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Tensile deformation of artificial muscles: Annealed nylon 6 lines
Polymer 177 (2019) 49–56
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Polymer journal homepage: www.elsevier.com/locate/polymer
Tensile deformation of artificial muscles: Annealed nylon 6 lines a
b
c
T
a,∗
Yi-Wei Huang , Wen-Shin Lee , Fuqian Yang , Sanboh Lee a b c
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, 300, Taiwan Department of Athletics, National Taiwan University, Taipei, 10617, Taiwan Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY, 40506, USA
H I GH L IG H T S
mechanical responses of artificial muscle and natural muscle. • Understand stages for the tensile deformation of chicken muscle were observed. • Four Young's modulus of artificial muscle varies with testing temperature. • The force of artificial muscle varies with annealing temperature. • Actuation • Young's modulus of non-twisted nylon 6 varies with testing temperature.
A R T I C LE I N FO
A B S T R A C T
Keywords: Chicken muscle fiber Artificial muscle Tension deformation Temperature effect
In this work, we study the tensile deformation of chicken muscle fibers and the temperature dependence of the tensile deformation of non-twisted nylon 6 lines and twisted nylon 6 (artificial muscles). Both the non-twisted nylon 6 lines and twisted nylon 6 are annealed at different temperatures of 150, 175, 190 and 200 °C. The chicken muscle fibers under tensile loading exhibit four deformation stages with the tensile load being a linear function of the elongation in each stage. The largest Young's modulus (the slope of the load versus the elongation curve) occurs at stage III. For the tensile deformation of the annealed non-twisted nylon 6 lines, the tensile load is proportional to the elongation. For the tensile deformation of the twisted nylon 6 (artificial muscles), there exist three stages with the tensile loading being a linear function of the elongation in stages I and III. The Young's modulus calculated from the load-elongation curves decreases with the increase of the testing temperature. For the testing conditions used in this work, the tensile deformation eventually leads to the fracture of both the chicken muscle fibers and the annealed non-twisted nylon 6 lines. The fracture stress of the annealed non-twisted nylon 6 lines decreases with the increase of the testing temperature.
1. Introduction
human muscle of the same length and mass, opens a new horizon to use thermal energy to produce large displacement linearly and/or rotationally for soft-artificial structures. This discovery has renewed the interest in exploring the potential applications of polymeric fibers in artificial muscles [2] and investigating the mechanical responses of the soft-artificial structures made from polymer fibers, since researchers have been seeking the possibility of using polymeric materials as the structural materials in soft-artificial structures for many decades [3–9]. To effectively fabricate soft-artificial structures from polymeric materials, it is of great importance to understand the mechanical responses of the polymeric materials under a variety of environments. Lin and Argon [10] studied the anisotropic behavior of textured nylon 6 under compression. Using melt spinning to make nylon 6 filament yarns from nylon 6, Ishibashi et al. [11] performed tension tests of the nylon 6
There is a great need to develop soft-artificial structures mimicking the functionality of biological structures in soft and smart machines. In the heart of soft-artificial structures are the actuation mechanisms, which depend on the materials used in the structures. There are a variety of materials, such as carbon nanotubes and shape memory polymers, which have the potential in the applications of soft-artificial structures. The challenges in the development of soft-artificial structures include the need of large stroke under the action of small stimulus, fast response and structural stability. The demonstration of polymeric fibers (especially nylon) as the structural materials for artificial muscles by Haines et al. [1], which can sustain weight of 100 times more than
∗
Corresponding author. E-mail address: [email protected] (S. Lee).
https://doi.org/10.1016/j.polymer.2019.05.070 Received 26 April 2019; Received in revised form 24 May 2019; Accepted 26 May 2019 Available online 28 May 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.
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it took more 15 min for the tension tests of the chicken muscle fibers at a loading rate of 0.001 N/min. Under such a loading rate, the experimental results cannot represent the intrinsic behavior of the chicken muscle fibers due to the change in the fraction of the composition of the chicken muscle fibers associated with the evaporation of water. For the loading rate of 1.5 N/min, the chicken muscle fibers fractured in 1 s, and there are no enough data available for the post-test analysis. Define threshold load as the force needed to overcome the intern force for artificial muscles to start to extend and thermal actuation force as the force needed to pull artificial muscles to the state at room temperature, respectively. Using the dead load approach, we measured the thermal actuation force of the artificial muscles and the non-twisted nylon 6 fibers in air at three different temperatures of 95, 120 and 145 °C to examine the annealing effect. The load-elongation curves were constructed from the measurement. Here, the elongation (length change) was referenced to the initial length at room temperature. The tensile tests of the artificial muscles and the non-twisted nylon 6 fibers were conducted in air at four different temperatures of 55, 70, 85, and 100 °C. The length of the artificial muscles for the tension test was limited to 1.5 ± 0.3 cm, which is due to the spatial confinement in TA Q800 DMA. The loading rates were 1 and 10 N/min. The geometrical dimensions of the artificial muscles, including wire diameter (d), outer diameter (D0) and the number of coils, as shown in Fig. 1c, were determined from the optical images of the specimens, which were taken prior to the tension test, on an optical microscope (OLYMPUS BX51, OLYMPUS Co., Shinjuku-Ku, Tokyo). For each set of experimental condition, tensile tests of at least four different samples were performed. The results reported in the work are the average values of the test results of the same group samples.
filament yarns, and revealed the effects of cooling and thinning on molecular orientation. Perkins and Porter [12] suggested that tensile deformation of nylon can cause the fragmentation of crystalline lamellae and the drawing out of polymer chains. Cherubini et al. [13] performed isothermal and isometric tests of coiled polymeric fiber made from nylon 6 at tensile state, and showed the decrease of the energy dissipation with the increase of the training temperature. Spinks [14] used spring mechanics to analyze the stiffness and the actuation mechanism of artificial muscles made from coiled-twisted polymer fibers. Zussman et al. [15] reported the scattering of the mechanical properties of electrospun nylon-6,6 nanofibers consisting of α-crystalline phase. All of these results demonstrate the dependence of the mechanical behavior of the soft-artificial structures on the microstructures and thermal history of the polymeric materials. However, there are few reports focusing on the fracture strength of the soft-artificial structures. It is of practical importance to understand the effects of microstructures and thermal history of polymeric materials on the structural integrity of soft-artificial structures made from the polymeric materials. Considering the important application of soft-artificial muscle made from nylon 6, we investigate the tensile deformation of non-twisted nylon 6 lines and artificial muscle (twisted nylon 6), which are annealed at four different temperatures. The tensile behavior of the nontwisted nylon 6 lines and twisted nylon 6 is compared to that of chicken muscle fibers. The study is focused on the effect of microstructures and thermal history on the fracture strength of the non-twisted nylon 6 lines and twisted nylon 6. 2. Experimental details For detailed information on the preparation of the materials used in this work, see the work by Huang et al. [16]. Briefly, chicken muscle fibers were obtained from chicken thighs in an aqueous solution of 400 mM mannitol and 50 mM potassium acetate. The artificial muscles were twisted nylon 6 of different coil lengths of 1.5 ± 0.3, 7.5 ± 0.5, and 13.5 ± 1.0 cm, which was made from monofilament nylon 6 line of 0.2 mm in diameter (100% polyamide-6 FCFC B248) (Formosa Chemicals & Fibre Corp, Taipei). The annealing of the twisted nylon 6 was conducted at four different temperatures of 150, 175, 190, and 200 °C for 20 min, and the training of the annealed-twisted nylon 6 was performed in the temperature range of 25–95 °C under the action of constant load of 20 g. Following the same procedure in the preparation of the artificial muscles, non-twisted nylon 6 lines were prepared with the annealing at four different temperatures of 150, 175, 190, and 200 °C and the training in the temperature range of 25–95 °C under the action of constant load of 20 g. Fig. 1 shows optical micrographs of a chicken muscle fiber, a non-twisted nylon 6 line and an artificial muscle (twisted nylon 6). The XRD (X-ray diffraction) analyses of the artificial muscles were performed on a Bruker D2 PHASER (Billerica, MA, USA) with Cu Kα (λ = 0.154 nm) at a voltage of 30 kV and a current of 10 mA. The diffraction intensities were counted at 0.02° per step. The spectrum was collected in an angular range of 10°–80°. A home-made tensile fixture, as described in detail in the previous work [16], was used to perform the tensile tests of the chicken muscle fibers, non-twisted nylon 6 lines and artificial muscles. The tension tests were performed on a TA Q800 DMA (TA Instruments, New Castle, DE). The study was focused on the dependence of fracture strength on loading rate. The tension tests of the chicken muscle fibers were conducted in air at room temperature. Optical imaging of the chicken muscle fibers was performed for the analysis of geometrical dimensions of the chicken muscle fibers after placing the chicken muscle fibers on the fixture. Two loading rates of 0.015 and 0.15 N/min were used. Prior to the tension test, the chicken muscle fiber was immersed in an aqueous solution consisting of 400 mM mannitol and 50 mM potassium acetate. Note that
