Ariya Akthakul

Noncircular pores on the surface of asymmetric polymer membranes: evidence of pore formation via spinodal demixing

Abstract To investigate the origin of noncircular pores in the selective layer of asymmetric poly(vinylidene fluoride) (PVDF), membranes cast by immersion precipitation, the mass transfer kinetics were controlled by casting membranes at different temperatures and casting solution concentrations. Interconnected surface pore morphologies were observed by scanning electron microscopy (SEM) for membranes cast at high concentration and/or temperature, supporting a mechanism of liquid–liquid demixing at the membrane surface by spinodal decomposition. The coarsening of this highly porous, interconnected morphology leads first to chain-like formations of discreet, noncircular pores and ultimately to scattered pores of circular shape and large size dispersity. Similar results for poly(sulfone) (PSF), membranes, which solidify via vitrification rather than crystallization, suggest a degree of universality in the mechanism of morphological development of asymmetric membrane surfaces. This new understanding of pore formation kinetics opens the possibility of tailoring membrane surfaces to achieve a high degree of porosity and pore size uniformity.

Surface acoustic modes in thin films on anisotropic substrates

Propagation of surface acoustic modes on the (001) and (111) surfaces of Si coated by a thin isotropic overlayer is studied theoretically and experimentally. It is shown that when a surface acoustic wave (SAW) coexists with a pseudosurface wave (PSAW) of the uncoated substrate, the second-order acoustic mode of the film/substrate system originates from a PSAW and the first-order one from a SAW. The polarization pattern of either mode varies from Rayleigh type (saggital plane polarization) to Love type (horizontal polarization) depending on the propagation direction and the product of the wave vector q and film thickness d. It is also shown that the isolated off-symmetry pure mode point within the PSAW branch disappears at some critical qd value. Experimentally, surface acoustic modes of Ti-coated Si wafers are measured with the impulsive stimulated thermal scattering (ISTS) technique based on laser generation and detection of acoustic waves at a specified wave vector. ISTS data are shown to be determined ...