8), (23) have been used as binders for the Terfenol particles with promising results.
In most of these papers, the thermosetting resin was used as binder in the process of compaction of the Terfenol-D to obtain the final material. Therefore, the particle alignment was done during the compaction process and as the compaction pressure increases the magnetostrictive properties deteriorate while the mechanical strength improves linearly (8). As thermosetting resins were used in the compaction process, this is an irreversible process and therefore the Terfenol-D particles cannot be reoriented anymore. One way to avoid this drawback it is through the use of thermoplastic polymers as polyurethane elastomers.
Thermoplastic polyurethane elastomers are a versatile group of multiphase segmented polymers that have excellent mechanical and elastic properties, good hardness, high abrasion and chemical resistance (24). They are block polymers whose chains are composed of alternating rigid urethane "hard" segments, formed by extending a terminal diisocyanate with a low molecular weight diol, which often has a glass transition, or crystalline melting point well above room temperature, and low glass transition temperature "soft" segments (polyether or polyester glycols). The ratio between the two segments gives the elastomer its unique set of mechanical properties. A small amount of excess of isocyanate groups is often used in polymerization, resulting in the formation of allophanate groups (25), (26) or polyurethane crosslinking. The degree of chemical crosslinking is responsible for the elastic behavior of polyurethanes.
Polyurethane elastomers can also be used as a polymeric matrix in various systems (27-30), e.g., in magnetostrictive composites, materials which change their dimensions when subjected to a magnetic field. The composites obtained show outstanding results and properties if they are compared with the monolithic material (31), (32) but respond in the same way in terms of magnetostrictive response at low applied magnetic fields.
Here, magnetostrictive composites (polyether urethane-Terfenol-D) were produced by combining the polymeric material with polycrystalline powders of the magnetostrictive material (Terfenol-D). A preferential orientation was induced in the composite to obtain the best magnetostrictive properties (33). Thermal and mechanical properties of the composites were compared with those of the matrix. To this end, polyether urethanes with different content of urethane segments were synthesized.
Synthesis of Polyurethanes and Composites
The synthesis of polyurethanes was carried out using diphenylmethane diisocyanate (MDI) and two different molecular weight polytetramethylene glycols (PTMG 1000 with [M.sub.n] = 1000 and PTMG650 with [M.sub.n] = 650). 1,4-Butanediol (BD) was used as chain extender. All products were supplied by Aldrich. The hard segment compositions for polyurethanes and the stoichiometric amounts of reactants used are shown in Table 1. The NCO/OH ratio for all compositions was 1.
TABLE 1. Synthesis conditions for polyurethane elastomers. Sample name wt% hard segment Polyol: isocyanate:BD (molar ratio) PUB 1000 (a) 25 1.0/1.2/0.23 30 1.0/1.5/0.51 35 1.0/1.8/0.85 40 1.0/2.2/1 2 45 1.0/2.7/1.7 50 1.0/3.2/2.2 55 1.0/3.8/2.8 60 1.0/4.7/3.7 PUB650 (b) 30 1.0/1.1/0.083 35 1.0/1.3/0.29 40 1.0/1.5/0.54 45 1.0/1.8/0.83 50 1.0/2.2/1.2 55 1.0/2.6/1.6 60 1.0/3.172.1 65 1.0/3.8/2.8 (a) From polytetramethylene glycol PTMG1OOO (b) From polytetramethylene glycol PTMG630.
The synthesis of polyurethanes was carried out in a two-step or prepolymer method (34) (Scheme 1). In a first step, the glycol reacts with an excess of diisocyanate, yielding a long-chain molecule with terminal isocyanate groups. In a second step, a copolymerization reaction with a difunctional chain extender produces the (final elastomer.
