Network space

Gelation and the attendent insolubility mentioned above are encountered in all of the nonlinear polymerizations listed in Table I and in many others likewise. Naturally these characteristics have been attributed to the restraining effects of three-dimensional, or space, network structures of infinite size within the polymer. This is the feature which distinguishes most nonlinear from linear polymers. 

Nonlinear addition polymers are readily obtained by copolymerizing a divinyl compound (e.g., divinylbenzene) with the vinyl monomer (e.g., styrene), as already mentioned. Products so obtained exhibit the insolubility and other characteristics of space-network structures and are entirely analogous structurally to the space-network polymers produced by the condensation of polyfunctional compounds. Owing to... 

Polymers of dienes such as butadiene frequently contain a substantial portion of gel which will not dissolve in a good solvent, though it may swell to a volume 20 to 100 or more times that of the polymer itself. This gel, which may comprise up to 90 percent or more of the polymer, consists of a space-network structure formed as a result of a very few cross-linkages provided by occasional (perhaps 1 in 1000 or less) diene units both double bonds of which have entered into the polymerization (see Chap. VI). 

While the condition of stoichiometric neutrality invariably must hold for a macroscopic system such as a space-network polyelectrolyte gel, its application to the poly electrolyte molecule in an infinitely dilute solution may justifiably be questioned. In a polyelectrolyte gel of macroscopic size the minute excess charge is considered to occur in the surface layer (the gel being conductive), which is consistent with the assumption that the potential changes abruptly at the surface. This change is never truly abrupt, for it must take place throughout a layer extending to a depth which is of the order of magnitude of the... 

What would a Flatland brain be like Could it really work Our 3-D brains are highly convoluted with microscopic neurons making complicated interconnections. Without this complex three-space network of nerve filaments—... 

Thermosetting space-network polymers can be prepared through the reaction of polybasic acid anhydrides with polyhydric alcohols. A linear polymer is obtained with a bifunctional anhydride and a bifunctional alcohol, but if either reactant has three or more reactive sites, then formation of a three-dimensional polymer is possible. For example, 2 moles of 1,2,3-propane-triol (glycerol) can react with 3 moles of 1,2-benzenedicarboxylic anhydride (phthalic anhydride) to give a highly cross-linked resin, which usually is called a glyptal ... 

If the monomers are bifunctional, as in the above example, then a linear polymer is formed. Terminating monofunctional groups will reduce the average degree of polymerisation. Polyfunctional monomers, such as glycerol and phthalic acid, are able to form branching points, which readily leads to irreversible network formation (see Chapter 9). Bakelite, a condensation product of phenol and formaldehyde, is an example of such a space-network polymer. Linear polymers are usually soluble in suitable solvents and are thermoplastic - i.e. they can be softened by heat without decomposition. In contrast, highly condensed network polymers are usually hard, are almost completely insoluble and thermoset - i.e. they cannot be softened by heat without decomposition. 

Fig. 2.8. Neutral networks in sequence space. The pre-image of the structure in the lower part of the figure is a connected neutral network spanning whole sequence space. Networks of this class are typical for frequent structures. The upper part of the figure shows an example of a parti-...

It is reasonable therefore to consider that fused silica resembles liquid water. Just as liquid water retains from the parent shucture (ice) the three-dimensional network but not the long-range periodicity of the network, one would expect that liquid silica also retains the continuity of the tetrahedra, i.e., the space network, but loses much of the periodicity and long-range order that are the essence of the crystalline state. This model of fused silica, based on keeping the extension of the network but losing the translational symmetry of crystalline silica, implies a low concentration of charge... 

Where both y- and 6-dendrites are formed the y-dendrites appear white and the area occupied by the 6-dendrites dark at low magnification (figure 1.8 a). The reason forthis is that, on cooling and quenching, the 6-dendrites are partly transformed to y which contains closely spaced networks of residual 6, figure 1.8 b. 

Deep space network—A system of large communications antennas around the world that provides continuous communications with spacecraft as the earth rotates. When one antenna turns out of range, the next takes over the task of staying in contact with the spacecraft. 

This sudden gel formation is usually interpreted as the formation of a small per cent of infinite space network. The formation and control of the gel phase are vital in the processing of random thermosets and structosets. For example, too rapid gelation can cause poor bond strength in laminated and bonded structures. Too slow gelation could cause the collapse of a foam. The amount of reaction required for gelation can be controlled by the functionality of prepolymer units, /pp. 

If each monomer molecule contains just two functional groups, growth can occur in only two directions, and a linear polymer is obtained, as in nylon 66 or Dacron. But if reaction can occur at more than two positions in a monomer, there is formed a highly cross-linked space network polymer, as in Glyptal, an alkyd resin. Dacron and Glyptal are both polyesters, but their structures are quite different and, as we shall see, so are their uses. 

Urea reacts with formaldehyde to form the urea-formahlehyde resins, highly important in molded plastics. Here, too, a space-network polymer is formed. 

The molecular structure of plastics is of two general kinds long molecules, either linear or branched and space-network molecules. 

In the regions intermediate between these limiting cases, normal modes of vibration "erode" at different rates and product distributions become sensitive to the precise conditions of the experiment. Intramolecular motions in different product molecules may remain coupled by "long-range forces even as the products are already otherwise quite separated" (Remade Levine, 1996, p. 51). These circumstances make possible a kind of temporal supramolecular chemistry. Its fundamental entities are "mobile structures that exist within certain temporal, energetic and concentration limits." When subjected to perturbations, these systems exhibit restorative behavior, as do traditional molecules, but unlike those molecules there is no single reference state—a single molecular structure, for example—for these systems. What we observe instead is a series of states that recur cyclically. "Crystals have extension because unit cells combine to fill space networks of interaction that define [dissipative structures] fill time in a quite... 

A network polymer [Fig. 1.3(d)], on the other hand, can be described as an interconnected branched polymer. For example, a three-dimensional or space network structure will develop, instead of the branched structure (XI), if styrene is copolymerized with higher concentrations of divinyl benzene. In a network structure, all polymer chains are linked to form one giant molecule. Thus, instead of being composed of discrete molecules, a piece of network polymer constitutes, essentially just one molecule. With the formation of network structure polymers acquire greater rigidity, dimensional stability, and resistance to heat and chemicals. Because of their network structure such polymers cannot be dissolved in solvents and cannot be melted by heat strong heating only causes decomposition. 

The amine has three distinct sites where it can react with the epoxide thus generating crosslinkages and forming a space-network polymer. 

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