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Benzoyl peroxide thermo-crosslinked poly(ethylene-co-vinyl acetate) foam with two-way shape memory effect
Materials Letters 264 (2020) 127343
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Benzoyl peroxide thermo-crosslinked poly(ethylene-co-vinyl acetate) foam with two-way shape memory effect Jin Hui a, Hong Xia b, Yaqin Fu c, Yiping Qiu d, Qing-Qing Ni b,e,⇑ a
Interdisciplinary Graduate School of Science and Technology, Shinshu University, Ueda 386-8567 Japan Department of Mechanical Engineering & Robotics, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan c Key Laboratory of Advanced Textile Materials and Manufacturing Technology Ministry of Education, Zhejiang Sci-Tech University, Hangzhou, Zhejiang 310018, China d Shanghai Key Laboratory of Advanced Micro & Nano Textile Materials, College of Textiles, Donghua University, Shanghai 201620, China e College of Textiles and Apparel, Quanzhou Normal University, Fujian 362000, China b
a r t i c l e
i n f o
Article history: Received 3 January 2020 Accepted 10 January 2020 Available online 11 January 2020 Keywords: Two-way shape memory foam Reversible Thermal crosslinking Pore size Lightweight porous actuators
a b s t r a c t Two-way reversible shape memory polymer foams with porous three-dimensional structures were prepared using salt-leaching technology based on benzoyl peroxide (BPO) thermo-crosslinked poly (ethylene-co-vinyl acetate) (PEVA). Various pore sizes of PEVA/BPO porous foam were obtained using different NaCl particle sizes. The PEVA/BPO foams with various pore sizes exhibited reversible shape changes that allowed contraction during cooling and expansion during heating. The ideal two-way shape memory performance can be clearly observed in the large pore size sample PEVA/BPO-450. The morphology was characterized by scanning electron micron microscopy (SEM) and X-ray microcomputed tomography scanning (lCT) analysis. The compression behavior of PEVA/BPO foam was also investigated. These properties of two-way reversible shape memory PEVA/BPO foams could qualify their use as lightweight porous actuators in artificial intelligence and aerospace applications. Ó 2020 Elsevier B.V. All rights reserved.
1. Introduction Shape memory polymer foams with three-dimensional (3D) porous structures have been investigated for aerospace [20], biomedical [1,21], and self-healing [2,3], applications. A wide range of applications can be expected due to the materials’ low mass, highly compressible, and self-deployable qualities, such as solar sails, and foldable microcar and airplane wings for specific flight requirements [4,5]. Numerous methods were used to fabricate porous foams, such as gas foaming [6], particulate leaching [7–9], electro-spinning [10], phase separation [11], emulsion templating [12,13], and solid-state foaming [14]. For the shape memory foam materials, most researches were focused on the one-way shape memory effect (1W-SME). Compared with an one-way shape memory polymer foam with the drawback that the programming step requires an external force for each cycle [15,16,22], which may limit their applications. The two-way shape memory effect (2W-SME) shows full reversibility during exposure to external stimuli in every cycle [17,23,24]. The
⇑ Corresponding author at: Department of Mechanical Engineering & Robotics, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan; College of Textiles and Apparel, Quanzhou Normal University, Fujian 362000, China. E-mail address: [email protected] (Q.-Q. Ni). https://doi.org/10.1016/j.matlet.2020.127343 0167-577X/Ó 2020 Elsevier B.V. All rights reserved.