A stoichiometric amount of the polyether glycol was charged into a five-neck reaction flask equipped with a mechanical stirrer and nitrogen inlet and kept at 70[degrees]C for 30 min to remove moisture before synthesis. Next, the isocyanate was added to the reactor and the mixture was stirred continuously. The reaction temperature was held at 70[degrees]C. After 3 h, the prepolymer with unreacted isocyanate ends was reacted with the appropriate amount of the chain extender. After approximately 1 min, the viscous mixture was poured into a preheated stainless steel mold and placed under pressure at 100[degrees]C for 24 h to obtain the final polymer.
The composites were prepared using the same experimental process by adding Terfenol-D powders 1 h after the reaction started, to get a homogenous material. The polycrystalline powders of Terfenol-D (random in shape) were supplied by Etrema Products, Inc., IA, having a particle size distribution of 0-300 pm. Composites with random distribution of particles were obtained. To produce oriented composites, a magnetic field of 0.5 T was applied during the cure process For aligning the magneto-strictive particles into the composite. Different contents of Terfenol-D (between 30 and 70 wt%) were added in the polymeric matrix PUB 1000 with hard segment contents of 50 and 60 wt%.
X-ray diffraction (XRD) diagrams were taken with a Phillips model diffractometer system PWI710, using CuKa radiation (40 kV, 15 mA). Polyurethanes were measured as films.
Differential scanning calorimetry (DSC) measurements were carried out using a METTLER DSC [822.sup.e] system to determine the glass transition temperature ([T.sub.g]) and the melting enthalpy ([DELTA][H.sub.m]). Samples (5-10 mg) were heated from -80[degrees]C to 250[degrees]C under nitrogen atmosphere in aluminum pans, with a heating rate of 10[degrees]C*[min.sup.-1]. Temperature and enthalpy calibration of DSC were obtained by using indium, lead, and tin standards.
Dynamic mechanical [hernial analysis (DMTA) properties were measured in a Polymer Laboratories MKII equipment. The measurements were obtained in the bending mode at a frequency of 3 Hz and temperature range from -100 to 170[degrees]C. The heating rate was 2[degrees]C*[min.sup-1]. The dimensions of the rectangular specimens were about 35 mm x 10 mm x 2 mm.
RESULTS AND DISCUSSION
Thermal and Dynamic-Mechanical Analysis of Polverrethanes
The thermal behavior of the elastomeric polyurethanes is related to the existence of a microphase separation caused by the different nature of the soft and hard segments and the degree of mixing between them.
Three transition regions have been found and detected by DSC for the polyurethanes, as shown in Figs. 1 and 2. The first transition appears at low temperatures, and corresponds to the glass transition ([T.sub.g]) of the amorphous soft segments. The glass transition of the hard segments is a small transition that appears between 50 and 100[degrees]C, and the third transition is attributed to the melting temperatures of the crystalline part of the hard segments. It is an endothermic transition of around 150[degrees]C, only significant for certain concentrations of the hard segment (between 40 and 60 wt%).
Table 2 summarizes the characteristics of the thermal transitions for the studied polyurethanes. The 7; values of the first transition increase with the content of the hard segment up to 40 and 45 wt% for polyurethanes PUB 1000 and PUB650, respectively, at which point [T.sub.g] values begin to decrease in contrast, melting peaks appear above these compositions as seen in Figs. 1 and 2. This behavior can be explained if we consider that there is no crystalline part in polyurethanes with a low content in hard segments, the hard and soft segments are mixed with a low degree of separation. With higher hard segment content, the interactions with the soft segments will gain relevance and [T.sub.g], values will increase. When hard segment content is enough to create an ordered structure, a separation of both hard and soft segments takes place and [T.sub.g] values will tend towards the [T.sub.g] value of the soft segment, or in other words to the pure polyol. The difference between the [T.sub.g] of the soft segment and the [T.sub.g] value of the pure soft segment is a measure of the relative number of hard blocks dissolved in the soft segment. Only when their length is below the critical length for micro-phase separation (35), the hard segments will dissolve within the soft segments.
