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Synergistic effect enhanced shape recovery behavior of metal-4D printed shape memory polymer hybrid composites
Journal Pre-proof Synergistic effect enhanced shape recovery behavior of metal-4D printed shape memory polymer hybrid composites Yang Liu, Fenghua Zhang, Jinsong Leng, Liyun Wang, Chase Cotton, Baozhong Sun, Tsu-Wei Chou PII:
S1359-8368(19)32991-9
DOI:
https://doi.org/10.1016/j.compositesb.2019.107536
Reference:
JCOMB 107536
To appear in:
Composites Part B
Received Date: 26 June 2019 Revised Date:
9 September 2019
Accepted Date: 13 October 2019
Please cite this article as: Liu Y, Zhang F, Leng J, Wang L, Cotton C, Sun B, Chou T-W, Synergistic effect enhanced shape recovery behavior of metal-4D printed shape memory polymer hybrid composites, Composites Part B (2019), doi: https://doi.org/10.1016/j.compositesb.2019.107536. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Synergistic effect enhanced shape recovery behavior of metal-4D printed shape memory polymer hybrid composites Yang Liu a,b, Fenghua Zhang c, Jinsong Leng c, Liyun Wang b, Chase Cotton d, Baozhong Sun a, and Tsu-Wei Chou b,∗ a
College of Textiles, Donghua University, Shanghai 201620, PR China
b
Department of Mechanical Engineering, University of Delaware, Newark, DE 19716,
USA c
Center for Composite Materials and Structures, Harbin Institute of Technology,
Harbin 150080, PR China d
Department of Electrical and Computer Engineering, University of Delaware, Newark,
DE 19716, USA Abstract The four-dimensional (4D) printing technology enables the convergence of three-dimensional (3D) printing and shape memory polymer (SMP). The aim of this research is to demonstrate the synergistic effect between a spring steel strip (SSS) and a 4D printed thermoplastic SMP on enhancing the shape recovery properties. The recovery time of the SMP/SSS hybrid composite was shortened by 39% as compared to that of the SMP specimen. The corresponding recovery force of the SMP/SSS hybrid specimen at 64 °C was 9 N, 199 times of that of the non-hybrid SMP specimen. For optimizing the design of hybrid composite specimens, the structural parameters, such as hybrid stacking configuration, SMP infill percentage and SSS thickness, have been also ∗
Corresponding author.
E-mail address: [email protected] (Tsu-Wei Chou).
1
investigated and their influences on the shape recovery properties of the hybrid composite specimens have been identified. The present results signify a promising approach to improve the performance of SMP based sensors and actuators using hybrid composites. Keywords: 4D printing; shape memory polymer; spring steel strip; synergistic effect. 1. Introduction Three-dimensional (3D) printing is a versatile technology due to its low material consumption and rapid fabrication of complex structures [1, 2]. Four-dimensional (4D) printing technology, the combination of 3D printing and stimulus-responsive materials, has demonstrated the tremendous potential in fabricating smart devices [3-5]. One of the most used stimulus-responsive materials is shape memory polymer (SMP) which has the capability of recovering from a temporary shape to its original shape under an external stimulus [6] (e.g., heat, light, electric and magnetic field). SMP has been widely applied in the fields of soft robotics [7], aerospace [8], tissue regeneration [4], etc. Thermoplastic SMPs have advantages of low cost and easy processing while the shape memory properties of the thermoplastic polymers are generally not as good as those of the thermoset polymers [9]. The efforts so far in improving the shape memory properties mainly focused on two approaches: one was to improve the elastic modulus of SMP through the addition of reinforcing filler, such as nano-carbon materials [10-13], short fibers [14], continuous fibers and fabrics [15-17]; the other was to enhance the storage of elastic strain energy of the composite by combining the SMP with an elastic material, for example, silicone elastomer [13, 18, 19] and tape spring [20, 21]. 2