3. Results It is known that nylon 6 possesses several crystalline phases with α and γ forms being the most stable and common phases [17]. The α phase consists of sheets of hydrogen-bonded chains in an anti-parallel form, and the γ phase is present in the form of parallel chains [18]. Fig. 2 shows XRD spectra of artificial muscles, which were annealed at four different temperatures. There are two characteristic peaks at 20.3–20.7° and 23.35–23.9°, which correspond to the (200) and [(002), (220)] planes of α phase, respectively [18]. In general, nylon 6 consists of crystalline phases and amorphous phase. Using the Gaussian deconvolution technique and the XRD spectra of Fig. 2, we obtain the crystallinity of nylon 6 as
Crystallinity (%) =
Acr Acr + Aam
(1)
from the areas enclosed the diffraction peaks and the abscissa for crystalline phase, Acr, and amorphous phase, Aam, respectively. Fig. 3 shows the variation of the crystallinity of the artificial muscles with the annealing temperature. The crystallinity of the artificial muscles increases slightly with the increase of the annealing temperature with the smallest crystallinity of 30% and the largest crystallinity of 35%. Thus, we can assume that there is no significant difference in the structures of the artificial muscles used in this work. Fig. 4 shows a typical force-displacement curve for the tension test of the chicken muscle fibers. It is evident that the tension deformation of the chicken muscle fibers consists of four stages with approximate constant slopes, i.e. the tension force is a linear function of the displacement (elongation) for each stage. In stage I, the slope of force versus displacement is very steep due to the deformation of the aluminum grip. In stage II, the tension activates the muscle cells, which contract to counter-balance the external force and are experiencing elongation. In stage III, all the muscle cells in the muscle fiber are activated and are at tensile state. There exists significant contraction force in the muscle cells to counter-balance the tensile force. At the transition from the stage III to the stage IV, local stress relaxation/damage in the 50
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(a)
(b)
(c) D0
d
100 m
400 m
Fig. 1. Optical micrographs of (a) a chicken muscle fiber, (b) a non-twisted nylon 6 line and (c) an artificial muscle.
Crystallinity of artificial muscle (%)
40
Annealing temp (oC): Intensity (a.u.)
200
190 175 150 0
10 20
30 40 50 60 70 2 (o)
80 90
35
30
25
20 140
150
160 170 180 190 o Annealing temperature ( C)
200
210
Fig. 3. Variation of the crystallinity of artificial muscles with annealing temperature.
Fig. 2. XDR spectra of artificial muscles.
muscles to reach the length at room temperature. Note that some artificial muscles even experiences contraction at small load, resembling the behavior of chicken muscle under the external stimulus from passive state to active state. Fig. 6 shows a typical force-displacement curve for the tension test of the artificial muscles. In contrast to the chicken muscle fibers, there are three stages for the tension of all the artificial muscles. The first stage is from the start of the tensile test to point A, which is due to the grip deformation. The second stage is from point A to point B with the slope smaller than that of the first stage. The third stage starts at point B with constant slope. All the artificial muscles did not break during the
muscle cells is present, and the force needed to produce the same length change decrease. Finally, the chicken muscle fiber breaks. The presence of the II, III and IV stages suggests that the tension causes the change in the configuration of the microstructures in the chicken muscle fibers. Fig. 5 shows the variation of the elongation of the artificial muscles with applied load at different temperatures for the measurement of thermal actuation force. It is evident that all the artificial muscles contracted upon heating, which can be referred to as the “shape memory” effect. It needs to increase the load to stretch the artificial 51
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0.03
0.4
IV
(a)
Testing Temp: 95 C
Displacement (cm)
Force (N)
0.2 0.02 III 0.01 II 0.0
-0.2 -0.4
Annealing Temp ( C): 150 175 190 200
-0.6 -0.8
I
0.00
0.0
0.2
0.4
0.6
0.8
1.0
-1.0
Displacement (cm)
0.00
0.05
0.10
0.15
0.20
0.25
Load (N)
Fig. 4. Typical force-displacement curve for the tension test of chicken muscle fibers.
0.5
Displacement (cm)
tests. It is worth mentioning that the slope of the force-elongation curve of the artificial muscles in stage III is a function of the loading rate, annealing temperature and testing temperature. Fig. 7 shows typical force-displacement curves for the tension test of the non-twisted Nylon 6 fibers annealed at different temperatures under different conditions. Similar to the artificial muscles made from the twisted Nylon 6 fibers, there is one single stage for the tension of all the non-twisted Nylon 6 fibers with the linear relationship between the tensile force and the displacement (elongation). All the non-twisted Nylon 6 fibers broke eventually. 4. Discussion Comparing Fig. 4 with Fig. 6, we note that there exists significant difference in the tensile behavior between the Nylon 6-based artificial muscles and the chicken muscle fibers. Such a difference suggests that artificial muscles simply made from twisted Nylon 6 cannot resemble the function of chicken muscle fibers. A combination of Nylon 6-based artificial muscles and shape memory polymers is needed in the design of artificial muscles, which can resemble the function of biological muscle fibers. However, if the resemblance of tensile behavior between the Nylon 6-based artificial muscles and the chicken muscle fibers is not important, the former can sustain a load of 100 times more than the latter. Also, the thermal actuation force of the former is larger than the fracture load of the latter by one order of magnitude. This implies that twisted Nylon 6 has potential to be artificial muscles. Using the force-displacement curves, as shown in Fig. 4, and approximating the chicken muscle fibers as cylindrical rods, we can calculate the true stress and true strain and determine the critical stresses from the transitions from stage II to stage III and from stage III to stage IV, respectively, and the fracture stress. Table 1 lists the critical stresses and the fracture stress for two different loading rates, in which σ1 and σ2 represent the critical stresses for the transitions from stage II to stage III and from stage III to stage IV, respectively, and σf is the fracture stress. It is evident that both the fracture stress and the critical stresses increase with the decrease of the loading rate. Such behavior is consistent with the phenomena that high strain rate can cause damage to muscles and fast transition of muscles at “passive” state to “activated” state corresponding to the transition from the stage II to the stage III. The fast transition of muscles at “passive” state to “activated” state can lead to fast consumption of chemical energy stored in muscles and the loss of the muscle strength, resulting in the early transition with small σ2 from the stage III to the stage IV. From the force-displacement curves, we calculate the Young's modulus of each stage, Ei (i = II, III and IV), i.e. the slope, which is also
(b) Testing Temp: 120 oC
0.0 -0.5 o
Annealing Temp ( C): 150 175 190 -1.5 200 0.00 0.05 0.10 0.15 0.20 0.25 0.30 -1.0
Load (N) 0.5 (c)
Testing Temp: 145 C
Displacement (cm)
0.0 -0.5 -1.0 Annealing Temp ( C): 150 -2.0 175 190 -2.5 200 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 -1.5
Load(N) Fig. 5. Variation of the elongation of artificial muscles with applied load at different temperatures for the measurement of thermal actuation force: (a) 95 °C, (b) 120 °C, and (c) 145 °C.
listed in Table 1. Here, the subscripts of II, III, and IV represent the stage II, III, and IV, respectively. The stage III has the largest Young's modulus. For the same stage, the smaller the loading rate, the larger is the 52
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6
18
5 III
Force (N)
Force (N)
4 3 B
2 II
1 0
I
12 o
Testing temp ( C): 6
K
A
0
2 4 6 Displacement (cm)
0 0.0
8
16
Fig. 6. Typical force-displacement curve for the tension test of artificial muscles.
