Polydimethylsiloxane cross-linking

Alexandru, M. Cazacu, M. Vlad, A. Macsim, A. M. Hitruc, G. lonita, D., Polydimethylsiloxane Cross-linked by 4,4 -(l,3-phenylenefioxy)-Dianiline Within In Situ Generated Silica Network. Polym-Plast. Tech. Eng. 2011,50, 539-546. 

The organotin sdanolate can then react with the polydimethylsiloxane diol by either attack on the SiOC bond or by sdanolysis of the SnOC bond (193,194). Other metal catalysts include chelated salts of titanium and tetraalkoxytitanates. Formation of a cross-linked matrix involves a combination of the three steps in equations 24—26. 

The most widely used sUicones are polymers of methyl(hydrogen)sUoxane and of dimethylsiloxane. Polydimethylsiloxane is the basic polymer used in sUicone repeUents. If the polymer is terminated with methyl groups it is inert however, if it is terminated with hydroxyl groups, it can be cross-linked. Continuous, durable coatings result from the use of curable blends of polydimethyl siloxane and polymethyl(hydrogen)sUoxane. The sUicone finish encapsulates individual fibers. 

A typical condensation system involves the reaction of a silanol-terminated polydimethylsiloxane with a multi-functional organosilicon cross-linking agent such as Si(RO)4 Figure 29.8). Pot life will vary from a few minutes to several hours, depending on the catalysts used and the ambient conditions. Typical catalysts include tin octoate and dibutyl tin dilaurate. 

Figure 8. SANS measurements of R /Fg° and R /Rg° versus A of tetrafunctional polydimethylsiloxane networks. Mw and Mn are weight and number average molecular weights before crosslinking, cp is the polymer fraction in solution before cross-linking. Data from Ref. 20.

There are now a number of techniques which may be used to prepare elastomeric networks of known structure Q-8). Two particularly useful and convenient ones involve the multi-functional end-linking of hydroxyl-terminated (4-16) or vinyl-terminated polydimethylsiloxane (PDMS) chains (3,17-21), and the cross-linking of PDMS chains through vinyl side groups present in known amounts and in known locations along the chains (4,18,22-25). A typical reaction of this type is... 

Networks with tri- and tetra-functional cross-links produced by end-linking of short strands give moduli which are more in accord with the new theory if quantitative reaction can be assumed (3...13) However, the data on polydimethylsiloxane networks, may equally well be analyzed in terms of modulus contributions from chemical cross-links and chain entangling, both, if imperfect reaction is taken into account (J 4). Absence of a modulus contribution from chain entangling has therefore not been demonstrated by end-linked networks. 

Dense membranes are a special type of polymeric membranes. Jacobs et al. published on the use of polydimethylsiloxane (PDMS) dense membranes in the hydrogenation of dimethylitaconate and acetophenone using standard homogeneous catalysts (see Section 4.6.1)[48]. The membranes were homemade from a PDMS solution in hexane, which was cross-linked in a vacuum oven at 100°C. The membranes were able almost completely to retain unmodified Ru-BINAP dissolved in isopropanol. However, as mentioned earlier, these applications will strongly depend on the size, i.e. molecular weight, of the substrate to be converted in order to guarantee a sufficient difference in size of the product and the catalyst to be retained. 

For the most common siloxanes, polydimethylsiloxanes (PDMS), R=Me. Thus, most common cyclic siloxanes would be referred to as Dn, for example, D3, D4, D5,. .. linear oligomers are often called MD M, for example, MM, MDM, MD2M,. .. and, T and Q units constitute parts of many highly branched oligomers and polymers, or cross-linked networks. 

Recently, Genzer and coworkers [85] presented an interesting new approach for the preparation of stable silane-based SAM systems. As a substrate, cross-linked polydimethylsiloxane (PDMS) was oxidized by UV/ozone treatment to yield a thin sihcon dioxide surface. The surface was then treated with fluorinated alkyltrichlo-rosilanes from the gas phase while being mechanically stretched by a certain length Ax. After modification, the elastomer was allowed to relax resulting in a mecdianically assembled monolayer (MAM) at the surface (Fig. 9.8). 

Room temperature-vulcanizing (RTV), silicon rubbers make use of the room temperature reaction of certain groups that can be placed on polydimethylsiloxanes, which react with water. When exposed to water, such as that normally present in the atmosphere, cross-links are formed creating an elastomeric product. 