The concept of adopting a metallic spring sheet for enhancing shape memory performance was highly attractive because of its higher stored strain energy and ease in composite fabrication relative to the elastic polymer counterpart. The above concept has been demonstrated in the optimized designs of tape spring hinges with curved cross-section [20] as well as an intelligent hinge system composed of two thermoset SMP shells and one spring sheet [21]. Therefore, the purpose of this research is to further demonstrate the benefit of synergistic effect [22] on improving the shape memory performance by coupling a spring steel strip (SSS) with a 4D printed thermoplastic SMP. This synergistic behavior, also known as “hybrid effect”, had been extensively studied in traditional hybrid fiber composites where high modulus (low elongation) fibers were combined with low modulus (high elongation) fibers. Thus, the focus of this study is on the hybrid effect of the SSS/SMP composite specimens on improving shape recovery rate and recovery force, in terms of hybrid stacking configuration, SMP infill percentage and thickness of the metallic strip. 2. Materials and methods 2.1. Filament material and specimen fabrication The thermoplastic polylactic acid (PLA) used in this work is a semi-crystalline shape memory polymer. The PLA filament was prepared by the Harbin Institute of Technology. The angle-ply laminated specimens were printed using PLA filament by fused deposition modeling method in a QIDI Tech 3D printer (Qidi Technology Co., China). The laminated specimens were composed of eleven 45° and -45° layers with a mid-plane symmetry and the size was 60 mm (length) × 11 mm (width) × 2.5 mm 3
(thickness). The infill percentages of 4D printed SMP specimens were chosen to be 50%, 75% and 100%. The nozzle temperature of 200 °C and the printing speed of 50 mm/s were chosen, respectively. The 1095 spring steel strips (SSS, McMaster-Carr, USA) were cut into the sizes of 60 mm (length) × 10 mm (width) × 0.14 mm (thickness), 0.20 mm (thickness) and 0.25 mm (thickness). The 10 mm ends of the SSS were glued to the as-printed specimen with a high strength epoxy for rapid assembly. The rough surfaces and voids in the SMP specimen induced by the printing process also enhanced bonding strength. 2.2. Characterization of shape recovery behavior The shape recovery ratio was characterized based on the specimen free–shape memory process. The specimen was first heated at the 90 °C in an oven and then it was bent to a “L”-type intermediate shape controlled with a mold. The intermediate shape was maintained when the specimen was cooled down to room temperature, and unloaded. Upon heating to 90 °C again, the specimen without external load almost recovered to its original shape. The shape recovery process was recorded with a video camera (Sony FDR-AX100). The bending angles of the specimen shapes were measured using the image processing and analysis (ImageJ) software. Besides, five consecutive heating-cooling cycles of free shape memory behavior of the hybrid composite specimen with interval of 15 minutes were conducted to study its reusable reliability. 2.3. Characterization of recovery force The specimen was heated at 90 °C in an oven and bent to an intermediate shape with a curvature radius of 35 mm. The intermediate shape was maintained as cooled down to 4
room temperature. The recovery force of the deformed specimen was measured using a RSA-G2 dynamic mechanical analyzer instrument (TA Instruments, USA) with an iso-strain temperature ramp from 25 °C to 90 °C at a rate of 5 °C/min. 3. Results and discussion In order to elucidate the hybrid or synergistic effect on the shape recovery process of an SSS/SMP composite, a model system is schematically shown in Fig. 1a. Here, the laminated specimen is first heated and deformed at high temperature (above SMP glass transition temperature). Then, deformations of the SSS and the SMP are maintained and the elastic strain energy of the SSS is stored as the composite is cooled down to room temperature. Upon re-heating to high temperature, the modulus of the SMP rapidly decreases, the SSS bounces back and the elastic strain energy of the SSS is released to facilitate the shape recovery. In order to substantiate the above reasoning, the experiments on the shape recovery of a non-hybrid (SMP) specimen and a hybrid (SSS/SMP) composite under the same applied load were carried out. The results in Fig. 1b-c demonstrated that the SSS/SMP specimen could lift the 100 g weight during the shape recovery process while the SMP specimen failed to do so. The elastic strain energy of the SSS started to release at 44 s, contributing to lift the weight (Fig. 1c). For structure optimization, the synergistic effect on shape recovery behaviors of 4D printed SMP/SSS specimens with different hybrid stacking configurations, SMP infill percentages and SSS thicknesses were investigated (Fig. 2). The SMP infill percentages of 50%, 75% and 100% gave rise to different sizes and contents of through-the-thickness pores as can be seen in Fig. 2a. The bending angle of the 5
