Crosslinking linear chains

Gelation processes, such as crosslinking linear chains or condensation of /-functional monomers A/ (where A reacts with A) with /> 2 are quite different from either linear condensation polymers or hyperbranched polymers. Linear condensation polymers (made from AB monomers, -where A only reacts with B) and hyperbranched polymers (made from... 

Figure 3.3.5B. Microporous polymer in the form of a gel particle formed by crosslinking linear chains of monomer crosslinks are shown by heavy lines.

The most investigated examples are to be formd in the precipitation of polyelectrolytes by metal ions. Here, networks are formed by the random crosslinking of linear polymer chains, and the theory requires some modification. The condition for the formation of an infinite network is that, on average, there must be more than two crosslinks per chain. Thus, the greater the length of a polymer chain the fewer crossUnks in the system as a whole are required. 

We can make polyurethanes via one- or two-step operations. In the single-stage process, diols and isocyanates react directly to form polymers. If we wish to make thermoplastic linear polymers, we use only diisocyanates. When thermosets are required, we use a mixture of diisocyanates and tri- or polyisocyanates residues of the latter becoming crosslinks between chains. In the first step of the two-stage process, we make oligomers known as prepolymers, which are terminated either by isocyanate or hydroxyl groups. Polymers are formed in the second step, when the isocyanate terminated prepolymers react with diol chain extenders, or the hydroxyl terminated prepolymers react with di- or polyisocyanates. 

A crosslinked system, including units of the diene type, can be considered as formed by linear chains (the initial chains) linked by randomly distributed bridging links. 

For low extents of crosslinking the curves lie between the limits of the curve near the gel point and that for linear chains. This behavior is understood when recalling that at low crosslinking the system mainly consists of linear chains. The highly crosslinked chain on the other hand approaches a molar mass independent master curve that, as expected, lies in the region of hard sphere behavior. 

The side groups and the repeating structure of the side groups change the chemical and physical properties of the polymer, and this defines the chemical and physical characteristics of the different polypeptide molecules. Not all natural macromolecules, however, are polymers. For example, insulin is a natural macromolecule with a molecular weight of 5733 kg/kg-mol. Insulin has long linear chains that are connected by 21 sulfur crosslinks. When it is decomposed 51 residual molecules result. Insulin is not a polymer because it does not have repeating units of monomers. 

The name of a crosslinking monomer molecule having two or more different types of polymerizable groups, each serving as a monomeric unit for one of two or more different linear chains, is cited with the name of each of the chains with the symbol v. 

Crosslinks can be controlled by the number of unsaturated sites in the polyester prepolymer. Theoretically if each molecule has only two reaction sites, then infinite, almost linear, chains could be obtained. Hence, average functionability and molecular weight distribution in the prepolymer are extremely important. Plasticizers can be used to advantage in adjusting the average properties of the binder as obtained in the solid propellant formulation. 

The 13C NMR crosslink density results were compared with the crosslink density obtained by the mechanical measurements. In the determination of the crosslink density by mechanical methods, the contributions of the topological constraints on the results were neglected and the density was expressed as G/2RT. The 13C and mechanical-crosslink densities were obtained for both sulfur and dicumyl peroxide (DCP)-cured samples to ensure the effect of wasted crosslinks (pendent or intramolecular type sulfurisations), which are expected in the typical sulfur-vulcanisation of NR. In the major range of crosslink densities, the crosslink densities for those two systems are described by the same linear function with a slope of 1.0. Based on these observations, it is shown that the crosslink density of the sulfur-vulcanised NR as determined by 13C is identical with the true crosslink density, and the influence of the wasted or ineffective crosslinks (pendent and cyclic crosslinks) and chain ends is negligible. However, this conclusion seems to be only valid if the effect of topological constraints or entrapped entanglements on the mechanical modulus is negligible which is rarely the case in real systems. 

In equation 1, a dicarboxylic acid derivative called an anhydride (literally without water ) reacts with ethylene glycol, a di-alcohol, to form a linear polyester and the byproduct water. Ethylene glycol is the major ingredient in most automotive radiator fluids. In equation 2, the ethylene glycol is replaced with a tri-alcohol, glycerol. As the reaction proceeds, the polyester does not form linear chains, but rather becomes crosslinked as the three OH groups react with phthalic anhydride, building up a three-dimensional structure. 

Polymerization can produce linear chains, but other structures can exist as well. As shown in Fig. 15.3 branched and crosslinked structures can be formed. Linear and branched structures can be shaped and reshaped simply by heating and are called thermoplastics. In the case of a crosslinked structure a three-dimensional network is formed that cannot be reshaped by heating. This type of structure is called a thermoset. 

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