14
Ftherm = Fs + k|εc |
1.0 1.5 2.0 2.5 Displacement (mm)
(b)
Loading Temp: 70 C Loading rate: 1 N/min
Force (N)
10 8
o
Testing Temp ( C):
6
150 175 190 200
4 2 0 0.0
0.5
1.0
1.5 2.0 2.5 3.0 Displacement (mm)
3.5
4.0
Fig. 7. (a) Force-displacement curves of the tension tests of non-twisted Nylon 6 fibers annealed at 190 °C at a loading rate of 10 N/min and at different temperatures, and (b) force-displacement curves for the tension tests of nontwisted Nylon 6 fibers annealed at different temperatures of 150, 175, 190, and 200 °C at a loading rate of 1 N/min and a temperature of 70 °C.
(2)
where Fs is the threshold load, k is the slope (load/strain), and εc is the contraction strain. Fig. 8d shows the thermal actuation force at different annealing temperatures. In general, the thermal actuation force decreases with the increase of the annealing temperature. For the same annealing temperature, the thermal actuation force increases with the increase of the testing temperature. For the artificial muscles made from twisted nylon 6 lines, the relationship between the spring constant, K, and the shear modulus, G, of the nylon 6 lines can be expressed as
Gd4 8(D0 − d )3Nc
0.5
55 70 85 100 3.0 3.5
12
Young's modulus. Small loading rate allows more conversion of chemical energy to mechanical energy to increase the mechanical strength of the muscles during the tensile deformation. From Fig. 5, we note that there exists a “threshold load” for some artificial muscles to start to extend under an external load, and there exists linear relationship between the applied load and the elongation of the artificial muscles when the applied load is larger than the threshold load. Fig. 8a–c shows the variations of the threshold load, the slope (load/strain) for the linear relationship between the applied load and the elongation of the artificial muscles and the contraction strain at zero load for different annealing temperatures. It is evident that there are no clear trends for the threshold load and the slope (load/strain). For the contraction strain, increasing the testing temperature leads to the increase of the contraction strain, suggesting that more stored energy is released through the disorder of polymer chains. From the contraction strain, the slope (load/strain) and the threshold load, we calculate the thermal actuation force, Ftherm, as
K=
(a) Annealing Temp: 190 C Loading rate: 10 N/min
the twisted nylon 6 lines at the third stage which were annealed at different temperatures for two different loading rates. It is evident that, for the same artificial muscle, the Young's modulus decreases with the increase of the testing temperature, qualitatively in accord with the results for the tensile deformation of polymers reported in literature [19]. Such behavior can likely be attributed to the loose structure of polymer chains and the decrease of the resistance to the relative motion of polymer chains in polymer at high temperature. For the same testing temperature, the artificial muscle annealed at 200 °C has the smallest Young's modulus, and there is no significant difference in the Young's modulus between the artificial muscles annealed at 150 and 175 °C. This trend is likely associated with the extent of crystallinity in the nylon 6, as shown in Fig. 3. The artificial muscle annealed at 200 °C has the largest fraction of crystallinity, which has more transition zone between the polymer chains and the region with high crystallinity, allowing easy deformation across the transition zone. Comparing Fig. 9a with Fig. 9b, we note that the Young's modulus of the artificial muscle increases with the increase of the loading rate for the same testing temperature. This result is consistent with the rate dependence of the Young's modulus of polymer. The larger the strain/ loading rate, the larger is the elastic modulus. According to the work by Wang et al. [19], the temperature
(3)
where d is the diameter of the nylon 6 line and D0 is the outer diameter of the twisted nylon 6 lines, as shown in Fig. 1c. The parameter Nc is equal to N – 2 with N being the number of coils in the artificial muscle. Both d and D0 can be measured from the optical images (Fig. 1c) by Image J, and K is the slope of the third stage of the force-displacement curves. Using linear regression to curve-fitting the force-displacement curves and Eq. (3), we can calculate the shear modulus of the twisted nylon 6 lines for given N and the Young's modulus, E, of the twisted nylon 6 lines from E = 2G(1 + v) with v (= 0.4) being the Poisson ratio. Fig. 9 shows the temperature dependence of the Young's modulus of 53
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Table 1 Test results of chicken muscle fibers. Loading rate (N/min)
σ1 (MPa)
σ2 (MPa)
σf (MPa)
EII (MPa)
EIII (MPa)
EIV (MPa)
0.015 0.15
0.081 ± 0.038 0.031 ± 0.013
0.30 ± 0.12 0.17 ± 0.06
0.41 ± 0.18 0.29 ± 0.11
0.082 ± 0.036 0.055 ± 0.020
1.85 ± 0.96 1.21 ± 0.40
0.39 ± 0.13 0.31 ± 0.10
thermal softening coefficient. Such behavior can be attributed to the retardation of the motion of polymer chains. Increasing the loading rate leads to the increase to the stretch of polymer chains and the decrease of the mobility of polymer chains, resulting in small thermal softening coefficient. Fig. 11 shows the temperature dependence of the Young's modulus of the non-twisted nylon 6 lines, which were annealed at different temperatures for two different loading rates. Similar to the artificial muscle, the Young's modulus of the non-twisted nylon 6 lines decreases with the increase of the testing temperature. Such behavior reveals the decrease of the resistance to the relative motion of polymer chains in polymer at high temperature. For the same testing temperature, the non-twisted nylon 6 lines annealed at 200 °C has the smallest Young's modulus at the loading rate of 10 N/min, and there is no significant difference in the Young's modulus between the non-twisted nylon 6 lines annealed in the temperature range of 150 and 190 °C in contrast to the artificial muscles. For the loading rate of 1 N/min. the non-twisted nylon 6 lines annealed at 150 °C generally has the largest Young's modulus, and there is no significant difference in the Young's modulus between non-twisted nylon 6 lines annealed in the temperature range of 175 and 200 °C. The reason for such behavior is unclear, and it might be associated with the stretch of polymer chains under tension during the
dependence of the Young's modulus of polymer can be expressed as
E = E0 (
ε˙ m ) exp[−λ (T − T0)] ε˙ 0
(4)
where E0 and ε˙ 0 are the Young's modulus and strain rate of the reference state at a temperature of T0, ε˙ is the strain rate at a temperature of T, m is the strain-rate index, and λ is the thermal softening coefficient. Using Eq. (4) to curve-fit the results in Fig. 9, we obtain the thermal softening coefficients for the artificial muscles annealed at different temperatures. The fitting results are also included in Fig. 9. It is evident that Eq. (4) can be used to describe the temperature dependence of the Young's modulus of the artificial muscles. Fig. 10 shows the variation of the thermal softening coefficient of the artificial muscles with annealing temperature for two loading rates. It is interesting to note that the thermal softening coefficient increases first with increasing the annealing temperature, reaches maximum at the annealing temperature of 190 °C, and then decreases with increasing the annealing temperature. The reason for such behavior is unclear, and it might be related to the change in the microstructure of the annealed nylon 6 lines. From Fig. 8, we note that there exists the dependence of the thermal softening coefficient of the artificial muscles on the loading rate. The larger the loading rate, the smaller is the
3
(a)
o
Testing temperature ( C): 95 120 145
0.15
0.1
0.05
Slope (Load/strain) (N)
Threshold load (N)
0.2
(b) Testing temperature (oC) 95 2.5 120 145 2 1.5 1 0.5
0
175 190 Annealing temperature (oC)
0.35
(c)
0 -5 -10 o
Testing temperature ( C) 95 120 145
-15 -20
0
200
150
175 190 Annealing temperature (oC)
Thermal actuation force (N)
Contraction strain (%)
5
150
(d)
175 190 Annealing temperature (oC)
200
Testing temperature (oC):
0.3
95 120 145
0.25 0.2 0.15 0.1 0.05 0
200
150
150
175 190 Annealing temperature (oC)
200
Fig. 8. Effects of annealing temperature on the “shape memory” of artificial muscles: (a) threshold load, (b) slope (load/strain), (c) contraction strain, and (d) thermal actuation force. 54
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1.5
1.5
o
Annealing Temp ( C): 200 190 175 150
Young's modulus (GPa)
Young's modulus (GPa)
o
Annealing Temp( C): 200 175 190 150 1
0.5
1
0.5
Loading rate: 1 N/min
Loading rate: 10 N/min 0 50
60
70 80 90 o Temperature ( C)
100
0 50
110
60
70 80 90 o Temperature ( C)
(a )
100
110
(b)
Fig. 9. Temperature dependence of the Young's modulus of artificial muscles (twisted nylon 6 lines) annealed at different temperatures at the loading rate of (a) 10 N/min and (b) 1 N/min.
