Cross-link length

Sulfur-containing chemicals such as dimorpholinyl disulfide (DTDM) and tetraethylthiuram disulfide (TMTD) are not only effective accelerators, but they can also be used as sulfur donors. As such, they are effective ia controlling sulfur cross-link length to form primarily moao- and disulfide cross-links. These short cross-links are more thermally stable than conventional sulfur curing and thereby provide better heat and set resistance. 

Accelerator type Scorch safety Cure rate Cross-link length... 

Hg. 8.1 (a) Model of a network junction. The multiplicity k and cross-link length are two fundamental... 

Likewise, for performance, sometimes a sulfur donor is needed as well. Elemental sulfur gives rise to variable cross-link lengths, regulated to some extent by sulfur-to-accelerator ratio. In some applications, particularly those which require very high temperature and or oxidative stability, one may need a preponderance of cross-links of very low rank, mostly monosulfidic. For this purpose, organic sulfur donors may be useful (24). 

By combining random flight statistics from Chap. 1 with the statistical definition of entropy from the last section, we shall be able to develop a molecular model for the stress-strain relationship in a cross-linked network. It turns out to be more convenient to work with the ratio of stretched to unstretched lengths L/Lq than with y itself. Note the relationship between these variables ... 

As the length and frequency of branches increase, they may ultimately reach from chain to chain. If all the chains are coimected together, a cross-linked or network polymer is formed. Cross-links may be built in during the polymerisation reaction by incorporation of sufficient tri- or higher functional monomers, or may be created chemically or by radiation between previously formed linear or branched molecules (curing or vulcanisation). Eor example, a Hquid epoxy (Table 1) oligomer (low molecular weight polymer) with a 6-8 is cured to a cross-linked soHd by reaction of the hydroxyl and... 

Table 3. Effect of Increasing Cross-Link Number and Length ...

P-SO consists of heHcal chain molecules (26) of unknown length (21). a-SO is also a polymer similar to beta-SO, but probably in a layered cross-linked stmcture (20,21). Melting points, or more precisely triple points, of 32.5°C and 62.2°C have been given to the P- and a-polymers respectively... 

Termination. The conversion of peroxy and alkyl radicals to nonradical species terminates the propagation reactions, thus decreasing the kinetic chain length. Termination reactions (eqs. 7 and 8) are significant when the oxygen concentration is very low, as in polymers with thick cross-sections where the oxidation rate is controlled by the diffusion of oxygen, or in a closed extmder. The combination of alkyl radicals (eq. 7) leads to cross-linking, which causes an undesirable increase in melt viscosity. 

Bis-Pyndoxal Tetraphosphate. A second class of bifunctional reagents, described in 1988, involves two pyridoxal groups linked by phosphates of different lengths (89). As shown in Table 4, the yield of intramolecularly cross-linked hemoglobin increases dramatically with increasing length of the phosphate backbone. It is beheved that the site of reaction of (bis-PL) is between the amino-terminal amino group of one P-chain and the... 

Elastomers are a special sort of cross-linked polymer. First, they are really linear polymers with just a few cross-links - one every hundred or more monomer units - so that a molecule with a DP of 500 might have fewer than five cross-link points along its length. And second, the polymer has a glass temperature which is well below room temperature, so that (at room temperature) the secondary bonds have melted. Why these two features give an elastomer is explained later (Chapter 23). 

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