An Overview and Basic Scientific Concepts

The total free volume Vf consists of an agglomeration of micro volumes whose specific distribution varies with the polymeric configurations and depends on external pressure. For polypropelene, the dimensions of the micro diameters range between 0.5 and 4.0 A (Deng and Jean 1993), while in other cases the micro volumes may attain diameters of up to 8 A. For epoxy, the average pore diameter was estimated to vary between 2.7 and 5 A (Soles et al. 2000 Soles and Yee 2000). It follows that some of these micro volumes are too small to admit H2O water molecules whose kinetic diameter varies between 1.5 and 3 A. 

However, it should be borne in mind that polymers in general and epoxies in particular consist of a large family of cross-linked resins with disparate chemical properties. 

Weitsman, Fluid Effects in Polymers and Polymeric Composites, Mechanical Engineering Series, DOI 10.1007/978-1-4614-1059-1 3, Springer Science-rBusiness Media, LLC 2012 

Water molecules that diffuse between polymeric chains tend to induce swelling, thus increasing interchain spacings and enhancing their freedom of motion. This phenomenon, called plasticization is reflected in an accelerated viscoelastic response. 

While there is no direct correlation between Vf and the magnitude of the saturation weight gain M, the latter is highly affected by resin polarity. Nevertheless, for cross-linked polymers, polarity and free volume are inter-related because both are concentrated about cross-linking sites (Soles and Yee 2000). 

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