Chemistry Reference
In-Depth Information
architecture, noncovalent strategies allow for the tuning of materials properties
because of the responsiveness of these interactions and ultimately the material
toward external stimuli. For example, the employment of metal coordination for non-
covalent cross-linking results in materials that are sensitive toward redox reactions or
metal-ligand displacement agents, whereas the use of hydrogen bonding interactions
yields thermally sensitive materials. Clearly, noncovalent cross-linking strategies
offer a route toward responsive materials with tunable properties that are otherwise
not accessible.
Network Formation Using Hydrogen Bonding.
A variety of hydrogen
bonding interactions have been employed by scientists to cross-link polymeric
materials including the described one-point, two-point, three-point, four-point, and
six-point hydrogen bonding donor/acceptor recognition moieties. Polymer networks
based on hydrogen bonding can be broadly classified into three classes: 1) polymer
networks based on self-associating hydrogen bonds, 2) polymer networks syn-
thesized via the use of complementary linker molecules, and 3) polymer networks
based on complementary polymer blends (Fig. 5.11). This chapter follows this classi-
fication to describe the polymer networks based on hydrogen bonding that are
reported in the literature.
Self-Associative Polymer Network.
In self-associative polymer networks (often
called one component systems), the hydrogen bonding recognition units that are
covalently attached to the polymer backbone have an appreciable tendency for
self-association, that is, self-dimerize, which leads to interchain cross-linking of
the polymers. As a result, the system is inherently cross-linked and does not
require any external cross-linking agents for network formation (Fig. 5.11a).
Because the cross-linking is based on dimerization phenomena, to achieve effective
Figure 5.11 The three distinct classes of hydrogen bonded polymer networks: (a) self-
associative polymer network, (b) polymer network synthesized via the use of complementary
linker molecules, and (c) complementary polymer blends.