0.02
annealing. The tension-induced stretch of polymer chains during the annealing alters the configuration of polymer chains, which limits the motion of polymer chains during the tensile testing. From Fig. 11, we note that the temperature dependence of the Young's modulus of the non-twisted lines does not follow Eq. (4). There exists internal stress, as introduced in the annealing, which needs to be incorporated in Eq. (4). Fig. 12 shows the temperature dependence of the fracture stress of the non-twisted nylon 6 lines, which were annealed at different temperatures for two different loading rates. Similar to the Young's modulus, the fracture stress of the non-twisted nylon 6 lines decreases with the increase of the testing temperature, suggesting the easy separation and breakage of polymer chains at high temperature. There is a good deal of scatter of the fracture stress associated with the length of polymer chains and the weak points in the non-twisted nylon 6 lines. For the loading rate of 10 N/min, the non-twisted nylon 6 lines annealed at 150 °C generally have the largest fracture stress, and the nontwisted nylon 6 lines annealed at 200 °C generally have the lowest fracture stress. For the loading rate of 1 N/min, the non-twisted nylon 6 lines annealed at 175 °C generally have the largest fracture stress, and the non-twisted nylon 6 lines annealed at 200 °C generally have the lowest fracture stress.
Loading rate (N/min): 10 1
o
-1
(C )
0.015
0.01
0.005 140
150
160 170 180 190 200 o Annealing temperature ( C)
210
Fig. 10. Variation of the thermal softening coefficient of artificial muscles with annealing temperature.
2.5
2.5
2
Young's modulus (GPa)
Young's modulus (GPa)
o
Annealing Temp ( C): 200 190 175 150
1.5
1 50
Loading rate: 10 N/min 60
70
80
90
100
2 o
1.5
1
110
o
Annealing Temp( C): 200 190 175 150 Loading rate: 1 N/min 50
60
70
80
90
100
110
o
Temperature ( C)
Temperature ( C)
(a )
(b )
Fig. 11. Temperature dependence of the Young's modulus of the non-twisted nylon 6 lines annealed at different temperatures at the loading rate of (a) 10 N/min and (b) 1 N/min. 55
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o
Annealing Temp ( C): 150 175 190 200
0.50 0.45 0.40 0.35 0.30 50
(b)0.55 Fracture stress (GPa)
Fracture stress (GPa)
(a)0.55
70 80 90 o Temperature ( C)
0.50 0.45 0.40 0.35
Loading rate: 10 N/min 0.30 50 60 70 80 90 o Temperature ( C)
Loading rate: 1 N/min 60
o
Annealing Temp ( C): 55 70 85 100
100
(a)
100
(b)
Fig. 12. Temperature dependence of the fracture stress of the non-twisted nylon 6 lines annealed at different temperatures at the loading rate of (a) 1 N/min and (b) 10 N/min.
5. Summary
References
It is known that nylon 6 lines thermally annealed at relatively high temperature have the potential in the applications of soft-smart structures and machines through the storage/release of strain energy in different thermal condition. Studying the effect of thermal annealing on the isothermal-mechanical responses of the non-twisted and twisted nylon 6 lines can shed insights into the mechanical stability of the softsmart structures and machines and determine the service conditions applicable to the soft-smart structures and machines. It is of great importance to understand the differences in the mechanical responses between natural muscles and artificial muscles. The following is the summary of the main results obtained from this work.
[1] C.S. Haines, M.D. Lima, N. Li, G.M. Spinks, J. Foroughi, J.D.W. Madden, S.H. Kim, S.L. Fang, M.J. de Andrade, F. Goktepe, O. Goktepe, S.M. Mirvakili, S. Naficy, X. Lepro, J.Y. Oh, M.E. Kozlov, S.J. Kim, X.R. Xu, B.J. Swedlove, G.G. Wallace, R.H. Baughman, Artificial muscles from fishing line and sewing thread, Science 343 (2014) 868–872. [2] J. Fan, G. Li, High performance and tunable artificial muscle based on two-way shape memory polymer, RSC Adv. 7 (2017) 1127–1136. [3] T. Mirfakhrai, J.D. Madden, R.H. Baughman, Polymer artificial muscles, Mater. Today 10 (2007) 30–38. [4] R. Baughman, Conducting polymer artificial muscles, Synth. Met. 78 (1996) 339–353. [5] S. Chiba, M. Waki, R. Kornbluh, R. Pelrine, Innovative wave power generation system using electroactive polymer artificial muscles, OCEANS 2009-EUROPE, IEEE, 2009, pp. 1–3. [6] M.C. Yip, G. Niemeyer, On the control and properties of supercoiled polymer artificial muscles, IEEE Trans. Robot. 33 (2017) 689–699. [7] P. Mukhopadhyay, N. Fujita, A. Takada, T. Kishida, M. Shirakawa, S. Shinkai, Regulation of a real-time self-healing process in organogel tissues by molecular adhesives, Angew. Chem. Int. Ed. 49 (2010) 6338–6342. [8] F. Carpi, R. Kornbluh, P. Sommer-Larsen, G. Alici, Electroactive polymer actuators as artificial muscles: are they ready for bioinspired applications? Bioinspiration Biomimetics 6 (2011) 045006. [9] K. Tominaga, H. Hashimoto, W. Takashima, K. Kaneto, Training and shape retention in conducting polymer artificial muscles, Smart Mater. Struct. 20 (2011) 124005. [10] L. Lin, A.S. Argon, Deformation resistance in oriented nylon 6, Macromolecules 25 (1992) 4011–4024. [11] T. Ishibashi, K. Aoki, T. Ishii, Studies on melt spinning of nylon 6. I. Cooling and deformation behavior and orientation of nylon 6 threadline, J. Appl. Polym. Sci. 14 (1970) 1597–1613. [12] W.G. Perkins, R.S. Porter, Solid-state deformation of polyethylene and nylon and its effects on their structure and morphology, J. Mater. Sci. 12 (1977) 2355–2388. [13] A. Cherubini, G. Moretti, R. Vertechy, M. Fontana, Experimental characterization of thermally-activated artificial muscles based on coiled nylon fishing lines, AIP Adv. 5 (2015) 067158. [14] G.M. Spinks, Stretchable artificial muscles from coiled polymer fibers, J. Mater. Res. 31 (2016) 2917–2927. [15] E. Zussman, M. Burman, A.L. Yarin, R. Khalfin, Y. Cohen, Tensile deformation of electrospun nylon-6,6 nanofibers, J. Polym. Sci., Polym. Phys. Ed. 44 (2006) 1482–1489. [16] Y.-W. Huang, W.-S. Lee, Y.-F. Chuang, W. Cao, F. Yang, S. Lee, Time-dependent deformation of artificial muscles based on Nylon 6, Mater. Sci. Eng. C 98 (2019) 445–451. [17] Z. Zhang, M.H. Litt, L. Zhu, Unified understanding of ferroelectricity in n-nylons: is the polar crystalline structure a prerequisite? Macromolecules 49 (2016) 3070–3082. [18] S.M. Aharoni, n-Nylons, Their Synthesis, Structure, and Properties, Wiley, New York, 1997. [19] Z. Wang, Y. Zhou, P. Mallick, Effects of temperature and strain rate on the tensile behavior of short fiber reinforced polyamide‐6, Polym. Compos. 23 (2002) 858–871.
