different mechanics depending on the speed or strength of applied deformation.
These properties can be controlled by using reversible cross-links whose dynamic
properties themselves depend on externally addressable parameters such as pH or
temperature; the resulting hybrid networks can be used as tunable stimuli-responsive
materials.
A beautiful example of this is found in [53], where a hybrid organogel is
synthesized that, owing to the addition of transient, supramolecular cross-links to a
covalent polymer gel, displays a dramatically increased extensibility. This occurs
with remarkably little change to the linear modulus of the material. The effect
disappears when the material is deformed at a rate comparable to, or faster than,
the typical bond lifetime: At these high deformation rates, the reversible links have
no time to adapt to the changing state of strain which causes them to act, effectively,
as permanent, irreversible connections. In Sect. 4.2, we will revisit these observations and provide evidence from models to support the scenario.
In summary, reversible and hybrid (reversible/permanent) networks have opened
the doors to new combinations of mechanical properties that are difficult to achieve
in permanently cross-linked or entangled polymer materials. In what follows, we
describe some of the computational approaches that permit computational investigations into the origins of these properties and the extent to which their beneficial
contributions may be optimized.
2.2.2 Vitrimers
Vitrimers are a revolutionary class of polymer, combining the malleability and
recyclability of thermoplastics with the insolubility and creep resistance of thermosets [57, 58]. Their unique connectivity-preserving bond exchange mechanism
[59, 60] with well-controlled exchange rate makes their cross-links dynamic. At
low exchange rates, they operate like thermosets, while at high rates, they are
malleable like thermoplastics. These activable bond swaps allow vitrimers to release
internal stresses without losing shape, unlike cross-linked elastomers or gels. This
unique bond swapping provides also a welding strategy [61] or grants them responsiveness to light, pH, voltage, metal ions, redox chemicals and mechanical stimuli
[62–64]. Interestingly, vitrimer topology plays a crucial role in determining their
dynamics, granting them additional tunability while designing smart materials. The
reason is that loops affect equilibrium elastic properties of any cross-linked network
[65], but since topology in vitrimers is dynamic, dramatic differences appear in stress
relaxation from different (although mechanically similar) molecular topologies. In a
past work, we showed that novel vitrimers can be designed explicitly considering
defects as a means to control separately mechanical and dynamical properties
[66]. Strongly connected to both topology and dynamics is also their self-healing
ability. The ideal self-healing material needs a fully controllable long-time solid-like
mechanical behaviour, the ability to heal without any form of external intervention,
and lastly the healing process has to be effective to the point that the location of the
damage can no longer be identified, possibly even do so repeatedly for multiple
72
C. Raffaelli et al.
These properties can be controlled by using reversible cross-links whose dynamic
properties themselves depend on externally addressable parameters such as pH or
temperature; the resulting hybrid networks can be used as tunable stimuli-responsive
materials.
A beautiful example of this is found in [53], where a hybrid organogel is
synthesized that, owing to the addition of transient, supramolecular cross-links to a
covalent polymer gel, displays a dramatically increased extensibility. This occurs
with remarkably little change to the linear modulus of the material. The effect
disappears when the material is deformed at a rate comparable to, or faster than,
the typical bond lifetime: At these high deformation rates, the reversible links have
no time to adapt to the changing state of strain which causes them to act, effectively,
as permanent, irreversible connections. In Sect. 4.2, we will revisit these observations and provide evidence from models to support the scenario.
In summary, reversible and hybrid (reversible/permanent) networks have opened
the doors to new combinations of mechanical properties that are difficult to achieve
in permanently cross-linked or entangled polymer materials. In what follows, we
describe some of the computational approaches that permit computational investigations into the origins of these properties and the extent to which their beneficial
contributions may be optimized.
2.2.2 Vitrimers
Vitrimers are a revolutionary class of polymer, combining the malleability and
recyclability of thermoplastics with the insolubility and creep resistance of thermosets [57, 58]. Their unique connectivity-preserving bond exchange mechanism
[59, 60] with well-controlled exchange rate makes their cross-links dynamic. At
low exchange rates, they operate like thermosets, while at high rates, they are
malleable like thermoplastics. These activable bond swaps allow vitrimers to release
internal stresses without losing shape, unlike cross-linked elastomers or gels. This
unique bond swapping provides also a welding strategy [61] or grants them responsiveness to light, pH, voltage, metal ions, redox chemicals and mechanical stimuli
[62–64]. Interestingly, vitrimer topology plays a crucial role in determining their
dynamics, granting them additional tunability while designing smart materials. The
reason is that loops affect equilibrium elastic properties of any cross-linked network
[65], but since topology in vitrimers is dynamic, dramatic differences appear in stress
relaxation from different (although mechanically similar) molecular topologies. In a
past work, we showed that novel vitrimers can be designed explicitly considering
defects as a means to control separately mechanical and dynamical properties
[66]. Strongly connected to both topology and dynamics is also their self-healing
ability. The ideal self-healing material needs a fully controllable long-time solid-like
mechanical behaviour, the ability to heal without any form of external intervention,
and lastly the healing process has to be effective to the point that the location of the
damage can no longer be identified, possibly even do so repeatedly for multiple
72
C. Raffaelli et al.