specimen shape at recovery time t was defined by the specimen top surface based on the schematic of Fig. 2b. Fig. 2c and 2d show the time variations of shape recovery process of the SMP specimen and the SSS/SMP specimen, respectively. The SSS/SMP specimen almost recovered to its original shape at 70 s while the SMP specimen showed much poorer recovery performance at 70 s. Furthermore, the shape fixity ratio (Rf) and shape recovery ratio (Rt) corresponding to the shape recovery time t were calculated using the following equations. R f (%) =
Rt (%) =
θ o − θi ×100 θ o − 90
θt − θi ×100 θ o − θi
(1)
(2)
Here, θo, θi and θt denote the bending angles of the specimen shape at the original state, the intermediate state (t = 0 s) and the recovery time t, respectively. The angle θo was 180°. The Rf of the specimens were summarized in Table S1. The Rf values of the SSS/SMP specimens were smaller than those of the SMP specimens, which could be attributed to the slight bounce back of the SSS. The time variations of the shape recovery ratio of the hybrid specimens with different hybrid stacking configurations (SSS/SMP and SMP/SSS), SMP infill percentages and SSS thicknesses are shown in Fig. 2e, 2f and 2g, respectively. As a result of the release of elastic strain energy of the SSS, the SSS/SMP and SMP/SSS specimens showed shorter final shape recovery times as compared to that of the non-hybrid SMP specimen (Fig. 2e). The final recovery time of the SMP/SSS specimen with 50% infill was shortened by 39% as compared to that of the SMP 6
specimen. During its release of elastic strain energy, the SSS either “pulled” (SSS/SMP) or “pushed” (SMP/SSS) the SMP layer back to its original shape (Fig. 2d and Fig. S1). Fig. 2f shows that the final shape recovery times of the SMP specimens and the SSS/SMP specimens decreased with the decreases in SMP infill percentage. It can be attributed to the fact that the specimens with smaller infill percentage would be heated to the recovery temperature more rapidly due to the smaller SMP mass and larger heat conduction contact surface. As shown in Fig. 2g, the final recovery time of the SSS/SMP specimens decreased as the SSS thickness increased. The thicker SSS would store more elastic energy and it contributed to faster shape recovery as the elastic strain energy was released. In order to demonstrate the reusable reliability of the hybrid composite, its free shape memory behavior was studied under five consecutive heating-cooling cycles. Fig. S2 of the Supplementary Material shows that for each cycle, the hybrid specimen nearly recovered to its original shape and the interfacial bond between SSS and SMP appeared to be intact. The final recovery time and final recovery ratio of the specimen with five shape memory cycles were 70.4 s (±4.6 s) and 87.4% (±1.8%), respectively, which confirm the reliability of the synergistic effect on facilitating the shape recovery. Recovery force is another key characteristic of the shape memory properties of 4D printed SMP specimens. The recovery forces of the pre-deformed specimens were measured in a 3-point bending mode with temperature ramp from 25 °C to 90 °C (Fig. 3a). The effects of hybrid stacking configuration, SMP infill percentage and SSS thickness on the recovery force of the hybrid specimens were studied (Fig. 3b-d). Fig. 7
3b shows that the recovery force of the SSS/SMP specimens with 50% infill and 0.20 mm SSS thickness increased from 25 °C to 64 °C (SMP glass transition temperature [5], Tg) and kept at a stable level from 64 °C to 90 °C. The recovery forces of the SMP/SSS and SSS/SMP specimens with 50% infill and 0.20 mm SSS thickness at Tg increased 198 times and 137 times, respectively, comparing to that of the SMP specimen. The results of Fig. 3c show that there was no significant difference in the recovery forces of the SSS/SMP specimens with 50%, 75% and 100% SMP infills. As shown in Fig. 3d, the recovery forces of the SSS/SMP specimens rapidly increased with increasing SSS thickness in the order of 0.14 mm, 0.20 mm and 0.25 mm, indicating the dominating influence of the SSS thickness in the improvement of the recovery force.
