Extent of cross-linking

The relative selectivities of strongly acid and strongly basic polystyrene resins, with about 8 per cent DVB, for singly charged ions are summarised in Table 7.2. It should be noted that the relative selectivities for certain ions may vary with a change in the extent of cross-linking of the resin for example, with a 10 per cent DVB resin the relative selectivity values for Li+ and Cs+ ions are 1.00 and 4.15, respectively. 

In summary, none of the individual components necessary for recombinant resilin curing were cytotoxic, and there were no leachables from the cured resilin that caused cell death. The cured resilin polymer was not a good surface for cell adhesion, but cells can survive and proliferate in the resilin on a gelatine bead. The curing of recombinant resilin in the presence of cells on beads has no effect on the cells ability to migrate and proliferate with new tissue formation. The resilin is seen to degrade with time, but it is believed that this could be controlled by the type and extent of cross-linking. 

AnUine, however, is too toxic for use in mbber products. Its less toxic reaction product with carbondisulfide, thiocarbanihde, was introduced as an accelerator in 1907. Further developments led to guanidine accelerator [4]. Reaction products formed between carbon disulfide and aliphatic amines (dithiocarbamates) were first used as accelerators in 1919 [5]. These were and still are the most active accelerators in respect to both cross-finking rates and extent of cross-link formation. However, most dithiocarbamates accelerators give little or no scorch resistance and therefore cannot be used in aU applications. 

Radiation-induced cross-linking of chlorinated and chlorosulfonated high-density PE has been reported by Korolev et al. [262]. It has been observed that the extent of cross-linking is strongly dependent on the chlorine content in the sample, chlorosulfonated PE is cross-linked more readily than the chlorinated sample in air and inert atmosphere. 

These simplified relationships offer a clearer insight into the dependence of the equilibrium swelling ratio qm on the quality of the solvent as expressed by Xh on the extent of cross-linking. Because of the nature of the approximations introduced to obtain Eqs. (40) and (40 ), their use as quantitative expressions must be limited to networks of very low degrees of cross-linking in good solvents. 

These measurements for the first time allowed experimental access to the microscopic extent of cross-link fluctuations. The observed range of fluctuation is smaller than predicted by the phantom network model, for which... 

The best results were obtained with dipentylamine-modified polyborazylenes. The dipentylamino substituents limit the extent of cross-linking during thermolysis. As a consequence, dipentylamine-modified polyborazylene was soluble in hydrocarbons... 

Figure 9. Dependence of the Mooney-Rivlin constant on extent of cross-linking. Mn is 21,600 PDMS. Key , 2C, X, 2CS (3).

The gel point is defined as the point at which the entire solid mass becomes interconnected. The physical characteristics of the gel network depends upon the size of particles and extent of cross-linking prior to gelation. Acid-catalysis leads to a more polymeric form of gel with linear chains as intermediates. Base-catalysis yields colloidal gels where gelation occurs by cross-linking of the colloidal particles. 

Dextran Gels Proteins and nucleotides can be separated by using cross-linked dextran gels available in various types and particle sizes. The molecular weight of dextran-gels vary considerably depending upon the extent of cross-linked nature. 

A comparison with cross-linker 4a proves the underlying dynamics are controlled by metal-ligand dissociation. Ligand exchange kinetics for 4a are substantially faster than for 4b but the association thermodynamics are very similar, and the effect of those kinetics is dramatic. At 5% cross-linker, the dynamic viscosity of lOOmgmL 4a-PVP is only 6.7 Pa s, a factor of 80 less than that of the isostmctural network 4b PVP. Although the association constants are not identical, the effect of the thermodynamics would be to increase the viscosity of 4a PVP relative to 4b PVP, the opposite direction of that observed. The kinetics dominate even the extent of cross-linking 5% 4a PVP is less viscous by a factor of 5 than is 2% 4b PVP. 

The assumption that volume remains constant during deformation is not entirely correct the change in volume is related to the extent of cross-linking, etc. For most polymers this change in volume AKis related to Poisson s ratio P as follows ... 

Phenolic There are various kinds of phenolic resins. The ones that are produced from phenol-formaldehyde condensation are veiy weak acid exchangers, where the phenolic -OH groups are the fixed-ionic groups. The formaldehyde content decides the extent of cross-linking in the resin. On the other hand, phenolsulfonic acid resins contain both strong acid -S03H and weak acid -OH groups. 

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