Two‐Way Reversible Shape Memory Properties of Benzoyl Peroxide Crosslinked Poly(ethylene‐co‐vinyl acetate) under Different Stress Conditions

Abstract

wide range of applications, including as actuators [17] and sensors, [18] and in aerospace technology, [19] textiles, [20] and biomedical applications. [21,22] The types of the shape memory effect (SME) include the one-way shape memory effect (1W-SME), two-way shape memory effect (2W-SME) and multishape memory effect (multi-SME). One-way shape memory polymers only remember their shape at high temperature, and programming once leads to only one shape memory cycle, which limits their application as actuators and for artificial intelligence. Multishape memory polymers [23,24] also show the 1W-SME. Two-way shape memory polymers [25][26][27] exhibit the complete reversibility in heating and cooling that is required for many applications. Several pioneering works have contributed to production of materials with two-way shape memory behavior, including laminated composites, [28][29][30][31][32] liquid crystal elastomers, [33,34] and crosslinked crystalline polymers. [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49] Laminated polymer composites comprising two layers bonded with an adhesive agent are limited by the weak interlayer strength and cyclicity. Furthermore, the complicated synthesis of liquid crystal elastomers is still challenge, which limits their mass production.The thermal crosslinking reaction using a crosslinking agent to crosslink semicrystalline polymers has attracted much attention because of the good two-way shape memory reversibility, low cost, and simple synthesis process. Mather's group reported Two-way (reversible) chemically crosslinked semicrystalline shape memory polymers are synthesized using poly(ethylene-co-vinyl acetate) (PEVA) with benzoyl peroxide (BPO). The two-way shape memory effect (2W-SME) is achieved under both constant stress and stress-free conditions. It is found that the stress-free 2W-SME can be achieved by the relationship between the initial prestretching strain (R prestretch ) and recovery strain (R rec ). Under the same prestretching stress, the stress-free two-way shape memory behavior can be controlled by variation of R rec using a different setting temperature (T set ) in the recovery process. More importantly, the driving force and recovery force, as one of the key indicators for two-way shape memory materials, are investigated, and they significantly change depending on the BPO content. The sample with high BPO content shows excellent high-temperature creep resistant performance. A highly crosslinked structure can suppress viscous flow and provides sufficient force to allow the sample to recover its initial shape after crystal melting. Therefore, the PEVA/BPO samples are able to contract during heating. The presence of an oriented crystal structure with high applied stress that causes sample elongation during cooling is also investigated. These findings for PEVA/BPO two-way shape memory polymers will contribute to their applications as soft actuators in various fields.The ORCID identification number(s) for the author(s) of this article can be found under http...

Converse two-way shape memory effect through a dynamic covalent network design

“…15 The introduction of a physical or chemical molecular mechanism can overcome this limitation. [16][17][18][19][20][21][22][23][24][25][26] For the physical mechanism, many 2W-SMPs usually have a dual crystalline phase [16][17][18][19] or an extra noncovalent interaction. 20 The high temperature transition phase or the noncovalent interaction can trap internal stress corresponding to network anisotropy, while the low temperature crystalline phase serves as the driving phase for reversible actuation.…”

“…23 In contrast, the chemical mechanism requires unique network designs with active functional groups or chemical bonds that can undergo additional crosslinking. [24][25][26][27][28][29] Despite such progress, all known thermal induced 2W-SMPs exhibit the same actuation behavior, i.e. CIE and HIC in the programming direction (Fig.…”

“…23 In contrast, the chemical mechanism requires unique network designs with active functional groups or chemical bonds that can undergo additional crosslinking. 24–29…”

Triple one‐way and two‐way shape memory poly(ethylene‐co‐vinyl acetate)/poly(ε‐caprolactone) immiscible blends

“…The 1W‐SMPs need external reprogramming of the temporary shape for reuse, whereas the 2W‐SMPs are able to automatically repeat shape transformation under the stimulus, which is more desirable for practical applications 18,19 . In recent decades, many polymers have been used to realize the two‐way shape memory effect (2W‐SME), containing poly(ε‐caprolactone) (PCL), 20–22 poly(ethylene‐co‐vinyl acetate) (EVA), 23,24 and poly(cyclooctene) (PCO) 25 . The essence of 2W‐SME is the elongation induced by oriented crystallization upon cooling and the contraction induced by melting upon reheating.…”

“…The co‐continuous structure is formed by the proper blending of immiscible EVA and PCL, and the triple 2W‐SMPs are prepared by the chemical crosslinking of blends. EVA has the advantages of superlative chemical stability, aging resistance, easy processing, so forth 24,35 . PCL is a low‐toxicity, fully bio‐/hydrolytic degradable, and biocompatible polymer 36 .…”