When tested in other polymers, maleimides did not affect the rate of cross-linking in polydimethylsiloxane, polyisobutylene, and polyvinylchloride. In polyethylene, the addition of 5 wt.% of m-phenylene dimaleimide increased the G(X) from 1.8 to 7.2. In the polyvinylacetate the effect was even more pronounced the dose for gelation was reduced by about a factor of 50. Contrary to the cross-link enhancing effect found for m-phenylene dimaleimide, cross-linking was retarded in polyvinyl acetate by the addition of monomaleimides. When analyzing the mechanism of the reaction it was concluded that monomaleimides are not expected to affect cross-linking in saturated polymers. ... 

There are a large variety of elastomers based on a chain -Si-O-Si- with different groups attached to the Si atom, that affect the properties of the polymer. Technologically, the most widely used silicone elastomers are those with all methyls on the silicone atoms, i.e., polydimethylsiloxanes (PDMSs), or ones with less than 0.5 mol% of vinyl substitution for the methyls. Irradiation of PDMS produces hydrogen, methane, and ethane. The gas yield at room temperature correlates with the concentration of cross-links formed. This can be expected since double bonds cannot be formed. 

The tensile strength of polydimethylsiloxane irradiated by a dose of 60 kGy (6 Mrad) is 15% lower than that of PDMS cross-linked by peroxide, but, when 0.14 mol percent of vinyl unsaturation is substituted for the methyl groups, the tensile strength is 30% higher. Irradiation of PDMS containing 55 phr silica filler by a dose of 40 kGy exhibited an optimum tensile strength of 8 MPa (1160 psi) and excellent thermal stability at 523 (482°F).209... 

On irradiation polydimethylsiloxane (PMDS) rapidly hardens, due to cross-linking, with evolution of ethane, methane and hydrogen. The introduction of aromatic substituents into the siloxane structure is found to increase resistance to cross-linking (3-6). The relative ease with which crosslinks form in a series of polysiloxanes being ... 

The two component FID (and corresponding superposed broad line spectra), for some polydimethylsiloxanes (PDMS) arises from crystalline and amorphous components. Polymers highly crosslinked with dicumyl peroxide exhibit quite similar data, but after the removal of all unlinked material, the FID consists only of a single component, indicating an almost complete reduction in crystallinity due to the cross-linking 84). 

Only little work has been focused on the action of ozone onto non-hydrocarbon polymers. This area of research has been investigated by Karandinos et al. [148] who cross-linked polydimethylsiloxanes (PDMS) bearing silane groups Si-H after ozonization. These workers studied the reaction shown in Scheme 41. 

For structural integrity, SPME sorbents are most commonly immobilized by coating onto the outside of fused silica fibers or on the internal surface of a capillary tube. The phases are not bonded to the silica fiber core except when the polydimethylsiloxane coating is 7 pm thick. Other coatings are cross-linked to improve stability in organic solvents [135], De Fatima Alpendurada [136] has reviewed SPME sorbents. 

Silicone polymers commonly have cross-linked network structures created by reacting a reactive prepolymer with a cross-linking agent. For example, a hydroxy-terminated polydimethylsiloxane is cross-linked with a methoxy-substituted silane and a catalyst as follows ... 

Using results such as these, c.ie can predict the Tg of any polymer for which the group contributions are known. The glass transition temperature for polydimethylsiloxane, for example, is Tg = (1/2) 20 + (1/2) 280 = 150 K = -123 0. The approach was originally applied to linear polymers but has been extended to cross-linked polymers as well (24) ... 

The reaction is of practical importance in the vulcanization of silicone mbbers (see Rubbercompounding). Linear hydroxy-terminated polydimethylsiloxanes are conveniently cross-linked by reaction with methyldiethoxysilane or triethoxysilane [998-30-1]. Catalysts are amines, carboxylic acid salts of divalent metals such as Zn, Sn, Pb, Fe, Ba, and Ca, and organotin compounds. Hydroxy-terminated polysiloxanes react with Si—H-containing polysiloxanes to... 

Silicone Room Temperature Vulcanizing Gross-Linking. Condensation-cured polydimethylsiloxanes contain terminal silanol groups which condense with the silanols produced by ambient moisture hydrolysis of acyloxysilanes. Methyltriacetoxysilane, ethyltriacetoxysilane, and tetraacetoxysilane are the most commonly used cross-linking agents. 

The state-of-the-art silicone systems used in label stock application are normally solventless and thermal curing. Base polymers for these systems are vinyl-functionalized polydimethylsiloxanes having viscosities of around 200 - 600 mPa.s. Cross-linkers normally are hydride-functionalized polydimethylsiloxanes with a viscosity of around 25 mPa.s. These two components are cross-linked by a platinum catalyst, which can be the Karstedt catalyst. Additionally an inhibitor is added to the silicone mixture to prevent curing before it is applied on the substrate. These inhibitors ate... 

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