Network-forming materials

As a result of these reactions the materials eventually crosslink and become set] that is, they lose the ability to flow or to be dissolved. Cure most often is thermally activated hence the term thermoset, but network-forming materials whose cure is light- or radiation-activated are also considered to be thermosets (see Section 2.11, on differential photocalorimetry). Some thermosetting materials, such as certain adhesives, crosslink by a dual-cure mechanism, that is, by either heat or light activation. In contrast to the values for crosslinked elastomers or rubbers, the glass transition temperature of thermosets is generally above room temperature. 

C), and is much less reactive it is therefore safer and easier to handle, and is essentially non-toxic. The amorphous material can be transformed into various crystalline red modifications by suitable heat treatment, as summarized on the right hand side of Fig. 12.3. It seems likely that all are highly polymeric and contain three-dimensional networks formed by breaking one P-P bond in each P4 tetrahedron and then linking the remaining P4 units into chains or rings of P atoms each of which is pyramidal and 3 coordinate as shown schematically below ... 

Similarly, a composite of hydroxyapatite and a network formed via cross-linking of chitosan and gelatin with glutaraldehyde was developed by Yin et al. [ 169]. A porous material, with similar organic-inorganic constituents to that of natural bone, was made by the sol-gel method. The presence of hydroxyapatite did not retard the formation of the chitosan-gelatin network. On the other hand, the polymer matrix had hardly any influence on the high crystallinity of hydroxyapatite. 

According to Ref. [12], template for synthesis of nanomaterials is defined as a central structure within which a network forms in such a way that removal of this template creates a filled cavity with morphological or stereochemical features related to those of the template. The template synthesis was applied for preparation of various nanostructures inside different three-dimensional nanoporous structures. Chemically, these materials are presented by polymers, metals, oxides, carbides and other substances. Synthetic methods include electrochemical deposition, electroless deposition, chemical polymerization, sol-gel deposition and chemical vapor deposition. These works were reviewed in Refs. [12,20]. An essential feature of this... 

The paper first considers the factors affecting intramolecular reaction, the importance of intramolecular reaction in non-linear random polymerisations, and the effects of intramolecular reaction on the gel point. The correlation of gel points through approximate theories of gelation is discussed, and reference is made to the determination of effective functionalities from gel-point data. Results are then presented showing that a close correlation exists between the amount of pre-gel intramolecular reaction that has occurred and the shear modulus of the network formed at complete reaction. Similarly, the Tg of a network is shown to be related to amount of pre-gel intramolecular reaction. In addition, materials formed from bulk reaction systems are compared to illustrate the inherent influences of molar masses, functionalities and chain structures of reactants on network properties. Finally, the non-Gaussian behaviour of networks in compression is discussed. 

Materials known as interpenetrating polymer networks, IPN s, contain two or more polymers, each in network form (6 9), A practical restriction requires that at least one of the polymer networks has been formed (i.e. polymerized or crosslinked) in the immediate presence of the other. Two major types of synthesis have been explored, both yielding distinguishable materials with different morphologies and physical properties. 

If a large number of branches exist that connect all of the backbone molecules into a three-dimensional network, the material will not flow when heated, and it is considered a thermoset resin. Vulcanized rubber is an example where the sulfur linkages create a three-dimensional network, converting the precursor rubber into a solid thermoset material. Crosslinked backbone chains are shown in Fig. 2.8(e). When extruding many thermoplastics, the polymer can undergo chemical reactions to form small amounts of crosslinked material. Partial crosslinking is a problem with some PE resins that contain residual double bonds that are made using... 

Mass fraction of the network material resulting from a network-forming polymerization or crosslinking process. 

Note 1 The polymer network formed often displays rubberlike elasticity. However, a high concentration of crosslinks can lead to rigid materials. 

Fig. 5. Average concentrations of major and minor constituents in the different types of HT materials studied. Left Si02 (network-forming), AI2O3 (network-forming or modifying), and CaO (network-modifying). Right MgO, Na20, K20, and Fe203.

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