Calorimetric determination of porosity and pore size distribution (PSD): Effect of heat on porosity, pore form, and PSD in water-saturated polyacrylonitrile fibers

Abstract

High-resolution low-temperature heat flow differential scanning microcalorimetry (DSC) is used to monitor the temperature lowerings of freezing and melting of water and ice in this gel under equilibrium conditions. A new DSC technique (the author's Ψ-plot) is given for determining and comparing pore size distributions in water-saturated porous materials. Equations which fit the DSC curves are often simple Gaussian distributions when the DSC curves are transposed using the Ψ plot. Changes in internal porosity and PSD which result from heating never-dried polyacrylonitrile (PAN) fibers in water at 100°C for about 2 hr are studied. The results show that this treatment causes a 49% reduction in porosity and that the Kelvin radii of the pore spaces in the heated fibers is 0.77 that of the pore radii in the unheated fibers. These results, together with the nature of the hysteresis between temperature lowerings for freezing and melting and the results of a new DSC technique to investigate pore length continuity (based upon the rate of diffusion of solute into the pore spaces), together with an electron micrograph, strongly suggest that the pores are open interconnected cylinders with a preferred longitudinal orientation along the long axis in both the heated and unheated fibers and that the effect of heat has caused all the pore radii, within the PSD studied, to be reduced by the same precise fraction.