Cross linked chains

When -xylene is used as the monomer feed in a plasma polymer process, PX may play an important role in the formation of the plasma polymer. The plasma polymer from -xylene closely resembles the Gorham process polymer in the infrared, although its spectmm contains evidence for minor amounts of nonlinear, branched, and cross-linked chains as well. Furthermore, its solubiUty and low softening temperature suggest a material of very low molecular weight (15). 

Model Networks. Constmction of model networks allows development of quantitative stmcture property relationships and provide the abiUty to test the accuracy of the theories of mbber elasticity (251—254). By definition, model networks have controlled molecular weight between cross-links, controlled cross-link functionahty, and controlled molecular weight distribution of cross-linked chains. Sihcones cross-linked by either condensation or addition reactions are ideally suited for these studies because all of the above parameters can be controlled. A typical condensation-cure model network consists of an a, CO-polydimethylsiloxanediol, tetraethoxysilane (or alkyltrimethoxysilane), and a tin-cure catalyst (255). A typical addition-cure model is composed of a, ffl-vinylpolydimethylsiloxane, tetrakis(dimethylsiloxy)silane, and a platinum-cure catalyst (256—258). 

The tetramer exists in two-molal zirconium chloride and nitrate solutions, but it polymerizes into cross-linked chains on hydrolysis (190—191) in strong acid solutions, the hydroxyl bridges can be replaced by other anions to form trimers (192) and monomers (192—193). 

Figure 14.10 Sulfur cross-linked chains resulting from vulcanization of rubber.

The above results proved the potential viability of the adsorbed hydrophilic macromolecules as bonded phases in chromatography of biopolymers but it must be admitted that additional crosslinking of previously adsorbed macromolecules is usually needed in order to obtain stable composites. The cross-linked bonded polymeric phases, however, may suffer from the restricted flexibility of the chain segment and their steric repellency may be diminished. Moreover, the conformational adaptivity of cross-linked chains for binding with solutes is poorer than that of grafted or chemically bound macromolecules. 

Hydroxyl containing polymers may be cross-linked with diisocyanates. Fordyce and Ferry cross-linked styrene-maleic anhydride copolymers through the action of glycols. The copolymerization of divinyl with vinyl monomers may be looked upon as a method of cross-linking chain polymers. The cross-linkages are introduced simultaneously with the growth of the linear polymer chains, rather than afterwards, but this difference is secondary. 

A pair of cross-linked chain units may be regarded as a single tetra-functional unit according to the scheme applied to polyfunctional condensation. In order to demonstrate the correspondence between the two treatments, consider the condensation of tetrafunctional units A... 

Note Excess can lead to undesirable molecular branching and cross-linking. Chain branching can severely hamper the crystallization capability of PET and this may lead to a loss in performance properties. 

Kinetics of Intramolecular Cross-Linking and Conformational Properties of Cross-Linked Chains... 

Figure 1. Topological structures of cross-linked chains...

Thus this model study permitted to elucidate the main regularities of the process of intramolecular cross-linking. The most interesting of them are, firstly, the existence of a uniform relationship between the kinetics of the reaction and the equilibrium properties of partially cross-linked chains and secondly, the independence of the kinetics of cross-linking on the character of reactive groups distribution on the initial stage of the reaction. 

The hypothesis can be checked by the mathematical experiment, if one takes into consideration the probability of the intemolecular cross-linking. Let s assume that sol fraction contains only intramolecularly cross-linked chains while the formation of even one intermole-cular cross-linkage leads to the sol-gel transition. Because only the properties of sol fraction are of our interest we don t need to follow the intermolecularly cross-linked chains. It is rather natural to assume that the probability of the transition into the gel is proportional to the dimensions of the macromolecular coil. 

The double bonds in isoprene and chloroprene allow these compounds to be vulcanized. In vulcanization, sulfur attaches to the doubly bonded carbon to produce cross-linked chains ... 

In Chap. C we have discussed in some detail the application of the cascade theory to polycondensates in their unperturbed state. In Chap. D some experimental results were already given for cross-linked or vulcanisated linear chains. In this chapter we shall now outline in brief how cross-linking chain reactions or the vulcanization of preformed chains of an arbitrary length distribution can be treated by cascade theory. Second, we shall discuss how heterogeneities in branching or a rigidity of a certain domain can be taken into account. 

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