Networks and Gels

Of course, appropriate design of the (polymeric) hgand can also result in nonlinear MSPs. Thus, a range of ligand-containing polymeric species can be imagined (e.g.. Fig. 7.1), which upon addition of a metal ion can self-assemble into side chain, intra-/intercross-linked (e.g.. Fig. 7.2), or other more complex MSPs architecmres. 

An interesting example of using metals to cross-link conjugated polymers was reported by Kokil et al. (2003). An organic-soluble PPE (15) was complexed with 

Yount et al. (2005) reported the formation of organogels using poly(vinylpyridine) (PVP) and ditopic metallopincer cross-linkers. This study provided particularly pertinent information on the dynamic elements of MSPs and elegantly demonstrated how they control the material s properties. PVP dissolved in dimethylsulfoxide (DMSO) is cross-linked with either bis-Pd - (18a) or Pt -pincer compounds (18b, Fig. 7.13 Yount et al. 2005). Addition of 5% 18a -Pd to a PVP solution results in a viscous material (77 = 6.7 Pa s), whereas the corresponding PVP 18b Pd is a gel (77 = 550 Pa s). Changing R from methyl to ethyl does not affect the thermodynamics of the pyridine/Pd interaction however, the rate of exchange decreases by approximately 2 orders of magnitude. Further studies on these materials and their 

Mixing different cross-linkers (19b and 19c) yielded systems with a strong gel-weak gel transition, rather than a distinct sol-gel shift. When the concentration of each of the cross-linkers was above the critical percolation threshold, the kinetically slower cross-linker (19c) dictated the gel properties. Upon addition of enough of a competitive binding additive, such as DMAP, to drop the concentration of the active cross-linking units below their individual percolation thresholds but still allowing the total amount of both active cross-linkers (19b and 19c) to be above the percolation threshold results in a gel whose properties are now controlled by the kinetically faster cross-linker (19c). 

When the crosslinking reactions of Chapter 6 are driven far beyond the gel point, nearly all species are attached to the gel in a single macroscopic network polymer. Such networks, with either chemical or strong physical bonds, are important soft solids. If the glass transition and melting temperatures are below room temperature, the material is a rubber. Rubbers 

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Figure 7.12 Chemical structure of a linear poly(p-phenylene ethynylene) (PPE) and the ligand exchange process utilized hy Kokil et al. (2003) to yield a conjugated cross-linked MSP network and gel.

Dotson NA, Macosko CW, Tirrell M (1992) in Aharoni SM (ed) Synthesis, Characterization and theory of polymeric networks and gels. Pleunum Press, New York, p 319... 

Peglow et al. (4) prepared polymerizable mesogenic materials consisting of a dichromophoric dichroic network containing azo-chromophores, which were used for preparing dichroic polymer networks and gels. 

The free-radical copolymerization of a vinyl monomer with a small amount of a divinyl monomer offers one of the simplest routes to the preparation of polymer networks and gels. Examples are... 

The same characteristic shape of curve was found with the absolute viscosity levels as a function of time depending on fat content. Obviously, the fat droplets in the system have an impact on the reaction, although the source of the structure build up is thought to be protein-based. Therefore, the effect of the protein content was assessed. As expected, a higher level of the protein content in the continuous phase increases the probability of networking and gel formation as also shown in Figure... 

Actin monomers assemble to form actin filaments which then associate to form higher level structures like bundles, networks, and gels d mamically in cells. Assembly and disassembly takes place spontaneously with a variety of actin-binding proteins involved in these processes. It has been reported that activated structures preexist as spontaneously fluctuating structures even in the absence of external stimuli. In Listeria monocytogenes an intracellular... 

Linear mechanical properties of the networks and gels discussed in this chapter are measured with the same methods of linear viscoelasticity as the polymer liquids (melts and solutions) discussed in Chapters 8 and 9. The various methods are described here, with examples pertaining to each class of materials. 

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