Abstract
Dynamic crosslinking networks based on Diels–Alder (DA) chemistry and ionic interactions were introduced to maleic anhydride modified ethylene-vinyl acetate copolymer (mEVA) via in situ melt processing. The dual dynamic crosslinking networks were characterized by temperature-dependent FTIR, and the effects on the shape memory properties of mEVA were evaluated with dynamic mechanical thermal analysis and cyclic tensile testing. A crosslinking density was achieved at 2.36 × 10−4 mol·cm−3 for DA-crosslinked mEVA; as a result, the stress at 100% extension was increased from 3.8 to 5.6 MPa, and tensile strength and elongation at break were kept as high as 30.3 MPa and 486%, respectively. The further introduction of 10 wt % zinc methacrylate increased the dynamic crosslinking density to 3.74 × 10−4 mol·cm−3 and the stress at 100% extension to 9.0 MPa, while providing a tensile strength of 28.4 MPa and strain at break of 308%. The combination of reversible DA covalent crosslinking and ionic network in mEVA enabled a fixing ratio of 76.4% and recovery ratio of 99.4%, exhibiting an enhanced shape memory performance, especially at higher temperatures. The enhanced shape memory and mechanical performance of the dual crosslinked mEVA showed promising reprocessing and recycling abilities of the end-of-life products in comparison to traditional peroxide initiated covalent crosslinked counterparts.
Highlights
The shape memory effect is a phenomenon related to a physical state change of a polymer upon application of a stimulus
This resulted in dynamic DA crosslinking networks in modified ethylene-vinyl acetate copolymer (mEVA)
After the introduction of Zinc methacrylate (ZnMA) into DA1-crosslinked mEVA, the modulus increased greatly, especially above ~70 ◦ C, close to the melt temperature of mEVA, and no sharp decreases in storage modulus were observed between 130–150 ◦ C, demonstrating the strength of the ionic interactions compared to the Diels–Alder bonding
Summary
The shape memory effect is a phenomenon related to a physical state change of a polymer upon application of a stimulus. The second segment fixes the temporary conformation by freezing the temporary shape in place This is usually achieved through an amorphous/semicrystalline phase transition or intermolecular interactions. The polymer deforms when subjected to a temperature above its melting point [12,13,14] This enables the potential recycling or reprocessing of those chemically crosslinked polymers at the end of the service life. The introduction of multiple reversible crosslinking networks to EVA will potentially improve the shape memory and mechanical properties while retaining a good reprocessing ability of the polymer. To keep the original shape-fixing ability of EVA through the melt–crystalline transition, reversible dynamic crosslinking networks were introduced into maleic anhydride modified. The effects of the reversible DA bonds and the ionic crosslinks on the mechanical and shape memory performance of modified mEVA were discussed
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