How Shape Memory Effects can Contribute to Improved Self-Healing Properties in Polymer Materials

Abstract

Realization of self-healing polymer materials cannot rely on the wealth of active repair tools found in living systems but must focus entirely on the structural composition of the material and the properties of its constituents. Current challenges of the search for such compositions include healing of large-scale defects as well as the need for a healing process that is generated by the scission itself. Herein, we describe ionomer–rubber blends from poly­(ethylene-co-methacrylic acid) and peroxide cross-linked ethylene–propylene–diene monomer (EPDM) that combine three types of cross-links: covalent links of a network of EPDM, clusters of aggregated ionic groups, and crystalline domains of longer ethylene sequences in the ionomer. Above the melting point of the latter, the components mix homogeneously, indicated by the clarity of the samples and supported by small-angle X-ray scattering (SAXS) and NMR. At ambient conditions, the samples are hard like a thermoplastic material. Self-healing after mechanical damage is enabled by two types of structural memory related to a hierarchy of deformation- and defect-caused stresses and their relaxation paths. Because of the solid-like character of the materials, damage-caused stress is retained by the micro deformation and rupture of the aggregates on small scales and on large scaleby the macroscopic shape memory effect of the deformed covalent network. When the samples get annealed at an elevated temperature, the former enables mending of fracture-caused surfaces and the lattershape recovery. Based on a careful evaluation of the structural relaxation effects on the blends and their constituents (differential scanning calorimetry, NMR, and wide-angle X-ray scattering/SAXS), we demonstrate the repair of defects in the range of millimeters to centimeters by the defect-caused stresses. It is intrinsic to our concept that it holds only to damages such as scratches, small cuts, and microcracks, whereby the object is not fully fragmented, and that it will require thermal activation.

New Principles of Polymer Composite Preparation. MQ Copolymers as an Active Molecular Filler for Polydimethylsiloxane Rubbers

“…The control of polymers and polymer compositions' properties by regulating the structure of composition macromolecular components has been, and remains, the most important tool of chemists in their movement towards materials with superior properties [1,2]. Elastomeric materials based on polydimethylsiloxane (PDMS) rubbers are now widely used as rubbers, sealants, and coatings for various purposes [3,4].…”

Photothermal-Responsive Wormlike Polydopamine-Wrapped Ethylene-Vinyl Acetate Copolymer toward Triple-Action Self-Healing Anticorrosion Coating

“…Upon the local temperature was higher than T g , the relaxation of epoxy network could promote the release of the entropic energy, manifesting as the movement of separated two sides to the scratch center. This healing mechanism could be illustrated from thermodynamic term and was defined as “shape-memory-assisted self-healing (SMASH)”. − However, scratch closure did not represent complete healing of the damage due to the broken chains that had not been rebonded at the damaged area. During coating service in a harsh environment, the aggressive corrosion species will permeate into coatings through the minor scratch and cause metal substrate corrosion.…”

Self-Healing, Thermadapt Triple-Shape Memory Ionomer Vitrimer for Shape Memory Triboelectric Nanogenerator

“…Basically, multiple SMP networks can be obtained with the incorporation of broad thermal transition or several distinct thermal transitions and molecular switches, − such as coupling the dynamic transcarbamoylation reaction with two independent phase transitions . Apart from chemical cross-linking, physical cross-linking is also widely used as the molecular switch to fabricate SMPs. − The ion-rich nanodomains in the ionomers could formed a phase-separated morphology, imposing strong restriction on the segmental dynamics of surrounding polymer chains. , Those ionic aggregates could act as permanent cross-linking below the ionic transition temperature. Based on the strong ionic interaction, various types of ionomers with shape memory properties have been developed by Weiss’s group. , Besides, the ionic cross-linked polymers with the remote controllability and solid plasticity have been obtained by utilizing the different photothermal effect of different ions .…”