1. For the tensile deformation of the chicken muscle fibers, there exist four stages with the tensile force being a linear function of the elongation of chicken muscle fibers. Both the fracture stress and the critical stresses for the transition for one stage to the next stage increase with the decrease of the loading rate. The third stage has the largest Young's modulus. For the same stage, the smaller the loading rate, the larger is the Young's modulus. 2. For the artificial muscles made from twisted nylon 6, the Young's modulus of the same artificial muscles decreases with the increase of the testing temperature. For the same testing temperature, the artificial muscle annealed at 200 °C has the smallest Young's modulus, and there is no significant difference in the Young's modulus between the artificial muscles annealed at 150 and 175 °C. The Young's modulus of the twisted nylon 6 is an exponential function of the testing temperature. The thermal softening coefficient increases first with increasing the annealing temperature, reaches the maximum at the annealing temperature of 190 °C, and then decreases with increasing the annealing temperature. 3. For the non-twisted nylon 6 wires, both the Young's modulus and fracture stress for the same non-twisted nylon 6 lines decrease with the increase of the testing temperature. 4. For the artificial muscles made from twisted nylon 6, the thermal actuation force decreases with the increase of the annealing temperature. For the same annealing temperature, the thermal actuation force increases with the increase of the testing temperature. Acknowledgment This work was financially supported by the Ministry of Science and Technology, Taiwan.
56
Contents lists available at ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Tensile deformation of artificial muscles: Annealed nylon 6 lines a
b
c
T
a,∗
Yi-Wei Huang , Wen-Shin Lee , Fuqian Yang , Sanboh Lee a b c
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, 300, Taiwan Department of Athletics, National Taiwan University, Taipei, 10617, Taiwan Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY, 40506, USA
H I GH L IG H T S
mechanical responses of artificial muscle and natural muscle. • Understand stages for the tensile deformation of chicken muscle were observed. • Four Young's modulus of artificial muscle varies with testing temperature. • The force of artificial muscle varies with annealing temperature. • Actuation • Young's modulus of non-twisted nylon 6 varies with testing temperature.
A R T I C LE I N FO
A B S T R A C T
Keywords: Chicken muscle fiber Artificial muscle Tension deformation Temperature effect
In this work, we study the tensile deformation of chicken muscle fibers and the temperature dependence of the tensile deformation of non-twisted nylon 6 lines and twisted nylon 6 (artificial muscles). Both the non-twisted nylon 6 lines and twisted nylon 6 are annealed at different temperatures of 150, 175, 190 and 200 °C. The chicken muscle fibers under tensile loading exhibit four deformation stages with the tensile load being a linear function of the elongation in each stage. The largest Young's modulus (the slope of the load versus the elongation curve) occurs at stage III. For the tensile deformation of the annealed non-twisted nylon 6 lines, the tensile load is proportional to the elongation. For the tensile deformation of the twisted nylon 6 (artificial muscles), there exist three stages with the tensile loading being a linear function of the elongation in stages I and III. The Young's modulus calculated from the load-elongation curves decreases with the increase of the testing temperature. For the testing conditions used in this work, the tensile deformation eventually leads to the fracture of both the chicken muscle fibers and the annealed non-twisted nylon 6 lines. The fracture stress of the annealed non-twisted nylon 6 lines decreases with the increase of the testing temperature.
1. Introduction
human muscle of the same length and mass, opens a new horizon to use thermal energy to produce large displacement linearly and/or rotationally for soft-artificial structures. This discovery has renewed the interest in exploring the potential applications of polymeric fibers in artificial muscles [2] and investigating the mechanical responses of the soft-artificial structures made from polymer fibers, since researchers have been seeking the possibility of using polymeric materials as the structural materials in soft-artificial structures for many decades [3–9]. To effectively fabricate soft-artificial structures from polymeric materials, it is of great importance to understand the mechanical responses of the polymeric materials under a variety of environments. Lin and Argon [10] studied the anisotropic behavior of textured nylon 6 under compression. Using melt spinning to make nylon 6 filament yarns from nylon 6, Ishibashi et al. [11] performed tension tests of the nylon 6
There is a great need to develop soft-artificial structures mimicking the functionality of biological structures in soft and smart machines. In the heart of soft-artificial structures are the actuation mechanisms, which depend on the materials used in the structures. There are a variety of materials, such as carbon nanotubes and shape memory polymers, which have the potential in the applications of soft-artificial structures. The challenges in the development of soft-artificial structures include the need of large stroke under the action of small stimulus, fast response and structural stability. The demonstration of polymeric fibers (especially nylon) as the structural materials for artificial muscles by Haines et al. [1], which can sustain weight of 100 times more than
∗
Corresponding author. E-mail address: [email protected] (S. Lee).
https://doi.org/10.1016/j.polymer.2019.05.070 Received 26 April 2019; Received in revised form 24 May 2019; Accepted 26 May 2019 Available online 28 May 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.
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it took more 15 min for the tension tests of the chicken muscle fibers at a loading rate of 0.001 N/min. Under such a loading rate, the experimental results cannot represent the intrinsic behavior of the chicken muscle fibers due to the change in the fraction of the composition of the chicken muscle fibers associated with the evaporation of water. For the loading rate of 1.5 N/min, the chicken muscle fibers fractured in 1 s, and there are no enough data available for the post-test analysis. Define threshold load as the force needed to overcome the intern force for artificial muscles to start to extend and thermal actuation force as the force needed to pull artificial muscles to the state at room temperature, respectively. Using the dead load approach, we measured the thermal actuation force of the artificial muscles and the non-twisted nylon 6 fibers in air at three different temperatures of 95, 120 and 145 °C to examine the annealing effect. The load-elongation curves were constructed from the measurement. Here, the elongation (length change) was referenced to the initial length at room temperature. The tensile tests of the artificial muscles and the non-twisted nylon 6 fibers were conducted in air at four different temperatures of 55, 70, 85, and 100 °C. The length of the artificial muscles for the tension test was limited to 1.5 ± 0.3 cm, which is due to the spatial confinement in TA Q800 DMA. The loading rates were 1 and 10 N/min. The geometrical dimensions of the artificial muscles, including wire diameter (d), outer diameter (D0) and the number of coils, as shown in Fig. 1c, were determined from the optical images of the specimens, which were taken prior to the tension test, on an optical microscope (OLYMPUS BX51, OLYMPUS Co., Shinjuku-Ku, Tokyo). For each set of experimental condition, tensile tests of at least four different samples were performed. The results reported in the work are the average values of the test results of the same group samples.
