and provide good long-term integrity. To remedy the long-time liquidlike behaviour,
and to impart additional resilience, the chains may be permanently linked together
with chemical cross-links to form so-called network materials. In the end, the
dynamical mechanical response of the fully formed material is determined by a
combination of chain length, chain density, cross-linking functionality and density,
solvent properties, the mechanical properties and the topology of the individual
polymer strands in a complex interplay that continues to surprise and confuse.
But, a general challenge remains. The product-scale demands on resilience and
stability are, in general, at odds with the requirements on malleability, processibility
and recyclability. Improvements in one aspect occur at the detriment of the other;
because they rely on the same architectural and dynamic processes, optimization
must be accomplished respecting these trade-offs.
In recent years, the field of polymer materials has embraced two exciting new
avenues towards a reconciliation between these incompatible properties: composites
and reversibly linked materials. In composites, multiple materials that may, or may
not, be chemically connected to each other occupy the same space. The resulting
material properties are far richer than the simple sum of the constituents, and as a
result of the cross-talk between the components, the elastic and viscous responses of
the composite material become much more independent, to the point of being
individually addressable in design. In reversibly linked materials, the cross-linkers
themselves become dynamic either through non-covalent links or by reconfigurable
covalent connectivity. While the dynamical response of permanently linked materials is determined by the relaxational processes of the polymer chains, reversibly
linked materials there possess at least one fully separate relaxation time set by the
binding/unbinding/rebinding kinetics of the reversible cross-links.
Clearly, neither of these two mechanisms is new. Natural polymer materials, for
instance, make abundant use of both a heterogeneous composition and cross-linker
properties to tune elastic response. All proteinaceous linkers forge supramolecular
bonds, the dynamics of which are dictated by a cohesive energy scale that quantifies
the effective potential well responsible for the connection. In biology, such bonds
range from rapidly unbinding to effectively permanent, and the various cross-links
and their timescales play an important role in the dynamical remodelling of networks
and tissues. A prime example is the cytoskeleton, a dynamic composite structure
composed of actin filaments, microtubuli and intermediate filaments, which is the
internal framework of cells and determines cell shape and mechanical properties.
The cytoskeleton remodels continually and adapts its structure, and thereby the
shape of the cell, to changes in the environment.
This adaptive mechanical character, where relaxations are actively tuned and are
largely decoupled from elastic rigidity, where structure is constantly evolving in
response to external and intrinsic cues, all the while preserving extraordinary
mechanical performance, serves as inspiration for the design of novel polymeric
materials. Indeed, in the realm of synthetic polymer materials, it is becoming
increasingly clear that there, too, clever uses of composition and reversible linkage
unlock exceptional mechanical qualities. In this vastly enhanced design space,
optimization will be an even greater challenge than it has been. This is where
Rheology, Rupture, Reinforcement and Reversibility: Computational Approaches. . .
65
and to impart additional resilience, the chains may be permanently linked together
with chemical cross-links to form so-called network materials. In the end, the
dynamical mechanical response of the fully formed material is determined by a
combination of chain length, chain density, cross-linking functionality and density,
solvent properties, the mechanical properties and the topology of the individual
polymer strands in a complex interplay that continues to surprise and confuse.
But, a general challenge remains. The product-scale demands on resilience and
stability are, in general, at odds with the requirements on malleability, processibility
and recyclability. Improvements in one aspect occur at the detriment of the other;
because they rely on the same architectural and dynamic processes, optimization
must be accomplished respecting these trade-offs.
In recent years, the field of polymer materials has embraced two exciting new
avenues towards a reconciliation between these incompatible properties: composites
and reversibly linked materials. In composites, multiple materials that may, or may
not, be chemically connected to each other occupy the same space. The resulting
material properties are far richer than the simple sum of the constituents, and as a
result of the cross-talk between the components, the elastic and viscous responses of
the composite material become much more independent, to the point of being
individually addressable in design. In reversibly linked materials, the cross-linkers
themselves become dynamic either through non-covalent links or by reconfigurable
covalent connectivity. While the dynamical response of permanently linked materials is determined by the relaxational processes of the polymer chains, reversibly
linked materials there possess at least one fully separate relaxation time set by the
binding/unbinding/rebinding kinetics of the reversible cross-links.
Clearly, neither of these two mechanisms is new. Natural polymer materials, for
instance, make abundant use of both a heterogeneous composition and cross-linker
properties to tune elastic response. All proteinaceous linkers forge supramolecular
bonds, the dynamics of which are dictated by a cohesive energy scale that quantifies
the effective potential well responsible for the connection. In biology, such bonds
range from rapidly unbinding to effectively permanent, and the various cross-links
and their timescales play an important role in the dynamical remodelling of networks
and tissues. A prime example is the cytoskeleton, a dynamic composite structure
composed of actin filaments, microtubuli and intermediate filaments, which is the
internal framework of cells and determines cell shape and mechanical properties.
The cytoskeleton remodels continually and adapts its structure, and thereby the
shape of the cell, to changes in the environment.
This adaptive mechanical character, where relaxations are actively tuned and are
largely decoupled from elastic rigidity, where structure is constantly evolving in
response to external and intrinsic cues, all the while preserving extraordinary
mechanical performance, serves as inspiration for the design of novel polymeric
materials. Indeed, in the realm of synthetic polymer materials, it is becoming
increasingly clear that there, too, clever uses of composition and reversible linkage
unlock exceptional mechanical qualities. In this vastly enhanced design space,
optimization will be an even greater challenge than it has been. This is where
Rheology, Rupture, Reinforcement and Reversibility: Computational Approaches. . .
65