4. Conclusion The synergistic effect of 4D printed thermoplastic SMP and SSS on enhancing the shape memory performance was studied in this research. The shape recovery processes of a non-hybrid SMP specimen and a hybrid SSS/SMP composite under the same applied load were demonstrated to substantiate the synergistic effect. As compared to the SMP specimen, the final recovery time of the hybrid SSS/SMP specimen was shortened and the recovery force was significantly increased, resulting from the release the elastic strain energy of the SSS. The effects of hybrid stacking configuration, SMP infill percentage and SSS thickness on the shape recovery behavior and recovery force of the hybrid composite specimens were also investigated. The final shape recovery times of the SSS/SMP specimens were shortened with the decreases in SMP infill percentage while SMP infill percentage had few influences on the recovery force of the SSS/SMP 8
specimens. The hybrid composite specimens with the SSS stacked on the top of the SMP layer and larger SSS thickness gave rise to larger recovery forces.
Acknowledgements Yang Liu acknowledges the financial support from the China Scholarship Council (CSC).
References [1] Ngo TD, Kashani A, Imbalzano G, Nguyen KT, Hui D. Additive manufacturing (3D printing): A review of materials, methods, applications and challenges. Compos Part B Eng. 2018;143:172-96. [2] Quan Z, Wu A, Keefe M, Qin X, Yu J, Suhr J, et al. Additive manufacturing of multi-directional preforms for composites: opportunities and challenges. Mater Today. 2015;18(9):503-12. [3] Kuang X, Roach DJ, Wu J, Hamel CM, Ding Z, Wang T, et al. Advances in 4D Printing: materials and applications. Adv Funct Mater. 2019;29(2):1805290. [4] Miao S, Castro N, Nowicki M, Xia L, Cui H, Zhou X, et al. 4D printing of polymeric materials for tissue and organ regeneration. Mater Today. 2017;20(10):577-91. [5] Liu Y, Zhang W, Zhang F, Lan X, Leng J, Liu S, et al. Shape memory behavior and recovery force of 4D printed laminated Miura-origami structures subjected to compressive loading. Compos Part B Eng. 2018;153:233-42. [6] Liu T, Zhou T, Yao Y, Zhang F, Liu L, Liu Y, et al. Stimulus methods of multi-functional shape memory polymer nanocomposites: A review. Compos Part A-Appl S. 2017;100:20-30. [7] Jin B, Song H, Jiang R, Song J, Zhao Q, Xie T. Programming a crystalline shape memory polymer network with thermo-and photo-reversible bonds toward a single-component soft robot. Sci Adv. 2018;4(1):eaao3865. [8] Liu Y, Du H, Liu L, Leng J. Shape memory polymers and their composites in aerospace applications: a review. Smart Mater Struct. 2014;23(2):023001. [9] Xie F, Huang L, Leng J, Liu Y. Thermoset shape memory polymers and their composites. J Intell Mater Syst Struct. 2016;27(18):2433-55. [10] Du F-P, Ye E-Z, Yang W, Shen T-H, Tang C-Y, Xie X-L, et al. Electroactive shape memory polymer based on optimized multi-walled carbon nanotubes/polyvinyl alcohol nanocomposites. Compos Part B-Eng. 2015;68:170-5. [11] Wang X, Sparkman J, Gou J. Electrical actuation and shape memory behavior of polyurethane composites incorporated with printed carbon nanotube layers. Compos Sci Technol. 2017;141:8-15.
