Chemistry Reference
In-Depth Information
Figure 5.14 Schematic representation of polymer network formation based on the addition
of complementary linkers using (a) single hydrogen bond formation and bifunctional cross-
linking agents, (b) single hydrogen bond formation and trifunctional cross-linking agents,
and (c) three-point hydrogen bond formation and bifunctional cross-linking agents.
functional groups attached to the polymer chains. For example, the Weck research
group demonstrated that networks based on three-point hydrogen bonding complex
formation between 2,6-diaminopyridine and thymine (K
a
10
3
M
21
) exhibit lower
solution viscosities in chloroform compared to similar systems in which the
network is based on a stronger six-point complex formation between cyanuric acid
and isophthalamide wedge receptors (K
a
10
6
M
21
; Pollino et al. 2004).
Another important factor in tuning the network properties is the molecular archi-
tecture of the cross-linking agent; by varying the functionality of the cross-linking we
can control the cross-linking density of the network. As a result, the addition of 1
equiv (based on recognition sites NOT the linker to polymer ratio) of a tetrafunctional
cross-linking agent will result in greater cross-linking efficiency compared to the
addition of 2 equiv of a difunctional cross-linking agent. These advantages of the
two component cross-linking systems allow for tunability of network properties
when compared to the one component system just described. However, the number
of reports in the literature using such a two component system is limited when
compared to the one component systems.
Kawakami and Kato cross-linked an SSCLCP with side chain carboxylic acid
groups by using bis-pyridine (Kato 1996) or bis-imidazoyl compound as the cross-
linking agent (Fig. 5.15; Kawakami and Kato 1998). The addition of the bis-pyridine
caused interchain cross-linking through the one-point hydrogen bonding of the side
chain carboxylic groups with the bis-pyridine, resulting in cross-linking and
increased mesophase stability of the liquid crystalline state.
As explained earlier, the role of the cross-linking agent is key in two component
systems and affects the properties of the final network. The work of Thibault et al.
(2003) illustrates the importance of the cross-linking agent. They employed
bis-thymine based cross-linking agents with different linker lengths to reversibly
cross-link 2,6-diaminopyridine functionalized copolymers (Fig. 5.16) that formed
discrete micron-sized spherical polymeric aggregates (Thibault et al. 2003).
