Mat Porosity and Pore Size Distribution

Fractional void volume, or total interstitial porosity H, is defined in terms of the apparent density of the fiber mat, pmat (g/cm ), and bulk density of the polymer, Pp (g/cm ), of which it is made  

These simple measurements yield only an average estimate of II, but cannot be employed to assess the distribution of pore volumes in a fiber mat. Intrusion porosimetry yields this additional information. 

Various general techniques available for characterization of porous materials are illustrated in Fig. 5.1. (Some of the techniques are more appropriate for stiffer materials and may not apply to nanofibers of all organic polymers.) 

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The pore size distributions obtained by the mercury intrusion and liquid extrusion techniques are expected to be different. Mercury intrusion allows access to the pores from both sides of the mat and the entire pore volume is likely to be sampled. For pores that have large surface openings, the liquid extrusion generally tends to imderestimate the pore volumes relative to those measured by intrusion porosimetry. Liquid extrusion measurements yield the pressure needed to push the liquid past the most constricted part (or the throat ) of the pore. The pore volume of the channel is estimated based on the throat diameter. This also introduces directionality to the liquid extrusion measurement For a sample where porosity includes converging or diverging channels, the pore volumes (and pore dimensions) obtained from the liquid extrusion method will depend on the direction of the gas flow into the membrane. However, this is not expected to be a serious source of error in routinely characterizing nanofiber mats. 

The average porosity and the pore-size distribution of the nanofiber mat. 

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