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[12] Chen L, Li W, Liu Y, Leng J. Epoxy shape memory polymer reinforced by thermally reduced graphite oxide: Influence of processing techniques. J Appl Polym Sci. 2015;132(38):42502. [13] Liu Y, Zhang W, Zhang F, Leng J, Pei S, Wang L, et al. Microstructural design for enhanced shape memory behavior of 4D printed composites based on carbon nanotube/polylactic acid filament. Compos Sci Technol. 2019:107692. [14] Xie H, Li L, Deng X-Y, Cheng C-Y, Yang K-K, Wang Y-Z. Reinforcement of shape-memory poly (ethylene-co-vinyl acetate) by carbon fibre to access robust recovery capability under resistant condition. Compos Sci Technol. 2018;157:202-8. [15] Ivens J, Urbanus M, De Smet C. Shape recovery in a thermoset shape memory polymer and its fabric-reinforced composites. eXPRESS Polym Lett. 2011;5(3):254-61. [16] Fejős M, Karger-Kocsis J. Shape memory performance of asymmetrically reinforced epoxy/carbon fibre fabric composites in flexure. eXPRESS Polym Lett. 2013;7(6):528-34. [17] Li F, Scarpa F, Lan X, Liu L, Liu Y, Leng J. Bending shape recovery of unidirectional carbon fiber reinforced epoxy-based shape memory polymer composites. Compos Part A-Appl S. 2019;116:169-79. [18] Akbari S, Sakhaei AH, Kowsari K, Yang B, Serjouei A, Yuanfang Z, et al. Enhanced multimaterial 4D printing with active hinges. Smart Mater Struct. 2018;27(6):065027. [19] Zhang W, Zhang F, Lan X, Leng J, Wu AS, Bryson TM, et al. Shape memory behavior and recovery force of 4D printed textile functional composites. Compos Sci Technol. 2018;160:224-30. [20] Kim K-W, Park Y. Systematic design of tape spring hinges for solar array by optimization method considering deployment performances. Aerosp Sci Technol. 2015;46:124-36. [21] Wang C, Wang Y. The mechanical design of a hybrid intelligent hinge with shape memory polymer and spring sheet. Compos Part B-Eng. 2018;134:1-8. [22] Chou T-W. Microstructural design of fiber composites (hard cover): Cambridge University Press; 1992. (Soft cover, 2005)
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Fig. 1 Hybrid laminated composite of 4D printed shape memory polymer (SMP) and spring steel strip (SSS). (a) Schematic of shape memory process. Demonstrations of shape recovery process of (b) the SMP specimen and (c) the hybrid SSS/SMP specimen under 100 g applied load at 90 °C.
Fig. 2 Characterization of shape recovery behavior of the hybrid composite specimens. (a) Illustration of the infill percentages of 50%, 75% and 100%. (b) Measurement schematic of the bending angle of the specimen at recovery time t. Time variations of shape recovery process of (c) the SMP specimen and (d) the SSS/SMP specimen with an inset of schematic of shape recovery. Shape recovery ratio vs. time curves of the 11
SMP specimen and the hybrid composite specimens with (e) different hybrid stacking configurations (SSS/SMP and SMP/SSS), (f) different SMP infill percentages and (g) different SSS thicknesses. (The legends SSS/SMP and SMP/SSS denote the SSS locations below and above the SMP specimen, respectively. The legend T denotes the SSS thickness. For example, the legend SSS/SMP-50%-T0.20 denotes the SSS/SMP specimen with 50% SMP infill and 0.20 mm SSS thickness.)
Fig. 3 Characterization of recovery force of the hybrid composite specimens. (a) Schematic of the fixture for recovery force measurement. Recovery force vs. temperature curves of the SMP specimens and the hybrid composite specimens with (b) different hybrid stacking configurations, (c) different SMP infill percentages and (d) different SSS thicknesses.
12
S1359-8368(19)32991-9
DOI:
https://doi.org/10.1016/j.compositesb.2019.107536
Reference:
JCOMB 107536
To appear in:
Composites Part B
Received Date: 26 June 2019 Revised Date:
9 September 2019
Accepted Date: 13 October 2019
Please cite this article as: Liu Y, Zhang F, Leng J, Wang L, Cotton C, Sun B, Chou T-W, Synergistic effect enhanced shape recovery behavior of metal-4D printed shape memory polymer hybrid composites, Composites Part B (2019), doi: https://doi.org/10.1016/j.compositesb.2019.107536. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Synergistic effect enhanced shape recovery behavior of metal-4D printed shape memory polymer hybrid composites Yang Liu a,b, Fenghua Zhang c, Jinsong Leng c, Liyun Wang b, Chase Cotton d, Baozhong Sun a, and Tsu-Wei Chou b,∗ a
College of Textiles, Donghua University, Shanghai 201620, PR China
b
Department of Mechanical Engineering, University of Delaware, Newark, DE 19716,
USA c
Center for Composite Materials and Structures, Harbin Institute of Technology,
Harbin 150080, PR China d
Department of Electrical and Computer Engineering, University of Delaware, Newark,
DE 19716, USA Abstract The four-dimensional (4D) printing technology enables the convergence of three-dimensional (3D) printing and shape memory polymer (SMP). The aim of this research is to demonstrate the synergistic effect between a spring steel strip (SSS) and a 4D printed thermoplastic SMP on enhancing the shape recovery properties. The recovery time of the SMP/SSS hybrid composite was shortened by 39% as compared to that of the SMP specimen. The corresponding recovery force of the SMP/SSS hybrid specimen at 64 °C was 9 N, 199 times of that of the non-hybrid SMP specimen. For optimizing the design of hybrid composite specimens, the structural parameters, such as hybrid stacking configuration, SMP infill percentage and SSS thickness, have been also ∗
Corresponding author.