TABLE 2. Thermal transitions of PUB1000 and PUB650 polyurethanes. PUB1000: PTMG1000/MDI/BD wt% hard segment 25 30 35 40 45 50 55 [T.sub.g]([degrees]C) -53.7 -48.3 -47.5 -46.6 -51.6 -53.8 - 58.2 [T.sub.m]([degrees]C) -- -- -- 150.4 150.0 150.7 151.7 (a) [DELTA] [H.sub.m] -- -- -- 3.3 13.9 18.7 28.2 (J*[g.sup.-1]) PUB650: PTMG650/MDI/BD wt% hard segment 30 35 40 45 50 55 60 [T.sub.g]([degrees]C) -37.3 -29.7 -26.7 -18.8 -20.2 -21.0 -23.3 [T.sub.m]([degrees]C) -- -- -- 150.9 151.0 149.2 149.7 (a) [DELTA] [H.sub.m] -- -- -- 1.3 9.0 13.0 22.6 (J*[g.sup.-1]) wt% hard segment 60 [T.sub.g]([degrees]C) -59.0 [T.sub.m]([degrees]C) 150.6 (a) [DELTA] [H.sub.m] 33.0 (J*[g.sup.-1]) PUB650: PTMG650/MDI/BD wt% hard segment 65 [T.sub.g]([degrees]C) -37.0 [T.sub.m]([degrees]C) 153.4 (a) [DELTA] [H.sub.m] 32.7 (J*[g.sup.-1]) (a) Onset melting temperature
Melting enthalpies ([DELTA][H.sub.m]) and temperature ([T.sub.m]) values are also shown in Table 2. In general, a higher melting enthalpy reveals a more organized structure, a higher crystallinity degree (36), and thus a higher phase separation degree. A lineal relation between the AH1 values and the hard segment content can be seen.
Furthermore, evaluation of the melting peaks (Figs. 1 and 2) reveals that in some cases there are multiple peaks. This could be explained by the differences in crystal morphology that cause melting temperatures to be dissimilar (37).
[T.sub.g] values of both polyurethanes (Fig. 3) allow its to understand the influence of their structure on thermal properties. The PTMG 1000 polyol has a higher molecular weight, providing higher flexibility and lower [T.sub.g] values to the final polyurethane.
The crystalline part in PUB 1000 polyurethanes becomes apparent in the XRD diagrams. As observed in Fig. 4, the degree of crystallinity is higher with hard segment content increase; several peaks were obtained (19.4[degrees], 21.7[degrees], and 23.5[degrees]) for samples with the highest hard segment content. This is associated to the different sizes and morphologies of the crystals, as it has been studied for the melting endothermic peaks through the DSC technique. The X-ray diagram for PUB1.000 30 wt% is ascribed to an amorphous polymer; a single broad peak (amorphous halo) (38) around 20[degrees] for polyurethanes with low hard segment content was obtained.
The analysis of the dynamic-mechanical properties by DMTA corroborates the behavior of the systems. Figures 5-8 show the variation of the storage modulus, E', and tan [delta] with temperature for all the studied series and the values of [T.sub.g], corresponding to the glass transition of the soft segment, are shown in Table 3. From the analysis of the elastic modulus at low temperatures, before glass transition, E' for PUB 1000 is higher than for PUB650 due to a higher crystallinity, as deduced from calorimetric data. Regarding the elastic behavior of the polyurethanes after the glass transition, the storage modulus reach different values depending on the hard segment content in the sample (Figs. 5 and 7). With the increase of hard segment content, above 40 and 45 wt% of the hard segment for PUB 1000 and PUB650 polyurethanes, respectively, separation of both hard and soft segments takes place, with a high degree of separation, which results in an increase of the E'value. Analysis of tan [delta] curves and considering the [T.sub.g] values in Table 3, taken at the maximum of the tan [delta] peak, allows us to conclude that for both polyurethanes, [T.sub.g] values of the soft segment generally increase with hard segment content increases, and thus crystallinity increases too.