filament yarns, and revealed the effects of cooling and thinning on molecular orientation. Perkins and Porter [12] suggested that tensile deformation of nylon can cause the fragmentation of crystalline lamellae and the drawing out of polymer chains. Cherubini et al. [13] performed isothermal and isometric tests of coiled polymeric fiber made from nylon 6 at tensile state, and showed the decrease of the energy dissipation with the increase of the training temperature. Spinks [14] used spring mechanics to analyze the stiffness and the actuation mechanism of artificial muscles made from coiled-twisted polymer fibers. Zussman et al. [15] reported the scattering of the mechanical properties of electrospun nylon-6,6 nanofibers consisting of α-crystalline phase. All of these results demonstrate the dependence of the mechanical behavior of the soft-artificial structures on the microstructures and thermal history of the polymeric materials. However, there are few reports focusing on the fracture strength of the soft-artificial structures. It is of practical importance to understand the effects of microstructures and thermal history of polymeric materials on the structural integrity of soft-artificial structures made from the polymeric materials. Considering the important application of soft-artificial muscle made from nylon 6, we investigate the tensile deformation of non-twisted nylon 6 lines and artificial muscle (twisted nylon 6), which are annealed at four different temperatures. The tensile behavior of the nontwisted nylon 6 lines and twisted nylon 6 is compared to that of chicken muscle fibers. The study is focused on the effect of microstructures and thermal history on the fracture strength of the non-twisted nylon 6 lines and twisted nylon 6. 2. Experimental details For detailed information on the preparation of the materials used in this work, see the work by Huang et al. [16]. Briefly, chicken muscle fibers were obtained from chicken thighs in an aqueous solution of 400 mM mannitol and 50 mM potassium acetate. The artificial muscles were twisted nylon 6 of different coil lengths of 1.5 ± 0.3, 7.5 ± 0.5, and 13.5 ± 1.0 cm, which was made from monofilament nylon 6 line of 0.2 mm in diameter (100% polyamide-6 FCFC B248) (Formosa Chemicals & Fibre Corp, Taipei). The annealing of the twisted nylon 6 was conducted at four different temperatures of 150, 175, 190, and 200 °C for 20 min, and the training of the annealed-twisted nylon 6 was performed in the temperature range of 25–95 °C under the action of constant load of 20 g. Following the same procedure in the preparation of the artificial muscles, non-twisted nylon 6 lines were prepared with the annealing at four different temperatures of 150, 175, 190, and 200 °C and the training in the temperature range of 25–95 °C under the action of constant load of 20 g. Fig. 1 shows optical micrographs of a chicken muscle fiber, a non-twisted nylon 6 line and an artificial muscle (twisted nylon 6). The XRD (X-ray diffraction) analyses of the artificial muscles were performed on a Bruker D2 PHASER (Billerica, MA, USA) with Cu Kα (λ = 0.154 nm) at a voltage of 30 kV and a current of 10 mA. The diffraction intensities were counted at 0.02° per step. The spectrum was collected in an angular range of 10°–80°. A home-made tensile fixture, as described in detail in the previous work [16], was used to perform the tensile tests of the chicken muscle fibers, non-twisted nylon 6 lines and artificial muscles. The tension tests were performed on a TA Q800 DMA (TA Instruments, New Castle, DE). The study was focused on the dependence of fracture strength on loading rate. The tension tests of the chicken muscle fibers were conducted in air at room temperature. Optical imaging of the chicken muscle fibers was performed for the analysis of geometrical dimensions of the chicken muscle fibers after placing the chicken muscle fibers on the fixture. Two loading rates of 0.015 and 0.15 N/min were used. Prior to the tension test, the chicken muscle fiber was immersed in an aqueous solution consisting of 400 mM mannitol and 50 mM potassium acetate. Note that
3. Results It is known that nylon 6 possesses several crystalline phases with α and γ forms being the most stable and common phases [17]. The α phase consists of sheets of hydrogen-bonded chains in an anti-parallel form, and the γ phase is present in the form of parallel chains [18]. Fig. 2 shows XRD spectra of artificial muscles, which were annealed at four different temperatures. There are two characteristic peaks at 20.3–20.7° and 23.35–23.9°, which correspond to the (200) and [(002), (220)] planes of α phase, respectively [18]. In general, nylon 6 consists of crystalline phases and amorphous phase. Using the Gaussian deconvolution technique and the XRD spectra of Fig. 2, we obtain the crystallinity of nylon 6 as
Crystallinity (%) =
Acr Acr + Aam
(1)
from the areas enclosed the diffraction peaks and the abscissa for crystalline phase, Acr, and amorphous phase, Aam, respectively. Fig. 3 shows the variation of the crystallinity of the artificial muscles with the annealing temperature. The crystallinity of the artificial muscles increases slightly with the increase of the annealing temperature with the smallest crystallinity of 30% and the largest crystallinity of 35%. Thus, we can assume that there is no significant difference in the structures of the artificial muscles used in this work. Fig. 4 shows a typical force-displacement curve for the tension test of the chicken muscle fibers. It is evident that the tension deformation of the chicken muscle fibers consists of four stages with approximate constant slopes, i.e. the tension force is a linear function of the displacement (elongation) for each stage. In stage I, the slope of force versus displacement is very steep due to the deformation of the aluminum grip. In stage II, the tension activates the muscle cells, which contract to counter-balance the external force and are experiencing elongation. In stage III, all the muscle cells in the muscle fiber are activated and are at tensile state. There exists significant contraction force in the muscle cells to counter-balance the tensile force. At the transition from the stage III to the stage IV, local stress relaxation/damage in the 50
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(a)
(b)
(c) D0
d
100 m
400 m
Fig. 1. Optical micrographs of (a) a chicken muscle fiber, (b) a non-twisted nylon 6 line and (c) an artificial muscle.
Crystallinity of artificial muscle (%)
40
Annealing temp (oC): Intensity (a.u.)
200
190 175 150 0
10 20
30 40 50 60 70 2 (o)
80 90
35
30
25
20 140
150
160 170 180 190 o Annealing temperature ( C)
200
210
Fig. 3. Variation of the crystallinity of artificial muscles with annealing temperature.
Fig. 2. XDR spectra of artificial muscles.
muscles to reach the length at room temperature. Note that some artificial muscles even experiences contraction at small load, resembling the behavior of chicken muscle under the external stimulus from passive state to active state. Fig. 6 shows a typical force-displacement curve for the tension test of the artificial muscles. In contrast to the chicken muscle fibers, there are three stages for the tension of all the artificial muscles. The first stage is from the start of the tensile test to point A, which is due to the grip deformation. The second stage is from point A to point B with the slope smaller than that of the first stage. The third stage starts at point B with constant slope. All the artificial muscles did not break during the
muscle cells is present, and the force needed to produce the same length change decrease. Finally, the chicken muscle fiber breaks. The presence of the II, III and IV stages suggests that the tension causes the change in the configuration of the microstructures in the chicken muscle fibers. Fig. 5 shows the variation of the elongation of the artificial muscles with applied load at different temperatures for the measurement of thermal actuation force. It is evident that all the artificial muscles contracted upon heating, which can be referred to as the “shape memory” effect. It needs to increase the load to stretch the artificial 51
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0.03
0.4
IV
(a)
Testing Temp: 95 C
Displacement (cm)
Force (N)
0.2 0.02 III 0.01 II 0.0
-0.2 -0.4
Annealing Temp ( C): 150 175 190 200
-0.6 -0.8
I
0.00
0.0
0.2
0.4
0.6
0.8
1.0
-1.0
Displacement (cm)
0.00
0.05
0.10
0.15
0.20
0.25
Load (N)
Fig. 4. Typical force-displacement curve for the tension test of chicken muscle fibers.
