Crosslinking of Polysiloxanes

The siloxane polymers can be tailored to contain a few percent of vinyl groups and these can be crosslinked with polymers that contain Si-H units. We will look at these types of functional polymers in the following section. 

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Curing is achieved either by random radical crosslinking of polysiloxanes by heating with peroxides or by room temperature vulcanization techniques making use of reactive end groups of the precursors. 

Peroxide crosslinking of the copolymer is more efficient than that of the homopolymer (Table 9-1). The process becomes a chain reaction (but with short kinetic chain length) involving polymerization of the pendant vinyl groups on the polysiloxane chains in combination with coupling of polymeric radicals. The crosslinking of EPDM rubbers is similarly more efficient when compared to EPM rubbers since the former contain double bonds in the polymer chain. 

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 ... 

Di-tert-butyl-4-(dichloromethylsUylpropoxy)phenol (120a) and/or 2-hydroxy-4-(dichloromethylsilylpropoxy)benzophenone (120b) were bound in the presence of water into siloxane blocks of the butadiene-siloxane rubber. Products formed reduced oxidative crosslinking of butadiene blocks of the latter [154]. Polysiloxanes containing 2,2,6,6-tetramethylpiperidine moieties were prepared too. 

All of these mechanisms which affect crosslink density were confirmed by experimental studies. The classic case of a reactive particle filler is silica filled polysiloxane (Figure 6.25). Silica particles have numerous OH groups which react with the crosslinking component of polysiloxane. Modification of silica by silanes reduces reinforcement. 

These rubbers are based on atoms of silicon chains rather than carbon atoms. Their unique structure is responsible for their extreme temperature properties. The most common types of silicone rubbers are specfically polysilaxanes. The Si-O-Si bonds can rotate much more freely than the C-C bond or the C-O bond. So the silicone chain is much more flexible and less affected by temperature. Silicone rubber is vulcanised by the action of peroxides which crosslink the chains by abstracting hydrogen atoms from the methyl side groups, allowing the resulting free radicals to couple into a crosslink. Some varieties of polysiloxanes contain some vinyl methyl siloxane units, which permit sulfur vulcanisation at the double bonds. 

The process is initiated at terminal hydroxy groups and favoured by the spiral-like structure of polysiloxanes. Replacement of the hydroxy groups by methyl, or blocking them by chelation to copper, iron or zirconium acetylacetonates, considerably decreases the rate of decomposition of the polymer and increases its thermal stability (Table 9). However, pronounced crosslinking even at moderate temperatures was observed in the polymer stabilized by transition metal compounds. The effect of the metal additives during thermal ageing is associated with reactions leading... 

A similar synthetic way to LBL electrodeposition of PT film has been designed by Advincula and coworkers [338]. They described electrodeposition of polysiloxane precursor polymer to form crosslinked conjugated PT ultrathin film. In this study, the precursor polymer contained a polysiloxane backbone with pendant electro active thiophene monomer, which was electropolymerized by cyclic voltammetry. The morphology of PT film was transformed from a relatively globular to membrane-like shape with increasing the number of cycles. 

R. Xing, W.S.W. Ho, Crosslinked polyvinylalcohol-polysiloxane/fumed siUca mixed matrix membranes containing amines for CO2/H2 separation. Journal of Membrane Science 367 (2011) 91-102. 

The physical properties of polysiloxanes are relatively unaffected by temperature owing to their exceptionally weak inter-molecular forces. High-molecular chain-like compounds of this series are always liquids, and their glass transition temperatures may be as low as 150 K. They fail to meet the requirement of sufficient mechanical strength at room temperature. This weakness can be overcome by crosslinking the chains by "hot vulcanization." Peroxides of the usual type are added to the polydimethyl siloxanes followed by curing at 70 to 120 C (343 to 393 K) see Fig. 12. 

The basic elastomer for silicone rubber is a linear polysiloxane, buUt from dimethyl sUoxane units with a small amotmt methylvinyl sUoxane groups. The ends of the chains are formed from trimethyl sUoxane- or dimethylvinyl sUoxane groups. The structure of the basic elastomer is shown in Fig. 24.1. Peroxides are usuaUy used for the crosslinking of silicone rubbers (for example dicumyl peroxide or 2,4-dichlorobenzoyl peroxide). 

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