Primary interacting networks

In discussing the formation of interpenetrating networks, we may reasonably assume as a first approximation that the synthesis of each component occurs irrespective of the others, and that the kinetics of the process is determined by the concentration of one or another component. However, this is a rather rough approximation, which is more or less valid at low concentrations of the polyurethane component. As was shown before,51 when the content of polyurethane exceeds 50%, its network begins to work as a cage, preventing polyester formation because the primary polyurethane network hampers diffusion of the ester. This means that in such systems, mutual interference of the components occurs even in the absence of chemical interactions between them. 

Figure 21 shows the arrangement of the water molecules in the second hydration shell in the major groove of MPD 7. It contains all the water molecules associated with those of the primary layer indicated by broken circles, and those interacting with one another in a network. 

DMSO and water form a solution with nonideal behavior, meaning that the properties of the solution are not predicted from the properties of the individual components adjusted for the molar ratios of the components. The strong H-bonding interaction between water and DMSO is nonideal and is the primary driver for the very hygroscopic behavior of DMSO. Even short exposure of DMSO to humid air results in significant water uptake. Water and DMSO nonideal behavior results in an increase in viscosity on mixing due to the extensive H-bond network. 

These moment studies have been performed on polymer systems such as polyethylene (or on penetrants in polymer systems) in which the interacting spins (protons or fluorines) reside on the same or on adjacent atoms. This allows essentially no freedom of variation in the internuclear vectors upon deformation of the network. The primary informational content therefore relates to independent segmental orientation distributions. By placing single spins on alternate segments, there should be much greater sensitivity to changes in the chain extension upon bulk deformation. 

Self-consistent approaches in molecular modeling have to strike a balance of appropriate representation of the primary polymer chemistry, adequate treatment of molecular interactions, sufficient system size, and sufficient statistical sampling of structural configurations or elementary transport processes. They should account for nanoscale confinement and random network morphology and they should allow calculating thermodynamic properties and transport parameters. 

Cellulose microfibrils make up the basic framework of the primary wall of young plant cells (3), where they form a complex network with other polysaccharides. The linking polysaccharides include hemicellulose, which is a mixture of predominantly neutral heterogly-cans (xylans, xyloglucans, arabinogalactans, etc.). Hemicellulose associates with the cellulose fibrils via noncovalent interactions. These complexes are connected by neutral and acidic pectins, which typically contain galac-turonic acid. Finally, a collagen-related protein, extensin, is also involved in the formation of primary walls. 

An analogy between the situation just described and those involved in ion-solvent and ion-ion interactions can be drawn. The solvent water, for example, normally has a particular structure, the water network. Near an ion, however, the water dipoles are under the conflicting influences of the water network and the charged central ion. They adopt compromise positions that correspond to primary and secondary solvation (Chapter 2). Similarly, in an electrolytic solution, the presence of the central ion makes the surrounding ions redistribute themselves—an ionic cloud is formed (see Chapter 3). 

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