Three-dimensional network polymers

A three-dimensional network polymer such as vulcanized rubber, although incapable of dispersing completely, may nevertheless absorb a large quantity of a suitable liquid with which it is placed in contact. 

As shown originally by Malcolm Dole, polyethylene molecules may be cross-linked when subjected to high-energy radiation. These three-dimensional network polymers may be represented by the structure shown in Figure 1.5. 

Evidence supporting the original paradigm of lignin in wood as a random, three-dimensional network polymer is reviewed. More recent results which do not fit this simple picture are discussed. A modified paradigm is proposed in which lignin in wood is comprised of several types of network which differ from each other both ultrastructurally and chemically. When wood is deligni-fied, the properties of the macromolecules made soluble reflect the properties of the network from which they are derived. 

Behavior of Lignin Compatible with the Paradigm of a Random, Three-Dimensional Network Polymer... 

Such behavior is what would be expected for soluble fragments of a random three-dimensional network polymer. 

From the discussion in the previous sections we see that the behavior of the protolignin in wood and the pattern of its subsequent degradation and solution are compatible, in a broad sense, with the paradigm of a random three-dimensional network polymer. There emerge, however, several aspects of the problem which do not fit easily with the simple paradigm given above. In the following sections some of these anomalies are discussed. 

The paradigm of an infinite random three-dimensional network polymer implies a uniform chemistry throughout. The topochemical behavior... 

Of course, the picture given in the preceding paragraph cannot apply to the almost pure lignin in the true middle lamella. Here the lamellar thickness is usually more than 100 nm (83,84). The concept of a random three-dimensional network polymer would seem to be appropriate for such a thick layer. However, the true middle lamella probably contains less than 20% of the total lignin (83,84,102). Thus, lignin from the secondary wall will be the dominant fraction in most preparations from whole wood. 

Studies of preparation and properties of the so-called Simultaneous Interpenetrating Networks (SIN) occupy a special position among these works. SIN is a complex system of two or more three-dimensional network polymers which are chemically not bonded but are inseparable due to mechanical entanglement of chains. A detailed description of the preparation and properties of SIN is given by Lipatov and Ser-geeva . ... 

This process very rapidly raises the molecular weight and introduces crosslinks to set the resin to a rigid, three-dimensional network polymer incorporating the glass fiber reinforcing. Details of the mechanism of the cross-linking process are discussed more fully in Chap. 22. 

A three-dimensional network polymer, such as vulcanized rubber, does not dissolve in any solvent. It may nevertheless absorb a large quantity of a suitable liquid with which it is placed in contact and undergo swelling. The swollen gel is essentially a solution of solvent in polymer, although unlike an ordinary polymer solution it is an elastic rather than a viscous one. 

Crosslinked or three-dimensional network polymers - Crosslinked polymers may be represented as a three-dimensional network structure. They are crosslinked, that is they consist of insoluble and infusible three-dimensional giant molecules. Examples are thermosetting polymers such as crosslinked rubbers, resins, and so on. 

Multifunctional acrylates are shown in Table 1.10. Trimethylolpropane triacrylate gives a three-dimensional network polymer for screen ink, latex, and quick hardening ... 

Tris(N-acryloyloxyethyl)-l,3,5-triazine-2,4,6-trione gives a complicated three-dimensional network polymer for heat resistivity and quick hardening, that is waterproof ... 

Three-dimensional network polymers, as shown in Figure 6.8. 

The strength and thermal stability of three-dimensional network polymers are not attributed to stronger chemical bonds, nor even to a greater number of bonds. Instead, these properties result from the fact that the network polymer is better able than a linear polymer to preserve its structural integrity even after some bonds are broken. This advantage derives finom the differences in geometrical structure between... 

Resols on further heating change into resitols, a three-dimensional network polymer of low crosslink density as... 

Since only OH groups in the hydrolysis products are active toward condensation reaction to form =Si-0-Si= bonds, the stmcture or morphology of the resultant siloxane polymers should depend on the number of OH groups, x, in Si(OH) (OR)i. Namely, when x is 2, the hydrolysis may lead to linear siloxane polymers, and three-dimensional network polymers may be formed when x is 4. This reaction scheme teUs us that the fiber-drawing should be possible if Si(OR)4 is hydrolyzed with an imderstoichiometric amount of water (ideally, when the molar ratio of water to the alkoxysilane or r-value is 2) under the acidic condition. 

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