two-way shape memory polymer foams are smart materials that have the advantages of being lightweight and excellent active deformation, which was potentially applied in a wide range of applications, including lightweight porous polymeric actuators, sensors, and artificial intelligence. Mather and coworkers [18] proposed shape memory poly(e-caprolactone)-co-poly(ethylene glycol) foams. Their foam with salts could be UV cured and exhibited reversible actuation when compressed. Lendlein and coworkers proposed water-blown polyurethane foams that showed a reversible shape memory effect [19]. Despite the importance of two-way shape memory polymer foams, few relevant researchers have focused on this material. The aim of this study was to innovate a porous foam with a 2WSME. The poly(ethylene-co-vinyl acetate) (PEVA)/benzoyl peroxide (BPO) mixture was prepared using a solution method. Samples were foamed via a salt-leaching and thermo-crosslinked technology. The salt-leaching technology resulted in the formation of pores between the pore walls in the PEVA/BPO foam. This technology has the advantages of easy process and control of the porous structure, and the foaming process does not include any chemical blowing agents. Different 2W-SMEs could be achieved for PEVA/ BPO foams with various pore sizes. The morphology of samples with different pore sizes were observed. This finding of the two-way shape memory PEVA/BPO foams may contribute to their
2
J. Hui et al. / Materials Letters 264 (2020) 127343
2. Experimental
imen (cm3), mw is the weight of the water-saturated specimen in air (g), qsolid is the density of the PEVA (0.94 g/cm3 at 25 °C), qw is the density of the water (0.99705 g/cm3 at 25 °C), and P is the porosity of the specimen.
2.1. Fabrication of two-way shape memory PEVA/BPO foams
3. Results and discussion
Four different sizes of NaCl particles (0–50 lm, 50–110 lm, 110–160 lm, and 160–450 lm; Wako Pure Chemical Industries, Osaka, Japan) were obtained using a ball mill and sieves with different average mesh sizes. Eight grams of PEVA (18 weight percent [wt%] vinyl acetate; Sigma-Aldrich, Tokyo, Japan) containing 2 g BPO (Nippon Miractran, Atsugi, Japan) with four sizes of NaCl particles were completely dissolved in xylene, respectively (Fig. 1). The mixed solution was put in vacuum for 12 h at 85 °C to remove bubbles and then dried at 85 °C for 5 days. Further thermocrosslinking was performed to cure the sample in a vacuum electric furnace for 7 h at 200 °C and then it was immersed in distilled water to remove the salts (the distilled water was changed every 5 h). Two-way shape memory porous foams were obtained with different pore sizes of 0–50 lm, 50–110 lm, 110–160 lm, and 160–450 lm, which were denoted as PEVA/BPO-50, PEVA/BPO110, PEVA/BPO-160, PEVA/BPO-450, respectively.
The SEM images of the samples with different pore sizes exhibit an open porous structure with a high degree of pore interconnectivity (Fig. 2a–f). The pore sizes will have a great influence on the two-way shape memory properties; therefore, it was necessary to study the 2W-SME of porous structures in the PEVA/BPO foams. The pore wall areas (red area in Fig. 2h) between pores increased with the increase in the size of NaCl particles. The large pore wall areas can be formed with large pores (PEVA/BPO-450, Fig. 2a and h). Each sample was deformed to 40% compressive strain then released at a rate of 560 lm/min, the compressive stress of four samples were 0.16 MPa, 0.15 MPa, 0.07 MPa, and 0.05 MPa, respectively (Fig. 2g). The PEVA/BPO-450 sample with large pore size exhibited the largest compression resistance, which is probably attributed to the fact that the large pore walls area provided enough space for thermo-crosslinking as a result of crosslinked networks restricting the mobility of the molecular structure. That is, greater crosslinking density led to increased Young’s modulus. The sample with a 4–5 mm (diameter) was first heated from 25 °C to 85 °C above the crystalline melt temperature (Tm) at a rate of 2.5 °C/min, which represents the semi-crystalline transformed to amorphous in the foams, and compressed to a 40% compressive strain at a rate of 950 lm/min using a 3.5 mm-diameter compression probe. A constant stress was then maintained throughout the test. The temperature was cooled from high temperatures to 25 °C, and the samples were observed to contract along the compression direction because of the promotion of oriented crystal growth called as actuation strain (eact) as marked in Fig. 3a, ①–②. The temperature was increased to 80 °C again at a rate of 4.5 °C/min under constant compression stress and the crystalline melt led to the sample expanding to the original shape, which was called recovery strain (erec) as result of crosslinking in Fig. 3a, ②–③. This cooling–heating process was repeated for three cycles. The mark of ① in the curve represents the prestretching at high temperatures, ② is the strain deformation at low temperatures, and ③ is the strain deformation when the sample is reheated to 80 °C in every cycle. The actuation strain (eact) and recovery ratio (Rrec) are important indicators explaining the two-way shape memory performance (Table 1), where the recovery ratio (Rrec) which was defined by Rrec = erec/eact 100%.
applications as lightweight actuators in artificial intelligence, among others.
