Chemistry Reference
In-Depth Information
between fibres.
7
The density of the crosslinks together with their strength also
has a large effect on the viscoelastic properties of the gels,
70
and therefore, the
control of crosslink formation is one of the primary design parameters of
fibrillar gel networks.
A closer inspection of various fibrillar gel systems reveals that for most gel
systems the crosslinks between fibres consist of branched fibres or bundled
fibres, and intertwined fibres. Together, these different types of crosslinks are
also known as junction zones, and the branched and bundled types together are
grouped as microcrystalline domains. In the microcrystalline domains the fibres
are held together by similar intermolecular interactions as between fibre fila-
ments, whereas with intertwined fibres also mechanical interlocking contributes
to the stability of the crosslinks. For both cases, the formation of the crosslinks
takes place during formation of the gels,
71
and is therefore governed by the
same complicated interplay of thermodynamics and kinetics.
There are few examples of studies in which fibre branching has been con-
trolled at the kinetic level. Liu and coworkers
72
have been able to control fibre
branching of a steroid-based organogel system by adding a tiny amount of a
polymer (Figure 1.15). They have been able to show that during fibre growth,
the polymer adsorbs at the growing tip and causes a lattice mismatch, which
leads to branching. More recently, our group found that fibre morphology and
in particular branching of dynamic covalent gels can be tuned by controlling
the gel growth kinetics by catalytic action.
73
It should be noted that these
approaches are independent of the molecular design of the gelator, but are
aimed at influencing the gel formation mechanism.
A very different approach to generate and control crosslink formation
between fibres is the introduction of specific noncovalent or covalent bonds
between fibres. In an early example our group reported the covalent crosslinking
of fibrous networks of cyclohexyl bis-urea type of gelators (17), by polymer-
isation of appended methacrylate moieties (Figure 1.16).
52
This approach pre-
served the original network morphology and thereby did not offer control over
crosslinking density. Moreover, the gels lost their thermoreversibility, indicating
d
n
1
r
3
n
g
|
1
.
Figure 1.15
The adsorption of additives causes a crystallographic mismatch at a
growing fibre tip, leading to branching.
(Reprinted with permission from ref. 74. Copyright (2002) American
Chemical Society).
