Dimethylsiloxane Silicones Silicone rubber

Whilst the Tg of poly(dimethylsiloxane) rubbers is reported to be as low as -123°C they do become stiff at about -60 to -80°C due to some crystallisation. Copolymerisation of the dimethyl intermediate with a small amount of a dichlorodiphenylsilane or, preferably, phenylmethyldichlorosilane, leads to an irregular structure and hence amorphous polymer which thus remains a rubber down to its Tg. Although this is higher than the Tg of the dimethylsiloxane it is lower than the so that the polymer remains rubbery down to a lower temperature (in some cases down to -100°C). The Tg does, however, increase steadily with the fraction of phenylsiloxane and eventually rises above that of the of the dimethylsilicone rubber. In practice the use of about 10% of phenyldichlorosilane is sufficient to inhibit crystallisation without causing an excess rise in the glass transition temperature. As with the polydimethylsilox-anes, most methylphenyl silicone rubbers also contain a small amount of vinyl groups. 

Similar types of lamellar morphologies were observed for triblock copolymers of diphenylsiloxane and dimethylsiloxane having 40 wt% polydiphenylsiloxane, using electron microscopy, 47-148>. The lamellae thickness was approximately equal to the chain length of the rigid polydiphenylsiloxane blocks. These copolymers showed elastomeric properties comparable to those of conventional silica-reinforced, chemically crosslinked silicone rubbers. Tensile tests yielded an initial modulus of 0.5-1 MPa, tensile strength of 6-7 MPa and ultimate elongation between 400 and 800 %. 

Foly(dimethylsiloxane) (silicone rubber) membranes are fabricated by hydrolysis of alkox-ysilyl terminal groups of silicone-rubber precursors [oligo(dimethylsiloxane) derivatives and crosslinking agents], followed by condensation. Covalent bonding of neutral carriers carrying an alkoxysilyl group to silicone rubber is, therefore, feasible by simple reaction of the silicone-rubber precursor with alkoxysilylated neutral carriers, as schematically shown in Scheme 1 [44]. 

The design of bioeompatible (blood compatible) potentiometric ion sensors was described in this chapter. Sensing membranes fabricated by crosslinked poly(dimethylsiloxane) (silicone rubber) and sol gel-derived materials are excellent for potentiometric ion sensors. Their sensor membrane properties are comparable to conventional plasticized-PVC membranes, and their thrombogenic properties are superior to the PVC-based membranes. Specifically, membranes modified chemically by neutral carriers and anion excluders are very promising, because the toxicity is alleviated drastically. The sensor properties are still excellent in spite of the chemical bonding of neutral carriers on membranes. 

X-ray diffraction techniques can be used to establish the structure of crystalline polymers. Measurements are typically made on crystalline lamellar platelets grown from dilute solution, fibres or stretched films. Such methods have been applied to several different inorganic polymers. For example, based on measurements on stretched samples of silicone rubber, poly(dimethylsiloxane) (Me2SiO) has been shown to possess a helical conformation (Figure 8.4). ... 

The Norplant contraceptive implant is a set of six flexible, closed capsules made of a dimethylsiloxane/ methylvinylsiloxane copolymer containing levonorgestrel. The silicone rubber copolymer serves as rate-... 

Another utilized approach for preventing reversion involves crosslinking of the siloxanes. For Instance, a number of methyl silicone rubbers were crosslinked, (about 1 crosslink per 3,000 to 20,000 units (10-21) for crosslinked products mainly composed of dimethylsiloxane Refs. IJ and I8). DSC and TM was utilized to evaluate the thermal properties of the products. TM was utilized to determine crosslink density along with typical modulus properties. Both enhancement and decrease in thermal properties were observed depending on the mode and conditions of crosslinking. 

Implanted polymeric materials can also adsorb and absorb from the body various chemicals that could also effect the properties of the polymer. Lipids (triglycerides, fatty acids, cholesterol, etc.) could act as plasticizers for some polymers and change their physical properties. Lipid absorption has been suggested to increase the degradation of silicone rubbers in heart valves (13). but this does not appear to be a factor in nonvascular Implants. Poly(dimethylsiloxane) shows very little tensile strength loss after 17 months of implantation (16). Adsorbed proteins, or other materials, can modify the interactions of the body with the polymer this effect has been observed with various plasma proteins and with heparin in connection with blood compatibility. 

Most joint replacements utilize polymers to some extent. Finger joints usually are replaced with a poly(dimethylsiloxane) Insert and over h00,000 such replacements are made each year (l). More recently a poly(1, -hexadiene) polymer has been tried in this application (l). Many other parts of the hand, such as the bones, have also been replaced by silicone rubber. Other types of joints, such as the hip or the knee, often involve the contact of a metal ball or rider on a plastic surface which is usually made from high density, high molecular weight polyethylene. These metal and plastic parts are usually anchored in the body using a cement of poly(methyl methacrylate) which is polymerized in situ. Full and partial hip prostheses are implanted about... 

Polysiloxane or Silicone Rubber (Q). The polymer backbones of sihcone rubbers contain no carbon atoms, but they contain alternating silicon and oxygen atoms. The predominate structural unit is generally dimethylsiloxane, as follows ... 

Hexachloroplatinic acid and other platinum complexes are mainly used as soluble catalysts for the additive cure. The most active catalyst used recently for vulcanization of silicon rubber is the platinum-alkenylsiloxanes complex, mainly the platinum-vinylsiloxane complex (Karstedt s catalyst) (4). One important approach to the activated cure of silicone rubber makes use of various inhibitors or moderators added to the platinum catalyst to reduce, or temporarily inhibit, its catalytic activity in the presence of the alkenyl- and hydropolysiloxanes (see catalysis by Pt complexes). The catalyst is usually added to the reaction mixture in quantities related to the number of unsaturated (e.g., vinyl) substituents in the polysiloxane. Vinyl-terminated polydimethylsiloxane polymers (viscosity > 200 cSt) are typically cross-linked by methylhydrosiloxane-dimethylsiloxane copolymer with 15-50 mol% of polymethylhydrosiloxane. A typical catalyst is a platinum complex in alcohol, xylene, divinylsiloxanes, or cyclic vinylsiloxanes. The system is usually prepared in two parts (part A, vinylsiloxane -I- Pt (5-10 ppm) part B, hydrosiloxane -I- vinylsiloxane). Inhibitors stop the platinum catalyst they are volatile or react with silicone hydride cross-linker to become a part of the polymer network. Some of them are decomposed by heat or light (UV). A single-component system contains fugitive inhibitors of Pt. 

The medical grade silicone rubber Sylgard 184 consists of a crosslinked poly-dimethylsiloxane. In vitro and in vivo testing has proven the non-cytotoxicity in direct and indirect contact. There is no ocular toxicity and no acute foreign body reaction in contact with soft tissues. 

In an elastomer (rubber), the macromolecules are cross-linked. The cross-linking degree is moderate so that elastomers can be stretched easily. A transparent elastomer widely used in microfluidics is the silicone rubber poly-dimethylsiloxane (PDMS). 

Oligomeric dimethyldisiloxanes 15 when reacted with vinyl-containing dimethylsiloxanes 16 gives copolymers with the vinylsiloxyl group, which in the presence of methylstyrene 17 and chloroplatinic acid gives a silicone rubber 18 (Scheme 6). ... 

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