2.2. Characterization and evaluation The pore morphology and 3D structure of the PEVA/BPO foam were examined by scanning electron microscopy (SEM; SU-1510, 15 kV; Hitachi, Tokyo, Japan) and X-ray microcomputed tomography scanning (lCT; SkyScan 1272 Micro-CT, 40 kV, 250 lA; Bruker, Billerica, MA, USA). The compression property and 2W-SME were investigated using thermomechanical analysis (TMA/SS6100; SII Nano Technology, Tokyo, Japan). The density and porosity were calculated using the following equations:
qporous ¼
mporous mporous ¼ porous Vporous mqporous þ mw m q solid
w
mporous qsolid qw ¼ mporous qw þ ðmw mporous Þqsolid P¼
qsolid qporous 100% qsolid
ð1Þ
ð2Þ
where qporous is the density of the specimen (g/cm3), mporous is the weight of the specimen in air (g), Vporous is the volume of the spec-
Fig. 1. Schematic of two-way shape memory PEVA/BPO foam fabrication.
J. Hui et al. / Materials Letters 264 (2020) 127343
3
Fig. 2. SEM photographs of PEVA/BPO foams with various size of pores, a: PEVA/BPO-450; b: PEVA/BPO-160; c: PEVA/BPO-110; d: PEVA/BPO-50. Enlarged SEM images, e: PEVA/BPO-450; f: PEVA/BPO-50. g: compressive stress-strain curves for samples. h: The formation of pore wall area between different sizes of NaCl particles.
Fig. 3. Two-way shape memory behavior of samples with various pore size: a. PEVA/BPO-450; b. PEVA/BPO-160; c. PEVA/BPO-110; d. PEVA/BPO-50; e. two-way shape memory behavior of PEVA/BPO foam under constant compression; f. 3D X-ray lCT image of samples with different pore sizes.
4
J. Hui et al. / Materials Letters 264 (2020) 127343
Table 1 Two-way shape memory properties, density, and porosity of the PEVA/BPO foams.
eact (%)
Rrec
(%)
Sample
1st cycle
2nd cycle
3rd cycle
1st cycle
2nd cycle
3rd cycle
PEVA/BPO-450 PEVA/BPO-160 PEVA/BPO-110 PEVA/BPO-50
1.60 1.57 1.44 1.48
1.39 1.40 1.29 1.30
1.53 1.48 1.28 1.23
83.88 74.61 80.43 70.61
94.07 93.35 94.92 86.33
88.66 88.80 97.56 92.92
Thermomechanical analysis confirmed the clear 2W-SME of the PEVA/BPO foam, which shows that the strain change during cooling and heating was affected by different porous structures (Fig. 3a–d, Table 1). Under the same prestretching strain of 40%, we found that the large pore size sample (PEVA/BPO-450) showed an ideal two-way shape memory performance at the crosslinking temperature of 200 °C (Table 1). This was probably attributable to the fact that large pore wall areas with large pore sizes formed in the PEVA/BPO foam (Fig. 2h), which provided enough area for crosslinking reactions. The crosslinking reaction restricts crystallization and the molecular chains within the network, which led to increased Young’s modulus and restrained the plastic flow at high temperatures. Fig. 3e shows the two-way reversible shape memory process of PEVA/BPO foams. The pores were classified into four sizes, and the more-intuitive porous structures with different pore wall areas were observed in 3D microstructure images (Fig. 3f). 4. Conclusion Two-way reversible shape memory PEVA/BPO foams with different interconnected pores were fabricated using a salt-leaching and thermo-crosslinking technology. The different reversible shape changes in various pore sizes of PEVA/BPO foams were investigated upon exposure at low/high temperatures under constant compression conditions. The samples with large pore size exhibited ideal two-way shape memory behavior under the same prestretching strain at the crosslinking temperature of 200 °C. The morphology of porous PEVA/BPO foams was also investigated. Future work is needed to improve the material’s two-way shape memory actuation and recovery performance for practical applications of lightweight porous actuators in the artificial intelligence and smart bioengineering fields. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
q (g/cm3)
Porosity (%)
Pore size (lm)
0.1434 0.1529 0.1775 0.1865
