Rubbers silicone, cross-linking

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. 

Core, cross-linked silicone rubber inner shell, cross-linked acrylate elastomer outer shell, styrene-AN copolymers Lindner et al. 1990... 

Silicone polymers can be cross-linked (silicone rubber) by different mechanism (e.g. at high temperatures by peroxides, at room temperatures by condensation reaction). 

The two major types of peroxides used for cross-linking elastomers are diaJkyl peroxides and peroxyketals. A third class, diacyl peroxides as exemplified by benzoyl peroxide are used almost exclusively in cross-linking silicone rubbers. 

A polymer is a material composed of large macromolecules. These macromolecules are formed by chains of hundreds or thousands of connected (polymerized) monomer molecules. The three main classes of polymers are thermoplastics, elastomers and thermosets. They differ in the degree of cross-linking of their macromolecules -from no cross-linking (thermoplastics) to moderate cross-linking (elastomers, rubbers) to high cross-linking (thermosets). Thermoplastics commonly used in microfluidics include materials like polymethylmethacrylate (PMMA), polycarbonate (PC), polyethylene terephthalate (PET) or cyclic olefin copolymers (COC). Silicones (like poly-dimethylsiloxane, PDMS) are a typical class of elastomers. Thermosets include photoresist materials like SU-8 and others. 

Silicones are a large group of compounds that include large polymers containing silicon. Depending on the formula and the degree of polymerization and cross-linking of the polymers, they may be slippery liquids, waxes, or rubbers. 

Dibutyltin diacetate, dilaurate, and di-(2-ethylhexanoate) are used as homogeneous catalysts for room-temperature-vulcanizing (RTV) silicones. The dialkyltin compounds bring about the cross-linking of the oligomeric siloxanes, to produce flexible, silicone rubbers having a host of different uses, such as electrical insulators and dental-impression molds. Recent work has also shown (560) that various dibutyltin dicar-boxylates catalyze both the hydrolysis and gelation of ethyl silicate under neutral conditions. 

Ethylene-propylene and silicone rubbers are crosslinked by compounding with a peroxide such as dicumyl peroxide or di-t-butyl peroxide and then heating the mixture. Peroxide cross-linking involves the formation of polymer radicals via hydrogen abstraction by the peroxy radicals formed from the decomposition of the peroxide. Crosslinks are formed by coupling of the polymer radicals... 

Figure 18 Calculated stress-re la at ion curves for styrene-butadiene and silicone rubbers, both uncross-linked (from Figure 17) and cross-linked to vr - 50 x 10 6 mol/cm, and for SBR additionally cross-linked to v, = 100 and 200 x 10 mol/cml. The horizontal bars show the location of the equilibrium modulus for SBR. M - 200.1100. T - 29 K.

Figure 18.4. Cross-linking of silicone rubber via hydrosilylation...

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