particles with respect to the polymer segments. On even smaller scales, numerous
molecular mechanisms of the Payne effect and reinforcement for polymer
nanocomposites filled with model (mainly ideal, spherical) nanofillers are studied
by [14] and in a series of publications from Liu and Lyulin groups [15–20]. The role
of the direct particle-particle interactions and segmental orientation at the particlematrix interface were both investigated. Still, questions remain about the dominant
molecular mechanism and about the potential effects of a more realistic shape of the
filler particles. Overall, there are now two distinct proposals regarding the molecular
origins of the excess reinforcement (the reinforcement in addition to what is induced
by the mere presence of rigid nanoparticles): one attributes it exclusively to the
aggregation of the nanoparticles, whereas the other ascribes it mainly to the formation of filler-polymer interphases. The aggregation scenario, understandably, features more prominently at high filler fractions, whereas the formation of a
mesoscopic network of filler-matrix-filler bridges appears to be more prevalent at
lower filler fractions. Currently, despite strenuous effort from both the experimental
and the computational scientific community [9, 21–42], there is no consensus on the
molecular origin of the Payne effect. What is clear, however, is that filler-filler and
filler-matrix interactions at the scale of the fillers are of crucial importance both in the
reinforcement and in the Payne effect and that tuning these interactions is a prime
candidate for a mechanism by which to rationally design superior nanocomposites.
In Sect. 4.2, we present some of our recent findings for the role and effect of fillermatrix interactions in and out of equilibrium.
2.2 Reversible and Exchange Materials
2.2.1 Reversible Networks
As remarked in the introduction, most natural materials including the cytoskeleton
and the extracellular matrix are held together by non-permanent links. This results in
dynamic, transient connectivity that is used to reshape networks and tissues,
connecting, releasing and rebinding actin and collagen filaments as they remodel.
But also outside of living systems, transient links change mechanical properties in
important ways: In [43–45] it is shown that the association and dissociation of crosslinks strongly affect the mechanical behaviour of in vitro actin networks, causing
them to be able to shift, purposely, from rubberlike, purely elastic behaviour to
viscous flow in transiently bonded networks.
Such switchable properties are of clear interest outside of biology, and because of
their structural similarities to natural materials in animals and plants, synthetic
hydrogels have been the materials of choice to attempt to copy some of the natural
behaviours. One particularly important aspect of the mechanical properties of
hydrogels that requires improvement in general is their toughness; classical
hydrogels are generally brittle materials that fracture abruptly and absorb little
mechanical energy before doing so. As with natural materials, physical –
70
C. Raffaelli et al.
molecular mechanisms of the Payne effect and reinforcement for polymer
nanocomposites filled with model (mainly ideal, spherical) nanofillers are studied
by [14] and in a series of publications from Liu and Lyulin groups [15–20]. The role
of the direct particle-particle interactions and segmental orientation at the particlematrix interface were both investigated. Still, questions remain about the dominant
molecular mechanism and about the potential effects of a more realistic shape of the
filler particles. Overall, there are now two distinct proposals regarding the molecular
origins of the excess reinforcement (the reinforcement in addition to what is induced
by the mere presence of rigid nanoparticles): one attributes it exclusively to the
aggregation of the nanoparticles, whereas the other ascribes it mainly to the formation of filler-polymer interphases. The aggregation scenario, understandably, features more prominently at high filler fractions, whereas the formation of a
mesoscopic network of filler-matrix-filler bridges appears to be more prevalent at
lower filler fractions. Currently, despite strenuous effort from both the experimental
and the computational scientific community [9, 21–42], there is no consensus on the
molecular origin of the Payne effect. What is clear, however, is that filler-filler and
filler-matrix interactions at the scale of the fillers are of crucial importance both in the
reinforcement and in the Payne effect and that tuning these interactions is a prime
candidate for a mechanism by which to rationally design superior nanocomposites.
In Sect. 4.2, we present some of our recent findings for the role and effect of fillermatrix interactions in and out of equilibrium.
2.2 Reversible and Exchange Materials
2.2.1 Reversible Networks
As remarked in the introduction, most natural materials including the cytoskeleton
and the extracellular matrix are held together by non-permanent links. This results in
dynamic, transient connectivity that is used to reshape networks and tissues,
connecting, releasing and rebinding actin and collagen filaments as they remodel.
But also outside of living systems, transient links change mechanical properties in
important ways: In [43–45] it is shown that the association and dissociation of crosslinks strongly affect the mechanical behaviour of in vitro actin networks, causing
them to be able to shift, purposely, from rubberlike, purely elastic behaviour to
viscous flow in transiently bonded networks.
Such switchable properties are of clear interest outside of biology, and because of
their structural similarities to natural materials in animals and plants, synthetic
hydrogels have been the materials of choice to attempt to copy some of the natural
behaviours. One particularly important aspect of the mechanical properties of
hydrogels that requires improvement in general is their toughness; classical
hydrogels are generally brittle materials that fracture abruptly and absorb little
mechanical energy before doing so. As with natural materials, physical –
70
C. Raffaelli et al.