0.5
Displacement (cm)
tests. It is worth mentioning that the slope of the force-elongation curve of the artificial muscles in stage III is a function of the loading rate, annealing temperature and testing temperature. Fig. 7 shows typical force-displacement curves for the tension test of the non-twisted Nylon 6 fibers annealed at different temperatures under different conditions. Similar to the artificial muscles made from the twisted Nylon 6 fibers, there is one single stage for the tension of all the non-twisted Nylon 6 fibers with the linear relationship between the tensile force and the displacement (elongation). All the non-twisted Nylon 6 fibers broke eventually. 4. Discussion Comparing Fig. 4 with Fig. 6, we note that there exists significant difference in the tensile behavior between the Nylon 6-based artificial muscles and the chicken muscle fibers. Such a difference suggests that artificial muscles simply made from twisted Nylon 6 cannot resemble the function of chicken muscle fibers. A combination of Nylon 6-based artificial muscles and shape memory polymers is needed in the design of artificial muscles, which can resemble the function of biological muscle fibers. However, if the resemblance of tensile behavior between the Nylon 6-based artificial muscles and the chicken muscle fibers is not important, the former can sustain a load of 100 times more than the latter. Also, the thermal actuation force of the former is larger than the fracture load of the latter by one order of magnitude. This implies that twisted Nylon 6 has potential to be artificial muscles. Using the force-displacement curves, as shown in Fig. 4, and approximating the chicken muscle fibers as cylindrical rods, we can calculate the true stress and true strain and determine the critical stresses from the transitions from stage II to stage III and from stage III to stage IV, respectively, and the fracture stress. Table 1 lists the critical stresses and the fracture stress for two different loading rates, in which σ1 and σ2 represent the critical stresses for the transitions from stage II to stage III and from stage III to stage IV, respectively, and σf is the fracture stress. It is evident that both the fracture stress and the critical stresses increase with the decrease of the loading rate. Such behavior is consistent with the phenomena that high strain rate can cause damage to muscles and fast transition of muscles at “passive” state to “activated” state corresponding to the transition from the stage II to the stage III. The fast transition of muscles at “passive” state to “activated” state can lead to fast consumption of chemical energy stored in muscles and the loss of the muscle strength, resulting in the early transition with small σ2 from the stage III to the stage IV. From the force-displacement curves, we calculate the Young's modulus of each stage, Ei (i = II, III and IV), i.e. the slope, which is also
(b) Testing Temp: 120 oC
0.0 -0.5 o
Annealing Temp ( C): 150 175 190 -1.5 200 0.00 0.05 0.10 0.15 0.20 0.25 0.30 -1.0
Load (N) 0.5 (c)
Testing Temp: 145 C
Displacement (cm)
0.0 -0.5 -1.0 Annealing Temp ( C): 150 -2.0 175 190 -2.5 200 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 -1.5
Load(N) Fig. 5. Variation of the elongation of artificial muscles with applied load at different temperatures for the measurement of thermal actuation force: (a) 95 °C, (b) 120 °C, and (c) 145 °C.
listed in Table 1. Here, the subscripts of II, III, and IV represent the stage II, III, and IV, respectively. The stage III has the largest Young's modulus. For the same stage, the smaller the loading rate, the larger is the 52
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6
18
5 III
Force (N)
Force (N)
4 3 B
2 II
1 0
I
12 o
Testing temp ( C): 6
K
A
0
2 4 6 Displacement (cm)
0 0.0
8
16
Fig. 6. Typical force-displacement curve for the tension test of artificial muscles.
14
Ftherm = Fs + k|εc |
1.0 1.5 2.0 2.5 Displacement (mm)
(b)
Loading Temp: 70 C Loading rate: 1 N/min
Force (N)
10 8
o
Testing Temp ( C):
6
150 175 190 200
4 2 0 0.0
0.5
1.0
1.5 2.0 2.5 3.0 Displacement (mm)
3.5
4.0
Fig. 7. (a) Force-displacement curves of the tension tests of non-twisted Nylon 6 fibers annealed at 190 °C at a loading rate of 10 N/min and at different temperatures, and (b) force-displacement curves for the tension tests of nontwisted Nylon 6 fibers annealed at different temperatures of 150, 175, 190, and 200 °C at a loading rate of 1 N/min and a temperature of 70 °C.
(2)
where Fs is the threshold load, k is the slope (load/strain), and εc is the contraction strain. Fig. 8d shows the thermal actuation force at different annealing temperatures. In general, the thermal actuation force decreases with the increase of the annealing temperature. For the same annealing temperature, the thermal actuation force increases with the increase of the testing temperature. For the artificial muscles made from twisted nylon 6 lines, the relationship between the spring constant, K, and the shear modulus, G, of the nylon 6 lines can be expressed as
Gd4 8(D0 − d )3Nc
0.5
55 70 85 100 3.0 3.5
12
Young's modulus. Small loading rate allows more conversion of chemical energy to mechanical energy to increase the mechanical strength of the muscles during the tensile deformation. From Fig. 5, we note that there exists a “threshold load” for some artificial muscles to start to extend under an external load, and there exists linear relationship between the applied load and the elongation of the artificial muscles when the applied load is larger than the threshold load. Fig. 8a–c shows the variations of the threshold load, the slope (load/strain) for the linear relationship between the applied load and the elongation of the artificial muscles and the contraction strain at zero load for different annealing temperatures. It is evident that there are no clear trends for the threshold load and the slope (load/strain). For the contraction strain, increasing the testing temperature leads to the increase of the contraction strain, suggesting that more stored energy is released through the disorder of polymer chains. From the contraction strain, the slope (load/strain) and the threshold load, we calculate the thermal actuation force, Ftherm, as
K=
(a) Annealing Temp: 190 C Loading rate: 10 N/min
the twisted nylon 6 lines at the third stage which were annealed at different temperatures for two different loading rates. It is evident that, for the same artificial muscle, the Young's modulus decreases with the increase of the testing temperature, qualitatively in accord with the results for the tensile deformation of polymers reported in literature [19]. Such behavior can likely be attributed to the loose structure of polymer chains and the decrease of the resistance to the relative motion of polymer chains in polymer at high temperature. For the same testing temperature, the artificial muscle annealed at 200 °C has the smallest Young's modulus, and there is no significant difference in the Young's modulus between the artificial muscles annealed at 150 and 175 °C. This trend is likely associated with the extent of crystallinity in the nylon 6, as shown in Fig. 3. The artificial muscle annealed at 200 °C has the largest fraction of crystallinity, which has more transition zone between the polymer chains and the region with high crystallinity, allowing easy deformation across the transition zone. Comparing Fig. 9a with Fig. 9b, we note that the Young's modulus of the artificial muscle increases with the increase of the loading rate for the same testing temperature. This result is consistent with the rate dependence of the Young's modulus of polymer. The larger the strain/ loading rate, the larger is the elastic modulus. According to the work by Wang et al. [19], the temperature
(3)
where d is the diameter of the nylon 6 line and D0 is the outer diameter of the twisted nylon 6 lines, as shown in Fig. 1c. The parameter Nc is equal to N – 2 with N being the number of coils in the artificial muscle. Both d and D0 can be measured from the optical images (Fig. 1c) by Image J, and K is the slope of the third stage of the force-displacement curves. Using linear regression to curve-fitting the force-displacement curves and Eq. (3), we can calculate the shear modulus of the twisted nylon 6 lines for given N and the Young's modulus, E, of the twisted nylon 6 lines from E = 2G(1 + v) with v (= 0.4) being the Poisson ratio. Fig. 9 shows the temperature dependence of the Young's modulus of 53
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Table 1 Test results of chicken muscle fibers. Loading rate (N/min)
σ1 (MPa)
σ2 (MPa)
σf (MPa)
EII (MPa)
EIII (MPa)
EIV (MPa)
0.015 0.15
0.081 ± 0.038 0.031 ± 0.013
0.30 ± 0.12 0.17 ± 0.06
0.41 ± 0.18 0.29 ± 0.11
0.082 ± 0.036 0.055 ± 0.020
1.85 ± 0.96 1.21 ± 0.40
0.39 ± 0.13 0.31 ± 0.10
