Three dimensional structure of celgard® microporous membranes

Abstract

The unique three dimensional structure of Celgard® 2500 film has been shown by direct enhanced contrast transmission electron microscopy (TEM) of thin sections from the bulk and by high resolution scanning electron imaging (SEI) of the film surface. The TEM method developed includes treatment with an unsaturated surfactant, which then reacts with osmium tetroxide to form an electron dense stain. This is followed by standard microtomy and TEM observations made possible by the stability and contrast imparted by the heavy metal. Sectioning in all three dimensions — along the machine direction, across the machine direction and across the face of the membrane — gives a total view of the bulk microstructure. Preparation for high resolution SEI imaging includes metal coating with the Ion Beam Micro-Sputter gun, to form a coherent fine grain surface to dissipate charge, provide secondary electrons for imaging, and protect the sensitive polymer from beam damage. The SEI view of the surface completes the three dimensional characterization. The microporous structure of Celgard® 2500 is formed during extrusion, annealing and stretching of isotactic polypropylene. The elliptical pores formed during this process have been shown to be oriented with their major axes parallel to the machine direction and the film surface. The pores are grouped into rows or pore channels which traverse the thickness of the film at an angle to its surface and which oscillate in and out of the plane of a cross-section taken in the machine direction. The observed structure is morphologically uniform throughout the bulk of the film. However, a thin surface layer appears to exhibit a slightly smaller pore size and a lower two dimensional pore area than does the interior. Thus, the microporous structure provides a continuous, but tortuous or staggered, row configuration necessary for high fluid transport rates.