Chemistry Reference
In-Depth Information
by hydrogen bonding and by metal -ligand associations, for example, provide for two
accessible handles for addressing responsive materials.
In the case of the PVP/pincer SP networks, the onset of percolation, as defined by
the concentration of cross-linkers, depends on the total weight percentage of the SP
network and provides another degree of control in the systems. These and other
details of the percolation behavior will necessarily be somewhat system specific.
Nevertheless, although the present discussion is limited to one representative
example, the strategy to exploit sol -gel or other phase transitions as highly sensitive
regions of stimulus-responsive behavior in SP systems is general.
The behavior of SP networks is typically that of transient network models, in
which the independent relaxations of stress-bearing entanglements determine the
dynamic mechanical response of a network (Green and Tobolsky 1946; Lodge
1956; Yamamoto 1956; Cates 1987; Cates and Candau 1990; Tanaka and Edwards
1992; Turner and Cates 1992; Turner et al. 1993; Jongschaap et al. 2001). Without
considering the exact structure of the networks, the detailed mechanism of relaxation
(Leibler et al. 1991; Tanaka and Edwards 1992), or the extent of cooperativity in the
associations, individual dissociation events clearly dominate the mechanical proper-
ties: no significant averaging or summation of different components is observed.
The independence of the cross-links has significant consequences for the rational,
molecular engineering of viscoelastic properties. When entanglements are defined
by very specific interactions, the chemical control of properties follows. As long as
the strength of the association is great enough to render associated a significant
fraction of the crosslinkers, the dynamics of cross-link dissociation, rather than
further details of their thermodynamics, are the key design criterion. As a result,
quite complex viscoelastic behavior can be engineered given suitable knowledge of
the small molecules.
3.4. MECHANICAL PROPERTIES IN SPs AT INTERFACES
Among the areas of materials science in which SPs are likely to bring interesting and
important new material properties is the study of interfaces. Here, the nature of stress-
bearing interactions must be considered in the context of a nearby surface. Two
examples will be considered in this section: main-chain reversible polymer brushes
and cross-linked polymer brushes. These two examples are analogs of the main-
chain SPs and SP networks described above, but in which the polymer chains are
anchored to a surface. In the case of main-chain reversible polymer brushes, we
will focus on the transfer of force from one surface to another via SP bridges
(Fig. 3.9). In particular, we consider the consequences of a force normal to the
surfaces (adhesion), although lateral forces (e.g., friction) might also be influenced.
In the case of cross-linked polymer brushes, both normal and lateral forces will
be discussed.
Polymer bridging has important consequences in the material and life sciences,
including an influence on adhesion, tribology and polymer flow, microtubule forma-
tion and function, and cell surface interactions (de Gennes 1979; Fleer et al. 1993).
Bridges occur when a polymer chain is either physisorbed or covalently bound to
