Relation of Water Vapor Permeability of Elastomers to Molecular Structure

This communication is a quantitative correlation of water vapor permeability of nonpolar elastomers with molecular structure. Permeability of physical blends and of composite layers of elastomers are also discussed. Permeability—Elastomer Structure.—Steric hindrance and unsaturation influence water vapor permeability of elastomers. The low permeability of isobutylene—isoprene (butyl) rubber is attributed to the presence of bulky methyl groups in its isobutylene component. The high unsaturation in isoprene and butadiene is blamed for the high permeability of natural rubber and styrene butadiene rubber. Hence, permeability should be correlated quantitatively with a factor which can evaluate and combine these two structural parameters. Defining the factors A and B to express the relative molar concentrations of unsaturated and steric groups in the average monomer unit of elastomers as (unsaturation) A=1+x/X (steric groups) B=1+y/Y