Matrix, silicone rubber

Liquid membranes in this type of ion-selective electrodes are usually heterogeneous systems consisting of a plastic film (polyvinyl chloride, silicon rubber, etc.), whose matrix contains an ion-exchanger solution as a plasticizer (see Fig. 6.5). 

The membranes of the other ion-selective electrodes can be either homogeneous (a single crystal, a pressed polycrystalline pellet) or heterogeneous, where the crystalline substance is incorporated in the matrix of a suitable polymer (e.g. silicon rubber or Teflon). The equation controlling the potential is analogous to Eq. (6.3.9). 

This process involves the suspension of the biocatalyst in a monomer solution which is polymerized, and the enzymes are entrapped within the polymer lattice during the crosslinking process. This method differs from the covalent binding that the enzyme itself does not bind to the gel matrix. Due to the size of the biomolecule it will not diffuse out of the polymer network but small substrate or product molecules can transfer across or within it to ensure the continuous transformation. For sensing purposes, the polymer matrix can be formed directly on the surface of the fiber, or polymerized onto a transparent support (for instance, glass) that is then coupled to the fiber. The most popular matrices include polyacrylamide (Figure 5), silicone rubber, poly(vinyl alcohol), starch and polyurethane. 

These incorporate membranes fabricated from insoluble crystalline materials. They can be in the form of a single crystal, a compressed disc of micro-crystalline material or an agglomerate of micro-crystals embedded in a silicone rubber or paraffin matrix which is moulded in the form of a thin disc. The materials used are highly insoluble salts such as lanthanum fluoride, barium sulphate, silver halides and metal sulphides. These types of membrane show a selective and Nemstian response to solutions containing either the cation or the anion of the salt used. Factors to be considered in the fabrication of a suitable membrane include solubility, mechanical strength, conductivity and resistance to abrasion or corrosion. 

Jia and coworkers prepared thin-film composite zeolite-filled silicone rubber membranes by a dip-coating method [82]. The membranes have a thin silicalite-1/ silicone rubber mixed-matrix selective layer on top of a porous polyetherimide support. 

Hirata and Date [62] built an insoluble precipitate (Cuj S) into a silicone rubber or epoxide resin matrix and deposited the mixture on a metal wire, thus... 

In practice, three types of membrane based on silver halides are used. The oldest type is based on silver halide precipitate in a matrix of silicone rubber. 

The Ag2 S ISEs used in practice are of three kinds with a heterogeneous membrane containing Ag2 S precipitate (preferably precipitated from excess sulphide or by the action of H2 S [228]) in a matrix of silicon rubber [346] or a thermoplastic material [248], with a pressed pellet as a membrane [235,... 

Cd ion-selective electrodes The usual version of the Cd " ISE contains a membrane of a sintered or pressed mixture of CdS and Ag2 S [121, 325,408]. Membranes from sintered Ag2S, CuS and CdS mixtures [157] have also been proposed, similarly as forPb ISEs. CdS precipitate in a polyethylene matrix [250] or a CdS-Ag2S precipitate mixture in a silicone rubber matrix [153] can also be used for Cd ISEs. Cd ISEs can be calibrated using a metal diethylenetriamine buffer [66]. Similar substances interfere in the response of the Cd ISE as for the Hg, Ag and Cu electrodes. 

In the electrode with CuS alone, which has negligible conductivity, the precipitate ISE with a silicone rubber matrix has better properties than the electrode with a pressed pellet [314], The ISE with a mixture of CuS and Agi S finds broad application [325]. If the membrane is prepared by pressing, the grains of these two compounds combine to form jalpaite, Agi.55C%.4sS [180], This substance is a mixed conductor with transport numbers of Ag, 0.69 Cu(I), 0.30 and electrons, 0.01, at 25 C [175]. The sintered electrode also contains Ag1.2Cuo.8S or Ago.93Cu1.07S. Oxidation of these phases leads to considerable deterioration in the electrode function [180]. Good electrodes... 

Fig. 7.4. Fracture toughness (O) and flexural strength ( ) of silicone rubber coated carbon fiber-epoxy matrix composites as a function of coating thickness. After Hancox and Wells (1977).

Graftcopolymerization onto silicone rubber is rather difficult to achieve and is often accompanied by unwanted changes in physico-mechanical properties of the polymer caused by initiating agents. To overcome the problem, silica was introduced into the rubber matrix as an active filler capable of binding cationic compounds such a cationic compound being y-aminopropyltriethoxysilane. Schematically the pathway for heparinization of the latter may be presented as follows ... 

An alternative method of representing the movement of an individual molecule by computational techniques is shown in Figure 2.4 [9], This figure shows the movement of three different permeate molecules over a period of 200 ps in a silicone rubber polymer matrix. The smaller helium molecule moves more frequently and makes larger jumps than the larger methane molecule. Helium, with a molecular diameter of 2.55 A, has many more opportunities to move from one... 

Lopour P et al. (1993) Silicone rubber-hydrogel composites as polymeric biomaterials. IV Silicone matrix-hydrogel filler interaction and mechanical properties. Biomaterials 14(14) 1051—1055... 

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