84.74 83.73 81.12 80.16
160–450 110–160 50–110 0–50
Acknowledgement This work was supported by the China Scholarship Council (CSC) and the JSPS [JSPS Kakenhi 15H01789 and 26420721]. References [1] A. Metcalfe, A.C. Desfaits, I. Salazkin, L.H. Yahia, W.M. Sokolowski, J. Raymond, Biomaterials 24 (3) (2003) 491–497. [2] G. Li, D. Nettles, Polymer 51 (3) (2010) 755–762. [3] I.M. Van Meerbeek, B.C. Mac Murray, J.W. Kim, S.S. Robinson, P.X. Zou, M.N. Silberstein, R.F. Shepherd, Adv. Mater. 28 (14) (2016) 2801–2806. [4] S.M. Hasan, L.D. Nash, D.J. Maitland, J. Polym. Sci., Part B: Polym. Phys. 54 (14) (2016) 1300–1318. [5] H. Janik, M. Marzec, Mater. Sci. Eng. C 48 (2015) 586–591. [6] H.J. Kim, I.K. Park, J.H. Kim, C.S. Cho, M.S. Kim, Tissue. Eng. Regen. Med. 9 (2) (2012) 63–68. [7] C. Gaillard, J.F. Despois, A. Mortensen, Mater. Sci. Eng. A 374 (1–2) (2004) 250– 262. [8] L. Lu, S.J. Peter, M.D. Lyman, H.L. Lai, S.M. Leite, J.A. Tamada, A.G. Mikos, Biomaterials 21 (15) (2000) 1595–1605. [9] C.J. Liao, C.F. Chen, J.H. Chen, S.F. Chiang, Y.J. Lin, K.Y. Chang, J. Biomed. Mater. Res. 59 (4) (2002) 676–681. [10] L.F. Tseng, P.T. Mather, J.H. Henderson, In 2012 38th Annual Northeast Bioengineering Conference (NEBEC), IEEE, 2012, pp. 227–228. [11] Y.S. Nam, T.G. Park, Biomaterials 20 (19) (1999) 1783–1790. [12] K. Lissant (Ed.), Emulsions and Emulsion Technology, CRC Press, 1974. [13] S.D. Kimmins, N.R. Cameron, Adv. Funct. Mater. 21 (2) (2011) 211–225. [14] X. Wang, W. Li, V. Kumar, Biomaterials 27 (9) (2006) 1924–1929. [15] K. Hearon, P. Singhal, J. Horn, I.V.W. Small, C. Olsovsky, K.C. Maitland, D.J. Maitland, Polym. Rev. 53 (1) (2013) 41–75. [16] L. Santo, Prog. Aerosp. Sci. 81 (2016) 60–65. [17] J. Hui, H. Xia, H. Chen, Y. Qiu, Y. Fu, Q.Q. Ni, Mater. Lett. 258 (2020) 126762. [18] R.M. Baker, J.H. Henderson, P.T. Mather, J. Mater. Chem. B 1 (38) (2013) 4916– 4920. [19] E. Zharinova, M. Heuchel, T. Weigel, D. Gerber, K. Kratz, A. Lendlein, Polymers 8 (12) (2016) 412. [20] Q. Fabrizio, S. Loredana, S.E. Anna, Mater. Lett. 69 (2012) 20–23. [21] L. De Nardo, R. Alberti, A. Cigada, L.H. Yahia, M.C. Tanzi, S. Farè, Acta. Biotech. 5 (5) (2009) 1508–1518. [22] W.M. Huang, C.W. Lee, H.P. Teo, J. Intell. Mater. Syst. Struct. 17 (8–9) (2006) 753–760. [23] M. Behl, K. Kratz, U. Noechel, T. Sauter, A. Lendlein, PNAS 110 (31) (2013) 12555–12559. [24] M. Farhan, T. Rudolph, U. Nöchel, W. Yan, K. Kratz, A. Lendlein, ACS Appl. Mater. Inter. 9 (39) (2017) 33559–33564.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Benzoyl peroxide thermo-crosslinked poly(ethylene-co-vinyl acetate) foam with two-way shape memory effect Jin Hui a, Hong Xia b, Yaqin Fu c, Yiping Qiu d, Qing-Qing Ni b,e,⇑ a
Interdisciplinary Graduate School of Science and Technology, Shinshu University, Ueda 386-8567 Japan Department of Mechanical Engineering & Robotics, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan c Key Laboratory of Advanced Textile Materials and Manufacturing Technology Ministry of Education, Zhejiang Sci-Tech University, Hangzhou, Zhejiang 310018, China d Shanghai Key Laboratory of Advanced Micro & Nano Textile Materials, College of Textiles, Donghua University, Shanghai 201620, China e College of Textiles and Apparel, Quanzhou Normal University, Fujian 362000, China b
a r t i c l e
i n f o
Article history: Received 3 January 2020 Accepted 10 January 2020 Available online 11 January 2020 Keywords: Two-way shape memory foam Reversible Thermal crosslinking Pore size Lightweight porous actuators