thermal softening coefficient. Such behavior can be attributed to the retardation of the motion of polymer chains. Increasing the loading rate leads to the increase to the stretch of polymer chains and the decrease of the mobility of polymer chains, resulting in small thermal softening coefficient. Fig. 11 shows the temperature dependence of the Young's modulus of the non-twisted nylon 6 lines, which were annealed at different temperatures for two different loading rates. Similar to the artificial muscle, the Young's modulus of the non-twisted nylon 6 lines decreases with the increase of the testing temperature. Such behavior reveals the decrease of the resistance to the relative motion of polymer chains in polymer at high temperature. For the same testing temperature, the non-twisted nylon 6 lines annealed at 200 °C has the smallest Young's modulus at the loading rate of 10 N/min, and there is no significant difference in the Young's modulus between the non-twisted nylon 6 lines annealed in the temperature range of 150 and 190 °C in contrast to the artificial muscles. For the loading rate of 1 N/min. the non-twisted nylon 6 lines annealed at 150 °C generally has the largest Young's modulus, and there is no significant difference in the Young's modulus between non-twisted nylon 6 lines annealed in the temperature range of 175 and 200 °C. The reason for such behavior is unclear, and it might be associated with the stretch of polymer chains under tension during the
dependence of the Young's modulus of polymer can be expressed as
E = E0 (
ε˙ m ) exp[−λ (T − T0)] ε˙ 0
(4)
where E0 and ε˙ 0 are the Young's modulus and strain rate of the reference state at a temperature of T0, ε˙ is the strain rate at a temperature of T, m is the strain-rate index, and λ is the thermal softening coefficient. Using Eq. (4) to curve-fit the results in Fig. 9, we obtain the thermal softening coefficients for the artificial muscles annealed at different temperatures. The fitting results are also included in Fig. 9. It is evident that Eq. (4) can be used to describe the temperature dependence of the Young's modulus of the artificial muscles. Fig. 10 shows the variation of the thermal softening coefficient of the artificial muscles with annealing temperature for two loading rates. It is interesting to note that the thermal softening coefficient increases first with increasing the annealing temperature, reaches maximum at the annealing temperature of 190 °C, and then decreases with increasing the annealing temperature. The reason for such behavior is unclear, and it might be related to the change in the microstructure of the annealed nylon 6 lines. From Fig. 8, we note that there exists the dependence of the thermal softening coefficient of the artificial muscles on the loading rate. The larger the loading rate, the smaller is the
3
(a)
o
Testing temperature ( C): 95 120 145
0.15
0.1
0.05
Slope (Load/strain) (N)
Threshold load (N)
0.2
(b) Testing temperature (oC) 95 2.5 120 145 2 1.5 1 0.5
0
175 190 Annealing temperature (oC)
0.35
(c)
0 -5 -10 o
Testing temperature ( C) 95 120 145
-15 -20
0
200
150
175 190 Annealing temperature (oC)
Thermal actuation force (N)
Contraction strain (%)
5
150
(d)
175 190 Annealing temperature (oC)
200
Testing temperature (oC):
0.3
95 120 145
0.25 0.2 0.15 0.1 0.05 0
200
150
150
175 190 Annealing temperature (oC)
200
Fig. 8. Effects of annealing temperature on the “shape memory” of artificial muscles: (a) threshold load, (b) slope (load/strain), (c) contraction strain, and (d) thermal actuation force. 54
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1.5
1.5
o
Annealing Temp ( C): 200 190 175 150
Young's modulus (GPa)
Young's modulus (GPa)
o
Annealing Temp( C): 200 175 190 150 1
0.5
1
0.5
Loading rate: 1 N/min
Loading rate: 10 N/min 0 50
60
70 80 90 o Temperature ( C)
100
0 50
110
60
70 80 90 o Temperature ( C)
(a )
100
110
(b)
Fig. 9. Temperature dependence of the Young's modulus of artificial muscles (twisted nylon 6 lines) annealed at different temperatures at the loading rate of (a) 10 N/min and (b) 1 N/min.
0.02
annealing. The tension-induced stretch of polymer chains during the annealing alters the configuration of polymer chains, which limits the motion of polymer chains during the tensile testing. From Fig. 11, we note that the temperature dependence of the Young's modulus of the non-twisted lines does not follow Eq. (4). There exists internal stress, as introduced in the annealing, which needs to be incorporated in Eq. (4). Fig. 12 shows the temperature dependence of the fracture stress of the non-twisted nylon 6 lines, which were annealed at different temperatures for two different loading rates. Similar to the Young's modulus, the fracture stress of the non-twisted nylon 6 lines decreases with the increase of the testing temperature, suggesting the easy separation and breakage of polymer chains at high temperature. There is a good deal of scatter of the fracture stress associated with the length of polymer chains and the weak points in the non-twisted nylon 6 lines. For the loading rate of 10 N/min, the non-twisted nylon 6 lines annealed at 150 °C generally have the largest fracture stress, and the nontwisted nylon 6 lines annealed at 200 °C generally have the lowest fracture stress. For the loading rate of 1 N/min, the non-twisted nylon 6 lines annealed at 175 °C generally have the largest fracture stress, and the non-twisted nylon 6 lines annealed at 200 °C generally have the lowest fracture stress.
Loading rate (N/min): 10 1
o
-1
(C )
0.015
0.01
0.005 140
150
160 170 180 190 200 o Annealing temperature ( C)
210
Fig. 10. Variation of the thermal softening coefficient of artificial muscles with annealing temperature.
2.5
2.5
2
Young's modulus (GPa)
Young's modulus (GPa)
o
Annealing Temp ( C): 200 190 175 150
1.5
1 50
Loading rate: 10 N/min 60
70
80
90
100
2 o
1.5
1
110
o
Annealing Temp( C): 200 190 175 150 Loading rate: 1 N/min 50
60
70
80
90
100
110
o
Temperature ( C)
Temperature ( C)
(a )
(b )
Fig. 11. Temperature dependence of the Young's modulus of the non-twisted nylon 6 lines annealed at different temperatures at the loading rate of (a) 10 N/min and (b) 1 N/min. 55
Polymer 177 (2019) 49–56
Y.-W. Huang, et al.
o
Annealing Temp ( C): 150 175 190 200
0.50 0.45 0.40 0.35 0.30 50
(b)0.55 Fracture stress (GPa)
Fracture stress (GPa)
(a)0.55
70 80 90 o Temperature ( C)
0.50 0.45 0.40 0.35
Loading rate: 10 N/min 0.30 50 60 70 80 90 o Temperature ( C)
Loading rate: 1 N/min 60
o
Annealing Temp ( C): 55 70 85 100
100
(a)
100
(b)
Fig. 12. Temperature dependence of the fracture stress of the non-twisted nylon 6 lines annealed at different temperatures at the loading rate of (a) 1 N/min and (b) 10 N/min.
5. Summary
References
It is known that nylon 6 lines thermally annealed at relatively high temperature have the potential in the applications of soft-smart structures and machines through the storage/release of strain energy in different thermal condition. Studying the effect of thermal annealing on the isothermal-mechanical responses of the non-twisted and twisted nylon 6 lines can shed insights into the mechanical stability of the softsmart structures and machines and determine the service conditions applicable to the soft-smart structures and machines. It is of great importance to understand the differences in the mechanical responses between natural muscles and artificial muscles. The following is the summary of the main results obtained from this work.
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1. For the tensile deformation of the chicken muscle fibers, there exist four stages with the tensile force being a linear function of the elongation of chicken muscle fibers. Both the fracture stress and the critical stresses for the transition for one stage to the next stage increase with the decrease of the loading rate. The third stage has the largest Young's modulus. For the same stage, the smaller the loading rate, the larger is the Young's modulus. 2. For the artificial muscles made from twisted nylon 6, the Young's modulus of the same artificial muscles decreases with the increase of the testing temperature. For the same testing temperature, the artificial muscle annealed at 200 °C has the smallest Young's modulus, and there is no significant difference in the Young's modulus between the artificial muscles annealed at 150 and 175 °C. The Young's modulus of the twisted nylon 6 is an exponential function of the testing temperature. The thermal softening coefficient increases first with increasing the annealing temperature, reaches the maximum at the annealing temperature of 190 °C, and then decreases with increasing the annealing temperature. 3. For the non-twisted nylon 6 wires, both the Young's modulus and fracture stress for the same non-twisted nylon 6 lines decrease with the increase of the testing temperature. 4. For the artificial muscles made from twisted nylon 6, the thermal actuation force decreases with the increase of the annealing temperature. For the same annealing temperature, the thermal actuation force increases with the increase of the testing temperature. Acknowledgment This work was financially supported by the Ministry of Science and Technology, Taiwan.
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