Benzoyl peroxide thermo-crosslinked poly(ethylene-co-vinyl acetate) foam with two-way shape memory effect
Graphical abstract
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], [8], [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 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(ε-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 2W-SME. 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 applications as lightweight actuators in artificial intelligence, among others.
Section snippets
Fabrication of two-way shape memory PEVA/BPO foams
Four different sizes of NaCl particles (0–50 μm, 50–110 μm, 110–160 μm, and 160–450 μm; 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
Results and discussion
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
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
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.
Acknowledgement
This work was supported by the China Scholarship Council (CSC) and the JSPS [JSPS Kakenhi 15H01789 and 26420721].
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