Elastomers cross-linking

As pointed out earlier, an elastomer cross-linked above its gel point will not dissolve in a solvent, but will absorb it and swell. The swelling will continue until the forces of swelling balance the retroactive forces of the extended chains of the network. The cross-link density can then be calculated from the degree of swelling using the Flory-Rehner equation ... 

In contrast, organophilic PV membranes are used for removal of (volatile) organic compounds from aqueous solutions. They are typically made of rubbery polymers (elastomers). Cross-linked silicone rubber (PDMS) is the state-of-the-art for the selective barrier [1, 43, 44]. Nevertheless, glassy polymers (e.g., substituted polyacetylene or poly(l-(trimethylsilyl)-l-propyne, PTMSP) were also observed to be preferentially permeable for organics from water. Polyether-polyamide block-copolymers, combining permeable hydrophilic and stabilizing hydrophobic domains within one material, are also successfully used as a selective barrier. 

If a modulus is plotted as a function of temperature, a very characteristic curve is obtained which is different in shape for the different types of polymer amorphous (glassy) polymers, semi crystalline polymers and elastomers (cross-linked amorphous polymers). 

In order to get an estimate of the adhesion energy between a pseudo-brush and an elastomer (cross-linked in the dry state), we can use the concentration profile calculated [65] for the case of a pseudo-brush exposed to a melt (Nc monomers per chain)... 

Elastomers Cross-linked solids. Reinforcement necessary for property performance Resins Highly branched cross-linked solids or fluids... 

A preformed linear polymer (in molten, solution, or emulsion states), or A polymer network, physically (e.g., semi crystalline linear polymer) or chemically (e.g., thermosets, elastomers) cross-linked. 

Peroxyketals. Peroxyketals, such as n-butyl-4,4-di-( -butylperoxy) valerate and l,l-di-(f-butyl peroxy)-3,3,5-trimethylcyclohexane, are high-temperature peroxides used in selective applications for PE and elastomer cross-linking and in the curing of unsaturated polyester. 

As pointed out by Hajji et al. [32], polymer blend nanocomposite systems can be prepared by various synthesis routes because of their ability to combine in different ways to introduce each phase. The organic component can be introduced as (1) a precursor, which can be a monomer or an oligomer (2) a preformed linear polymer (in molten, solution, or emulsion states) or (3) a polymer network, physically (eg, semicrystalline linear polymer) or chemically (eg, thermosets, elastomers) cross-linked. The mineral part can be introduced as (1) a precursor (eg, tetraethyl orthosilicate) or (2) preformed nanoparticles. Organic or inorganic polymerization generally becomes necessary if at least one of the starting moieties is a precursor. 

Figure 7.1 schematically shows the preparation of networks by cross linking in solution followed by removal of the solvent. Success in obtaining elastomers with fewer entanglements is supported by the observation that such networks come to elastic equilibrium much more rapidly than elastomers cross linked in the dry state. Table 7.1 shows results on PDMS networks cross linked in solution by means of y radiation. - Note the continual decrease in the time required to reach elastic equilibrium, and in the extent of stress relaxation as measured by the ratio of equilibrium to initial values of the reduced stress, [f ], upon decrease in the volume... 

Removal of the solvent has the additional effect of putting the chains into a "supercontracted state (figure 7.3). Experiments on strain-induced crystallization carried out on such solution cross-linked elastomers indicate that the decreased entangling is less important than the supercontraction of the chains. Crystallization required larger values of elongation compared to elastomers cross linked in the dry state. - The most recent work in this area has focused on the unusually high extensibilities of such elastomers. -102-107... 

Erman, B. Mark, J. E., Interpretation of Stress-Strain Isotherms for Elastomers Cross-Linked in Solution. Macromolecules 1987,20, 2892-2897. 

A final technique that has been utilized to chemically remove residual styrene in PS is radiation treatment. Both beta (e-beam) and gamma radiation have been tried. E-beam appears to be the most effective form of radiation and is most suitable for continuous use (319,320). The e-beam ruptures C—H bonds, resulting in the formation of PS radicals. These radicals are very reactive and scavenge unreacted monomer. However, if no styrene is in the vicinity of the PS radical, it can do other things such as couple with another PS radical or react with oxygen. Currently, electron beam treatment of polymers is used commercially for elastomer cross-linking (wire/cable coatings) but not for monomer reduction. 

Depolymerization (or reversion) occurs essentially at high temperatures, only in linear polymers having weak monomer-monomer bonds, or in tridimensional polymers having weak cross-link junctions (see Table 12.2). These are linear polymers containing the weakest aliphatic C-C bonds, i.e. involving tetrasubstituted carbon atoms, e.g. polyisobutylene (PIB), poly(methyl methacrylate) (PMMA), poly(or-methyl styrene) (PorMS), etc. These are also linear polymers containing heteroatoms, e.g. poly(oxy methylene) (POM), poly(ethylene terephthalate) (PET), poly(vinyl chloride) (PVC), etc., but also sulphur vulcanized elastomers. Cross-linking predominates mainly in unsaturated linear polymers, i.e. essentially polybutadiene and its... 

Verwey GC, WamM M (1997) Nematic elastomers cross-linked by rigid rod linkers. Macromolecules 30 4196-4204... 

Abstract Smart composites based on carbonyl iron powder, micro and nano size Fe304 in ethylene - propylene and acrylonitrile - butadiene rubber were manufactured and studied. Elastomer samples with various volume fractions of magnetic particles were tested. To improve dispersion of applied fillers in polymer matrix, ionic liquids were added during the process of composites preparation. To align particles in elastomer, cross-linking process took place in magnetic field. 

A sample of elastomer, cross-linked at room temperature, is swollen afterward to 10 times its original volume. Then it is stretched. What value of the front factor should be used in the calculation of the stress ... 

Cross-Linking in Solution. This approach (5) did produce elastomers that had fewer entanglements, as indicated by the observation that such networks came to elastic equilibrium much more rapidly than elastomers cross-linked in the dry state. When a network is cross-linked in solution and the... 

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