E-mail address: [email protected] (Tsu-Wei Chou).
1
investigated and their influences on the shape recovery properties of the hybrid composite specimens have been identified. The present results signify a promising approach to improve the performance of SMP based sensors and actuators using hybrid composites. Keywords: 4D printing; shape memory polymer; spring steel strip; synergistic effect. 1. Introduction Three-dimensional (3D) printing is a versatile technology due to its low material consumption and rapid fabrication of complex structures [1, 2]. Four-dimensional (4D) printing technology, the combination of 3D printing and stimulus-responsive materials, has demonstrated the tremendous potential in fabricating smart devices [3-5]. One of the most used stimulus-responsive materials is shape memory polymer (SMP) which has the capability of recovering from a temporary shape to its original shape under an external stimulus [6] (e.g., heat, light, electric and magnetic field). SMP has been widely applied in the fields of soft robotics [7], aerospace [8], tissue regeneration [4], etc. Thermoplastic SMPs have advantages of low cost and easy processing while the shape memory properties of the thermoplastic polymers are generally not as good as those of the thermoset polymers [9]. The efforts so far in improving the shape memory properties mainly focused on two approaches: one was to improve the elastic modulus of SMP through the addition of reinforcing filler, such as nano-carbon materials [10-13], short fibers [14], continuous fibers and fabrics [15-17]; the other was to enhance the storage of elastic strain energy of the composite by combining the SMP with an elastic material, for example, silicone elastomer [13, 18, 19] and tape spring [20, 21]. 2
The concept of adopting a metallic spring sheet for enhancing shape memory performance was highly attractive because of its higher stored strain energy and ease in composite fabrication relative to the elastic polymer counterpart. The above concept has been demonstrated in the optimized designs of tape spring hinges with curved cross-section [20] as well as an intelligent hinge system composed of two thermoset SMP shells and one spring sheet [21]. Therefore, the purpose of this research is to further demonstrate the benefit of synergistic effect [22] on improving the shape memory performance by coupling a spring steel strip (SSS) with a 4D printed thermoplastic SMP. This synergistic behavior, also known as “hybrid effect”, had been extensively studied in traditional hybrid fiber composites where high modulus (low elongation) fibers were combined with low modulus (high elongation) fibers. Thus, the focus of this study is on the hybrid effect of the SSS/SMP composite specimens on improving shape recovery rate and recovery force, in terms of hybrid stacking configuration, SMP infill percentage and thickness of the metallic strip. 2. Materials and methods 2.1. Filament material and specimen fabrication The thermoplastic polylactic acid (PLA) used in this work is a semi-crystalline shape memory polymer. The PLA filament was prepared by the Harbin Institute of Technology. The angle-ply laminated specimens were printed using PLA filament by fused deposition modeling method in a QIDI Tech 3D printer (Qidi Technology Co., China). The laminated specimens were composed of eleven 45° and -45° layers with a mid-plane symmetry and the size was 60 mm (length) × 11 mm (width) × 2.5 mm 3
(thickness). The infill percentages of 4D printed SMP specimens were chosen to be 50%, 75% and 100%. The nozzle temperature of 200 °C and the printing speed of 50 mm/s were chosen, respectively. The 1095 spring steel strips (SSS, McMaster-Carr, USA) were cut into the sizes of 60 mm (length) × 10 mm (width) × 0.14 mm (thickness), 0.20 mm (thickness) and 0.25 mm (thickness). The 10 mm ends of the SSS were glued to the as-printed specimen with a high strength epoxy for rapid assembly. The rough surfaces and voids in the SMP specimen induced by the printing process also enhanced bonding strength. 2.2. Characterization of shape recovery behavior The shape recovery ratio was characterized based on the specimen free–shape memory process. The specimen was first heated at the 90 °C in an oven and then it was bent to a “L”-type intermediate shape controlled with a mold. The intermediate shape was maintained when the specimen was cooled down to room temperature, and unloaded. Upon heating to 90 °C again, the specimen without external load almost recovered to its original shape. The shape recovery process was recorded with a video camera (Sony FDR-AX100). The bending angles of the specimen shapes were measured using the image processing and analysis (ImageJ) software. Besides, five consecutive heating-cooling cycles of free shape memory behavior of the hybrid composite specimen with interval of 15 minutes were conducted to study its reusable reliability. 2.3. Characterization of recovery force The specimen was heated at 90 °C in an oven and bent to an intermediate shape with a curvature radius of 35 mm. The intermediate shape was maintained as cooled down to 4