TABLE 3. Dynamic-mechanical transitions of the synthesized polyurethanes. PUB1000: PTMG/MDI/BD wt% hard segment 25 30 35 40 45 50 55 [T.sub.g]([degrees]C) -30.0 -27.1 -17.8 -23.3 -21.7 -21.2 -22.3 PUB650: PTMG/TDI/BD wt% hard segment 30 35 40 45 50 55 60 [T.sub.g]([degrees]C) -11.3 -8.3 - 1.4 0.9 8.5 7.9 12.0 wt% hard segment 60 [T.sub.g]([degrees]C) -15.4 PUB650: PTMG/TDI/BD wt% hard segment 65 [T.sub.g]([degrees]C) 18.2
The height of the tan [delta] peak is related to the amount of the amorphous material present (39). As the hard segment content increases the intensity of the tan [delta] peak decreases and the peak widens; crystallinity appears increasing the microphase separation (40). Therefore, the degree of crystallinity determines the shape and the position of the relaxation process. This behavior, already studied in other polymers (41), (42), has been qualitatively explained by the Yamafuji and Ishida theory (43) for the micro-Brownian movement of the main chain.
The increasing asymmetry of tan [delta] at temperatures above the peak value, suggests a marked influence of the crystallites on the noncrystalline phase of the polymer and is associated to an increase in the distribution of the relaxation times. This is caused by constriction of the amorphous phase among increasingly larger crystalline areas, leading to more restricted chain movements.
When comparing [T.sub.g] values in PUB 1000 and PUB650 polyurethanes, the [T.sub.g] value is lower in the case of PUB 1000 for the same hard segment content, probably clue to a longer chain providing easier movement.
Thermal and Dynamic-Mechanical Behavior of the Magnetostrictive Composites
To create the magnetostrictive composites, the PUB 1000 polyurethane was selected because of its mechanical properties. Furthermore, the values of the storage modulus of PUB 1000 are the highest and, in contrast, the values of the glass transition temperatures for all compositions are below 0[degrees]C and suitable for magnetostrictive studies at room temperature. Besides, it is possible to obtain materials without significant imperfections and easy moldability.
Composites with different content of oriented magnetostrictive particles were prepared using PUB 1000 50% and PUB 100060% polyurethanes. For these composites, thermal and dynamic-mechanical properties were studied. In Figs. 9 and 10, the thermograms of the neat polyurethanes are compared to composites containing 30-70 wt% of Terfenol-D magnetostrictive particles, respectively. Melting temperatures of the polyurethane composites, defined as the onset temperature of the melting process, are hardly affected by the incorporation of Terfenol-D in the polymeric matrix as is shown in Figs. 9 and 10. As can be seen on Table 4 the degree of crystallinity of the synthesized polyurethanes, which is directly related to their melting enthalpy, decrease when the Terfenol-D is added to the polyurethane matrix.
TABLE 4. DSC and DMTA analysis for the PUB1OOO 50% and PUB1000 60% magnetostrictive composites with different contents of Tcrfenol-D. DSC DMTA bending Sample [T.sub.g]([degrees]C) [DELTA] [H.sub.m] [T.sub.g] (J*[g.sup.-1]) (a) ([degrees]C) PUB1000 50% -53.8 18.7 -21.2 PUB1000 -49.0 15.6 -17.2 50%-Terfenol-D 30% PUB1000 -48.9 17.0 -11.5 50%-Terfenol-D 50% PUB1000 -45.8 16.2 4.6 50%-Terfenol-D 70% PUB1000 60% -59.0 33 -12.1 PUB1000 -35.1 19.4 40.1 60%-Terfenol-D 50% PUB1000 -27.2 19.2 67.2 60%-Terfenol-D 70% (a) Normalized lo the polymer mass.
