nonpermanent, reversible – links between the polymer strands in (supramolecular)
hydrogels are a promising avenue towards materials that are easier to process and
recycle and that are less brittle than permanently connected or purely entangled
systems. The mechanism that underpins the improved mechanical performance has
been suggested [2] to be related to the fact that physical cross-links are generally
weaker than chemical cross-links and permit the materials to dissipate more mechanical energy before ultimately failing due to delocalized dissipation (spreading the
typically concentrated mechanical loads at the tip of a propagating crack over larger
volumes) and the potential to reform previously dissociated bonds – the same
mechanism that gives rise to some of the self-healing qualities of reversible
hydrogels. The binding and rebinding of cross-linking agents can change more
than just the connectivity of a polymer network; the orientational distribution, too,
can adapt as the material remodels. Synthetically mimicking the self-healing mechanism observed in biological tissues is a long-term goal of materials science in
general [46–49], holding great promise to improve, enhance or altogether change the
mechanical quality of man-made materials.
Thus, in systems with dynamical, physical bonds, the mechanical and rheological
properties can be controlled to yield materials that are dynamical, anisotropic and
responsive to external stimuli, in a manner similar to tissues and cells, the properties
of which respond to changes in the environment and to active, internal cues.
The classic review [50] on supramolecular polymers describes a number of
examples of physical interactions that may be used to synthesize physically linked
polymer gels. Arrays of hydrogen bonds (also important in organizing the interactions between proteins) are a powerful way to create very stable and mechanically
strong physical gels [50] – the ureidopyrimidinone-based supramolecular polymers
are particularly interesting in this respect and show mechanical properties that are
extremely temperature dependent: response ranges from viscoelastic at room temperatures to fluid-like at more elevated temperatures [51]. This behaviour is strongly
reminiscent of the fluid-solid transitions achievable in reconstituted biopolymer
networks. In addition to hydrogen bonding, other interactions may be used to create
transient bonds over a broad range of strengths: π À π interactions, hydrophobic
interactions and metal-ligand coordination bonds among them [50].
However, a merely transiently cross-linked elastomer has relatively poor mechanical properties, e.g. it flows at long timescales and the toughness is usually low.
Recent, more advanced hydrogel and elastomer chemistries demonstrated that a
combination of permanent and transient cross-links can dramatically improve the
toughness of a material while retaining the capacity to self-heal. This has led to
greatly increased interest in ‘dual cross-linked’ materials – both hydrogels and
elastomers – over the recent years [47, 52–55]. In those systems, the transient
cross-links are hypothesized to act as sacrificial bonds, whose dissociation provides
the material with a parallel channel for energy dissipation [56]. As a result, the
material can absorb more energy before physically rupturing, leading to enhanced
toughness of the material. Permanent bonds imprint the original network structure
and permit the network to remain elastic even at large and slow deformations.
Interestingly, this behaviour is strongly rate-dependent: materials may display very
Rheology, Rupture, Reinforcement and Reversibility: Computational Approaches. . .
71
hydrogels are a promising avenue towards materials that are easier to process and
recycle and that are less brittle than permanently connected or purely entangled
systems. The mechanism that underpins the improved mechanical performance has
been suggested [2] to be related to the fact that physical cross-links are generally
weaker than chemical cross-links and permit the materials to dissipate more mechanical energy before ultimately failing due to delocalized dissipation (spreading the
typically concentrated mechanical loads at the tip of a propagating crack over larger
volumes) and the potential to reform previously dissociated bonds – the same
mechanism that gives rise to some of the self-healing qualities of reversible
hydrogels. The binding and rebinding of cross-linking agents can change more
than just the connectivity of a polymer network; the orientational distribution, too,
can adapt as the material remodels. Synthetically mimicking the self-healing mechanism observed in biological tissues is a long-term goal of materials science in
general [46–49], holding great promise to improve, enhance or altogether change the
mechanical quality of man-made materials.
Thus, in systems with dynamical, physical bonds, the mechanical and rheological
properties can be controlled to yield materials that are dynamical, anisotropic and
responsive to external stimuli, in a manner similar to tissues and cells, the properties
of which respond to changes in the environment and to active, internal cues.
The classic review [50] on supramolecular polymers describes a number of
examples of physical interactions that may be used to synthesize physically linked
polymer gels. Arrays of hydrogen bonds (also important in organizing the interactions between proteins) are a powerful way to create very stable and mechanically
strong physical gels [50] – the ureidopyrimidinone-based supramolecular polymers
are particularly interesting in this respect and show mechanical properties that are
extremely temperature dependent: response ranges from viscoelastic at room temperatures to fluid-like at more elevated temperatures [51]. This behaviour is strongly
reminiscent of the fluid-solid transitions achievable in reconstituted biopolymer
networks. In addition to hydrogen bonding, other interactions may be used to create
transient bonds over a broad range of strengths: π À π interactions, hydrophobic
interactions and metal-ligand coordination bonds among them [50].
However, a merely transiently cross-linked elastomer has relatively poor mechanical properties, e.g. it flows at long timescales and the toughness is usually low.
Recent, more advanced hydrogel and elastomer chemistries demonstrated that a
combination of permanent and transient cross-links can dramatically improve the
toughness of a material while retaining the capacity to self-heal. This has led to
greatly increased interest in ‘dual cross-linked’ materials – both hydrogels and
elastomers – over the recent years [47, 52–55]. In those systems, the transient
cross-links are hypothesized to act as sacrificial bonds, whose dissociation provides
the material with a parallel channel for energy dissipation [56]. As a result, the
material can absorb more energy before physically rupturing, leading to enhanced
toughness of the material. Permanent bonds imprint the original network structure
and permit the network to remain elastic even at large and slow deformations.
Interestingly, this behaviour is strongly rate-dependent: materials may display very
Rheology, Rupture, Reinforcement and Reversibility: Computational Approaches. . .
71