a b s t r a c t Two-way reversible shape memory polymer foams with porous three-dimensional structures were prepared using salt-leaching technology based on benzoyl peroxide (BPO) thermo-crosslinked poly (ethylene-co-vinyl acetate) (PEVA). Various pore sizes of PEVA/BPO porous foam were obtained using different NaCl particle sizes. The PEVA/BPO foams with various pore sizes exhibited reversible shape changes that allowed contraction during cooling and expansion during heating. The ideal two-way shape memory performance can be clearly observed in the large pore size sample PEVA/BPO-450. The morphology was characterized by scanning electron micron microscopy (SEM) and X-ray microcomputed tomography scanning (lCT) analysis. The compression behavior of PEVA/BPO foam was also investigated. These properties of two-way reversible shape memory PEVA/BPO foams could qualify their use as lightweight porous actuators in artificial intelligence and aerospace applications. Ó 2020 Elsevier B.V. All rights reserved.
1. Introduction Shape memory polymer foams with three-dimensional (3D) porous structures have been investigated for aerospace [20], biomedical [1,21], and self-healing [2,3], applications. A wide range of applications can be expected due to the materials’ low mass, highly compressible, and self-deployable qualities, such as solar sails, and foldable microcar and airplane wings for specific flight requirements [4,5]. Numerous methods were used to fabricate porous foams, such as gas foaming [6], particulate leaching [7–9], electro-spinning [10], phase separation [11], emulsion templating [12,13], and solid-state foaming [14]. For the shape memory foam materials, most researches were focused on the one-way shape memory effect (1W-SME). Compared with an one-way shape memory polymer foam with the drawback that the programming step requires an external force for each cycle [15,16,22], which may limit their applications. The two-way shape memory effect (2W-SME) shows full reversibility during exposure to external stimuli in every cycle [17,23,24]. The
⇑ Corresponding author at: Department of Mechanical Engineering & Robotics, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan; College of Textiles and Apparel, Quanzhou Normal University, Fujian 362000, China. E-mail address: [email protected] (Q.-Q. Ni). https://doi.org/10.1016/j.matlet.2020.127343 0167-577X/Ó 2020 Elsevier B.V. All rights reserved.
two-way shape memory polymer foams are smart materials that have the advantages of being lightweight and excellent active deformation, which was potentially applied in a wide range of applications, including lightweight porous polymeric actuators, sensors, and artificial intelligence. Mather and coworkers [18] proposed shape memory poly(e-caprolactone)-co-poly(ethylene glycol) foams. Their foam with salts could be UV cured and exhibited reversible actuation when compressed. Lendlein and coworkers proposed water-blown polyurethane foams that showed a reversible shape memory effect [19]. Despite the importance of two-way shape memory polymer foams, few relevant researchers have focused on this material. The aim of this study was to innovate a porous foam with a 2WSME. The poly(ethylene-co-vinyl acetate) (PEVA)/benzoyl peroxide (BPO) mixture was prepared using a solution method. Samples were foamed via a salt-leaching and thermo-crosslinked technology. The salt-leaching technology resulted in the formation of pores between the pore walls in the PEVA/BPO foam. This technology has the advantages of easy process and control of the porous structure, and the foaming process does not include any chemical blowing agents. Different 2W-SMEs could be achieved for PEVA/ BPO foams with various pore sizes. The morphology of samples with different pore sizes were observed. This finding of the two-way shape memory PEVA/BPO foams may contribute to their
2
J. Hui et al. / Materials Letters 264 (2020) 127343
2. Experimental
imen (cm3), mw is the weight of the water-saturated specimen in air (g), qsolid is the density of the PEVA (0.94 g/cm3 at 25 °C), qw is the density of the water (0.99705 g/cm3 at 25 °C), and P is the porosity of the specimen.