In Table 4, the To values and melting enthalpies for magnetostrictive composites are shown. The glass transition temperature increased with the increase in Terfenol-D content. The oriented Terfenol-D in the polyurethane matrix forms a chained structure, leading to an increase of up to 8[degrees]C in the [T.sub.g] value (DSC) for PUB 1000 5004) polyurethane matrix -(70 wt% of Terfenol-D) and around 32[degrees]C for PUB1000 60% with the same magnetostrictive particle content. This indicates that Terfenol-D changed the kinetics of the glass transition caused by a decrease in the movement of the soft chains of the polymer by magnetostrictive particles. The same conclusion is reached with [DELTA][H.sub.m] values. For both composites, values were lower than those obtained with the corresponding polyurethane matrices, being more pronounced in the hardest matrix. This decrease in crystallinity implies a higher microphase separation between hard and soft segments and, therefore, a higher glass transition temperature.
Figures 11 and 12 show the temperature dependence of elastic modulus and loss factor, tan .6, of unfilled and filled magnetostrictive polyurethanes at temperatures between - 100[degrees]C and 150[degrees]C. All samples studied show only one very broad relaxation peak around the corresponding glass transition temperature. Maximum peak values of the various samples shifted to higher temperatures with increasing magnetostrictive volume fraction in the composite. [T.sub.g], values obtained by this technique (Table 4) show the same behavior as those obtained with DSC measurements, that is, the [T.sub.g] values increase as the Terfenol-D content increases. However, values are quite different because of the width of high peaks.
The peak area increased with increased Terfenol-D fraction in the composites. Furthermore, a broadening of the damping peaks with the increasing magnetostrictive alloy content was also seen. The broad peak of relaxation of the pure polyurethane already shows that there was no sharp relaxation time of the glass transition but there was a widely distributed relaxation spectrum. The increased width of the relaxation peak indicates a broader distribution of relaxation times by the addition of Terfenol-D to the polyurethane and therefore, further relaxation modes might be introduced (42).
The value of the E' storage modulus at low temperatures was up to four times lower than that of the polyurethane matrix and was lower for composites with higher Terfenol-D content. This may be a consequence of the increasing amorphous content, with a lower mechanical modulus value than the crystalline part. After the glass transition, the storage modulus tends to stabilize, reaching the same values in all cases.
Composite materials based on polyurethanes containing different volume percentages of magnetostrictive filler were developed and their thermal and mechanical properties at different conditions investigated. The synthesized polyurethanes, PUB 1000 and PUB650, were semicrystalline for hard segment content between 40 and 45 wt%, respectively. Calorimetric results show that the glass transition temperature increases with increasing Terfenol-D content indicating that these particles cause a decrease in the movement of the soft chains of the polymer.
Mechanical measurements show that the storage modulus of the developed magnetostrictive composites became lower with the increment of Terfenol-D volume fraction in all investigated compositions. This decrease in E' results from a reduction of the high modulus crystalline phase content. In contrast, the damping peak area increases with increasing fraction of the magnetostrictive filler. Therefore, it has been demonstrated that Terfenol-D is an effective filler for polyurethane matrices increasing damping properties without harmful influences on important characteristics of polyurethanes, such as the thermal behavior.
Correspondence to: L.M. Leon; e-mail: firstname.lastname@example.org
Contract grant sponsor: Basque Country Government (ETORTEK programme, ACTIMAT project).
Published online in Wiley Online Library (wileyonlinelibrary.com).
[c] 2012 Society of Plastics Engineers
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J.L. Vilas, J.M. Laza, C. Rodriguez, M. Rodriguez, LM. Leon
Dpto. Qulmica Fisica, F. de Ciencia y Tecnologla, Universidad del Pals Vasco (UPV/EHU), Apdo. 644, 48080 Bilbac, Spain
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|Author:||Vilas, J.L.; Laza, J.M.; Rodriguez, C.; Rodriguez, M.; Leon, L.M.|
|Publication:||Polymer Engineering and Science|
|Date:||Apr 1, 2013|
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