Multiple and two-way reversible shape memory polymers: Design strategies and applications

Several reversible switching structures have been employed, such as crystallization/melting transition, vitrification/glass transition, liquid crystal anisotropic/isotropic transition, reversible molecule cross-linking reactions, and supramolecular association/disassociation.

“Several reversible switching structures have been employed, such as crystallization/melting transition, vitrification/glass transition, liquid crystal anisotropic/isotropic transition, reversible molecule cross-linking reactions, and supramolecular association/disassociation”

According to the nature of both reversible thermal transition and cross-linking, one-way SMPs can be divided into chemically and physically cross-linked polymers.

“According to the nature of both reversible thermal transition and cross-linking, one-way SMPs can be divided into chemically and physically cross-linked polymers”

Net-points determine the permanent shape and can be formed by molecular entanglement, a crystalline phase, chemical cross-linking, an interpenetrated network, or a cyclodextrin polymer's inclusion.

“Net-points determine the permanent shape and can be formed by, e.g., molecular entanglement, a crystalline phase, chemical cross-linking, an interpenetrated network, or a cyclodextrin polymer's inclusion”

According to Wang et al., polymers belonging to these classes generally contain three phases: a chemically cross-linked phase or a phase within a transition with the highest transition temperature holding the permanent shape; a semi-crystalline or elastic amorphous phase determining shifting-geometry and supplying a stretching tensile force when compressed; and a semi-crystalline or liquid crystal

“According to Wang et al. [46], the polymers belonging to these classes generally contain three phases: (1) a chemically cross-linked phase or a phase within a transition with the highest transition temperature, holding the permanent shape; (2) a semi-crystalline or elastic amorphous phase, determining the shifting-geometry and supplying a stretching tensile force when compressed due to the”

Recently, two-way SMPs containing dynamic covalent bonds have been prepared by using a method similar to the two-stage cross-linking method.

“Recently, two-way SMPs containing dynamic covalent bonds have been prepared by using a method similar to the two-stage cross-linking method”

SMPs cross-linked by dynamic covalent bonds are advantageous due to their reprocessibility, good mechanical properties, and good recoverability.

“SMPs cross-linked by dynamic covalent bonds are advantageous due to their reprocessibility, good mechanical properties, and good recoverability”

Thermally-induced SMPs may be directly triggered by heating with hot gas or water, but indirect heating has also been used.

“Finally, it is worth mentioning that thermally-induced SMPs may be directly triggered by heating with hot gas or water, but indirect heating has been also used (see [3,33,46] and references therein).”

Other types of SMPs that are not activated by temperature changes exist, such as light-induced or chemo-responsive SMPs.

“Obviously, other types of SMPs that are not activated by temperature changes exist (e.g., light-induced or chemo-responsive SMPs), but will be not discussed in the present manuscript (for details, see [3,33,46] and references therein).”

SMPs with functional fillers may be triggered by light, electricity, magnetic fields, microwaves, or ultrasound, but they are still intrinsically triggered by heat because different forms of energy are converted into heat through the fillers.

“For example, SMPs with functional fillers may be triggered by light, electricity, magnetic fields, microwaves, or ultrasound, but they are still intrinsically triggered by heat, since different forms of energy are converted into heat through the fillers (see [33,46,47,170-172] and references therein).”