2.1. Fabrication of two-way shape memory PEVA/BPO foams
3. Results and discussion
Four different sizes of NaCl particles (0–50 lm, 50–110 lm, 110–160 lm, and 160–450 lm; Wako Pure Chemical Industries, Osaka, Japan) were obtained using a ball mill and sieves with different average mesh sizes. Eight grams of PEVA (18 weight percent [wt%] vinyl acetate; Sigma-Aldrich, Tokyo, Japan) containing 2 g BPO (Nippon Miractran, Atsugi, Japan) with four sizes of NaCl particles were completely dissolved in xylene, respectively (Fig. 1). The mixed solution was put in vacuum for 12 h at 85 °C to remove bubbles and then dried at 85 °C for 5 days. Further thermocrosslinking was performed to cure the sample in a vacuum electric furnace for 7 h at 200 °C and then it was immersed in distilled water to remove the salts (the distilled water was changed every 5 h). Two-way shape memory porous foams were obtained with different pore sizes of 0–50 lm, 50–110 lm, 110–160 lm, and 160–450 lm, which were denoted as PEVA/BPO-50, PEVA/BPO110, PEVA/BPO-160, PEVA/BPO-450, respectively.
The SEM images of the samples with different pore sizes exhibit an open porous structure with a high degree of pore interconnectivity (Fig. 2a–f). The pore sizes will have a great influence on the two-way shape memory properties; therefore, it was necessary to study the 2W-SME of porous structures in the PEVA/BPO foams. The pore wall areas (red area in Fig. 2h) between pores increased with the increase in the size of NaCl particles. The large pore wall areas can be formed with large pores (PEVA/BPO-450, Fig. 2a and h). Each sample was deformed to 40% compressive strain then released at a rate of 560 lm/min, the compressive stress of four samples were 0.16 MPa, 0.15 MPa, 0.07 MPa, and 0.05 MPa, respectively (Fig. 2g). The PEVA/BPO-450 sample with large pore size exhibited the largest compression resistance, which is probably attributed to the fact that the large pore walls area provided enough space for thermo-crosslinking as a result of crosslinked networks restricting the mobility of the molecular structure. That is, greater crosslinking density led to increased Young’s modulus. The sample with a 4–5 mm (diameter) was first heated from 25 °C to 85 °C above the crystalline melt temperature (Tm) at a rate of 2.5 °C/min, which represents the semi-crystalline transformed to amorphous in the foams, and compressed to a 40% compressive strain at a rate of 950 lm/min using a 3.5 mm-diameter compression probe. A constant stress was then maintained throughout the test. The temperature was cooled from high temperatures to 25 °C, and the samples were observed to contract along the compression direction because of the promotion of oriented crystal growth called as actuation strain (eact) as marked in Fig. 3a, ①–②. The temperature was increased to 80 °C again at a rate of 4.5 °C/min under constant compression stress and the crystalline melt led to the sample expanding to the original shape, which was called recovery strain (erec) as result of crosslinking in Fig. 3a, ②–③. This cooling–heating process was repeated for three cycles. The mark of ① in the curve represents the prestretching at high temperatures, ② is the strain deformation at low temperatures, and ③ is the strain deformation when the sample is reheated to 80 °C in every cycle. The actuation strain (eact) and recovery ratio (Rrec) are important indicators explaining the two-way shape memory performance (Table 1), where the recovery ratio (Rrec) which was defined by Rrec = erec/eact 100%.
applications as lightweight actuators in artificial intelligence, among others.
