Surface Morphology of the Polyurethane-Based Pervaporation Membranes Studied by Atomic Force Microscopy. I. Effect of the Polyurethane Molecular Structure

Atomic force microscopy (AFM) was employed to study the surface morphologies of the polyurethane-based pervaporation membranes and to evaluate the effect of the polyurethane (PU) molecular structure. Segmented poly(tetramethylene oxide)-based PUs varying in the length of both segments as well as in the structure of the hard segments were tested. Small spherical or extended hard domains, on the size range of tens of nm, and their large, interconnected associates for a higher hard segment volume fraction, have been observed at relatively flat surface of the diamine-based PUs. The surface of the diol-based PUs appears to be generally much coarser and shows globular or circular structures of a few micron size which are thought to be composed of the hard segment lamellae. The morphology variations observed by AFM were found to correlate well with the small-angle X-ray scattering (SAXS) data revealing the necessity of the periodic arrangement in space of the segmental level domains for the formation of a higher...