Gas Permeability and Optical Sensing

Permeability (P) is defined as the product of solubility (S) and diffusivity (D) of the gas into the polymer membrane and is temperature and pressure dependent. The permeability, diffusivity and solubility values of different gases in polydimethylsiloxane (PDMS) are collected in Table 22.1. 

The effect of molecular structure on the permeability of the silicone film has been studied extensively in the past [62, 59, 63-66]. In one of the pioneering studies of silicone permeability, Robb demonstrated that the substitution of the methyl group by a phenyl one in the PDMS significantly reduces the gas permeability of the material The introduction of such a 

Gas Temp. [°C] Permeability [Barter] Solubility [cm (STP)/cm atm] Diflfusivityx 10 [cm7s] Ref. 

As expected, the degree of crosslinking in the cured sih-cone has an effect over the permeability of the polymer. A certain degree of crosslinking is needed for a proper stabihty and durability of the membrane but, at the same time, an excess of crosslinking will decrease the permeability. The higher the crosslinking, the lower the siloxane permeability [47,66,68]. 

The addition of filler into the silicone matrix also has a noticeable effect in the gas dynamics, reducing permeabihty in proportion to its volume fraction [62, 69]. Fillers enhance the solubility of gases but decrease the diffusivity in such a way that the permeability coefficient in the reinforced elastomers is smaller than in the filler-free silicone as can be observed in Table 22.2. This behavior has been demonstrated not only for PDMS but also for polyfmethylvinyl siloxane) (PMVS) [70]. 

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