2.2. Characterization and evaluation The pore morphology and 3D structure of the PEVA/BPO foam were examined by scanning electron microscopy (SEM; SU-1510, 15 kV; Hitachi, Tokyo, Japan) and X-ray microcomputed tomography scanning (lCT; SkyScan 1272 Micro-CT, 40 kV, 250 lA; Bruker, Billerica, MA, USA). The compression property and 2W-SME were investigated using thermomechanical analysis (TMA/SS6100; SII Nano Technology, Tokyo, Japan). The density and porosity were calculated using the following equations:
qporous ¼
mporous mporous ¼ porous Vporous mqporous þ mw m q solid
w
mporous qsolid qw ¼ mporous qw þ ðmw mporous Þqsolid P¼
qsolid qporous 100% qsolid
ð1Þ
ð2Þ
where qporous is the density of the specimen (g/cm3), mporous is the weight of the specimen in air (g), Vporous is the volume of the spec-
Fig. 1. Schematic of two-way shape memory PEVA/BPO foam fabrication.
J. Hui et al. / Materials Letters 264 (2020) 127343
3
Fig. 2. SEM photographs of PEVA/BPO foams with various size of pores, a: PEVA/BPO-450; b: PEVA/BPO-160; c: PEVA/BPO-110; d: PEVA/BPO-50. Enlarged SEM images, e: PEVA/BPO-450; f: PEVA/BPO-50. g: compressive stress-strain curves for samples. h: The formation of pore wall area between different sizes of NaCl particles.
Fig. 3. Two-way shape memory behavior of samples with various pore size: a. PEVA/BPO-450; b. PEVA/BPO-160; c. PEVA/BPO-110; d. PEVA/BPO-50; e. two-way shape memory behavior of PEVA/BPO foam under constant compression; f. 3D X-ray lCT image of samples with different pore sizes.
4
J. Hui et al. / Materials Letters 264 (2020) 127343
Table 1 Two-way shape memory properties, density, and porosity of the PEVA/BPO foams.
eact (%)
Rrec
(%)
Sample
1st cycle
2nd cycle
3rd cycle
1st cycle
2nd cycle
3rd cycle
PEVA/BPO-450 PEVA/BPO-160 PEVA/BPO-110 PEVA/BPO-50
1.60 1.57 1.44 1.48
1.39 1.40 1.29 1.30
1.53 1.48 1.28 1.23
83.88 74.61 80.43 70.61
94.07 93.35 94.92 86.33
88.66 88.80 97.56 92.92
Thermomechanical analysis confirmed the clear 2W-SME of the PEVA/BPO foam, which shows that the strain change during cooling and heating was affected by different porous structures (Fig. 3a–d, Table 1). Under the same prestretching strain of 40%, we found that the large pore size sample (PEVA/BPO-450) showed an ideal two-way shape memory performance at the crosslinking temperature of 200 °C (Table 1). This was probably attributable to the fact that large pore wall areas with large pore sizes formed in the PEVA/BPO foam (Fig. 2h), which provided enough area for crosslinking reactions. The crosslinking reaction restricts crystallization and the molecular chains within the network, which led to increased Young’s modulus and restrained the plastic flow at high temperatures. Fig. 3e shows the two-way reversible shape memory process of PEVA/BPO foams. The pores were classified into four sizes, and the more-intuitive porous structures with different pore wall areas were observed in 3D microstructure images (Fig. 3f). 