Biological membranes can achieve remarkably high permeabilities, while maintaining ideal selectivities, by relying on well-defined internal nanoscale structures in the form of membrane proteins. Here, we apply such design strategies to desalination membranes. A series of polyamide desalination membranes—which were synthesized in an industrial-scale manufacturing line and varied in processing conditions but retained similar chemical compositions—show increasing water permeability and active layer thickness with constant sodium chloride selectivity. Transmission electron microscopy measurements enabled us to determine nanoscale three-dimensional polyamide density maps and predict water permeability with zero adjustable parameters. Density fluctuations are detrimental to water transport, which makes systematic control over nanoscale polyamide inhomogeneity a key route to maximizing water permeability without sacrificing salt selectivity in desalination membranes.

No abstract

A new optical technique for characterizing the grain structure of ordered block copolymer thin films has been developed. The technique, which we refer to as guided wave depolarized light scattering (GWDLS), is an adaptation of previous work wherein polarized light was used to characterize the grain structure in bulk block copolymer samples [Newstein et al. Macromolecules 1998, 31, 64]. Spin-casting and vacuum-annealing were used to prepare thin films of an ordered poly(α-methylstyrene-block-isoprene) copolymer on a flat, fused silica substrate. The ordered phase consisted of poly(α-methylstyrene) cylinders in a polyisoprene matrix. A plane-polarized laser beam was coupled into and out of a transverse magnetic (TM) mode of the film, which acts as a planar waveguide. The angle of incidence is adjusted to optimize the coupling efficiency into a particular mode of the waveguide. The path length traveled by the guided wave between the input and output coupling points was approximately 1 cm. The polarization of some of the incident light changes due to encounters with randomly oriented, optically anisotropic grains. This results in the coupling of light into propagating transverse electric (TE) modes in the sample. We show that the TE beam intensity of annealed samples with well-developed grain structure is significantly larger than that obtained from unannealed samples with poorly developed grain structure. The GWDLS results are consistent with atomic force microscopy results obtained from the annealed and unannealed films.

We demonstrate the use of guided wave depolarized light scattering (GWDLS) with diffraction gratings for studying the properties of soft block copolymer films in the vicinity of order−disorder transitions. The order−disorder transition temperature (T O D T) of 500 nm thick poly(α-methylstyrene-block-isoprene) films is determined by this technique. Atomic force microscopy on quenched samples is used to verify our conclusion. Disordering and ordering dynamics of these films were examined in real time as the films were heated and cooled across the T O D T. Our work opens the door for using depolarized light scattering for locating transitions between optically isotropic and optically anisotropic block copolymer phases in thin films.

No abstract