Moisture Sorption and Diffusion

The diffusivity and solubility of water in polymers differs from that of simple gases to the extent that the water molecule interacts with the polymer. Based on this interaction, the polymer molecules are classified as hydrophilic or hydrophobic. The solubility of water in hydrophobic polymers, such as polyolefins and some polyesters, is very low and the diffusivity follows the rules of the other simple gases mentioned in Section 4.2.1. On the other hand, hydrophilic polymers, such as cellulose, poly(vinyl alcohol), and proteins, interact strongly with water, and water diffusivity increases with its content. A good approximation for this situation is (Van Krevelen, 1990) 

The solubility of water in polymers can be estimated from contributions from individual groups of the structural unit. Table 4.15 presents these molar water contributions of various groups (Van Krevelen, 1990) in a manner similar to the permachor. The solubility in g/g or cm (STP)/g can easily be calculated from the contributions of Table 4.15. The molar volume of the structural unit is needed in the case where the solubility is needed in cm (STP)/cm. Finally, the heat of sorption is about 25 kJ/mol for nonpolar polymers and about 40 kJ/mol for polar polymers. 

Example 4.9. Estimation of the Saturation Moisture Content of Polymers 

Estimate the saturation moisture content of poly(methyl acrylate) (PMA) and polystyrene (PS) at 25 °C and relative humidity of 50%. Compare these values with experimental values from Crank and Park (1968, p. 263). 

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Burnett, D. J., Garcia, A. R., and Thiehnann, R 2006. Measuring moisture sorption and diffusion kinetics on proton exchange membranes using a gravimetric vapor sorption apparatus. J. Power Sources. 160, 426-430. 

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