Liquid extrusion porosimetry

In this technique a nanofiber mat is supported on a porous membrane and a layer of a wetting hquid (the contact angle between the hquid and the polymer is zero) is placed on its top surface. Gas pressure is applied over the liquid column and is gradually increased on the face of the mat until the gas is able to push the liquid through the largest of the pores in the mat, overcoming 

The volume of hquid displaced, V, can be related to the change in interfacial area at the liquid/polymer interface (Jena and Gupta 1999)  

Any blind pores (e.g., surfaee porous features on individual nanofibers. Fig. 5.3) that do not aUow flow-through of hquid cannot be assessed by the technique. The fraction of bhnd pores can therefore be indirectly estimated by comparing hquid extrusion data to the intrusion porosimetric data (which quantifies both open and bhnd pores). 

Hie pore volume distribution function Fy of a mat, ddetmined by liquid extnision (or by memuiy intiusion), is expiessed as follows (Jena and Gupta 1999)  

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. 

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Cortez Tomello, P.R., Caracciolo, P.C., Cuadrado, T.R., Abraham, G.A. Structural characterization of electrospun micro/nanofibrous scaffolds by liquid extrusion porosimetry a comparison with other techniques. Mater. Sci. Eng. C 41, 335-342 (2014). doi 10.1016/j. msec.2014.04.065... 

In mercury porosimetry the liquid is forced into the pores with the aid of high pressure. The volume of intruded liquid is plotted against the applied pressure as shown in Fig. 4. The large increase in volume at low pressure indicates the filling of the extra particle volume. The intrusion and extrusion curves do not coincide vdiich has been attributed to the shc of the pores (i.e. narrcw neck and wide-l30dy pores) and also to the entrapment of mercury in the porous network (66). The pressure at vdiich penetration occurs can )3e related to the dimension of the pore entrance by the Washburn equation... 

The high pressures used in mercury porosimetry gives erroneous results with deformable material such as fabric. A more general version of liquid porosimetry, based on the same principles, uses a variety of liquids and can be carried out in the extrusion mode [79,80]. In this technique, a pre-saturated specimen is placed on a microporous membrane supported on a rigid porous plate in an enclosed chamber. The gas pressure within the chamber is increased in steps causing the liquid to flow out of the pores. The amount of liquid removed is monitored by a top-loading recording balance. One also has the... 

Liquid intrusion/extrusion method should apply ideally to membranes containing cylindrical pores. When this is not the case, the pore size distributions obtained refer actually to equivalent or effective pore sizes. In Fig. 12 the air-liquid displacement distribution is presented along with the Hg-porosimetry one for the AOl membrane. A Ti02 /a-Al2 O3 layer on a stainless steel woven wire cloth made by Synthesechemie, whose pores are far from being cylindrical, as shown in Fig. 13, gives a different pore size distribution by air-liquid displacement and Hg-porosimetry distributions, as shown in Fig. 14. 

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