4. Conclusion Two-way reversible shape memory PEVA/BPO foams with different interconnected pores were fabricated using a salt-leaching and thermo-crosslinking technology. The different reversible shape changes in various pore sizes of PEVA/BPO foams were investigated upon exposure at low/high temperatures under constant compression conditions. The samples with large pore size exhibited ideal two-way shape memory behavior under the same prestretching strain at the crosslinking temperature of 200 °C. The morphology of porous PEVA/BPO foams was also investigated. Future work is needed to improve the material’s two-way shape memory actuation and recovery performance for practical applications of lightweight porous actuators in the artificial intelligence and smart bioengineering fields. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
q (g/cm3)
Porosity (%)
Pore size (lm)
0.1434 0.1529 0.1775 0.1865
84.74 83.73 81.12 80.16
160–450 110–160 50–110 0–50
Acknowledgement This work was supported by the China Scholarship Council (CSC) and the JSPS [JSPS Kakenhi 15H01789 and 26420721]. References [1] A. Metcalfe, A.C. Desfaits, I. Salazkin, L.H. Yahia, W.M. Sokolowski, J. Raymond, Biomaterials 24 (3) (2003) 491–497. [2] G. Li, D. Nettles, Polymer 51 (3) (2010) 755–762. [3] I.M. Van Meerbeek, B.C. Mac Murray, J.W. Kim, S.S. Robinson, P.X. Zou, M.N. Silberstein, R.F. Shepherd, Adv. Mater. 28 (14) (2016) 2801–2806. [4] S.M. Hasan, L.D. Nash, D.J. Maitland, J. Polym. Sci., Part B: Polym. Phys. 54 (14) (2016) 1300–1318. [5] H. Janik, M. Marzec, Mater. Sci. Eng. C 48 (2015) 586–591. [6] H.J. Kim, I.K. Park, J.H. Kim, C.S. Cho, M.S. Kim, Tissue. Eng. Regen. Med. 9 (2) (2012) 63–68. [7] C. Gaillard, J.F. Despois, A. Mortensen, Mater. Sci. Eng. A 374 (1–2) (2004) 250– 262. [8] L. Lu, S.J. Peter, M.D. Lyman, H.L. Lai, S.M. Leite, J.A. Tamada, A.G. Mikos, Biomaterials 21 (15) (2000) 1595–1605. [9] C.J. Liao, C.F. Chen, J.H. Chen, S.F. Chiang, Y.J. Lin, K.Y. Chang, J. Biomed. Mater. Res. 59 (4) (2002) 676–681. [10] L.F. Tseng, P.T. Mather, J.H. Henderson, In 2012 38th Annual Northeast Bioengineering Conference (NEBEC), IEEE, 2012, pp. 227–228. [11] Y.S. Nam, T.G. Park, Biomaterials 20 (19) (1999) 1783–1790. [12] K. Lissant (Ed.), Emulsions and Emulsion Technology, CRC Press, 1974. [13] S.D. Kimmins, N.R. Cameron, Adv. Funct. Mater. 21 (2) (2011) 211–225. [14] X. Wang, W. Li, V. Kumar, Biomaterials 27 (9) (2006) 1924–1929. [15] K. Hearon, P. Singhal, J. Horn, I.V.W. Small, C. Olsovsky, K.C. Maitland, D.J. Maitland, Polym. Rev. 53 (1) (2013) 41–75. [16] L. Santo, Prog. Aerosp. Sci. 81 (2016) 60–65. [17] J. Hui, H. Xia, H. Chen, Y. Qiu, Y. Fu, Q.Q. Ni, Mater. Lett. 258 (2020) 126762. [18] R.M. Baker, J.H. Henderson, P.T. Mather, J. Mater. Chem. B 1 (38) (2013) 4916– 4920. [19] E. Zharinova, M. Heuchel, T. Weigel, D. Gerber, K. Kratz, A. Lendlein, Polymers 8 (12) (2016) 412. [20] Q. Fabrizio, S. Loredana, S.E. Anna, Mater. Lett. 69 (2012) 20–23. [21] L. De Nardo, R. Alberti, A. Cigada, L.H. Yahia, M.C. Tanzi, S. Farè, Acta. Biotech. 5 (5) (2009) 1508–1518. [22] W.M. Huang, C.W. Lee, H.P. Teo, J. Intell. Mater. Syst. Struct. 17 (8–9) (2006) 753–760. [23] M. Behl, K. Kratz, U. Noechel, T. Sauter, A. Lendlein, PNAS 110 (31) (2013) 12555–12559. [24] M. Farhan, T. Rudolph, U. Nöchel, W. Yan, K. Kratz, A. Lendlein, ACS Appl. Mater. Inter. 9 (39) (2017) 33559–33564.