room temperature. The recovery force of the deformed specimen was measured using a RSA-G2 dynamic mechanical analyzer instrument (TA Instruments, USA) with an iso-strain temperature ramp from 25 °C to 90 °C at a rate of 5 °C/min. 3. Results and discussion In order to elucidate the hybrid or synergistic effect on the shape recovery process of an SSS/SMP composite, a model system is schematically shown in Fig. 1a. Here, the laminated specimen is first heated and deformed at high temperature (above SMP glass transition temperature). Then, deformations of the SSS and the SMP are maintained and the elastic strain energy of the SSS is stored as the composite is cooled down to room temperature. Upon re-heating to high temperature, the modulus of the SMP rapidly decreases, the SSS bounces back and the elastic strain energy of the SSS is released to facilitate the shape recovery. In order to substantiate the above reasoning, the experiments on the shape recovery of a non-hybrid (SMP) specimen and a hybrid (SSS/SMP) composite under the same applied load were carried out. The results in Fig. 1b-c demonstrated that the SSS/SMP specimen could lift the 100 g weight during the shape recovery process while the SMP specimen failed to do so. The elastic strain energy of the SSS started to release at 44 s, contributing to lift the weight (Fig. 1c). For structure optimization, the synergistic effect on shape recovery behaviors of 4D printed SMP/SSS specimens with different hybrid stacking configurations, SMP infill percentages and SSS thicknesses were investigated (Fig. 2). The SMP infill percentages of 50%, 75% and 100% gave rise to different sizes and contents of through-the-thickness pores as can be seen in Fig. 2a. The bending angle of the 5
specimen shape at recovery time t was defined by the specimen top surface based on the schematic of Fig. 2b. Fig. 2c and 2d show the time variations of shape recovery process of the SMP specimen and the SSS/SMP specimen, respectively. The SSS/SMP specimen almost recovered to its original shape at 70 s while the SMP specimen showed much poorer recovery performance at 70 s. Furthermore, the shape fixity ratio (Rf) and shape recovery ratio (Rt) corresponding to the shape recovery time t were calculated using the following equations. R f (%) =
Rt (%) =
θ o − θi ×100 θ o − 90
θt − θi ×100 θ o − θi
(1)
(2)
Here, θo, θi and θt denote the bending angles of the specimen shape at the original state, the intermediate state (t = 0 s) and the recovery time t, respectively. The angle θo was 180°. The Rf of the specimens were summarized in Table S1. The Rf values of the SSS/SMP specimens were smaller than those of the SMP specimens, which could be attributed to the slight bounce back of the SSS. The time variations of the shape recovery ratio of the hybrid specimens with different hybrid stacking configurations (SSS/SMP and SMP/SSS), SMP infill percentages and SSS thicknesses are shown in Fig. 2e, 2f and 2g, respectively. As a result of the release of elastic strain energy of the SSS, the SSS/SMP and SMP/SSS specimens showed shorter final shape recovery times as compared to that of the non-hybrid SMP specimen (Fig. 2e). The final recovery time of the SMP/SSS specimen with 50% infill was shortened by 39% as compared to that of the SMP 6
specimen. During its release of elastic strain energy, the SSS either “pulled” (SSS/SMP) or “pushed” (SMP/SSS) the SMP layer back to its original shape (Fig. 2d and Fig. S1). Fig. 2f shows that the final shape recovery times of the SMP specimens and the SSS/SMP specimens decreased with the decreases in SMP infill percentage. It can be attributed to the fact that the specimens with smaller infill percentage would be heated to the recovery temperature more rapidly due to the smaller SMP mass and larger heat conduction contact surface. As shown in Fig. 2g, the final recovery time of the SSS/SMP specimens decreased as the SSS thickness increased. The thicker SSS would store more elastic energy and it contributed to faster shape recovery as the elastic strain energy was released. In order to demonstrate