(ionic liquid) gel (PIL), which showed shape memory behavior because of a similar
mechanism [26].
In summary, the diversified stimuli of CD-guest interactions provide a good
opportunity to develop SMPs with novel stimulus sensitivities other than thermal
induction. However, it should be noted that most SMPs based on CD ICs are
hydrogels because the inclusion between CDs and guest groups always requires
the help of water [27].
22.3 SHPs Based on CDs-Guest Interactions
22.3.1 General Aspect
According to the self-healing mechanism, self-healing can be divided into two types,
extrinsic and intrinsic. Extrinsic self-healing systems require external healing agents.
The healing agents and catalyst are typically embedded in the polymer matrix in the
form of microcapsules or capillary networks. When a mechanical damage was
exerted to the capsules or capillary networks, they were ruptured, and the healing
agents were released to the damaged region and repaired damage. This extrinsic
healing method is fast and efficient, and the self-healing process is autonomous. But
the healing is not repeatable at a given place. Intrinsic self-healing materials are
usually based on dynamic covalent bond chemistry or non-covalent bond chemistry.
Commonly using dynamic chemical reactions requires external stimulus intervention. While non-covalent bond may induce autonomous self-healing behavior since
they are easily broken and regenerated at ambient conditions. CD-guest interactions
are one of typical supramolecular interactions. Due to high selectivity and dynamic
equilibrium of host-guest interactions, CD-guest-based SHPs have three advantages:
(1) repeatable healing process requiring no external energy, (2) high healing rate, and
(3) long storage time, which means the broken parts still show healing property after
long time.
In general, the extent of “healing” can be expressed as stress recovery efficiency.
Healing efficiency = mechanical value/mechanical value (pristine). In some studies,
the area of crack recovery was also used to evaluate the healing effect of materials.
22.3.2 Self-Healing Hydrogels Based on CD-Guest Interactions
In 2011, Harada et al. firstly explored CD-guest interactions as self-healing mechanism [28]. This supramolecular hydrogel is formed by mixing poly(acrylic acid)
(PAA) possessing β-CD as a host polymer and PAA with possessing Fc as a guest
polymer. Two cut (PAA-6-β-CD /PAA-Fc) hydrogel pieces were able to rejoin and
the crack sufficiently healed to form one gel due to the reformation of host-guest
interaction between the side chains of both polymers (Fig. 3).
Later, various polymeric hydrogels with CD-guest groups have been explored
[29–31]. For example, β-CD functionalized poly(acrylamide) compolymerized with
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mechanism [26].
In summary, the diversified stimuli of CD-guest interactions provide a good
opportunity to develop SMPs with novel stimulus sensitivities other than thermal
induction. However, it should be noted that most SMPs based on CD ICs are
hydrogels because the inclusion between CDs and guest groups always requires
the help of water [27].
22.3 SHPs Based on CDs-Guest Interactions
22.3.1 General Aspect
According to the self-healing mechanism, self-healing can be divided into two types,
extrinsic and intrinsic. Extrinsic self-healing systems require external healing agents.
The healing agents and catalyst are typically embedded in the polymer matrix in the
form of microcapsules or capillary networks. When a mechanical damage was
exerted to the capsules or capillary networks, they were ruptured, and the healing
agents were released to the damaged region and repaired damage. This extrinsic
healing method is fast and efficient, and the self-healing process is autonomous. But
the healing is not repeatable at a given place. Intrinsic self-healing materials are
usually based on dynamic covalent bond chemistry or non-covalent bond chemistry.
Commonly using dynamic chemical reactions requires external stimulus intervention. While non-covalent bond may induce autonomous self-healing behavior since
they are easily broken and regenerated at ambient conditions. CD-guest interactions
are one of typical supramolecular interactions. Due to high selectivity and dynamic
equilibrium of host-guest interactions, CD-guest-based SHPs have three advantages:
(1) repeatable healing process requiring no external energy, (2) high healing rate, and
(3) long storage time, which means the broken parts still show healing property after
long time.
In general, the extent of “healing” can be expressed as stress recovery efficiency.
Healing efficiency = mechanical value/mechanical value (pristine). In some studies,
the area of crack recovery was also used to evaluate the healing effect of materials.
22.3.2 Self-Healing Hydrogels Based on CD-Guest Interactions
In 2011, Harada et al. firstly explored CD-guest interactions as self-healing mechanism [28]. This supramolecular hydrogel is formed by mixing poly(acrylic acid)
(PAA) possessing β-CD as a host polymer and PAA with possessing Fc as a guest
polymer. Two cut (PAA-6-β-CD /PAA-Fc) hydrogel pieces were able to rejoin and
the crack sufficiently healed to form one gel due to the reformation of host-guest
interaction between the side chains of both polymers (Fig. 3).
Later, various polymeric hydrogels with CD-guest groups have been explored
[29–31]. For example, β-CD functionalized poly(acrylamide) compolymerized with
592
S. Zhang et al.