the reusable reliability of the hybrid composite, its free shape memory behavior was studied under five consecutive heating-cooling cycles. Fig. S2 of the Supplementary Material shows that for each cycle, the hybrid specimen nearly recovered to its original shape and the interfacial bond between SSS and SMP appeared to be intact. The final recovery time and final recovery ratio of the specimen with five shape memory cycles were 70.4 s (±4.6 s) and 87.4% (±1.8%), respectively, which confirm the reliability of the synergistic effect on facilitating the shape recovery. Recovery force is another key characteristic of the shape memory properties of 4D printed SMP specimens. The recovery forces of the pre-deformed specimens were measured in a 3-point bending mode with temperature ramp from 25 °C to 90 °C (Fig. 3a). The effects of hybrid stacking configuration, SMP infill percentage and SSS thickness on the recovery force of the hybrid specimens were studied (Fig. 3b-d). Fig. 7
3b shows that the recovery force of the SSS/SMP specimens with 50% infill and 0.20 mm SSS thickness increased from 25 °C to 64 °C (SMP glass transition temperature [5], Tg) and kept at a stable level from 64 °C to 90 °C. The recovery forces of the SMP/SSS and SSS/SMP specimens with 50% infill and 0.20 mm SSS thickness at Tg increased 198 times and 137 times, respectively, comparing to that of the SMP specimen. The results of Fig. 3c show that there was no significant difference in the recovery forces of the SSS/SMP specimens with 50%, 75% and 100% SMP infills. As shown in Fig. 3d, the recovery forces of the SSS/SMP specimens rapidly increased with increasing SSS thickness in the order of 0.14 mm, 0.20 mm and 0.25 mm, indicating the dominating influence of the SSS thickness in the improvement of the recovery force.
4. Conclusion The synergistic effect of 4D printed thermoplastic SMP and SSS on enhancing the shape memory performance was studied in this research. The shape recovery processes of a non-hybrid SMP specimen and a hybrid SSS/SMP composite under the same applied load were demonstrated to substantiate the synergistic effect. As compared to the SMP specimen, the final recovery time of the hybrid SSS/SMP specimen was shortened and the recovery force was significantly increased, resulting from the release the elastic strain energy of the SSS. The effects of hybrid stacking configuration, SMP infill percentage and SSS thickness on the shape recovery behavior and recovery force of the hybrid composite specimens were also investigated. The final shape recovery times of the SSS/SMP specimens were shortened with the decreases in SMP infill percentage while SMP infill percentage had few influences on the recovery force of the SSS/SMP 8
specimens. The hybrid composite specimens with the SSS stacked on the top of the SMP layer and larger SSS thickness gave rise to larger recovery forces.
Acknowledgements Yang Liu acknowledges the financial support from the China Scholarship Council (CSC).
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Fig. 1 Hybrid laminated composite of 4D printed shape memory polymer (SMP) and spring steel strip (SSS). (a) Schematic of shape memory process. Demonstrations of shape recovery process of (b) the SMP specimen and (c) the hybrid SSS/SMP specimen under 100 g applied load at 90 °C.
Fig. 2 Characterization of shape recovery behavior of the hybrid composite specimens. (a) Illustration of the infill percentages of 50%, 75% and 100%. (b) Measurement schematic of the bending angle of the specimen at recovery time t. Time variations of shape recovery process of (c) the SMP specimen and (d) the SSS/SMP specimen with an inset of schematic of shape recovery. Shape recovery ratio vs. time curves of the 11
SMP specimen and the hybrid composite specimens with (e) different hybrid stacking configurations (SSS/SMP and SMP/SSS), (f) different SMP infill percentages and (g) different SSS thicknesses. (The legends SSS/SMP and SMP/SSS denote the SSS locations below and above the SMP specimen, respectively. The legend T denotes the SSS thickness. For example, the legend SSS/SMP-50%-T0.20 denotes the SSS/SMP specimen with 50% SMP infill and 0.20 mm SSS thickness.)
Fig. 3 Characterization of recovery force of the hybrid composite specimens. (a) Schematic of the fixture for recovery force measurement. Recovery force vs. temperature curves of the SMP specimens and the hybrid composite specimens with (b) different hybrid stacking configurations, (c) different SMP infill percentages and (d) different SSS thicknesses.
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